Molecular Design of Benzodithiophene-Based Organic Photovoltaic

Review pubs.acs.org/CR

Molecular Design of Benzodithiophene-Based Organic Photovoltaic Materials Huifeng Yao,†,‡ Long Ye,†,‡ Hao Zhang,†,‡ Sunsun Li,†,‡ Shaoqing Zhang,† and Jianhui Hou*,†,‡ †

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China ABSTRACT: Advances in the design and application of highly efficient conjugated polymers and small molecules over the past years have enabled the rapid progress in the development of organic photovoltaic (OPV) technology as a promising alternative to conventional solar cells. Among the numerous OPV materials, benzodithiophene (BDT)-based polymers and small molecules have come to the fore in achieving outstanding power conversion efficiency (PCE) and breaking 10% efficiency barrier in the single junction OPV devices. Remarkably, the OPV device featured by BDT-based polymer has recently demonstrated an impressive PCE of 11.21%, indicating the great potential of this class of materials in commercial photovoltaic applications. In this review, we offered an overview of the organic photovoltaic materials based on BDT from the aspects of backbones, functional groups, alkyl chains, and device performance, trying to provide a guideline about the structure-performance relationship. We believe more exciting BDT-based photovoltaic materials and devices will be developed in the near future.

CONTENTS 1. Introduction 2. Synthesis of BDT Units 3. Design Strategies Used in BDT-Based Photovoltaic Materials 3.1. Backbone Modulation 3.1.1. D−A Combination 3.1.2. π-Bridges 3.1.3. Terpolymers 3.1.4. Regioregular Structures 3.2. Flexible Side Chains Optimization 3.2.1. Alkyl Chain Configuration 3.2.2. Alkyl Chain Substitution Positions 3.2.3. Functional Side Chains 3.3. Functional Substitutions 3.3.1. Electron-Withdrawing Substitutions 3.3.2. Electron-Donating Substitutions 3.4. Two-Dimensional Conjugated Benzodithiophene (2D-BDT) 4. BDT-Based Photovoltaic Copolymers 4.1. Copolymers Based On BDT and TT 4.2. Copolymers Based On BDT and BT 4.3. Copolymers Based On BDT and TPD/BDD 4.4. Copolymers Based On BDT and Qx 4.5. Copolymers Based On BDT and DPP/IID 4.6. Copolymers Based On BDF or DTBDT 5. BDT-Based Photovoltaic Small Molecules 5.1. Side Chain Engineering of BDT-Based Photovoltaic Small Molecules

5.2. Electron-Withdrawing Units used in the Backbone of BDT-Based Photovoltaic Small Molecules 5.3. Photovoltaic Small Molecules Based On BDT Derivatives 6. Application of BDT-Based Polymer Donors in Fullerene-Free PSCS 7. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

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1. INTRODUCTION Organic semiconductors have received considerable attention from both the academic and industrial communities due to their numerous applications, which include light-emitting diodes, field-effect transistors, photodetectors, nonvolatile memory, batteries, supercapacitors, solar cells and thermoelectric generators.1−3 In particular, organic solar cells or so-called organic photovoltaics (OPVs) have the potential for harnessing low-cost solar energy due to their advantages in materials and manufacturing processes.4 The efficiency of converting the input solar power (Pin) to electric power is defined as the power conversion efficiency (PCE) (PCE = VOC × JSC × FF/Pin), which

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Received: March 15, 2016 Published: June 2, 2016 © 2016 American Chemical Society

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DOI: 10.1021/acs.chemrev.6b00176 Chem. Rev. 2016, 116, 7397−7457

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Figure 1. Schematic of a typical OPV and the basic relationships between material properties and photovoltaic parameters.

is directly proportional to three parameters: open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF) under standard solar irradiation (AM 1.5G 100 mW/cm2). In 1986, Tang et al. demonstrated the first breakthrough in the OPV field by inventing bilayer OPV devices.5 Until 1995, Heeger and Friend demonstrated the bulk heterojunction (BHJ) concept in OPVs using polymer:fullerene and polymer:polymer blends, respectively, in which the donor/acceptor interface area was considerably enlarged and nanoscale pathways for charge generation and transport could be formed.6,7 In a typical BHJ OPV device, a heterojunction consisting of a p-type organic donor and an n-type organic acceptor is the photoactive part for converting solar light to electricity (Figure 1). Thus far, the PCEs of single, double, and triple heterojunction OPV devices using conjugated polymers as the donors and fullerene derivatives as the acceptors have surpassed 10.5%,8−13 11%,14 and 11.5%,15,16 respectively. The design, synthesis, and application of novel organic materials with superior photovoltaic properties play vital roles in improving the PCEs of these OPV devices. For a highly efficient BHJ OPV, its active layer should possess the following intrinsic features: (a) a broad absorption spectrum with a high extinction coefficient to utilize more solar photons; (b) a bicontinuous network with nanoscale phase separation to facilitate exciton diffusion and charge separation; (c) a suitable molecular energy level alignment between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels to afford a sufficient driving force for efficient charge separation with minimum energy loss; and (d) a high charge mobility to facilitate charge transport.17−25 On the basis of these requirements, the chemical structures and distinct properties of the organic molecules should be finely tuned. In 2008,26 the benzo[1,2-b:4,5-b′]dithiophene (BDT) unit was first used in the synthesis of photovoltaic polymers and became one of the most successful building blocks in the synthesis of highly efficient photovoltaic materials, which have been proven to match the above requirements well.27−29 The rigid and planar conjugated structure of BDT makes it attractive for achieving highly tunable molecular energy levels and optical band gaps as well as high hole mobilities. In recent years, hundreds of photovoltaic polymers and small molecules have been developed using BDT or its analogues as building blocks. Although there is still a lack of standard procedures for the fabrication and measurement of OPV devices,30−34 OPV devices with BDT-based materials achieved several record PCEs certified by the National Renewable Energy Laboratory (NREL) or other well-recognized institutes. As illustrated in Figure 2, the photovoltaic performance of OPV devices with BDT-based small molecules or polymers kept pace with the development in the field of OPVs. Therefore, summarizing the critical achievements in BDT materials will provide a general guideline for the

Figure 2. Advances in the PCEs of single junction OPVs based on BDT polymers (red line/red ●) and small molecules (blue line/blue ■) reported in the literature over the past few years; the black line/▲ signifies the data from the NREL Web site showing the cell efficiency records for OPVs based on all types of organic materials.

molecular design strategies of high-performance photovoltaic materials. In this review, we aim to summarize the molecular design strategies, chemical structures, and photovoltaic properties of BDT-containing conjugated polymers and small molecules developed over the past few years. First, the general synthesis routes of BDT units and polymers are presented. Second, the design strategies, including backbone modulation, side chain optimization, and functional substitutions, are summarized and discussed with several representative examples. Third, a wide range of BDT-based polymers and small molecules are introduced by category, and certain representative materials are highlighted in detail. Then we present a short review about the versatile application of BDT-based polymers, particularly the emerging fullerene-free PSCs. Lastly, after a brief summary of the developments in all aspects, several fundamental challenges and future prospects of this class of materials are proposed.

2. SYNTHESIS OF BDT UNITS In the early 1980s, BDT-based molecules had already been synthesized and their electrical conducting properties had been studied.35,36 Afterward, several organic molecules containing BDT units were developed and applied in organic field effect transistors (OFETs). For example, in 1997, Dodabalapur and coworkers reported a dimer-BDT molecule and achieved a hole mobility of 0.04 cm2 V−1 s−1 in an OFET device.37 Xu et al. improved the hole mobility of a BDT-based polymer to 0.25 cm2 V−1 s−1 in 2007, which was one of the top values for polymerbased OFETs.38 7398


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Figure 3. BDT units with different substitutions.

Scheme 1. Common Synthetic Procedures for the BDT Monomers: (i) Oxalyl Chloride; (ii) Diethylamine; (iii) n-Butyllithium then Water; (iv) Alkyne Lithium; (v) SnCl2, HCl; (vi) Pd/C, H2; (vii) Zn, NaOH, H2O; (viii) Bromoalkane, TBAB; (ix) Aromatic Lithium; (x) n-Butyllithium, Chlorotrimethylstannane or 2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane; and (xi) PdCatalyst

In 2008, Hou and Yang et al. first introduced BDT units in the design of photovoltaic polymers, and the optical and electronic properties of the BDT-based polymers could be easily tuned, suggesting that BDT was a promising unit for conjugated photovoltaic polymers.26 Over the years of investigations, a series of BDT units have been developed by incorporating different substituents in the central benzene core, such as alkyl, tri-isopropylsilylethynyl, alkoxy, alkylthio, aromatic groups, etc. (Figure 3), and different synthetic routes were developed in various studies.27,39,40 In this section, the general synthesis methods of the commonly used alkyl-, alkoxy-, and aromaticsubstituted BDT units are introduced. As depicted in Scheme 1, the intermediate of benzo[1,2-b:4,5b′]dithiophene-4,8-dione played a vital role in the synthesis of BDT units with different substitutions, which could be easily prepared in three steps. Starting with thiophene-3-carboxylic acid, thiophene-3-carbonyl chloride could be obtained using either thionyl chloride or oxalyl chloride. Then, the amino compound could be prepared through an amination reaction with diethyl amine, and the product could be purified by distillation. Subsequently, the benzo[1,2-b:4,5-b′]dithiophene4,8-dione was obtained via a reaction between the amide and n-

butyllithium in tetrahydrofuran (THF) at 0 °C followed by recrystallization from acetic acid. The yield of the first two steps was approximately 90%, and the last step yielded 75%. Alkyl-substituted BDT units could be synthesized in three steps from benzo[1,2-b:4,5-b′]dithiophene-4,8-dione. First, the dione was reacted with alkynyl lithium or alkynyl magnesium halide to form the diol compound, which could be reduced by SnCl2 to produce alkynyl-substituted BDT. Subsequently, the alkyl-substituted BDT could be obtained via catalytic hydrogenation with the assistance of Pd. When the alkynyl lithium was replaced by aromatic lithium, similar procedures were performed to prepare the aromatic-substituted BDT units. The alkoxysubstituted BDT could be synthesized by a one-pot two-step reaction. First, the dione compound was reduced to diol using zinc in a sodium hydroxide solution. Then, an excessive amount of alkyl bromide was added along with a small amount of KI (potassium iodide) and tetrabutylammonium bromide (TBAB), which was used as a catalyst. The alkoxy-substituted BDT could be easily purified by recrystallization or silica gel column chromatography at a yield of approximately 70%−90%. Stille coupling and Suzuki coupling were the two most popular reactions used to prepare the conjugated photovoltaic materials; 7399


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in the development of polymer solar cells. However, these materials typically delivered relatively low PCEs, and further improvement was limited by their poor absorption properties and fixed HOMO levels, which restricted the JSC and VOC of the solar cell devices. D−A copolymerization is one of the most successful methods for designing high-efficiency photovoltaic polymers because the optical and electronic properties can be tuned easily by controlling the intermolecular charge transfer (ICT) from the donor to the acceptor.47 The D−A alternating approach is also called push−pull method, and the electron density of the polymer always has a rearrangement from D to A. This class of polymer often demonstrates a resonance form between −D−A− and −D+−A+−, increasing the double-bond character of the single bonds in the polymer backbone, thus affecting the absorption and molecular energy properties.3 The narrowed bandgap of D−A copolymer can be easily understood using molecular orbital theory (Figure 5). The initial orbitals of

hence, BDT boronic acid ester compounds and stannanes were needed for reactions with n-butyllithium followed by the addition of chlorotrimethylstannane or 2-isopropoxy-4,4,5,5-tetramethyl1,3,2-dioxaborolane.

3. DESIGN STRATEGIES USED IN BDT-BASED PHOTOVOLTAIC MATERIALS The development of a highly efficient OPV device has always been accompanied by innovations in device architecture, finetuning of blend morphology, and application of interface engineering. In addition to the methods mentioned above, the design of new photovoltaic materials is one of the driving forces to improve the PCE to over ∼10%. Because the three parameters, VOC, JSC, and FF, of OPV devices have close relationships with each other, to obtain a high PCE, molecular design strategies should be carefully considered to achieve a simultaneous improvement in these parameters. Over the past few decades, thousands of organic conjugated materials have been developed and applied in OPVs. Subtle variations in the molecular structures of conjugated molecules lead to significant changes in the photovoltaic properties. Therefore, understanding the relationship between molecular structure and photovoltaic performance remains a challenge. In this section, we will use BDT-containing polymers as an example to summarize the molecular design strategies for polymer solar cell (PSC) applications, such as backbone modulation, side chain optimization, and functional substitutions. Additionally, creating BDT with two-dimensional side groups can be considered a feasible and efficient method to optimize the photovoltaic performance of the resulting conjugated polymers. Thus, the correlation between the molecular structure of the conjugated molecule and the photovoltaic performance of the OPV device can be determined.

Figure 5. Molecular orbital hybridization of the D−A copolymer.

the donor and acceptor experience a rearrangement during the formation of the covalent bond between the two moieties. The energy level of one of the of the newly occupied molecular orbitals is higher than the initial HOMO level, and the energy level of one of the unoccupied molecular orbitals is lower than the LUMO level before combination, which results in a narrowed bandgap. In 2003, Andersson and co-workers reported an alternating copolymer based on benzothiadiazole and fluorene, which exhibited a considerable PCE of 2.2%.48 Afterward, numerous photovoltaic polymers with D−A structures were designed and applied in polymer solar cells, and a few of the D−A copolymers achieved milestone PCEs in the development of PSCs. In 2008, a series of copolymers based on BDT unit were synthesized by copolymerizing with different acceptors.26 As depicted in Figure 6, the new BDT-containing polymers with different acceptor units provide an excellent example of the optimization of the polymer backbone via varied D−A combinations. When different acceptors, such as thiophene, benzothiadiazole (BT), thieno[3,4-b]pyrazine (TPz), etc., were used in the copolymers, the bandgaps and molecular energy levels of these BDT-based polymers were tuned effectively. The absorption edges of these polymers ranged from 600 to 1100 nm, the corresponding optical bandgaps ranged from 2.1 to 1.1 eV, and the HOMO levels could be tuned from −4.56 to −5.16 eV (Table 1). Although the PCEs of the PSC devices based on fullerene derivates (without special instructions, the PSC devices were fabricated by fullerene derivates) were very low, this study provided an effective method to modulate the bandgap and the molecular energy level of the photovoltaic polymer by selecting different D−A combinations.

3.1. Backbone Modulation

The conjugated backbone (Figure 4) of the copolymer determines the fundamental properties of the resulting polymer,

Figure 4. Typical molecular structure of a photovoltaic polymer.

such as the absorption spectra and the molecular energy levels. Over the past decade, photovoltaic polymers based on different donor−acceptor (D−A) combinations have been developed to realize well-balanced broad absorption spectra and appropriate energy levels. In addition, π-bridges, such as thiophene, furan, thieno[3,2-b]thiophene, etc., have been widely used to further optimize the backbone conformation for better photovoltaic performance, and extended π systems were also designed for the same purpose. Recently, terpolymers (i.e., ternary component polymers) with three components were developed by researchers for a broad absorption spectrum and a higher photovoltaic efficiency.41 Furthermore, the regioregularity of the D−A polymers with asymmetric units played important roles in effecting their photovoltaic performance.42−44 3.1.1. D−A Combination. In early studies on photovoltaic polymers, researchers focused their attention on homopolymers, such as MEH-PPV45 and P3HT,46 which played significant roles 7400


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Figure 6. D−A copolymers P1−P8 based on BDT and varied acceptor units.

Table 1. Summary of the Optical and Electronic Properties as well as the Device Performances of P1−P8 polymer

Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P1 P2 P3 P4 P5 P6 P7 P8

2.13 2.03 2.06 1.97 1.63 1.70 1.05 1.52

−5.16 −5.07 −5.05 −4.56 −4.78 −5.10 −4.65 −4.88

− 0.56 0.75 0.37 0.60 0.68 0.22 0.55

− 1.16 3.78 2.46 1.54 2.97 1.41 1.05

− 38 56 40 26 44 35 32

− 0.25 1.60 0.36 0.23 0.90 0.11 0.18

26 26 26 26 26 26 26 26

Figure 7. Molecular structures of P9−P14.

Table 2. Optical, Electronic, and Photovoltaic Performances of P9−P14 polymer

Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P9 P10 P11 P12 P13 P14

1.6 1.75 1.8 1.8 1.73 1.44

−5.15 −5.45 −5.56 −5.23 −5.35 −5.30

0.74 0.92 0.85 0.86 0.90 0.73

14.5 14.5 9.81 10.68 13.52 14.0

68.97 64 66 72.27 70.07 65

7.4 9.4 5.5 6.67 8.55 6.6

55 70 73 76 82 86

ester-substituted TT unit and used it as an acceptor building block to copolymerize with the BDT units, thus preparing a series of copolymers based on BDT and TT.53,54 TT units with fluorine atom modification have become popular units in highperformance polymers. P9 (known as PTB7) was one of the most studied donor materials developed in 2010, and numerous impressive device results were achieved using this polymer. P9 has a strong absorption from 550 to 750 nm, and the cyclic voltammetry (CV) measurement indicated that the introduction of the fluorine atom in PTB7 decreased its HOMO and LUMO to −5.15 and −3.31 eV, respectively.55,56 Using [6,6]-phenyl C61

Combined with various acceptors, the BDT-containing polymers with suitable absorption spectra and molecular energy levels possess the potential to obtain high-efficiency PSC devices, and many impressive PCEs have been achieved by several research groups. Figure 7 summarizes the copolymers based on BDT units and some acceptor units, and their basic optical and electronic properties as well as photovoltaic performances are listed in Table 2. Thieno[3,4-b]thiophene (TT) units are known for their stabilized quinoidal structures, which have wide applications in designing low bandgap polymers.49−52 Yu et al. developed the 7401


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DOT:PSS/P11:PC71BM/LiF/Al was used to investigate the photovoltaic performance of P11, a VOC of 0.85 V, a JSC of 9.81 mA/cm2, and a FF of 66% were obtained with an active area of 1.0 cm2, resulting in a PCE of 5.5%. Driven by the further improvement in the PCE, Beaujuge and co-workers carefully optimized the alkyl chains on BDT and TPD due to their considerable effect on photovoltaic properties.75 Lastly, the PBDT-TPD polymer with a branched 2-ethylhexyl on BDT and linear heptyl on TPD provided the best PCE of 8.5%, with a VOC of 0.97 V, a JSC of 12.6 mA/cm2, and an FF of 70%. The P12 copolymer, which is based on BDT and benzo-[1,2c:4,5-c′]dithiophene-4,8-dione (BDD), was developed by Hou and co-workers in 2012.76 Two clear absorption peaks could be observed in the P12 film. One of the peaks, located at 622 nm, could be ascribed to the interchain π−π* transition of the molecular π−π stacking. Furthermore, the strong π−π stacking absorption peak was observed in the solution state at ambient temperature, and the absorption intensity decreased when the temperature increased, suggesting that strong aggregation occurred in the solution at low temperatures, which was proven to have a significant influence on the photovoltaic performance of the corresponding PSC device. Using PC61BM as an acceptor material, an optimal PCE of 6.67% with a high FF of 72% was achieved by manipulating the processing temperature. The photovoltaic efficiency of the PBDT-BDD was further improved to 8.75% when a novel cathode interface layer of a-ZrAcac was used in the device.77 Due to its two strong electronegative nitrogen atoms, quinoxaline (Qx) is a typical electron-deficient unit, and several conjugated D−A copolymers based on Qx or its derivatives were developed and applied in PSC devices.78−80 Because there are two fused six-member rings in Qx, the Qx-containing D−A polymers should have less quinoid properties.81 The absorption edge of P13 was located at 716 nm, and the polymer had a deep HOMO level of −5.35 eV.82 When this polymer was used to fabricate the PSC devices, a PCE of 8.55% with a VOC of 0.90 V, a JSC of 13.52 mA/cm2, and an FF of 70.02% was achieved. Because PBDT-Qx had good solubility in several common solvents, when a green solvent known as anisole was used as a processing solvent to fabricate the cell device, the PSC device yielded an excellent PCE of 8.37%, thus demonstrating its potential in large-scale productions.83 Diketopyrrolopyrrole (DPP) was first developed by Farnum and co-workers with a diphenyl-DPP structure in 1974.84 Afterward, the DPP units were widely used to construct high mobility organic molecules for OFETs. In recent years, the DPP units were developed to construct photovoltaic polymers due to their strong electron-deficient properties. Copolymers based on BDT and DPP always have narrow optical bandgaps of approximately 1.45 eV and HOMO levels at −5.29 eV.85 Hou and co-workers synthesized P14 with different alkyl side chain on DPP and achieved similar results.85 In Yang’ group, a better PCE of 6.6% with a VOC of 0.73 V, a JSC of 14.0 mA/cm2, and an FF of 65% was recorded for P14-based PSC device.86 Because of their broad absorption spectra, the PBDT-DPP polymers were good candidates for tandem solar cells. Using P3HT/ICBA as the front cell and PBDT-DPP/PC71BM as the rear cell, an inverted tandem polymer solar cell was fabricated by Yang and coworkers, which achieved a high PCE of 8.6% with a VOC of 1.56 V.87 Due to the development of D−A copolymers, the PCE of the PSCs have been improved to over 10%. The D−A structure has become a standard model for designing photovoltaic polymers,

butyric acid methyl ester (PC61BM) or [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as the acceptor material in BHJs, they fabricated conventional solar cell devices with an ITO/ PEDOT:PSS/P9:PCBM/Ca/Al structure to investigate their photovoltaic properties, and either dichlorobenzene (DCB)/ diiodooctane (DIO) or chlorobenzene (CB)/DIO was used as processing solvents comparatively for a better BHJ morphology. After optimization of the devices, the device using PC71BM as an acceptor material and CB/DIO as a processing solvent yielded an impressive PCE of 7.4% with a VOC of 0.74 V, a JSC of 14.50 mA/ cm2, and an FF of 68.97%. Impressively, Cao and co-workers improved the PCE of the P9-based device to 9.2% by applying an inverted device structure and new interface material.57 In addition to P9, the combination of BDT and TT units yielded several high-performance copolymers, such as PBDTTT-C-T,58 PBDT-TS1,59 PTB7-Th,60 etc. For instance, when the alkoxyl side chains on BDT were replaced by the 2D conjugated thienyl side chains, the resulting polymer PTB7-Th showed very impressive photovoltaic performance and became one of the most studied polymers. The achievements of these polymers are highlighted by a recent review.61 2,1,3-Benzothiadiazole (BT) is one of the strongest acceptor units used in D−A copolymers, and several conjugated polymers have been developed by combining it with different donor units, such as carbazole,62−64 2,7-silafluorene,65 cyclopenta[2,1-b;3,4b′]dithiophene,66 dithdithienosilole,67 and indacenodithiophene.68 In addition to the polymers mentioned above, copolymers based on BDT and BT units were commonly studied due to their outstanding photovoltaic performances.69 For example, Jones et al. reported the P10 polymer in 2014, and the influence of the molecular weight on the photovoltaic performance was studied.70 A UV−vis measurement indicated that P10 possessed an absorption edge of approximately 710 nm, thus indicating an optical bandgap of 1.75 eV; furthermore, its HOMO and LUMO levels were −5.45 and −3.65 eV, respectively, which were measured using CV measurement. Inverted PSC device architectures using ZnO as an interface layer were first fabricated to investigate their photovoltaic performance, and a high PCE of 8.5% was achieved with a VOC of 0.92 V, a JSC of 14.5 mA/cm2, and an FF of 64% when the number-average molecular weight of P10 increased to 112 kDa. Then, the ZnO layer was modified by fullerene, thus resulting in a higher PCE of 9.4%. Because of its excellent photovoltaic properties, studies on the PBDT-BT family of copolymers can provide insight into the rules for designing high-efficiency photovoltaic polymers. A thienopyrroledione (TPD) unit possesses a simple symmetric planar structure with an alkyl-substituted imide fused on thiophene. The imide provides the unit with a relatively strong electron-withdrawing property, and the solubilities of polymers based on TPD can be tuned by introducing different alkyl chains on the pyrrole ring. In 2010, n-alkylthieno[3,4c]pyrrole-4,6-dione (TPD) was simultaneously developed and applied as a building block for organic photovoltaic materials by a few research groups.71−74 They synthesized the copolymers using BDT as the donor units, which yielded good PCEs of approximately 4%−7%. For example, Leclerc and co-workers synthesized the P11 polymer with branched 2-ethylhexyl on BDT and linear octyl on TPD, which possessed an absorption edge of 685 nm (optical bandgap 1.8 eV), and the HOMO and LUMO were estimated to be −5.56 and −3.75 eV, respectively, which were measured by the CV.73 The number-average weight was measured as 13 kDa using high-temperature size-exclusion chromatography. When a device structure of ITO/PE7402


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Figure 8. Modulation of molecular conformation via fused or linked thiophene units.


Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P15 P16 P17 P18 P19 P20 P21 P22 P23

1.61 1.59 1.59 1.67 1.72 1.70 1.79 1.73 1.69

−5.29 −5.21 −5.04 −5.22 −5.24 −5.29 −5.27 −5.09 −5.40

0.81 0.73 0.69 0.76 0.79 0.72 0.96 0.80 0.79

12.27 16.63 16.35 5.32 13.56 10.12 9.10 11.83 11.9

61.98 64.13 66.3 42.31 69.10 60.0 51.8 66.6 50

5.93 7.79 7.48 1.71 7.4 4.37 4.9 6.3 4.7

88 88 89 90 90 91 92 93 92

and their basic optical and electronic properties as well as photovoltaic performances are listed in Table 3. Considering the connecting angle between the conjugated units, the backbones of conjugated polymers are usually one of two types, zigzagged or straight; these structures have considerable influences on the crystalline properties of the polymers. For instance, Hou et al. synthesized the conjugated polymer P15, which had a zigzagged backbone with an included angle of 36°.88 The grazing incident X-ray diffraction (GIXRD) measurement indicated a weak (010) reflection peak at 1.62 Å−1, which corresponded to a π−π stacking distance of 3.88 Å. Then, they modulated the zigzagged-backbone conformation of the polymer to a straight linear molecular structure via fused thiophene units near the BDT, and the new P16 polymer indicated a stronger and closer π−π stacking compared to P15. The impact of the molecular conformation on the interchain aggregation could be interpreted by the varied packing modes existing in different conformations. In the zigzagged-backbone type, the ordered interchain packing only formed under the favorable mode, whereas the straight linear molecule had only one general packing mode. Because of the optimized molecular conformation, the PSC devices based on P16 resulted in a high PCE of 7.79% without using any additives whereas the P15 had a relatively low PCE of 5.93% after the optimization of DIO. Incorporating π-bridges in the backbone of the conjugated polymer via a single bond is another simple method used to tune

and several novel donor and acceptor units have been developed as building blocks to construct D−A polymers. Although certain D−A copolymers already have excellent properties, many of them need further optimization. Therefore, to enhance the photovoltaic performance of D−A copolymers, π-bridges, terpolymers and regioregular structures were developed to optimize the backbone of the polymers. 3.1.2. π-Bridges. Generally, π-bridges are chromophores with small sizes, which are widely used to modulate the backbones of conjugated molecules. Inserting a π-bridge between the donor and acceptor units will affect the conformation of the conjugated molecule chain, thus resulting in a change in the optical absorption, molecular energy levels, hole mobility, as well as the blend morphology with the acceptor material, which are all related to the photovoltaic performance of PSC devices. Therefore, it is important to optimize the polymer conformation by rationally selecting the π-bridge (e.g., thiophene, furan, selenophene, benzene, or thieno[3,2-b]thiophene). Besides the π-bridge units linked by single bonds, a few extended fused πsystems, such as dithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT), which has two fused thiophene units in addition to the BDT, were also developed to tune the backbone of the polymer. Numerous outstanding photovoltaic polymers were designed via the structural optimization of πbridges in different D−A combinations. Figure 8 summarizes some copolymers based on BDT units and some acceptor units, 7403


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Figure 9. Molecular structures of P24−P35 with varied π-bridges.

than P18 without the thiophene unit. The hole mobility of the two polymer blends with PC71BM were measured using the SCLC method, and the result suggested that the mobility of the P19 blend was 4 orders of magnitude higher than that of the P18, which resulted in the fast recombination of the carriers existing in the P18-based device. Li et al. introduced the thieno[3,2b]thiophene as the bridge in the backbone of PBDT-BT, and a high hole mobility of 0.38 × 10−2 cm2 V−1 s−1 was obtained from the OFET device fabricated using the new P20 polymer.91 The different molecular backbone conformations of the polymers always affect the morphologies of the blends when mixed with the acceptors. As previously discussed, in comparison with the P21 polymer with the zigzagged backbone conformation, P23, which had a straight linear structure, had a different morphology when blended with PC71BM.92 The AFM and TEM images indicated that a large and regular molecular aggregation of the P23 was formed in the blend because the polymer had a straight linear backbone conformation. Heteroatom replacement is another effective strategy commonly used in designing highly efficient photovoltaic polymers, and the subtle modification of the polymer backbone using this method could often cause significant changes in the optical and electronic properties for the resulting material. As a homologue of thiophene, selenophene is often used to optimize the optical and electronic properties of the conjugated polymers. The relatively low aromaticity of selenophene will enhance its ground state quinoid resonance character; thus, Se-containing polymers generally have broad absorption spectra. The absorption edge of the P22 polymer extended to 732 nm compared to 692 nm for P21.93 Hence, the absorption properties, hole mobility, and morphology of the

the molecular conformation. Compared with P15, the new P17 polymer with thiophene bridges also possessed a straight linear conformation, and the π−π stacking distance of P17 was measured to be 3.51 Å, which was one of the smallest values for conjugated polymers.89 The devices based on P17 achieved a high PCE of 7.81%. Both the interchain packing of the conjugated polymer as well as the absorption properties and molecular energy levels will be affected by the change in the backbone conformation. As observed in the two examples mentioned above, the P16 and P17 polymers, with closer molecular stacking distances, had red-shifted absorption spectra and upshifted HOMO levels. Therefore, a comprehensive investigation should be performed when designing a novel photovoltaic polymer by inserting π-bridges. Furthermore, the conformation of the molecular backbone has a considerable influence on the hole mobility for conjugated polymers, which affects the transport of the carriers. For instance, although P18 and P19 have similar HOMO levels and absorption ranges, the PSC devices based on them indicated significant differences in the photovoltaic performance.90 A high PCE of 7.4% was obtained for the P19-based PSC device, whereas a low PCE of 1.77% was achieved for the P18-based PSC device. The EQE of the P19-based device was as high as 71%, whereas P18 had an extremely low EQE of 29%, suggesting a large difference in the generation and transport of the charge carriers. The introduction of thiophene relieved the direct electron communication between the BDT and TT units, which caused a change in the absorption coefficient and hole mobility of the polymer. As observed by the UV−vis and the using density functional theory (DFT) calculation, the P19 polymer with the thiophene π-bridge possessed a higher absorption coefficient 7404


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Table 4. Optical, Electronic, and Photovoltaic Performance of P24−P35 polymer

Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P24 P25 P26 P27 P28 P29 P30 P31 P32 P33 P34 P35

1.46 1.49 1.43 1.51 1.38 1.70 1.77 1.71 1.88 1.84 1.82 1.89

−5.30 − −5.05 −5.26 −5.25 −5.47 −6.00 −5.90 −5.56 −5.39 −5.47 −5.42

0.73 0.63 0.63 0.77 0.69 0.78 0.84 0.82 0.87 0.81 0.88 0.83

13.7 13.9 16.25 10.9 16.8 3.73 9.18 10.87 7.2 6.11 9.30 10.60

65 55 6.18 56 62 52 67 59 58 67 68 65

6.5 4.8 6.18 4.7 7.2 1.52 5.30 5.29 3.6 3.30 5.55 5.77

96 97 85 96 96 101 102 102 103 104 104 104

Figure 10. Molecular structures of the terpolymers P36−P43.

et al. inserted a furan unit into the backbone of the DPPcontaining polymer PDPP2FT, which demonstrated improved solubility when compared with the PDPP3T polymer containing the thiophene unit.98 Then, Ma and co-workers developed conjugated polymers based on a 2D-BDT and furan-DPP unit, and the new P27 polymer depicted a blueshifted absorption spectrum and a downshifted HOMO level when compared with its thiophene equivalent.99 Then they synthesized a series of furan-bridged polymers to further investigate the influence of furan bridge on the photovoltaic performance of polymers by incorporating BT, TPD, and DPP as acceptor units, and PCE around 5% were obtained in the PSC devices.100 Replacing thiophene with selenophene in the conjugated backbone always results in a broad absorption spectra and improved charge transport properties for conjugated polymers. The P28

conjugated photovoltaic polymers could be tuned effectively by selecting appropriate π-bridges. In addition to the examples mentioned above, π-bridges such as thiophene, furan, selenophene, and thieno[3,2-b]thiophene are widely used to tune the photovoltaic performances of conjugated polymers with varied D−A structures, such as PBDTDPP, PBDT-Qx, and PBDT-TPD (Figure 9 and Table 4). Thiophene is one of the most common π-bridges used in DPPcontaining polymers, and PDPP3T achieved a PCE of 6.71% after the optimization of the blend morphology.94 The polymers based on 2D-BDT and DPP using thiophene as the π-bridge were developed by several research groups, and PCEs of approximately 3−7% were achieved.85,86,95,96 Additionally, bithiophene and thienothiophene were developed as π-bridges in the backbone of PBDT-DPP copolymers.85,97 In 2010, Frechet 7405


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x

Egopt (eV)

HOMO (eV)

VOC (eV)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P36 P37 P38 P39 P40 P41 P42 P43

0.5 0.25 0.2 0.5 0.25 1 0.25 0.025

1.60 1.52 1.8 1.34 1.40 1.61 1.57 1.66

−5.20 −5.47 −5.39 −5.32 −5.35 −5.28 −5.08 −5.25

0.71 0.78 0.78 0.72 0.74 0.79 0.66 0.72

6.41 14.13 12.20 14.7 13.99 16.74 13.4 16.16

51.7 48 68 56 54 63 63.5 65

2.35 5.29 6.46 6.0 5.61 8.36 5.61 7.52

105 106 107 108 109 110 111 112

copolymer based on selenium-substituted DPP and BDT was designed and synthesized by Yang and co-workers, and the new polymer showed an absorption spectrum that is red-shifted by nearly 50 nm compared with P24.96 Furthermore, the HOMO level of P28 increased to −5.25 from −5.30 eV for P24. Due to the P28 copolymer’s advantage of a broad absorption spectrum, the PSC device resulted in a high JSC of 16.8 mA/cm2, whereas P24 had a JSC of 13.7 mA/cm2. Benzene-DPP was developed at an early stage; when the benzene unit was used in the backbone of the conjugated polymer P29, its absorption spectrum was blueshifted compared with its thiophene equivalent.101 When thienothiophene was used as the π-bridge in the backbone of PBDT-DPP, the P26 polymer had a red-shifted absorption spectrum compared to P24 for its extended π conjugation.85 Furthermore, thiophene and thienothiophene were used as πbridges to optimize the structures of the copolymers based on BDT and Qx, and the same trends in the absorption spectra and molecular levels were observed (i.e., the more planar and linear molecular structure of the thienothiophene-containing polymer possessed a narrowed optical bandgap but increased HOMO level compared to the thiophene-containing polymer).102 After the reported results of the conjugated polymer PBDT-TPD, Leclerc and co-workers used thiophene as π-bridges to further optimize the physical-chemical properties of the polymers, and the influence of alkyl chains on the π-bridge was also studied.103 However, due to the poor morphology of the new polymers, they did not achieve a good photovoltaic performance. In addition to the thiophene unit, bithiophene and thienothiophene were also used as π-bridges in the backbones of the PBDT-TPD polymers. Yang and co-workers designed and synthesized a series of P33− P35 copolymers using these different π-bridges, and the PSC devices based on these polymers achieved varied PCEs of 3%− 8%, suggesting the considerable influence of π-bridges on the photovoltaic properties for the conjugated polymers.104 Because π-bridges have a significant effect on the properties of the photovoltaic polymers, a comprehensive investigation should be performed when selecting one suitable π-bridge for its unique characteristics. The optimization of the backbone of the conjugated polymers through π-bridges will yield more highly efficient photovoltaic polymers. 3.1.3. Terpolymers. Along with the great success of the D− A alternating copolymers, a large variety of electron-rich and electron-deficient building blocks were developed to construct the conjugated polymers. These building blocks have distinctive features. For example, certain donors/acceptors possess chromophores with the absorption of photons in the long wavelength region, whereas certain units have deep HOMO levels and the characteristics of these units allow the resulting copolymers to be unique in different applications. The D−A alternating copolymers have achieved considerable success in designing highly efficient photovoltaic polymers by utilizing one

donor and one acceptor combination. It is interesting and promising to construct copolymers with three or more building blocks. Terpolymers with three units in the backbones of the conjugated polymers are good examples to further utilize the properties of the varied donors/acceptors.41 Because the terpolymers have complicated molecular structures, a comprehensive consideration should be performed on the parameters when designing photovoltaic terpolymers, such as solubility, absorption spectra, molecular energy levels, charge mobility, and blend morphology, etc. Figure 10 summarizes the terpolymers based on BDT units and some acceptor units, and their basic optical and electronic properties as well as photovoltaic performances are listed in Table 5. Ternary copolymerization is an easy and effective method used to tune the optical and electronic properties of the conjugated copolymers. In 2010, Wei et al. reported a D−A copolymer based on pyridopyrazine (PP) and BDT, which exhibited an absorption band at approximately 550−750 nm.105 When the third component, thiophene (x = 0.5), was introduced into the conjugated backbone of PP-BDT, an additional absorption band at approximately 350−600 nm was observed for P36 and the absorption edge had a slight redshift. Concurrently, the HOMO level of the terpolymer was upshifted compared to that of the D− A copolymer. Similarly, thiophene units were used as the third component in the molecular backbones of the D−A polymers PBDT-DPP and PBDT-TPD. P37 was developed by Tan and coworkers, and the influence of the ratios of the three components on the properties of the terpolymers was investigated.106 In their study, the authors used a long conjugated side group, dithienylbenzodiathiazole, to modify the thiophene unit, which had a strong absorption at 300−600 nm. When the DPP content decreased from 75% to 50% or 25%, the absorption band belonging to DPP decreased gradually, whereas the intensity of the short wavelength absorption peak increased. Furthermore, the HOMO levels of the copolymers deceased to −5.47 from −5.29 eV. The PSC device resulted in a best PCE of 5.29% (VOC = 0.78 V, JSC = 14.47 mA/cm2, and FF = 0.48) for the balanced optical and electrical properties (x = 0.25). Furthermore, Tajima and co-workers reported a terpolymer P38 based on BDT, TPD, and thiophene, and the thiophene unit had a tris(thienylenevinylene) conjugated side chain.107 Compared with the D−A copolymer PBDT-TPD, the introduction of the third component leads to a 25% improvement in the PCE for the P38based PSC device. By taking advantage of the complementary absorption properties of the two different components, a full visible light absorption could be easily realized. For example, the primary absorption peak of the PBDT-BT polymers were always located at 400−600 nm, whereas the PBDT-DPP polymers had absorption bands at 600−900 nm. Due to the complementary absorption properties of the DPP and DTBO, the terpolymer P39 exploited in Wei’s group indicated a full visible light 7406


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P44 P45 P46 P47 P48 P49 P50 P51 P52

1.59 1.55 1.51 1.51 − 1.54 1.46 1.59 1.58

−5.28 −5.28 −5.30 −5.30 − −4.64 −4.63 −5.20 −5.20

0.75 0.76 0.80 0.80 − 0.60 0.60 0.89 0.90

14.39 15.68 17.72 17.99 − 17.77 15.21 7.25 10.82

61.53 64.97 68.57 70.55 − 63.13 48.58 60.56 60.81

6.60 7.79 9.74 10.20 − 6.70 4.45 3.91 5.92

43 43 44 44 116 117 117 42 42

Figure 11. Molecular structures of the random and regioregular polymers P44−P52.

absorption.108 When P39 was used to fabricate the PSC device, a high JSC value of 17 mA/cm2 was achieved. As the number of copolymerization monomers increased to three, the molecular structures of the terpolymers became more complex than that of the D−A copolymers. The ratio of the three monomers plays a vital role in determining the properties of the final terpolymers. A series of terpolymers, PBDT-DPP-TPD, with varied ratios (x = 1, 0.9, 0.75, 0.5, 0.25, 0.1, and 0) of monomers were synthesized by Kim and co-workers, and the influence of the composition of the random terpolymer on the optical and electronic properties was studied systematically.109 When the content of the TPD increased from 0 to 100%, the band gaps in the terpolymers gradually became larger, and the HOMO levels simultaneously became deeper. When these terpolymers were used to fabricate the PSCs, the VOC changed from 0.73 to 0.97 V, whereas the JSC varied from 6.09 to 13.99 mA/cm2, and an optimal PCE of 5.61% was achieved for the P40 terpolymer with the optimal

composition (x = 0.75). Except for the optical and electronic properties of the polymers, the morphology and charge mobility could be tuned using ternary copolymerization. Cao et al. reported a series of terpolymers based on BDT and two types of TT (with or without fluorination) unit, and they determined that the size of the phase separation decreased as the fluorine content increased.110 Furthermore, the increasing fluorine content enhanced the hole mobility of the terpolymers from 9.27 × 10−6 to 2.75 × 10−5 cm2 V−1 s−1. Additionally, Choi and coworkers improved the charge mobility of P42 using ternary copolymerization.111 Pyrene was also used as the third component of the terpolymers due to its strong π−π interactions.112,113 For example, Park et al. investigated the pyrene-driven self-assembly behavior in the conjugated polymers, and the results suggested that the pyrene−pyrene interactions were beneficial for the formation of fibrous structures.112 As the pyrene content increased to 2.5%, the 7407


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P53 P54 P55 P56 P57 P58 P59 P60 P61 P62 P63 P64 P65 P66 P67 P68 P69 P70 P71 P72 P73 P74 P75 P76

1.58 1.59 1.62 1.61 1.60 1.59 1.61 − − 1.75 1.70 − − − − 1.85 1.80 1.82 1.64 1.68 1.75 − 1.58 1.58

−4.90 −4.94 −5.01 −5.01 −5.29 −5.29 −5.29 − − −5.48 −5.57 − − − − −5.61 −5.47 −5.56 −5.09 −5.11 −5.31 −5.06 −5.09 −5.24

0.58 0.60 0.68 0.62 0.81 0.81 0.81 0.96 0.97 0.87 0.81 0.92 0.90 0.97 0.94 1.00 0.92 0.95 0.67 0.75 0.90 0.61 0.66 0.75

12.5 12.8 10.3 7.74 17.55 16.52 16.22 11.1 12.6 8.1 9.7 6.8 9.1 3.6 6.3 9.79 8.92 8.94 9.63 11.82 10.81 14.88 13.53 12.42

65.4 66.3 43.1 47.0 67.2 62.3 56 62 70 56 67 51 42 48 65 63.0 55.0 53.2 54 65 53 57 58.55 47.78

4.76 5.10 3.02 2.26 9.52 8.37 7.36 6.6 8.3 3.9 5.4 3.2 3.4 1.7 3.8 6.17 4.51 4.50 3.48 5.76 5.16 5.17 5.23 4.45

54 54 54 54 122 122 122 75 75 71 71 75 75 125 125 126 126 126 127 127 127 128 129 130

7.79% was obtained for the regioregular P45-based PSC device, which was 19% higher than that of the random P44-based PSC device. Similar results were obtained by Hou and co-workers when they synthesized the regioregular PBDT-TS1 (P47), which achieved an improved photovoltaic performance of 10.2%.44 Jen and co-workers used PTB7-Th (P48) as an example to study the components of the random parts. As depicted in Figure 11, two major isomers (part A and part B) existed in the random P48, and the content of the dominant one (part B) accounted for 64% (x = 0.64).116 Because the 4- and 6-positions of the fluorinated TT unit have different chemical reaction activities, the two isomers were prepared by controlling the synthesis procedures, and they indicated distinct absorption properties, packing orders, and charge mobilities. Zhu et al. reported two copolymers, P49 and P50, that achieved PCEs of 6.70% and 4.45%, respectively, in the PSC devices.117 Watkins and co-workers synthesized the regioregular copolymer P52 based on BDT and monofluorinated BT.42 Due to its improved intermolecular π-stacking interactions and enhanced charge carrier mobility, the PCE of the PSC device based on P52 improved to 5.92% from 3.91% for the random PBDT-BT (P51). All of these reported results indicate that the molecular configuration should be carefully considered due to its significant effects on the photovoltaic properties. The backbone is one of the parts of conjugated photovoltaic polymers that determines the fundamental properties of the photovoltaic materials. Over the past few decades, several excellent D−A photovoltaic copolymers have been developed, and each one has its own unique characteristics. Additionally, different π-bridges, such as thiophene, furan, selenophene, benzene, and thieno[3,2-b]thiophene, were successfully used to further optimize the properties of the conjugated polymers. Terpolymers are an emerging feasible and facile method used to tune the backbone of the photovoltaic polymers. Furthermore, recent studies have demonstrated that the fine structure, such as the regioregularity of the backbone of the conjugated polymer,

PCE of the P43-based PSC device increased to 7.52% from 5.03%. The optimization of the backbones of the conjugated photovoltaic polymers by ternary copolymerization has advantages, such as easy preparation and comprehensive modulation of the properties of the resulting polymers. However, the fundamental understanding of the structure−property-performance relationship for the terpolymers is still in its early stages. Therefore, a rational selection of the three components and the detailed characterization of the molecular structure with multiple components should be considered in the near future. 3.1.4. Regioregular Structures. When asymmetric building blocks exist in the conjugated backbones, the relationship between the molecular structure and the device performance becomes more complicated. Early studies on the P3HT demonstrated that a regioregular structure was beneficial for improving the absorption and charge transport properties of the P3HT.114,115 Therefore, a high PCE of 4.4% was recorded for the PSC devices based on P3HT with a regioregularity of 95.2%, whereas the PCE was only 1.8% for the P3HT with a low regioregularity (90.7%). In addition to 3-hexyl-thiophene, several other asymmetric blocks, such as TT and monofluorinated benzothiadiazole, were developed to construct photovoltaic materials, and the influence of the regioregularity on the photovoltaic performance has been investigated in recent years. Basic optical and electronic properties as well as photovoltaic performances for P44-P56 are listed in Table 6. Although PBDT-TT photovoltaic polymers have been studied since 2009, there have been few studies addressing the regioregularity of this type of polymer. Recently, Lee and coworkers synthesized the regioregular PBDTTT-C-T (P45), which exhibited a red-shifted absorption spectrum and a higher degree of crystallinity than its random counterpart P44.43 The hole mobility of the regioregular polymer was one order magnitude higher than that of the random P44. A high PCE of 7408


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Figure 12. Molecular structures of P53−P76 with different flexible side chains.

depicted in Figure 12, P53−P56 are four copolymers that contain different alkyl side chains used in their research. These polymers had similar optical bandgaps of approximately 1.60 eV, and the HOMO levels changed from −4.90 to −5.01 eV. The PSC devices based on these polymers achieved a different VOC from 0.58 to 0.68 V. Chen et al. studied the π−π distances of the conjugated polymers using grazing incident wide-angle X-ray scattering (GIWAXS) and corrected the results with the difference in their molecular structures.121 P53 and P55 indicates the largest π−π stacking distance difference (i.e., 3.65 Å for P53 and 3.89 Å for P55). The large π−π distance of P55 may be attributed to the branched side chains on BDT because the branched side chains would occupy a larger space than the linear side chains. Although P53 and P54 possessed different side chains on the TT unit, where P53 had a linear alkyl side chain whereas P54 had a branched side chain, the π−π distances of P53 and P54 were nearly identical. Furthermore, they suggested that there was a positive correlation between the strength of the π−π interactions of the PBDT-TT polymers and the fill factor for the PSC device. Additionally, similar results were determined by Hou and co-workers when they introduced three types of side chains (i.e., octyl, 2-ethylhexyl, and 3,7-dimethyloctyl) on alkylthiothiophene-substituted BDT and synthesized three copolymers, P57−P59.122 The three alkyl side chains had minimal influence on the absorption spectra and the molecular energy levels of the polymers, whereas P57, which had a linear alkylthio side chain (octyl), demonstrated the strongest and tightest π−π stacking compared to the other polymers due to its reduced steric hindrance. The P57-based PSC device demonstrated the highest JSC value of 17.55 mA/cm2 and an FF of 67.2%, resulting in a high PCE of 9.52%.

played a crucial role in affecting the properties of the resulting polymers, which should be considered when optimizing the backbone in the near future. 3.2. Flexible Side Chains Optimization

Selecting a suitable flexible side chain for the conjugated organic molecule with a definite conjugated backbone is important due to its role in determining the intermolecular interactions between the polymer molecules and the polymer−fullerene, thus affecting their solubility, π-electron transport, and miscibility with fullerenes. Therefore, alkyl chain engineering parameters, such as length, shape (linear or branched), positions, and terminal groups, should be thoroughly determined to obtain a highperformance organic photovoltaic material. Over the past few decades, several types of flexible side chains have been used in designing conjugated polymers for OFET and OPV with the desired properties.118−120 3.2.1. Alkyl Chain Configuration. Solution processability is one of the advantages of organic solar cells, which can be achieved by modulating the flexible side chains of the welldefined conjugated backbones. Long and branched side chains will improve the solubility of the polymers in common organic solvents by reducing the π−π interchain interactions, which have a negative effect on the carrier transport. Thus, moderate length and bulkiness are required to balance the solubility and the intermolecular interactions of the conjugated polymers. Basic optical and electronic properties as well as photovoltaic performances are listed in Table 7. In 2009, after the reported results of P53 (PTB1), Yu and coworkers synthesized a series of copolymers based on BDT and TT by changing the side chains on the BDT and TT units.54 As 7409


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intermolecular π−π stacking. The optical and electronic properties of the conjugated polymers are highly dependent on the π-electron delocalization of the polymer, which can be affected by the different steric hindrance effects or the rotational angles caused by the alkyl chains at varied substitution positions. As depicted in Figure 12, Hou and co-workers designed and synthesized the P71−P73 copolymers with alkyl side chains at varied positions.127 The total number of carbon atoms in the side chains was kept the same to guarantee similar solubility in their study. A theoretical calculation revealed that when the alkyl chains occupied different positions of the thiophene unit, the dihedral angles between the thiophene and the BDT units changed from 52.7° and 58.2° to 87.8°, and the frontier orbital electron density distribution was affected. Due to the largest steric hindrance of the meta-alkyl chains, the resulting P73 polymer had a blueshifted absorption spectrum and a downshifted HOMO level compared to the P71 and P72 polymers. Furthermore, the intermolecular stacking in the P73 film was disturbed, thus resulting in its low hole mobility. When the three polymers were used to fabricate the PSC devices, they exhibited extremely different VOC, ranging from 0.67 V for the P71-based PSC device to 0.90 V for the P73-based PSC device. 3.2.3. Functional Side Chains. Additionally, unique functions could be realized by using flexible side chain optimization. For example, to improve the stability of the PSC devices, Tan and co-workers introduced the terminal bromine containing side chains on the TT units and synthesized the bromine-functionalized low bandgaps in the P74 polymer.128 After the UV light treatment, the bromine-containing group could be cross-linked without causing a significant impact on the packing and electronic properties of the conjugated polymers, thus improving the solvent resistance and the thermal stability of the corresponding PSC device. By changing the content of the bromine-containing TT unit, a higher PCE of 5.17% with better stability compared to its alkyl side chain equivalents was obtained for the P74-based PSC device. By changing the flexible alkyl side chains of the polymers, the conjugated polymers could also be dissolved in nonaromatic and nonchlorinated solvents. Due to its conjugated backbones, organic photovoltaic materials are commonly dissolved in aromatic or chlorinated solvents, such as toluene (TOL), chloroform (CF), chlorobenzene (CB), and 1,2-dichlorobenzene (o-DCB). To develop highly efficient conjugated polymers with environmentally friendly solvents, Hou and co-workers introduced the triethylene glycol monoether (TEG) side chain on the BDT unit and synthesized the P75 and P76 copolymers.129,130 Due to the enhanced solubility caused by the TEG side chain, the P75 copolymer could be dissolved in tetrahydrofuran (THF), 1,4-dioxane (DIOX), N,N-dimethylformamide (DMF), and N-methyl-2-pyrrolidone (NMP) et al.129 The PSC device based on the NMP achieved a PCE of 5.23% (VOC = 0.66 V, JSC = 13.53 mA/cm2, and FF = 58.55%). Using a novel donor material, P76, and a new synthesized acceptor, PC71B-DEG, they prepared the PSC device using anisole, and a PCE of 4.45% was obtained.130

Beaujuge and co-workers performed several studies to investigate the influence of the alkyl side chains on the photovoltaic performance of the PBDT-TPD polymers. In 2010, they synthesized a series of copolymers, P11, P62−P63, and the PSC device results suggested that the varied side chains on the TPD did not have a significant impact on VOC but had a noticeable influence on JSC.71 Specifically, the P11-based PSC device had a JSC of 11.5 mA/cm2, which was significantly higher than that of the P62-based device when the TPD had a linear octyl side chain instead of the branched 2-ethylhexyl. The higher JSC of the P11-based device was attributed to its smaller π−π distance of 3.6 Å. So et al. determined that the decreased π−π intermolecular interaction of P62 would cause lower hole mobility, higher bimolecular recombination, and higher energetic disorder.123 Furthermore, Beaujuge and co-workers replaced the branched 2-ethylhexyl side chain of the BDT with linear dodecyl or myristyl.75 They determined that this would induce a critical change in the polymer (P64−P65) self-assembly and preferential backbone orientation in thin films, which would cause a dramatic drop in the photovoltaic efficiency of the PSC devices. Using transient optical spectroscopy, Laquai et al. studied the relationship between the side chain pattern and the charge carrier dynamics in the PBDT-TPD:PCBM BHJ systems, and they determined that the varied side chain did not have a significant influence on the exciton dissociation and ultrafast charge generation steps; however, the nongeminate recombination and the subnanosecond geminate recombination would be suppressed when the branched side chain was used to modify the BDT unit.124 By fine-tuning the length of the side chain of the TPD from octyl to heptyl or hexyl, Beaujuge et al. prepared the P11, P61, and P60 polymers, and a high PCE of 8.5% was obtained for the P60-based PSC device.75 The side chains not only affected the intermolecular interactions of the resulting polymers but also had a strong influence on the intermolecular arrangements at the donor/ acceptor interfaces.125 Beaujuge and co-workers systematically studied the PBDT-TPD polymer family (P11, P62, and P65− P67) with varied side chains on the BDT and TPD units and discussed how the side chains affected the intermolecular interaction between the conjugated polymers with fullerenes. They suggested that the electron-accepting moiety (acceptor unit) of the copolymer should be docked with the fullerene in highly efficient polymer:fullerene systems, and the preferred intermolecular arrangement could be realized using side chain optimization. For example, branched side chains were more sterically hindered with fullerenes, whereas linear side chains were more sterically accessible with fullerenes. Therefore, in the polymer family of PBDT-TPD, the high-performance polymers should consist of branched side chain-substituted BDT units and linear side chain-substituted TPD units. Ma et al. synthesized a series of PBDT-TPD polymers (P68−P70) by incorporating different alkyl side chains on the thienyl-BDT, and their experimental results and theoretical calculation suggested the size and topology of the side chains had impacts on the polymer solubility, energy levels, and intermolecular packing and thus influenced the photovoltaic performance.126 Due to its improved morphology and fine-tuned energy levels of P68 with 2-ethylhexyl as the side chains, a maximum PCE of 6.17% with a high VOC of 1.00 V was obtained by using PC61BM as acceptor in the PSC device. 3.2.2. Alkyl Chain Substitution Positions. Additionally, the substitution positions played an important role in designing highly efficient photovoltaic polymers due to their influence on

3.3. Functional Substitutions

Introducing functional substitutions (electron-withdrawing or electron-donating) to the backbone and/or the side chains is a common and useful strategy to tune the optical and electronic properties of the conjugated polymers. Over the past few years, several functional atoms, such as fluorine, oxygen, and sulfur, and substituted groups, such as ester, carbonyl, sulfonyl, and cyano 7410


Chemical Reviews

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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P77 P78 P79 P80 P81 P82 P83 P84 P85 P86 P87 P88

1.63 1.58 1.58 1.75 1.73 1.60 1.65 1.64 1.64 1.98 2.00 1.84

−5.12 −5.09 −5.22 −5.41 −5.48 −4.90 −4.95 −5.15 −5.20 −5.29 −5.36 −5.67

0.74 0.68 0.80 0.68 0.75 0.56 0.60 0.74 0.78 0.70 0.79 0.97

13.0 14.59 15.73 11.1 9.1 12.2 14.3 14.4 15.2 11.14 11.83 10.92

61.4 62.6 74.3 42.2 39.4 66.7 65.7 67.7 72.4 55.2 72.9 71.4

5.90 6.21 9.35 3.2 2.7 4.5 5.6 7.2 8.6 4.30 6.81 7.52

54 58 60 131 131 132 132 132 132 133 133 134

Figure 13. Molecular structures of P77−P88 with different electron-withdrawing substitutions.

groups, were developed to design photovoltaic polymers with the desired properties. 3.3.1. Electron-Withdrawing Substitutions. The fluorine atom is the most electronegative atom with a small size in the periodic table, making it an excellent functional atom to modify the electronic properties of the conjugated polymers. Due to its strong electron-withdrawing ability and small size, introducing fluorine atoms to the conjugated molecules always results in simultaneously downshifted HOMO and LUMO levels without causing considerable steric hindrance. Furthermore, fluorination of the conjugated backbones (donor and/or acceptor segments) and/or the conjugated side chains have different effects on the

optical and electronic properties as well as the photovoltaic performance of the resulting polymers. Basic optical and electronic properties as well as photovoltaic performances for P77-P88 are listed in Table 8. In 2009, Yu and co-workers synthesized a series of copolymers based on BDT and TT, among which the introduction of the fluorine atom on the TT unit was an important discovery.54 Compared to its counterpart P54 without fluorination, the P77 copolymer containing a fluorine atom had decreased HOMO and LUMO levels; however, its optical bandgap remained unchanged. Due to its downshifted HOMO level, the P77-based PSC device obtained a high VOC of 0.74 V and a PCE of 5.9%, 7411


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Figure 14. Molecular structures of polymers P89−P100 with different substitutions.

Hou and co-workers incorporated fluorine atoms on the conjugated thienyl side chains and synthesized a set of copolymers, P82−P85, containing different numbers of fluorine atom from 0 to 3.132 They determined that the introduction of the fluorine atom on both the donor and acceptor units had a synergistic modulation effect on the molecular energy levels for the resulting D−A copolymers. Particularly, when compared with polymer P82 (0F) without fluorination, the VOC of the PSC device based on P83 (1F on the TT unit) indicated a 40 mV increase. Furthermore, the difluorination on the thienyl unit of BDT resulted in an increase in VOC by 180 mV compared to the P82-based PSC device. Interestingly, the VOC of the PSC device based on the trifluorinated polymer P84 was 220 mV higher than that of the P82-based PSC device. Due to the high VOC of the device based on P84, a remarkable PCE of 8.6% was achieved with a VOC of 0.78 V. As a strong electron-withdrawing substitution, the cyano group was also developed to modify the photovoltaic polymer (Figure 13). You and co-workers did a lot work to optimize the structures of copolymers based on BDT and benzotriazole (BTz).133,134 To reduce the HOMO levels of the PBDT-TAz copolymers, they synthesized the P87 copolymer with fluorinated TAz (triazoles). Although the VOC of the PSC device based on P87 reached 0.79 V, there was still room for improvement because the PBDT-BTz possessed a larger optical bandgap of approximately 2.0 V. Therefore, they introduced the cyano group on the TAz unit and prepared the P88 copolymer. The CV measurement indicated that the HOMO level of P88

whereas the P54-based PSC device yielded a VOC of 0.66 V and a PCE of 4.1%. In 2011, Hou and co-workers designed and synthesized the P78 copolymer based on thienyl-BDT and estersubstituted TT, which had a HOMO level of −5.09 eV and demonstrated a PCE of 6.21%.58 When the fluorine atom was introduced to P79, the HOMO level of P79 was downshifted to −5.22 eV; Chen and co-workers fabricated an inverted PSC device, and an impressive PCE of 9.35% was achieved in 2013.60 Huang and co-workers investigated the effects of the fluorine content on the photovoltaic properties of the resulting polymers using random copolymerization (P41).110 They synthesized a series of copolymers with increasing fluorine content, and it was determined that the crystalline properties of the polymers and the morphology of the blends with PCBM could also be affected by the content of the fluorine. In 2011, Yu and co-workers introduced the fluorine atoms on the backbones of the conjugated polymers.131 As depicted in Figure 13, they synthesized the fluorinated BDT unit and prepared a series of copolymers, P54, P9, and P80−P81, with varied fluorine atoms (0F−3F). The fluorination of the polymers lowered their HOMO and LUMO levels and simultaneously widened the optical bandgaps slightly by 0.1−0.2 eV. Furthermore, they determined that the perfluorination P81 (3F) introduced fluorophobicity for the PCBM molecules, which enhanced its crystallinity and increased its phase separation size to ca. 100 nm. Therefore, the PCE values of the PSC devices changed from 5.1% to 7.4%, 3.2%, and 2.7% when the number of fluorine atoms increased from 0 to 1, 2, or 3. 7412


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P89 P90 P91 P92 P93 P94 P95 P96 P97 P98 P99 P100

1.60 1.58 1.85 1.55 2.21 2.03 2.07 1.53 1.63 1.66 1.64 1.70

−5.07 −5.11 −4.80 −5.20 −5.31 −5.36 −5.41 −5.18 −5.50 −5.01 −5.16 −5.17

0.7 0.74 0.84 0.40 0.83 0.91 0.99 0.73 0.98 0.64 0.81 0.78

14.7 17.48 6.28 5.27 4.18 4.40 7.66 15.17 10.15 14.9 5.95 13.4

64.1 58.7 36.9 60.6 45 43 53 64.44 45 70.3 37.9 71.8

6.58 7.59 1.95 1.28 1.56 1.73 4.00 7.10 4.48 6.70 1.85 7.50

135 58 137 137 138 138 138 139 140 141 141 142

was as low as −5.7 eV, which was 0.28 eV lower than that of P87. The PSC device based on P88 indicated a very high VOC of 1.0 V, and a PCE of 5.53% was achieved when the active layer thickness was approximately 100 nm. When the thickness of the active layer increased to 300 nm, an impressive PCE of 8.4% was recorded for the P88-based PSC device. 3.3.2. Electron-Donating Substitutions. The chalcogens “O” and “S” are widely used to modify the optical and electronic properties of the photovoltaic materials. Because the alkyloxy has a much stronger electron-donating effect than the alkyl, previous studies on thiophene-based polymers have demonstrated that the alkyl-containing polymers would have deeper HOMO levels than their alkyloxy substituted counterparts. When the carboxyl was replaced by carbonyl, the resulting polymer would display a downshifted HOMO level. Therefore, the carboxyl and carbonyl substitutions should be carefully selected to modify the molecular structures of the conjugated photovoltaic materials (Figure 14). In 2009, the P55 copolymer, which was based on BDT and carboxyl-substituted TT and developed by Yu and co-workers, exhibited promising photovoltaic properties. The P55-based PSC device yielded high JSC and FF values; however, its VOC was only 0.6 V.54 Hou and co-workers replaced the carboxyl with carbonyl to modify the TT unit and synthesized the P89 copolymer, which simultaneously demonstrated downshifted HOMO and LUMO levels compared to P55.135,136 Benefit for its higher VOC of 0.7 V, the PSC device based on P89 yielded a PCE of 6.58%, whereas the P55-based PSC device obtained a PCE of 5.15%. As compared to P78, which was based on the BDT estersubstituted TT, P90 with carboxyl-substituted TT units had a similar optical bandgap but a downshifted HOMO level, thus leading to a higher VOC of 0.74 V for the P90-based PSC device.58 Replacing the alkyl chains with alkoxy chains always had a significant influence on the photovoltaic properties of the resulting polymers. For instance, in 2009, Hou et al. synthesized two polymers, P91 and P92, with an identical conjugated main chain but different side chains.137 The absorption spectra of the two polymers in solution and films demonstrated considerable differences. Specifically, the alkoxy-containing P92 polymer possessed a narrowed optical bandgap of 1.55 eV, which was 0.27 eV lower than that of P91. Furthermore, from solution to film, a small redshift (8 vs 51 nm) was observed for P92 and P91, respectively, suggesting that the alkoxy possessed a lesser steric hindrance than alkyl. Although the alkoxy-substituted polymer had a superior absorption property, its HOMO level was 40 mV higher than that of P91, thus causing its low VOC of 0.4 V in the corresponding PSC device.

Because sulfur is known to be a poorer electron donor than oxygen, the S-containing polymers always had lower HOMO and LUMO levels than the O-containing polymers. Ferraris et al. introduced the alkylthio substitution on the BDT unit and synthesized the P95 homopolymer and the P94 copolymer.138 When compared with their alkoxy analogue P93, P94, and P95 showed narrowed optical bandgap from 2.2 to 2.0 eV, and the HOMO levels of P94 and P95 were lowered to −5.36 and −5.41 from −5.31 eV. Meanwhile, the P95 film possessed a higher absorption coefficient than the P93 film. The PSC devices fabricated using P93−P95 exhibited a varied VOC from 0.83, 0.91, and 0.99 V, respectively. Li and co-workers conducted a further structural optimization of P79 (TB7-Th) by replacing the alkyl chain with alkoxy (P96) and alkylthio (P58) substitutions.139 P58 and P96 indicated a broad absorption spectra compared to P79, and the HOMO level of the three polymers were −5.30, − 5.18, and −5.41 eV for P79, P96, and P58, respectively. Due to its downshifted HOMO level, the P58-based PSC device yielded a PCE of 8.42% with a high VOC of 0.84 V, whereas the P96-based PSC device obtained a PCE of 7.1% with a VOC of 0.73 V. To further improve the VOC of the PSC device based on the PBDT-TT polymers, Hou and coworkers introduced the dialkylthio-substituted thiophene on the BDT unit and prepared the P97 copolymer, which demonstrated a very high VOC of 0.98 V in the PSC devices.140 However, due to the P97 copolymer’s blueshifted absorption spectrum and decreased hole mobility, the PCE of the PSC device fabricated by P97 was only 4.48%. Additionally, a sulfonyl substitution was used as an electronwithdrawing group; by changing the alkyl to alkylthio and then to sulfonyl, the HOMO levels of the PBDTDTTTs could be gradually decreased. As presented in Table 9, the VOC of the PSC devices increased to 0.56, 0.64, and 0.81 V for P82, P98, and P99, respectively.132,141 In 2014, Hou and co-workers used the metaalkoxy-phenyl group to modulate the structure of the thienylsubstituted copolymer PBDT-DTTT.142 The results suggested that replacing thienyl with a meta-alkoxy-phenyl substitution had a minimal effect on their optical absorption, molecular packing, device morphology, and charge transport properties but significantly lowered the HOMO level of the polymer. From P83 to P100, the VOC of the PSC devices changed from 0.6 to 0.78 V, whereas the JSC and FF changed slightly. 3.4. Two-Dimensional Conjugated Benzodithiophene (2D-BDT)

The 2D conjugated side chain was first used to modify the structure of polythiophene to obtain broad and strong visible absorption properties by Li and Hou et al.143,144 After the BDT 7413


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Figure 15. Common conjugated side chains used for 2D-BDT units.

Figure 16. 2D-BDT-based copolymers P101−P115 with different conjugated side chains.

years, several researchers designed and synthesized many conjugated units used as side chains in the 2D-BDT polymers (Figure 15). Thienyl-substituted BDT was the earliest 2D conjugated BDT unit, and several widely studied highly efficient

was developed to construct copolymers for the polymer solar cells, Hou and co-workers used the same concept to further optimize the structures of the BDT-based conjugated polymers, obtaining better photovoltaic performance.28 Over the past few 7414


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P101 P102 P103 P104 P105 P106 P107 P108 P109 P110 P111 P112 P113 P114 P115

1.55 1.58 1.39 1.55 1.53 1.78 1.78 1.78 1.74 1.67 1.73 1.75 1.73 1.70 1.68

−5.19 −5.29 −5.09 −5.31 −5.29 −5.46 −5.38 −5.40 −5.47 −5.4 −5.59 −5.47 −5.37 −5.31 −5.30

0.69 0.81 0.44 0.77 0.81 0.86 0.81 0.85 0.90 0.80 0.94 0.98 0.90 0.86 0.84

11.77 16.57 9.5 14.70 16.00 12.8 11.2 11.8 12.93 11.94 6.17 9.62 10.6 12.3 6.35

64.81 65.64 55 68 62.71 67 60 64 62.73 65.10 48.80 59.7 54.7 61.4 40.5

5.28 8.78 2.30 7.71 8.13 7.4 5.4 6.4 7.30 6.21 2.83 5.46 5.22 6.48 2.17

145 145 147 148 149 150 150 150 151 152 153 154 154 154 154

had a very compact intermolecular packing with a π−π stacking distance of 3.63 Å, which caused oversized aggregations when blended with PCBM, and the P101-based PSC device thus achieved a relatively low PCE of 5.28% (VOC = 0.69 V, JSC = 11.77 mA/cm2, and FF = 64.81%). Under the same conditions, the PSC based on P79 achieved a PCE of 9% (VOC = 0.78 V, JSC = 16.86 mA/cm2, and FF = 68.16%), whereas the P102-based PSC device achieved a PCE of 8.78% (VOC = 0.81 V, JSC = 16.57 mA/ cm2, and FF = 65.64%). Furthermore, a thiazolyl-substituted BDT unit was developed by Wong’s group, and the P103 copolymer with TT as an acceptor unit demonstrated a narrowed optical bandgap of 1.40 eV but a high HOMO level of −4.96 eV.147 The PSC device based on P103 depicted a relatively low PCE of 2.3% with a VOC of 0.44 V. Extending the conjugated surface could enhance the intermolecular π−π interaction for conjugated polymers. In 2013, Hwang et al. synthesized a thieno[3,2-b]thiophenesubstituted BDT unit and prepared the P104 copolymer using fluorinated TT unit as an acceptor unit, which indicated a low optical band gap of 1.55 eV and a low-lying HOMO level of −5.31 eV.148 The GIWAXS measurement demonstrated that the P104 film had a preferable face-on orientation with respect to the substrate. The inverted PSC device yielded a high PCE of 7.71% (VOC = 0.77 V, JSC = 14.70 mA/cm2, and FF = 68%). Moreover, a tandem PSC device fabricated using P104:PC71BM as the top cell and P3HT:IC60BA as the bottom cell indicated a maximum PCE of 8.66%. Recently, a rigid long conjugated side chain, thienylenevinylene (TVT), was introduced on the BDT unit by Hou group, and the copolymer P105 was synthesized.149 As compared to P9, which had a nonconjugated alkoxyl side chain, P105 with a prolonged conjugated side chain on BDT displayed a large redshifted absorption spectrum with an absorption onset of 810 nm. Moreover, because the TVT-containing BDT monomer had a strong absorption character in the range of 300−600 nm, the absorption spectrum of P105 in the short wavelength region was significantly enhanced. What’s more, P105 showed closer π−π stacking and increased hole mobility compared to P9. The PSC device based on P105 achieved a PCE of 8.13% (VOC = 0.81 V, JSC = 16 mA/cm2, and FF = 62.71%) when using PC71BM as the acceptor material. Benefiting from its panchromatic absorption property, when PC61BM was used as the acceptor material, the P105-based device also showed an inspiring PCE of 7.67%.

conjugated polymers, such as PBDTTT-C-T, PTB7-Th, and PBDT-TS1, were comprised of thienyl-substituted BDT units. As a homologous series of thiophene, furan and selenophene were also frequently used to modify the physicochemical properties of the thienyl-BDT containing polymers. Additionally, a benzene ring and thiazole were developed as 2D conjugated side chains to modify the structure of the BDT-based polymer. To extend the effects of the 2D conjugated side chain, several conjugated side chains with larger π-conjugated areas and increased conjugated lengths were developed, such as thienothiophene, thianaphthene, oligothiophene, and thiophenevinylthiophene (TVT). Generally, the introduction of 2D conjugated side chains on the BDT units had an impact on not only the absorption spectra of the resulting polymers but also their molecular energy levels, charge carrier mobilities, and crystalline properties, which resulted in a comprehensive effect on the photovoltaic performances of the PSC devices. Using 2D conjugated side chains with heteroatoms is an easy and promising method for tuning the optical and electronic properties of conjugated polymers. Replacing the “S” atom of thiophene with its homologous series of “O” or “Se” has frequently been used to tune the photovoltaic performance of the thienyl-BDT containing polymers. After the results of P79 (PTB7-Th) were reported by Chen’s group,60 Hou and coworkers introduced furan and selenophene to BDT as conjugated side chains, and copolymers P101 and P102, which are based on 2D-BDT and fluorinated TT, respectively, were prepared as indicated in Figure 16.145 The “O” atom has the smallest atomic radius of 64 ppm (104 pm for “S” and 118 pm for “Se”), and the C−O bond has the longest bond length but smallest C−O−C bond angle, which facilitates planarity between neighboring heterocycles.146 On the basis of theoretical calculations using DFT, the dihedral angles between the BDT and the conjugated side groups increased from 34° to 60° when the furan group was replaced by thiophene or selenophene, which had a significant effect on the absorption spectra, HOMO levels, crystallinities, and aggregation sizes of the resulting polymers. As listed in Table 10, the furan-substituted P101 copolymer indicated a narrowed optical band gap but a higher HOMO level than that of the other polymers (P79 and P102), and the selenophene equivalent P102 possessed the lowest HOMO level and the same optical bandgap as the thiophenebased P79 copolymer. The XRD measurement showed that P101 7415


Chemical Reviews

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Figure 17. Highly efficient polymers based on BDT and TT units.

Due to its weak electron-donating properties, large πconjugated area and good planarity of the benzene ring, a phenyl-substituted BDT was also developed by researchers. In 2013, Wei et al. introduced thiophene, furan, and benzene on the BDT units and synthesized three copolymers, P106−P108.150 A theoretical calculation using the DFT indicated that P108 had the largest torsional angle between the conjugated side chain and the BDT unit, whereas the furan-substituted BDT had the smallest torsional angle. The three polymers demonstrated similar optical bandgaps of approximately 1.78 eV; however, their HOMO levels measured using cyclic voltammograms varied from −5.46 to −5.38 eV. The photovoltaic properties of the three polymers were evaluated through the PSC devices, and the detailed parameters are listed in Table 10. The highest PCE of 7.4% was achieved for the P106-based PSC device. Yang and co-workers performed several studies to design a novel conjugated side chain for the BDT unit. To decrease the electron density of thiophene, which is a six-π-electron fivemembered aromatic compound, they introduced thianaphthene (thiophene fused with benzene) on the BDT unit.151 When compared with its thienyl-substituted counterpart, the HOMO level of the new P109 copolymer decreased from −5.36 to −5.47 eV. Due to the extended π-conjugated system of P109, its absorption spectrum had a slight redshift compared to that of the thienyl-based analogues. The deeper HOMO level of P109 resulted in a higher VOC of 0.90 V for the PSC device, and a maximum PCE of 7.30% was achieved using PC61BM as the acceptor materials, which was one of the best results for PC61BMbased PSCs at that time. Then, they attached trialkoxy phenyl on thiophene to improve the solubility of the P110 polymer, and the P110-based PSC device showed a PCE of 6.21%. 152 Furthermore, they introduced the terthiophene derivative on the BDT unit and synthesized the P111 copolymer, which exhibited a high thermal decomposition temperature of 437 °C, and the P111-based PSC device achieved a PCE of 3.57%.153 Recently, Cho et al. extended the π-conjugation of 2D-BDT polymers by introducing oligothienyl side chains on the BDT, and four copolymers (P112−P115) with alkylated thienyl, bithienyl, terthienyl, or quaterthienyl side chains were prepared.154 The polymers exhibited gradually red-shifted absorption spectra and upshifted HOMO levels as the number of thienyl units increased. The absorption intensity of the polymers at a short wavelength range from 350 to 550 nm was significantly enhanced when the number of thiophene units increased. GIXRD indicated an amorphous polymeric arrangement in the P112 film, whereas the others had enhanced crystalline properties, which can be ascribed to their different intramolecular steric hindrances. Due to the improved optical absorption property and higher carrier mobility of P114, the P114-based PSC device showed the highest JSC and PCE values out of the four polymers (PCE = 6.48% with a JSC = 12.5 mA/ cm2).

4. BDT-BASED PHOTOVOLTAIC COPOLYMERS 4.1. Copolymers Based On BDT and TT

Thieno[3,4-b]thiophene (TT) was frequently used in low band gap polymers due to its stable quinoidal structure. In 1997, alkylsubstituted poly(thieno[3,4-b]thiophene) was first synthesized as a soluble low band gap conducting polymer.52 Furthermore, Yang and co-workers prepared a PTT copolymer by copolymerizing TT and a thiophene unit in 2006 and used the low band gap polymer in near-infrared photodetectors.52 In the OPV field, TT is one of the most well-known building block used in several highly efficient photovoltaic polymers, such as PTB7,55 PBDTTT-C-T,58 PTB7-Th,60 and PBDT-TS1.59 All of these polymers achieved record PCEs in the OPV field and were widely studied by researchers (see Figure 17). In 2008, Yu et al. initially synthesized the P53 copolymer (Figure 12) based on BDT and TT.53 The maximum absorption peak of P53 was located at 690 nm with an absorption onset at 784 nm, and the electrochemical cyclic voltammetry measurement indicated that the HOMO and LUMO levels of the polymer were −4.90 and −3.20 eV (Table 10), respectively. When PC61BM was used as the acceptor in the BHJ solar cell device, a high PCE of 4.76% was achieved (VOC = 0.58 V, JSC = 12.5 mA/cm2, and FF = 65.4%). Due to the better absorption properties in the visible region of PC71BM, the PSC devices fabricated by P53 and PC71BM achieved a higher JSC of 15.0 mA/ cm2, and the PCE was boosted up to 5.30% with a VOC of 0.56 V and an FF of 63.3%. Because of its excellent photovoltaic performance, P53 was also a good model to further explore the charge transfer and charge separation steps using atomistic computer calculation. Lischka and co-workers used P53/ PC61BM as a bulk heterojunction model to conduct a detailed quantum chemical simulation.155 They determined that the charger transfer states were located below the bright interchain excitonic states, and they presented a simple model for the charge separation process. In addition to the PBDT-TT polymers discussed above, there are several PBDT-TT polymers that exhibit excellent photovoltaic properties. For example, when alkyl-substituted BDT was used as the donor unit, P116 had an optical bandgap of 1.60 eV and a HOMO level of 5.04 eV and the P116-based PSC device achieved a PCE of 5.85% (VOC = 0.72 V, JSC = 13.9 mA/cm2, and FF = 58.5%). By thoroughly understanding the relationship between the VOC of the PSC device and the HOMO level of the polymer, Hou and co-workers systematically tuned the VOC using a molecular design approach.135 Specifically, the P117 polymer with carbonyl and a fluorine atom-modified TT unit had a downshifted HOMO level located at −5.22 eV, which was approximately 200 mV lower than that of the ester-substituted P55. Therefore, the VOC of the PSC devices increased from 0.62 to 0.76 V, and an impressive PCE of 7.73% (certified as 6.77% by NREL in 2010) 7416


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Figure 18. Molecular structures of the copolymers based on BDT and TT units.


Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P116 P117 P118 P119 P120 P121 P122 P123

1.60 − 1.69 1.71 1.67 1.65 1.44 1.91

−5.04 −5.22 −5.60 −5.60 −5.61 −5.12 −4.95 −5.44

0.72 0.76 0.82 0.79 0.78 0.76 0.63 0.86

13.9 15.2 12.75 9.47 8.38 14.1 6.37 4.97

58.5 66.9 55 55 48 58 51.0 55

5.85 7.73 5.76 4.11 3.16 6.22 2.04 2.72

54 135 162 162 162 163 164 165

3.16% to 5.76%. In 2011, Hou and co-workers used the strong electron-withdrawing sulfonyl to modify the TT unit and synthesized the P121 copolymer, which showed a PCE of 6.22% with a VOC of 0.76 V.163 Because the PBDT-TT polymer family has excellent photovoltaic properties, many building blocks derived from BDT or TT were designed and synthesized by researchers. For example, Yu et al. synthesized the bi-TT unit and prepared the P122 copolymer,164 which possessed a broad absorption in the near-IR region with an absorption onset of 863 nm corresponding to an optical band gap of 1.44 eV, and the HOMO energy level of P122 was located at −5.19 eV. Furthermore, the effects of the dipole moment of the conjugated polymers on the charge separation process were studied. The theoretical calculation results suggested that P122 possessed a large electron−hole pair binding energy due to its minimized dipole moment, which resulted in its difficult charge separation. In 2012, Ding and coworkers synthesized an interesting electron-accepting unit, cyclopenta[2,1-b:3,4-c′]dithiophene-4-one, and the copolymer P123 copolymerized with BDT exhibited a PCE a 2.72% with a VOC of 0.86 V for the PSC device.165 Figure 18 summarizes the copolymers based on BDT units and TT units, and their basic optical and electronic properties as well as photovoltaic performances for P53-P76 are listed in Table 11. Generally, copolymers based on 2D-BDT and TT have superior photovoltaic properties compared to those of their counterparts with alkoxy side groups. In 2011, Hou and coworkers initially designed and synthesized copolymers based on 2D-BDT and TT by replacing the alkoxy group with an alkylthienyl group of the BDT unit.58 When compared to their alkoxy-substituted counterparts, copolymers with 2D side groups (P78 and P90) indicated an enhanced thermal stability,

was achieved for the P117-based PSC device, which was the highest value at that time. At almost the same time, Yu and co-workers developed the well-known P9 (PTB7)55,56 after an extensive structural optimization of PBDT-TT, and a PCE of 7.40% was achieved for the P9-based PSC device, which was one of the few PCE values over 7% reported in 2010. After the reported results of PTB7, its excellent properties attracted several researchers to further improve its photovoltaic performance and explore the working mechanism of the OPV. By increasing its molecular weight to 128 kDa with a low polydispersity (PDI) of 1.21, Cao and co-workers pushed the PCE of the P9-based device to 8.5%.156 Additionally, a theoretical calculation using DFT was used to explore the nanostructures and electronic properties of P9. The results suggested that P9 possessed a nonplanar conformation with an inter-ring torsional angle of approximately 25°, and the effective conjugation length of P9 was calculated as 147 Å.157 Different device structures and several interface materials were applied in the P9-based PSC devices to improve its PCE, and a PCE over 9% was reported by several groups.57,158,159 Among them, Cao and co-workers improved the PCE of the P9-based PSC device to 9.2% using an inverted cell device structure with MoO3 and PFN as the interlayer materials.57 Ge et al. further improved the PCE of the P9-based PSC device to 10.02% using a nonconjugated small molecule electrolyte.160 Furthermore, studies on the BHJ morphology of the PSC device, dissociation of the exciton, and transfer of the charges were all performed using P9/PCBM as a model.161 Lee et al. synthesized a triisopropylsilylethynyl-substituted BDT unit, and they used TT with different substitutions as the acceptor units to prepare the copolymers P118−P120.162 The PCEs of the PSC devices based on these polymers ranged from 7417


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achieved a high PCE of 9.35%, which was one of the best results at that time. Li et al. determined that a polymer based on an alkylthio-substituted BDT exhibited an improved VOC in the corresponding PSC device.139 Hou et al. designed and prepared the P57 copolymer with a linear alkylthio side chain, which had an optical band gap of 1.51 eV and a HOMO level of −5.33 eV.59 An impressive PCE of 9.48% was recorded for the P57-based PSC device, and a PCE over 10% was also obtained by further optimization of the PSC devices.166 Very recently, an impressive PCE of 9.67% was obtained for the P57-based PSC device using a novel environmentally friendly single-solvent (i.e., 2-methylanisole).167

improved absorption properties, downshifted molecular energy levels, and thus improved photovoltaic performances. For instance, P90 had an optical band gap of 1.58 eV and a HOMO of −5.11 eV. Due to its extended π-conjugated area, the hole mobility of the P90/PC71BM film blend reached 0.27 cm2 V−1 s−1, which was a 3 orders of magnitude enhancement over its alkoxyl substitution counterpart. These advantages of the 2DBDT polymers contributed to the improvement in their photovoltaic properties. The P90-based PSC device achieved the best PCE of 7.59% with a VOC of 0.74 V. P79 (PTB7-Th) was another successful application of 2D-BDT. When thienyl group was used to replace the alkoxyl of P9 (PTB7) by Chen and coworkers, the absorption spectrum was extended by 25 nm to a longer wavelength due to the improved coplanarity of P79.60 A CV measurement indicated that the HOMO level of P79 was 80 mV lower than that of P9. Furthermore, the GIWAXS showed that the intermolecular π−π stacking of the P79/PC71BM blend film was stronger than that of P9/PC71BM. Using a novel fullerene derivative-doped zinc oxide (ZnO-C60) nanofilm as the cathode interlayer, the P79-based inverted PSC device

4.2. Copolymers Based On BDT and BT

Benzothiadiazole (BT) units were widely applied for constructing conjugated D−A copolymers due to their strong electronwithdrawing properties, many of which had good applications in solar cell devices.69 From the view of the chemical structure of the BT unit, it could be modified by several methods. First, the “S” atom could be replaced by its congeners “O” or “Se”, and TAz 7418


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P124 P125 P126 P127 P128 P129 P130 P131 P132 P133 P134 P135 P136 P137 P138 P139 P140 P141 P142 P143 P144 P145 P146 P147 P148

1.7 1.7 1.67 1.70 1.73 1.95 1.96 1.78 2.03 1.92 1.93 1.82 1.74 1.91 1.77 1.55 2.0 1.95 1.83 1.81 2.02 1.51 2.00 1.95 1.97

−5.40 −5.54 −5.48 −5.31 −5.37 −5.17 −5.44 −5.21 −5.56 −5.44 −5.38 −5.42 −5.27 −5.34 −5.31 −5.81 −5.04 −5.06 −5.38 −5.32 − −5.47 −5.59 −5.40 −5.26

0.87 0.91 0.85 0.78 0.81 0.76 0.94 0.76 0.84 0.83 0.86 0.87 0.86 0.83 0.69 0.60 0.61 0.61 0.91 0.88 0.78 0.85 0.89 0.80 0.77

10.03 12.91 11.4 15.38 8.00 8.96 6.5 13.87 10.59 10.16 9.40 7.61 10.4 10.6 11.34 13.58 4.8 4.5 11.81 12.36 13.3 12.78 6.81 12.69 7.87

57.3 6.12 50.6 69.2 55 59 46 66.6 49 50.2 39 49.8 64.4 64.7 63.0 64 47 62 58.2 71.2 70.5 58.2 48 55 48

5.0 7.2 5.28 8.30 3.65 4.02 2.81 7.05 4.32 4.23 3.17 3.27 5.7 5.6 4.93 5.18 1.4 1.7 6.25 7.74 7.3 6.32 2.92 5.53 2.88

168 168 170 171 91 172 172 173 174 175 176 177 178 197 179 180 181 181 182 182 184 185 186 187 187

by P125 increased to 7.16% from 4.53% in the case of P124. When the alkoxy-modified benzodithiophene replaced the alkylsubstituted benzodithiophene to construct the P127 copolymer, the PSC devices achieved a high PCE of 8.30% in Jiang’s group.171 Recently, P20 and P128 with thienothiophene as the π spacer was developed by Wang et al., and their results suggested that the fluorination of the polymer could enhance the intra- and intermolecular interactions.91 In 2011, Wang et al. developed the P129 and P130 polymers with different π spacers and investigated their influence on the photovoltaic properties.172 As compared to P129, P130 with a furan spacer had a deeper HOMO level, and thus, a higher VOC of 0.94 V was achieved in the PSC device. Then, they compared the different π-bridges of furan, thiophene, and thienothiophene (P131).173 The theoretical calculations indicated that the polymer structures gradually changed from z-shaped to an almost straight line when the π-bridges varied from furan to thiophene and then to thienothiophene, and the absorption bandgaps of the polymers ranged from 1.96 to 1.78 eV when the HOMO levels changed from −5.44 to −5.21 eV. Additionally, the dithienybenzothiadiazole (DTBT) unit was developed as a side group to optimize the properties of the photovoltaic materials. Tan et al. attached the DTBT to the main chain of P132 using a vinylene group, which improved the coplanarity of the polymer backbone.174 The P132-based PSC device achieved a maximum PCE of 4.32%. Li and co-workers further investigated the photovoltaic properties of this type of polymer by selecting the fluorination BT unit.175 Zhu et al. synthesized two interesting copolymers, P134 and P135, with pending benzothiadiazole as a side group.176,177 Replacing “S” with “O”, “Se”, or “N” was proven to be an effective strategy for optimizing the properties of BT-containing polymers. By replacing the “S” with more electronegative “O”, D−A copolymers containing benzooxadiazole (BO) units would

was also developed by researchers. Moreover, the BT unit could be modified by the alkoxy groups or fluorine atoms. Additionally, different π spacers were used to tune the properties of the BTcontaining conjugated materials. Copolymers based on BDT and BT are a type of “weak donor” and “strong acceptor” D−A combination, which would result in a deep HOMO level, thus leading to a high VOC for the BHJ PSC devices. Figure 19 summarizes the molecular structures of P124-P148. You and co-workers performed several studies on copolymers based on fluorinated BT units. In 2011, they prepared P124 and P125 and applied them in PSC devices.168 The spectra of the polymers indicated an 80 nm blueshift when the solution temperature increased from 25 to 100 °C. In comparison, the fluorinated polymer P125 indicated an enhanced absorption coefficient and a downshifted HOMO level compared to P124 without fluorine. In the PSC devices, P125 exhibited a high PCE of 7.2% (VOC = 0.91 V, JSC = 12.9 mA/cm2, and FF = 61%) using PC61BM as the acceptor material. At almost the same time, Ding et al. also reported similar results.169 To further investigate the influence of the number of fluorine atom on the properties of the conjugated polymers, You and co-workers synthesized the P126 polymer using monofluorination BT and compared it with P124 (0F) and P125 (2F). From 0F to 2F, the VOC of the PSC devices increased from 0.81 to 0.91 V.170 Furthermore, the JSC and FF of the devices improved as the number of fluorine atom increased, which could be ascribed to the suppressed charge recombination. On the other hand, a quantum mechanical calculation indicated that the polymer with 2F had a larger change in the dipole moment between the ground and excited states (Δμge) compared to polymers with 0F and 1F, which may contribute to the suppressed geminate recombination. Furthermore, P125 displayed an enhanced face-on crystalline orientation and improved π−π stacking along with a higher domain purity compared to its counterparts. The PCE of the device fabricated 7419


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Wei et al. achieved a maximum PCE of 5.7% with a VOC of 0.86 V by blending PC61BM as the acceptor in the PSC device.178 Then,

possess lower HOMO levels than their BT equivalents. For example, the P136 polymer-based BDT and BO synthesized by 7420


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P149 P150 P151 P152 P153 P154 P155 P156 P157 P158 P159 P160 P161 P162 P163 P164 P165 P166 P167 P168 P169 P170 P171 P172 P173

1.75 1.73 1.63 1.65 1.70 1.73 1.72 1.78 1.69 1.65 1.75 1.53 1.63 1.67 1.78 1.91 1.93 1.56 1.60 1.85 1.79 1.62 1.58 1.93 1.53

−5.31 −5.26 −5.14 −5.11 −5.35 −5.39 −5.33 −5.28 −5.29 −5.40 −5.04 −5.48 −5.41 −5.32 −4.92 −5.26 −5.32 −5.29 −5.19 −5.44 −5.40 −5.46 −5.19 −5.23 −5.44

0.92 1.00 0.82 0.78 0.88 0.82 0.85 0.86 1.03 0.86 0.98 0.79 0.86 0.82 0.54 0.75 0.89 0.66 0.78 0.86 0.83 0.78 0.80 0.92 0.75

10.70 5.80 5.78 12.46 12.94 13.11 13.3 8.7 7.05 7.72 10.60 7.23 12.05 12.53 9.47 11.90 17.43 8.3 11.80 9.1 12.7 9.3 11.71 11.71 12.56

57.5 34.6 49.5 62.0 70.9 65.28 68.0 59.0 48 43 57.8 47.08 59.9 54.9 60.6 67.2 61.48 44.0 54 58.6 62.0 47.0 61.0 65.0 54.2

5.66 2.11 2.34 6.03 8.07 7.02 7.7 4.4 3.52 2.92 6.37 2.70 6.21 5.64 3.1 6.00 9.53 2.4 5.0 4.5 6.5 3.4 6.00 7.11 5.11

188 205 189 189 190 191 192 192 193 193 194 196 197 197 199 202 203 93 200 198 198 204 205 206 207

photovoltaic performance using P144, and a maximum PCE of 7.3% was obtained from the PSC devices.184 Pyridine is a more electron deficient unit than the benzene ring. When pyridine replaced the benzene ring in the BT unit, the thiadiazolopyridine (PyT) acted as a stronger acceptor than BT.185 As compared to the BT-containing polymer, P145, which was based on BDT and PyT, indicated a smaller bandgap and a decreased HOMO level, and a maximum PCE of 6.32% was achieved in the PSC devices. Additionally, the unsubstituted BDT and triisopropylsilylethynyl (TIPS)-substituted BDT were used in designing BTzcontaining conjugated polymers, and the photovoltaic parameters of the PSC devices are listed in Table 12.186,187 Two-dimensional side chain groups, such as furan, thiophene, selenophene, or benzene, were frequently introduced on BDT to modify the properties of the PBDT-BT polymers (Figure 20 and Table 13), and some very impressive photovoltaic results were obtained. For example, P10 could yield PCE of 9.4% in the PSC device. In 2010, Huo and co-workers synthesized the P149 copolymer-based thienyl-substituted BDT and BT, and the polymer had an optical bandgap of 1.75 eV and a deep HOMO level of −5.31 V.188 A PCE of 5.66% with a high VOC of 0.92 V was obtained for the PSC device. When thienothiophene was used as the π-bridge in P152, P151 and P152 depicted little differences in the absorption spectra and molecular energy levels. However, the hole mobility of P152 (1.97 × 10−3 cm2 V−1 s−1) was considerably higher than that of P151 (1.58 × 10−5 cm2 V−1 s−1).189 The photovoltaic properties of the two polymers were evaluated by blending PC61BM as the acceptor in the PSC devices. Due to the enhancement of the hole mobility of P152, the JSC of the device reached 12.46 mA/cm2, which was significantly higher than that of P151 (5.78 mA/cm2). The alkylphenyl-substituted BDT had a wide application in constituting conjugated photovoltaic polymers for its large πconjugated area and good planarity. The polymer P153

thienothiophene was used as a spacer to prepare the P138 polymer in Li’s group, which indicated an enhanced hole mobility of 0.023 cm2 V−1 s−1.179 In comparison with its BT counterpart P131, P138 had a broader absorption spectrum and a lower HOMO level by 0.1 eV. The inverted PSC device with a structure of ITO/PFN/polymer:PC61BM/MoO3/Al was used to investigate the performance of P138, and a maximum PCE of 7.05% was obtained with a VOC of 0.76 V. When the “S” atom changed to the “Se” atom, the benzoselenadiazole (BSe)containing polymer P139 was developed by Tajima and coworkers.180 Compared to its BT analogue, P139 had a red-shifted absorption spectrum along with a higher HOMO level. The PSC devices fabricated using P139 exhibited a PCE of 5.18% with a VOC of 0.60 V. Benzotriazole (BTz) is another BT derivative, which can be easily modified by an alkyl chain to improve the solubility of the resulting polymer. The more electron-rich “N” atom made BTz a slighter weaker acceptor unit, which resulted in a higher HOMO level compared to its BT-based counterpart. In 2010, Zou et al. designed and synthesized the P140 and P141 polymers based on the BTz unit; however, they achieved a relatively low PCE of approximately 1%−2% and a low VOC approximately 0.61 V.181 As a useful method to improve the HOMO level, alkylthiolsubstituted BDT and fluorinated BT were used to improve the properties of this type of polymer by Peng’s group.182 After the optimization of the π spacer, the VOC of the device fabricated by P143 increased to 0.88 V and a PCE of 7.74% was achieved. Furthermore, Ade et al. conducted a series of studies on copolymers based on BDT and BTz units. They determined that the morphology of the P144/PC61BM blend was insensitive to the processing conditions. It was believed that the low equilibrium miscibility of PC61BM in P144 played a crucial role in the morphological development of varied solvents.183 Then, they investigated the influence of molecular weight on the 7421


Chemical Reviews

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Figure 21. Molecular structures of P174−P186 based on BDT and TPD.

BDT and fluorinated BT, and the PSC device achieved a PCE exceeding 9%.195 Li and co-workers synthesized P160 based on alkoxyphenyl-substituted BDT and fluorinated BT, which exhibited a broad absorption at 300−800 nm.196 Dai and coworkers prepared P161 and P162 based on alkoxyl-substituted BT and fluorinated BT.197 The HOMO level of P161 was lower than that of P162 by 90 mV. A PCE of 6.21% with a VOC of 0.86 V was obtained from the PSC device fabricated using P161, whereas the P162-based PSC device yielded a PCE of 5.64% with a VOC of 0.82 V. Additionally, heteroatom substitution was widely studied by researchers when designing copolymers based on BDT and BT units.92,93,198−201 For example, Zou et al. synthesized the P163 polymer using BTz as the acceptor unit.199 Compared to its counterpart with BT or BO as the acceptor, P163 indicated a larger optical bandgap of approximately 1.82 eV and a high-lying HOMO level of −4.92 eV; thus, the VOC of the PSC device was only 0.54 V. Li and co-workers designed and synthesized the polymer P164 composed of thienyl-substituted BDT and fluorinated BTz, which showed a maximum PCE of 6.0% in the PSC device.202 PCE of 4.74% was also recorded when the thickness of active layer increased to 400 nm. Recently, they incorporated alkylthio-substituted BDT into the polymer and synthesized P165, and a very impressive PCE of 9.53% was recorded by using a nonfullerene acceptor.203 “Se”, which is another chalcogen, has a larger size and is less electronegative than “S”. Due to their enhanced ground-state quinoid resonance and improved planarity, the Se-containing polymers always had narrow optical bandgaps.146 Wei and co-workers synthesized a series of Se-substituted copolymers with Se substitution on the side chain and/or a π spacer and/or an acceptor building block.93

developed in Hou’s group possessed an absorption edge of approximately 730 nm and a HOMO level of −5.35 eV.190 The authors determined that 0.5% of DIO would result in a significant improvement in the PCE from 5.09% to 8.07%; thus, they used a series of characterization techniques, including AFM, TEM, GIXD, and resonant soft X-ray scattering (RSoxS), to study the change in the blend morphology. The results revealed that the additive increased the crystallinity of the polymer, and a multilength scale morphology was formed after the addition of DIO, which was beneficial for exciton dissociation and charge carrier transport. A maximum PCE of 8.07% was achieved with a VOC of 0.88 V for the PSC device. Yang et al. synthesized the polymer P154 based on alkoxyphenyl BDT and DTBT, and the fluorine atom was used to decrease its HOMO level.191 The PSC device based on P154 exhibited a PCE of 7.02% with a VOC of 0.82 V. The fluorinated BT and alkoxy-substituted BT units were widely used to optimize the PBDT-BT polymers with 2D side chains. Janssen and co-workers prepared the P155 and P156 copolymers with thiophene or furan as the π spacer, and a maximum PCE of 7.7% was obtained for the P156-based PSC devices.192 Furthermore, the photovoltaic performance of the device was insensitive to a wide range of active layer thicknesses (i.e., from 100 to 250 nm). Stefan and co-workers synthesized the polymers P157 and P158 by extending the thienyl side chains, which showed moderated PCE around 3%−4% in the PSC devices.193 Jiang and Cho et al. developed the P159 polymer with a similar molecular structure and achieved a PCE of approximately 6% for the PSC device processed without any additives or post-treatments.171,194 Recently, Jo et al. synthesized a medium bandgap copolymer based on thienylthio-substituted 7422


Chemical Reviews

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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P174 P175 P176 P177 P178 P179 P180 P181 P182 P183 P184 P185 P186

1.93 − 1.88 1.86 1.78 1.85 1.79 1.86 1.88 −− 1.70 1.76 1.87

−5.67 − −5.56 −5.66 −4.85 −4.98 −5.41 −5.30 −5.48 −5.62 −5.52 −5.42 −5.40

1.0 − 0.89 0.66 0.9 0.98 1.05 0.92 0.87 1.00 0.91 0.96 0.87

2.0 − 7.6 1.2 7.8 8.7 10.6 10.94 9.20 14.32 3.22 13.20 10.90

39 − 57 26 42 48 60.0 60.4 62.8 52 45 55.88 63.0

0.8 − 3.9 0.2 3.0 4.7 6.7 6.08 5.03 7.59 1.33 7.14 6.04

211 103 103 103 214 214 216 217 217 218 219 220 221

polymers.206 After optimization of the alkyl side chains, P172 exhibited a PCE of 7.11% with a high VOC of 0.92 V. Ma et al. synthesized P173 by combining thienyl-substituted BDT as donor unit and PyT as an acceptor unit, which showed a PCE of 5.11% with a VOC of 0.75 V, a JSC of 12.56 mA/cm2, and a FF of 54.2%.207

The effects of the Se atom number on their absorption spectra, molecular energy levels, hole mobilities, and photovoltaic properties were studied. In comparison with their thiophene equivalents, the absorption onsets of the Se-containing polymers were red-shifted by 40−100 nm. The absorption onset of the allSe polymer P166 located at 792 nm and its HOMO was as high as −4.64 eV. A relatively low PCE of 2.4% with a VOC of 0.66 was obtained for the P166-based PSC device, thus suggesting that a careful selection of heteroatoms should be considered when designing highly efficient photovoltaic polymers. Chen and coworkers developed P167 based on monofluorine-substituted benzoselenadiazole and BDT, which indicated an optical band gap of 1.60 eV. A PCE of 5.0% with a VOC of 0.78 V was obtained for the corresponding PSC device.201 Several reports showed the replacement of thiophene with furan would improve the solubility of the polymer and result in a deeper HOMO level.98 Ge and co-workers developed three copolymers based on BDT and benzooxadiazole with furan as the π-bridge. They used alkoxy, thiophene, and furan-substituted BDT units as the electron-donor unit to construct copolymers P130, P168, and P169, respectively.198 Copolymers with 2D conjugated side groups (P168 and P169) had a broader absorption range compared to those with the alkoxy-substituted counterpart (P130). Specifically, the optical bandgaps of the three polymers were 1.91, 1.85, and 1.79 eV for P130, P168, and P169, respectively. The authors determined that the treatment of the polar solvent (such as methanol and ethanol) helped improve the PCE of the PSC devices. For the P169-based devices, the VOC, JSC, and FF values improved after the polar solvent treatment, and the PCE thus increased to 7.0% from 3.0%, which resulted from the increased hole mobility, more-balanced charge transport, and better interpenetrating morphology. A copolymer P170 with alkoxyphenyl-substituted BDT and benzooxadiazole was synthesized in Zou’s group.204 In comparison with its BOcontaining counterpart, P170 possessed a similar optical band gap but a deeper HOMO level. The P170-based PSC device achieved a PCE of 6.2% with a VOC of 0.89 V. Cao and co-workers applied naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT) when designing highly efficient copolymers in 2011.205 Compared with its BT counterpart P150, the NTcontaining P171 polymer demonstrated a pronounced redshifted absorption spectrum and higher hole mobility while maintaining a suitable energy level. P171 exhibited a promising photovoltaic performance with a PCE of 6%. Furthermore, they developed naphtho[1,2-c:5,6-c]bis(2-octyl-[1,2,3]triazole) (TZNT) to enlarge the π-conjugated area of the BT-containing

4.3. Copolymers Based On BDT and TPD/BDD

In 2010, a few groups designed and synthesized PBDT-TPD polymers.71−74 Due to the low-lying HOMO levels of the PBDTTPD polymers, the PSC devices based on this type of polymer always resulted in a relatively high VOC. In addition to the copolymers based on BDT building blocks, TPD units were also widely used as acceptors to construct other highly efficient polymer systems.208,209 Figure 21 summarizes the copolymers based on BDT units, and their basic optical and electronic properties as well as photovoltaic performances are listed in Table 14. PBDT-TPD polymers based on alkoxy-substituted BDT and alkyl-modified TPD were first synthesized and investigated, and the P11-based PSC device achieved an initial PCE of 5.5%. A further improvement in the PCE to over 8% was achieved by side chain engineering of the polymer and fine-tuning the blend morphology.75,210 Additionally, researchers synthesized the P174 copolymer with alky-substituted BDT and TPD and explored its photovoltaic properties.211 The P174 film possessed an optical bandgap of 1.93 eV, which was slightly larger than its alkoxy-substituted analogues (1.83 eV). The PSC device based on P174 and PC61BM showed a high VOC of 1.0 V. Studies incorporated thiophene spacers on both sides of the TPD unit (DTTPD) to tune the physicochemical properties of PBDTTPD. Leclerc et al. and Zhang et al. synthesized a series of PBDTDTTPD copolymers, and the impact of the alkyl side chains on the properties of the resulting copolymers was studied.103,212 Compared with PBDT-TPD without spacers, the nonsubstituted thiophene spacer-containing P175 polymer demonstrated similar absorption spectra, whereas the introduction of flexible alkyl side chains on thiophene would result in a blueshift of the absorption spectra. Furthermore, the effects of the π spacers on the molecular energy levels and blend morphology were observed. As discussed above, polymers P68 composed of thienyl-BDT and TPD developed by Ma’s group showed a maximum PCE of 6.17% with an impressive VOC of 1 V, which was the highest result with a VOC over 1 V at that moment.126 The high VOC can be attributed to its low-lying HOMO level of −5.61 eV, and this result demonstrated its potential application in a tandem solar 7423


Chemical Reviews

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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P187 P188 P189 P190 P191 P192 P193 P194 P195 P196 P197 P198 P199 P200 P201 P202 P203 P204

1.83 1.60 1.51 1.31 1.53 1.93 − − − − − − 1.78 1.79 1.92 1.86 1.78 1.81

−5.39 −5.59 −5.79 −5.37 −5.40 −5.63 −5.44 −5.25 −5.40 −5.42 −5.45 −5.36 −5.59 −5.60 −5.41 −5.40 −5.49 −5.32

0.89 0.88 0.41 0.72 0.80 0.92 0.85 0.62 0.72 0.73 0.79 0.88 0.96 0.97 0.87 0.83 0.87 0.86

8.0 6.70 1.53 13.5 6.50 9.62 7.06 3.39 8.76 10.68 1.2 12.37 11.00 8.28 2.64 1.81 4.75 13.13

67 47 44 54 60 62 54.7 53 62 62 45 62 58.5 41.7 33 30 45 71.8

4.8 2.74 0.28 5.3 3.12 5.50 3.28 1.13 3.90 4.84 0.27 6.78 6.18 3.35 0.73 0.45 2.18 8.12

211 222 222 223 224 226 227 228 228 228 228 228 229 230 231 231 232 233


7424


Chemical Reviews

Review

had an optimized absorption spectrum, the P187 film exhibited a disordered molecular arrangement, which resulted in a negative impact on its photovoltaic performance. Fréchet et al. replaced the imide of TPD with either the ester group or nitrile group to investigate the effects of electron-withdrawing substituents on the optical and electronic properties of the copolymers.222 In the PSC devices, when mixed with PC61BM, the authors determined that copolymers based on imide and ester (P188) acted as donor materials. However, when mixed with poly(3-(4-n-octyl)phenylthiophene), the nitrile-containing polymer P189 performed as an n-type material in the bilayer device. By incorporating a thiophene into the TPD, an unique unit known as n-octyl-2,7-dithia-5-azacyclopenta[a]pentalene-4,6dione (DTPD) was synthesized in Kwon’s group.223 P190 had a wide absorption range from 300 to 900 nm. The photovoltaic performance of the polymer was investigated using a BHJ solar cell with a device structure of ITO/PEDOT:PSS/polymer:PC71BM/TiO2/Al, and a PCE of 5.3% was obtained. Hou and co-workers incorporated a thiophene spacer between the BDT and the TID to further improve the photovoltaic performance of this type of copolymer.224 Compared to its counterpart, which was based on BDT and TPD, the new P191 polymer possessed a broad absorption spectrum and a higher HOMO level due to its strong quinoid structure, and the PSC device fabricated by P191 and PC61BM obtained a PCE of 3.12% with a VOC of 0.80 V. The bithiophene imide (BTI) unit had a structure consisting of an imide group in the middle of two thiophene spacers, which possessed a smaller steric hindrance than that of the TPD unit. When the polymers based on BTI and BDT were designed and applied to organic thin-film transistors, they exhibited a good carrier mobility over 0.1 cm2 V−1 s−1.225 Marks et al. synthesized the P192 copolymer based on BTI and BDT and applied it in the PSC device.226 P192 had an absorption onset of 740 nm and an electrochemical HOMO level of −5.60 eV. With the use of an inverted architecture with an ITO/ZnO/ polymer:PC71BM/MoOx/Ag structure, the P192-based device achieved a PCE of 5.50% with a VOC of 0.92 V. Yu and co-workers and You et al. conducted a series of studies on the relationship between the dipole moment and the photovoltaic performance for conjugated polymers, and they concluded that the increased dipole change (Δμge) was beneficial for improving the PCE of the devices.164,170 Therefore, they replaced one of the carbonyls with a sulfonyl group on the TPD unit to increase the local dipole moment change between the ground and excited states.227 However, the resulting P193 polymer exhibited a diminished photovoltaic performance. The authors believed the strong electron-withdrawing sulfonyl group was detrimental to charge separation, and the results demonstrated that the positive linear correlation between the PCE value and Δμge were suitable only in a certain range. A series of copolymers (P194−P197) based on BDT and the acceptors with varied electron-withdrawing capabilities were synthesized in Kim’s group.228 The theoretical calculation and experiment results suggested that a stronger electron-withdrawing acceptor unit would result in lower HOMO and LUMO levels of the resulting polymers, and the LUMO decreased more rapidly than the HOMO, which caused a narrowed bandgap. According to their findings, they synthesized P198 polymer, and the PSC device achieved a PCE of 6.78%. Ma and co-workers introduced the 2D conjugated thienyl side chain on BDT and synthesized polymer P199, and PCE about 6% was obtained in the PSC device with a D/A ratio of 1:0.5.229 Then they synthesized P200 by modifying the molecular structure of P199, and a PCE of 3.35% was recorded in the PSC device.230

cell. Similar results were obtained by Wei and co-workers.213 Additionally, furan and selenophene were incorporated on BDT as 2D side chains, and the effects of the ring substituents on the morphology of the blends with PCBM were studied by researchers.214,215 The DFT calculations indicated that the optimal torsion angles between the BDT unit and the adjoining ring substituents had considerable differences. The BDT with a furan substituent had a more planar structure than the BDT with thiophene and selenophene. The photovoltaic properties of the polymers were investigated using standard solar cell devices, and they showed very poor performance when the additive was not used. After the optimization of the DIO, the short-circuit current had a significant increase, and the PCE of the P68-based device reached 6.5%. Then, the authors examined the morphology of the polymers/PC71BM blends using a bright-field TEM, and the results suggest that the use of the DIO prevented the formation of large fullerene clusters, thus increasing the JSC. Beaujuge et al. modified the TPD with a carbonyl substitution to further decrease the HOMO and LUMO levels of the resulting P180 polymer.216 By fine-tuning the flexible side chains, the PSC device based on P180 achieved a PCE of 6.7% with a high VOC of 1.05 V. Wei and co-workers introduced the thiophene unit as the π spacer to tune the properties of the 2D PBDT-TPD polymers.217 When the unsubstituted thiophene was used, the resulting polymer had a poor solubility in most common solvents, so a hexyl-substituted thiophene spacer was used and P181 was synthesized. Although the DFT calculation results suggested that the coplanar structure of the BDT and TPD unit was distorted, the solubility and absorption coefficient improved. Compared to P68, P181 maintained a similar optical absorption but higher HOMO level. The PSC device based on P181 demonstrated a lower VOC but a higher JSC than that of P68, and a PCE of 6.08% was achieved for the P181-based PSC device. Recently, to increase the density of the acceptor units, Wei et al. introduced two TPD units in one repeating unit and synthesized the P183 copolymer.218 P183 possessed a low-lying HOMO level of −5.62 eV, and the PSC device fabricated by P183 achieved a PCE of 7.59% with a high VOC of 1 V. By extending the 2D conjugated side chain on BDT, Stefan and co-workers synthesized the polymer P184, which showed a PCE of 1.33% in the PSC device by using PC61BM as an acceptor.219 Jin and coworkers synthesized the alkoxyphenylthiophene-substituted BDT unit and prepared the P185 copolymer. 220 They determined that the PCE of the PSC devices had a significant improvement from 4.19% to 7.14% when the active layer was treated with methanol. Kang et al. introduced alkylthienylenevinylene thiophene (TVT) on BDT as a highly conjugated side group and synthesized the P186 polymer, which had an enhanced absorption intensity at short wavelengths (∼400 nm).221 The PSC devices based on P186 achieved a higher PCE of 6.04% compared to its thienyl-substituted equivalent (4.82%) due to its increased JSC value (from 8.05 to 10.9 mA/cm2). Basic optical and electronic properties as well as photovoltaic performances are listed in Table 15. As shown in Figure 22, several novel TPD-like building blocks were developed and applied when designing photovoltaic polymers. Olson and co-workers designed and synthesized the P187 copolymer based on thienoisoindoledione (TID) and BDT.211 When the benzene ring was inserted into the middle of the TPD, the quinoid structure of the copolymer was stabilized by the aromatic resonance and intramolecular charge transfer, which resulted in a red-shifted absorption spectrum as compared to its analogue based on TPD and BDT (P11). Although P187 7425


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Additionally, benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) units were widely used when designing photovoltaic

conjugated polymers. In 2012, Aso and co-workers synthesized the copolymers P201 and P202.231 P201 demonstrated a larger 7426


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P205 P206 P207 P208 P209 P210 P211 P212 P213 P214 P215 P216 P217 P218 P219 P220 P221 P222 P223 P224 P225 P226 P227

1.78 1.68 1.81 1.85 1.62 1.38 1.67 1.74 1.73 1.72 1.82 1.78 1.80 1.80 1.81 1.72 1.77 1.72 1.64 1.66 1.68 1.59 1.64

−5.41 −5.43 −5.48 −5.62 −5.11 −5.16 −5.52 −5.12 −5.12 −5.35 −5.41 −5.51 −4.98 −5.07 −5.08 −5.20 −5.87 −5.31 −5.05 −5.19 −5.19 −5.34 −5.97

0.77 0.65 0.81 0.91 0.65 0.72 0.76 0.71 0.76 0.83 0.95 0.94 0.83 0.87 0.91 0.86 0.96 0.85 0.64 0.76 0.80 0.70 0.98

5.0 6.3 4.74 10.61 12.5 11.47 17.9 7.00 10.13 11.71 10.82 11.28 6.37 6.52 6.84 12.77 6.5 11.3 12.56 13.05 13.60 11.10 10.4

50.3 55.2 42 62 54 62 57.6 61.5 64.3 40 61.4 64.7 54 55 59 69.93 54 75.7 70.05 63.34 67.68 62.5 48

1.9 2.3 1.55 6.08 4.39 5.12 7.8 3.06 5.0 3.90 6.3 6.9 2.9 3.1 3.7 7.68 3.4 7.25 5.63 6.25 7.39 4.86 4.9

235 235 236 236 237 238 239 81 81 241 242 243 244 244 244 245 246 247 82 82 82 248 249

0.91 V for the P208-based PSC device, a high PCE of 6.08% was obtained. Cao et al. synthesized a series of copolymers based on alkylthienyl-substituted Qx.237 After optimization of the side chains, P209 achieved the best PCE of 4.39% with a relatively low VOC of 0.65 V. Sharma and co-workers introduced thiadiazoloquioxaline to prepare the P210 copolymer, which possessed a low bandgap of 1.38 eV.238 Because P210 exhibited superior solubility in THF, the authors fabricated the PSC device using THF/DIO as a processing solvent, and a notable PCE of 5.12% was recorded. Chou et al. synthesized the P211 copolymer based on BDT and meta-phenoxyl-substituted Qx.239,240 They determined that by increasing the molecular weight form 37 to 71 kDa, the PCEs of the P211-based PSC devices increased to 8% from 7%. In 2012, Hou and co-workers incorporated the thienyl-BDT when constructing the P213 polymer.81 As compared to its alkoxyl-substituted counterpart P212, P213 showed a broader absorption spectrum and a higher hole mobility. The PSC device based on P213 obtained a PCE of 5%, which was 60% higher than that of the P212-based PSC device, suggesting that the application of the 2D conjugated side chains would be a feasible approach to improve the photovoltaic performance of the Qxbased polymers. Furthermore, Li et al. prepared P214 without the thiophene spacer, which achieved a PCE of 3.9%.241 Additionally, alkoxy groups were introduced to modify the Qx unit, and the P215 and P216 polymers were designed and synthesized by Zhu’s group.242,243 P215 exhibited very low-lying HOMO level of −5.51 eV, which resulted in a VOC over 0.9 V for the PSC device and a PCE of 6.31% was obtained. P216 showed broad absorption spectrum and higher carrier mobility than P215; hence, the PCE was raised to 6.9% due to the enhanced JSC and FF values. To further investigate the effects of the fluorine substituents on the device performance, Luscombe and co-workers synthesized three polymers, P217−P219, with different numbers of fluorine atoms.244 The HOMO levels of the three polymers were located

optical bandgap (1.92 eV) than P202 (1.86 eV), and they had similar HOMO levels around −5.40 eV. However, both of the polymers showed very low PCEs for the PSC devices. Ding et al. prepared the P203 copolymer, and the PSC device showed a PCE of 4.33%.232 Very recently, Sun and co-workers developed the P204 copolymer based on alkylphenyl-BDT and BDD, which indicated an optical bandgap of 1.82 eV and a HOMO level of 5.32 eV.233 The P204-based PSC device achieved a high PCE of 8.12%. 4.4. Copolymers Based On BDT and Qx

Due to its strong electronegativity, Qx and its derivatives were widely used as electron moieties to build novel photovoltaic polymers by copolymerizing with thiophene, fluorene, and IDT electron-rich units.234 In comparison with BT, Qx has more substitution positions, which could be used to improve the solubility of the resulting polymers. For example, alkyl, alkylphenyl, and alkylthienyl substitutions were frequently used as side chains to modify the Qx units. Additionally, the fluorine atom and alkoxyl groups were commonly seen in the Qxcontaining polymers. Among the numerous Qx-based D−A polymers, polymers based on BDT and Qx demonstrated excellent photovoltaic performance. Figure 23 summarizes the molecular structures and electronic properties as well as photovoltaic performances for P205-P227 are listed in Table 16. In 2008, the first BDT-Qx polymer P5 reported by Hou and co-workers only achieve a very low PCE of 0.23%.26 In 2012, Park et al. synthesized P205 and P206 based on phenylsubstituted Qx and BDT.235 Due to its enhanced intermolecular interaction of P206 with the thiophene spacer, the P206-based PSC device showed improved JSC and FF values, and a maximum PCE of 2.3% was thus obtained. Hwang et al. reported the P207 and P208 polymers based on triisopropylsilylethynyl (TIPS)substituted BDT and Qx with or without fluorination.236 P208 indicated downshifted HOMO and LUMO levels compared to P207 without causing a considerable influence on their absorption properties. Due its improved VOC from 0.79 to 7427


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at −4.98, − 5.07, and −5.08 eV for P217, P218, and P219, respectively. Due to its improved VOC for the P219-based PSC device, a maximum PCE of 3.7% with a high VOC of 0.91 V was achieved. Wang et al. systematically studied the effects of the 2D conjugated side chains on the photovoltaic performance of the PBDT-Qx polymer and synthesized a series of PBDT-Qx polymers with alkoxyphenyl or alkylthienyl on the BDT and/or Qx units.245 Among which, P220 achieved the best PCE of 7.68%

due to its broad absorption spectrum and close molecular packing. Andersson et al. introduced two alkyl side chains on the BDT unit and synthesized the P221 polymer.246 Even though P221 demonstrated a blueshifted absorption spectrum compared to its one alkyl side-substituted analogue, the P221-based PSC device achieved a very high VOC of 0.96 V. Recently, Cao et al. synthesized a series of polymers based on monofluorinated Qx and BDT.247 By fine-tuning the side chains on the thienyl 7428


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P228 P229 P230 P231 P232 P233 P234 P235 P236 P237 P238 P239 P240 P241 P242 P243 P244 P245 P246 P247 P248 P249 P250 P251 P252 P253

1.45 1.36 1.50 1.44 1.46 1.52 1.44 1.47 1.37 1.29 1.46 1.39 1.29 1.82 1.53 1.54 1.71 1.66 − − − − 1.59 1.60 1.58 1.05

−5.15 −5.10 −5.30 − −5.35 −5.33 −5.16 −5.24 −5.26 −5.31 −5.47 −5.25 −5.20 −5.39 −5.51 −5.11 −5.24 −5.38 −5.81 −5.75 −5.62 −5.62 −5.38 −5.52 −5.64 −4.96

0.71 0.62 0.66 0.76 0.76 0.77 0.69 0.72 0.78 0.78 0.88 0.74 0.68 0.68 0.98 0.71 0.77 0.80 0.85 0.79 0.72 0.66 0.85 0.88 0.92 0.38

9.4 5.2 15.9 16.21 13.6 7.42 10.46 12.64 5.69 2.67 6.59 13.17 9.89 4.11 6.2 7.93 11.71 10.20 6.40 9.38 14.96 11.82 10.48 8.95 11.30 11.74

61 43 48.9 65 60 59 48.9 56.5 58.09 52.36 54.1 57.28 50 44.8 53 34 71 65 52 66 68 52 65.8 60 68 57.2

4.1 1.4 5.12 8.00 6.2 3.3 3.5 5.1 2.60 1.09 3.14 5.58 4.04 1.2 3.2 1.91 6.41 5.36 2.80 4.85 7.31 3.98 5.86 4.76 7.04 2.55

259 259 261 262 86 86 263 263 264 153 140 265 265 266 267 275 277 278 279 279 279 279 280 281 281 280

Ding et al. synthesized a series of copolymers based on alkylsubstituted BDT and DPP, and the influence of the alkyl side chains on the charge transport and the photovoltaic properties of the conjugated polymers were studied, as indicated in Figure 24.259 P229 demonstrated a broader absorption spectrum and a higher-lying HOMO level than P228. In the OFET devices, P229 demonstrated a considerably higher hole mobility than P228 (1.6 × 10−3 vs 3.6 × 10−4 cm2 V−1 s−1) due to its improved interchain interactions. However, in the PSC devices, P228 achieved a PCE of 4.1%, whereas P229 achieved a PCE of 1.4%, suggesting that the effects of the alkyl side chains on the performances of the OFETs and PSCs were quite different. Furthermore, copolymers based on alkoxyl-substituted BDT and DPP were frequently seen in the PSCs, which showed PCEs of approximately 2%− 3%.101,260 Recently, Morse et al. further optimized the side chain length on the BDT and DPP, and the PCE was improved to 4.35% for the P230-based PSC device.261 A further improvement in the PCE to 5.12% was obtained for the optimized inverted device structure. In 2014, Hwang et al. synthesized P231 by incorporating TIPS-substituted BDT and DPP, which achieved a low bandgap of 1.44 eV.262 As a result, P231 showed an OFET hole mobility of up to 0.12 cm2 V−1 s−1, and an extraordinary PCE of 8.0% with a VOC of 0.76 V was recorded for the PSC device. The 2D-conjugated side chains were widely used to improve the photovoltaic performance of PBDT-DPP polymers. Except for the thiophene unit (see P14, P24), the benzene ring, furan, and other conjugated side chains were developed when constructing PBDT-DPP polymers. For instance, Yang and coworkers prepared the P232 and P233 copolymers using phenylsubstituted BDT as the donor unit and thiophene or furan as the π-bridges.86 Compared with their thienyl-BDT counterparts, phenyl-containing polymers indicated deeper HOMO levels.

substitution, the P222-based PSC device achieved the best PCE of 7.25%. Furthermore, Hou and co-workers designed four polymers, P13 and P223−P225, with increasing fluorine atoms from 0 to 4.82 The results suggested that the introduction of fluorine atoms did not have a significant impact on their optical bandgaps (less than 0.1 eV) but had a distinct influence on the molecular energy levels, ranging from −5.05 to −5.35 eV. The PSC devices fabricated using these four polymers demonstrated similar JSC and FF values with VOC increasing from 0.64 to 0.90 V, and a maximum PCE of 8.55% was obtained for the P13-based (4F) PSC device. In addition to the Qx units discussed above, there were several other Qx derivatives developed by researchers. For example, pyrene-fused Qx was synthesized by Zhu and co-workers, and the P226-based PSC device achieved a PCE of 4.86%.248 Wang et al. synthesized the P227-based pyrrole-fused Qx and BDT. Due its low-lying HOMO level, the P227-based PSC device showed a maximum PCE of 4.9% with a high VOC of 0.98 V.249 4.5. Copolymers Based On BDT and DPP/IID

Early in 1974, the DPP unit was synthesized by researchers, which possessed a strong electron-deficient amide group and a planar conjugated backbone.84 In OFET devices, the DPP-based molecules showed excellent transport properties for both the electron and the hole.250−252 Over the past few years, photovoltaic materials based on DPP have emerged and demonstrated good applications for its broad absorption properties and high carrier mobility.95,253 PCEs over 8% have been achieved by PSC devices fabricated by DPP-based polymers.254−257 Furthermore, polymers containing BDT as the donor unit and DPP as the acceptor unit gained considerable attention, especially due to their successful application in multiple junction solar cells.86,96,258 Here, a few representative BDT-DPP-based polymers have been summarized. 7429


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Recently, 2D-BDT was used to improve the photovoltaic performance of PBDT-IID polymers. Chou et al. synthesized the P250 polymer based on thienyl-BDT, which exhibited a high hole mobility of 0.1 cm2 V−1 s−1 in the OFET device.280 When it was applied in the PSC device, a promising PCE of 5.86% with a VOC of 0.85 V was obtained. Peng et al. introduced the fluorine atom on the IID unit and synthesized the P252 copolymer.281 As compared to its counterpart P251 without fluorination, P252 showed a more planar configuration. P252 possessed a smaller optical bandgap and a deeper HOMO level as well as a higher carrier mobility compared to P251. The P252-based PSC device achieved a notable PCE of 7.04% with a high VOC of 0.92 V. Chou and co-workers prepared the P253 copolymer based on thienoisoindigo and BDT, which showed a very low optical bandgap of 1.05 eV.280 Furthermore, P253 demonstrated excellent ambipolar mobility in the OFET devices. However, the P253-based PSC device achieved a poor photovoltaic performance.

After optimization of the PSC devices, P232 exhibited the best PCE of 6.2% with a VOC of 0.76 V. Ge et al. introduced furanylBDT to construct the PBDT-DPP polymers and synthesized P234 and P235 with different π-bridges.263 As compared to their alkoxyl-substituted analogues, P234 and P235 showed narrowed optical bandgaps and downshifted HOMO levels. P235 using furan as the π-bridge achieved a maximum PCE of 5.1% with a VOC of 0.72 V. To extend the conjugated side chain, Lee et al. introduced benzothiophene on BDT and synthesized the P236 polymer.264 P236 showed a lower HOMO level of −5.26 eV and a broader absorption spectrum with a bandgap of 1.37 eV, and a PCE of 2.6% was recorded for the P236-based PSC device. Polymer P237, which was developed by Yang and co-workers, possessed a terthiophene group as the side chain on BDT, which showed a HOMO level of −5.31 eV and an optical bandgap of 1.29 eV.153 However, the P237-based PSC device showed a relatively low PCE of approximately 1%. To improve the VOC of the PSC devices based on the DPP-containing polymer, Hou et al. introduced a dialkylthio-substituted BDT to construct the P238 polymer, which showed a low-lying HOMO level of −5.47 eV (130 mV lower than its thienyl-BDT counterpart).140 A high VOC of 0.88 V was obtained for the P238-based PSC device. In addition to the PBDT-DPP polymers mentioned above, several interesting molecules (P239−P242) have been developed by researchers. For example, modifications on the BDT or DPP units using thiazole as the bridge were all studied, and the photovoltaic parameters of the PSC devices are summarized in Table 17.265−267 Isoindigo (IID) was another strong electron-withdrawing unit widely used for the construction of low bandgap D−A polymers.268,269 After molecules based on IID were first reported by Reynolds et al. in 2010, it was soon recognized as a superior building block due to its planar conjugated backbone, electrondeficient character, and outstanding absorption property.270−272 The IID-based polymers always exhibited low bandgaps from 1.3 to 1.8 eV as well as low-lying HOMO levels. A high PCE over 8% and a high mobility over 3 cm2 V−1 s−1 were successfully achieved for IID-based polymers.273,274 Here, we summarized some representative PBDT-IID polymers and discussed their applications in PSCs. In 2011, Zhang et al.275 and Pan et al.276 independently reported the P243 polymer based on BDT and IID. They both selected the alkoxyl-substituted BDT to be used as the donor unit to construct the P243 copolymer, which indicated an optical bandgap of 1.54 eV and a HOMO level of −5.20 eV. Due to the low JSC and FF values, P243-based PSC devices showed PCEs approximately 1%−2%. Polymer P244 developed by Sun and coworkers exhibited a PCE of 6.41% by applying PCBM as donor in the PSC device.277 In 2013, Andersson’s group reported the P245 polymer based on alkyl-substituted BDT and IID.278 The P245-based PSC device achieved an impressive PCE of 5.4% with a VOC of 0.80 V. In 2014, Wang and co-workers conducted a systematic study on the photovoltaic properties of IID-based polymers using different numbers of thiophene spacers.279 They prepared four polymers, P246−P249, containing different thiophene spacers from 0 to 3. P248 and P249 showed red-shifted spectra in the solid state, and the HOMO levels of P246−P249 were located at −5.81, − 5.75, − 5.62, and −5.62 eV, respectively. For the planar polymer structure of P248 using bithiophene as the π spacer, the PSC device based on P248 achieved the best PCE of 7.4%.

4.6. Copolymers Based On BDF or DTBDT

Because BDT-based polymers have achieved considerable success in designing highly efficient photovoltaic materials, several block units derived from BDT were developed and applied to construct photovoltaic polymers and small molecules, such as benzodifuran (BDF), BDSe, dithienobenzodithiophene (DTBDT), naphthodithiophenes, anthradithiophene, tetrathienoanthracene, and BDT isomers (Figure 25).282−291 Polymer

Figure 25. Building blocks derived from BDT units.

solar cells fabricated using copolymers based on both BDF and DTBDT all achieved PCEs over 9%; in this section, we used BDF and DTBDT as examples and summarized their application in constructing highly efficient photovoltaic materials. Furan is a common renewable product, which can be prepared using vegetable residues in the food and agricultural industries. When compared to thiophene, the more electronegative oxygen of furan affects the electronic and optoelectronic properties. Moreover, because the atomic radius of the oxygen atom is smaller than that of the sulfur atom, the furan-containing polymers have increased planarity and conjugation due to its reduced steric hindrance. Figure 26 summarizes the copolymers based on BDF. As shown in Figure 27, Biewer et al. synthesized the phenylethynyl-substituted BDF unit and prepared three polymers, P254−P256.292,293 The three polymers exhibited similar optical bandgaps of approximately 2.0 eV, whereas their HOMO levels increased from −5.39 to −5.30 or −5.10 eV for P256 to P255 or P254. Due to the relatively low JSC and FF 7430


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Figure 26. Molecular structures of P254−P262 based on BDF.



Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P254 P255 P256 P257 P258 P259 P260 P261 P262

2.05 2.04 2.07 1.73 1.64 1.6 1.7 1.4 1.3

−5.10 −5.30 −5.39 −5.63 −5.49 −5.7 −5.7 −5.5 −5.6

0.75 0.81 0.83 0.85 0.75 0.74 0.64 0.69 0.66

4.0 2.24 3.71 2.89 1.74 1.66 1.31 7.0 7.4

34 44 39 59 48 48.6 36 60 47

1.05 0.79 1.19 1.44 0.65 0.59 0.30 2.89 2.28

293 292 292 294 294 295 295 296 296

[1,2-b:5,4-b′]difuran (anti-BDF) units and prepared the two copolymers by copolymerizing with the isoindigo unit.294 As summarized in Table 18, P257 had a considerably larger optical bandgap of 1.73 eV compared to P258 (1.64 eV), whereas P257 possessed a deeper HOMO level compared to P258, suggesting

values, PSC devices fabricated using these polymers only achieved a PCE of approximately 1%. Nonsubstituted BDF units were developed when synthesizing conjugated polymers. Zhang and co-workers designed the synthesized benzo[1,2-b:5,4-b′]difuran (syn-BDF) and benzo7431


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P263 P264 P265 P266 P267 P268 P269 P270 P271

1.6 1.73 1.93 1.70 1.60 1.68 1.61 1.63 1.56

−5.10 −5.11 −4.99 −5.19 −5.33 −5.08 −5.11 −5.20 −5.11

0.78 0.69 0.44 0.82 0.62 0.73 0.80 0.78 0.68

11.77 9.87 4.92 5.04 9.17 9.94 5.84 13.51 14.4

54.6 65.3 57.5 70.0 58.4 60.9 55.6 61.0 62

5.01 4.45 1.24 2.88 3.3 4.42 2.60 6.42 6.1

297 298 298 298 299 300 300 301 302



Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P272 P273 P274 P275 P276 P277 P278 P279 P280

1.53 1.52 1.53 1.60 1.61 − − 1.51 1.50

−5.03 −5.11 −5.07 −5.44 −5.25 −5.05 −5.04 −4.98 −5.12

0.54 0.63 0.61 0.87 0.59 0.64 0.64 0.78 0.65

13.13 13.88 15.75 5.88 14.21 14.6 16.8 13.04 12.11

60.03 59.81 53.82 44.59 60.70 66 64 61.55 64

4.26 5.23 5.17 2.28 5.08 6.13 6.87 6.26 5.01

303 303 303 303 303 283 283 305 306

that the syn and anti structures of BDF had significant influences on the optical and electronic properties of the conjugated polymers. Jeffries-El and co-workers synthesized the alkyl-substituted BDF unit and prepared the P259 and P260 copolymers. When the alkyl side chains were introduced into the different positions of the thiophene spacer, the two resulting polymers demonstrated a distinct absorption spectra and photovoltaic performance. Specifically, the optical bandgap of P259 was 1.6 eV, while that of P260 was 1.7 eV.295 Both polymers showed photovoltaic responses in the PSC devices; however, the PCEs were lower than 1%. Then, they prepared a series of copolymers using thiophene or furan as π-bridges, and these polymers depicted similar optoelectronic properties.296 The furan-containing polymer P261 achieved the best PCE of 2.89%. Basic optical and electronic properties as well as photovoltaic performances are listed in Table 19.

In 2012, Hou and co-workers initially synthesized the alkoxysubstituted BDF unit and used it to synthesize photovoltaic polymers.297 By copolymerizing with the DTBT unit, they prepared the low bandgap P263 polymer, which had an optical bandgap of 1.6 eV and a HOMO level of −5.1 eV. A promising PCE of 5.01% was achieved for the PSC device, demonstrating its potential in designing highly efficient polymers for PSCs. Li and co-workers selected three different electron-deficient groups, BT, BO, and BTz, as acceptor units and synthesized the copolymers P264−P266.298 The optical bandgaps of the three copolymers ranged from 1.70 to 1.93 eV, whereas their HOMO levels changed from −4.99 to −5.11 eV. The PSC device based on P264 achieved the highest PCE of 4.45%. Zou and co-workers prepared the P267 copolymer based on BDF and monomerfluorinated BT, and a PCE of 3.3% was achieved for the PSC device.299 7432


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P281 P282 P283 P284 P285 P286

1.97 1.65 1.70 1.67 1.68 1.83

−5.41 −5.33 −5.31 −5.25 −5.30 −5.43

0.97 0.87 0.80 0.83 0.83 0.92

11.2 9.14 9.81 5.28 8.74 13.28

68 59.2 58.7 51.0 61.2 77.4

7.4 4.71 4.61 2.22 4.44 9.43

307 308 309 310 310 233

improved hole mobilities. A best PCE of 6.87% with a VOC of 0.64 V was recorded for the all-Se polymer P278; however, due to the relatively high-lying HOMO levels, the PSC devices based on these polymers were lower than 0.7 V, thus limiting the PCEs. The 2D conjugated substitutions, such as thiophene and furan, were introduced to BDF as side chains by researchers. Hou and co-workers synthesized the P279 copolymer based on thienylBDF and carbonyl substituted TT.305 Compared with its alkoxysubstituted counterpart, P279 showed narrowed optical bandgap and downshifted HOMO level. Furthermore, the hole mobility of the P279/PC71BM blend reached 0.128 cm2 V−1 s−1. The VOC of the PSC device based on P279 was improved to 0.78 V, and a PCE of 6.26% was obtained, suggesting that the 2D conjugated side chains had a universal role in designing the BDT and BDF containing polymers. Biewer et al. introduced furan on the BDF and synthesized the P280 copolymer, which exhibited a narrowed optical bandgap but higher HOMO level. A maximum PCE of 5.23% with a VOC of 0.65 V was achieved for the P280based PSC device.306 Figure 29 summarizes the molecular structues, and their energy levels, band gaps, as well as photovoltaic performances are listed in Table 21. In addition to the BT and TT units, there were many other units, such as TPD, BDD, DPP, and Qx, that were used to construct the BDF-containing photovoltaic polymers. For instance, following their work of PBDT-TPD, Beaujuge and co-workers synthesized a series of copolymers based on BDF and TPD.307 As compared to PBDT-TPD, P281 showed a slightly larger optical bandgap of approximately 1.97 eV and the HOMO level of P281 was lower than that of PBDT-TPD by 0.12 eV. In their study, varied flexible side chains were used to investigate their influence on photovoltaic performance. By selecting 2ethylhexyl on BDF and n-octyl on TPD, the P281 obtained a maximum PCE of 7.4% for the PSC device.

Li and co-workers used thiophene and furan rings as side chains to replace the alkoxy side chains, and they synthesized the P268 and P269 copolymers.300 P268 using thiophene as a side chain showed larger optical bandgap and higher HOMO level than P269. However, the hole mobility of P268 reached 9.0 × 10−3 cm2 V−1 s−1, which was much higher than that of P269. The PSC device based on P268 achieved a PCE of 4.42%, whereas the P269-based device achieved a PCE of 2.6%. Zou et al. designed an interesting thieno[2,3-f ]benzofuran unit and prepared the P270 copolymer by copolymerizing with the DTBT unit, which achieved a PCE of 6.42%.301 Furthermore, Li and co-workers prepared a P271 copolymer by adopting the monofluorinated BT as the acceptor unit, and the PSC device achieved a PCE of 6.1%.302 Hou and co-workers synthesized a series of copolymers constructed using BDF and TT, and varied functional groups, such as carbonyl, ester, fluorine, sulfuryl, and cyano, were introduced on the TT units to finely tune the molecular energy levels of the P272−P276 polymers (Figure 28 and Table 20).303 The P275 and P276 copolymers with sulfuryl and cyano groups had blueshifted absorption spectra compared to other polymers. On the basis of the electrochemical measurements, they arranged the electron-deficient abilities of these functional groups in order: carbonyl < fluorine + ester < fluorine + carbonyl < sulfuryl < cyano + ester. The VOC of the PSC devices based on these polymers ranged from 0.54 to 0.87 V, and the PCEs ranged from 2.28% to 5.23%. Additionally, Li et al. prepared a copolymer based on BDF and carbonyl-substituted TT with different alkyl side chains, which demonstrated an improved VOC from 0.54 to 0.66 V for the PSC device.304 Yu et al. designed the benzodiselenophene unit and synthesized the P277 and P278 copolymers by copolymerizing with the selenopheno[3,4b]selenophene and thieno[3,4-b]thiophene units.283 The selenium-containing polymers showed narrowed bandgaps and 7433


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Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

P287 P288 P289 P290 P291 P292 P293 P294 P295 P296 P297 P298 P299 P300 P301

1.59 1.67 1.66 1.77 1.84 1.63 1.66 1.48 1.47 1.45 1.50 1.58 1.58 1.85 1.85

−5.15 −5.24 −5.18 −5.20 −5.23 −5.29 −5.53 −5.13 −5.19 −5.25 −5.35 −5.38 −5.51 −4.82 −5.36

0.68 0.89 0.65 0.30 0.71 0.72 0.79 0.62 0.65 0.75 0.75 0.70 0.70 0.83 0.92

9.85 13.0 12.2 0.96 7.23 15.77 6.64 6.41 12.61 13.9 9.80 12.58 14.98 15.1 14.1

54 65.3 63.0 28.8 54.6 59.4 53.9 60.9 63.4 61.3 65 50.28 60.0 54.3 75

3.64 7.6 5.0 0.083 2.8 6.74 2.83 2.42 5.19 6.39 4.75 4.43 6.28 6.81 9.74

284 311 312 312 312 313 313 314 314 314 316 317 318 319 320

Li et al. synthesized the naphtho[2,3-c]thiophene-4,9-dione unit and prepared the P282 copolymer.308 When compared to its BDT-based counterpart, the absorption spectrum of P282 was

slightly red-shifted, and its HOMO level was downshifted. The PSC device fabricated by P282 yielded a PCE of 4.71%. 7434


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Figure 31. Molecular structures of S1−S27.

with a VOC of 0.8 V. P284 and P285 were prepared by An and coworkers by selecting fluorinated Qx as the acceptor units.310 P285 showed a similar absorption spectrum but downshifted HOMO level by 50 mV compared to P284. The P285-based PSC device yielded a PCE of 4.44% with a VOC of 0.83 V.

Tang et al. optimized the synthesis route of 4,8-functionalized BDF units with different substitutions and prepared a series of copolymers, which showed varied optical and electronic properties.309 They used P283 as an example to investigate their photovoltaic properties, and a PCE of 4.61% was obtained 7435


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Table 23. Optical, Electronic, and Photovoltaic Performances of S1−S27 molecules

Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 S23 S24 S25 S26 S27 S28

1.83 1.74 1.79 1.74 1.60 1.60 1.59 1.61 1.72 1.76 1.76 1.78 1.80 1.77 1.73 1.97 1.86 1.82 1.79 1.75 1.88 1.82 1.83 1.76 1.77 1.82 1.80 1.87

−5.11 −5.02 −5.08 −5.07 −5.16 −5.16 −5.18 −5.19 −5.02 −5.06 −5.07 −5.13 −5.15 −5.51 −5.18 −5.36 −5.34 −5.25 −5.13 −5.18 −5.15 −5.15 −5.33 −5.19 −5.11 −5.34 −5.37 −5.20

0.93 0.93 0.94 0.91 0.92 0.92 0.91 1.03 0.93 0.96 0.92 0.91 0.90 0.94 0.97 0.97 0.90 0.90 0.908 0.968 0.96 0.97 0.92 0.94 0.87 0.90 0.95 0.88

9.77 12.21 12.56 14.45 8.58 11.05 9.47 10.07 13.17 11.92 12.09 12.93 12.20 12.5 13.45 9.1 10.5 7.88 9.65 8.80 10.32 8.67 6.89 8.0 9.94 13.90 11.86 6.30

59.9 65.0 70.0 73.0 64.8 66.4 48.2 54.7 66.3 59.4 72.1 71.0 70.0 69.0 70.5 52.0 46.3 63.7 60.1 63.6 59.0 60.0 63.0 70.0 65.0 74.0 70.0 75.0

5.44 7.38 8.26 9.95 5.11 6.75 4.15 5.67 8.12 6.79 8.02 8.70 8.01 8.1 9.2 4.62 4.37 4.51 5.27 5.42 5.84 5.03 4.0 5.26 5.64 9.3 7.93 4.16

330 331 332 327 333 333 333 333 334 334 335 336 336 326 337 338 339 340 341 341 342 342 343 344 344 345 346 347

an extremely low efficiency of nearly 0%, which may be caused by its low mobility. Recently, Kim et al. synthesized the P292 and P293 copolymer using thienyl-substituted DTBDT as the donor unit.313 Due to P292’s flat structure, highly crystalline properties were observed in its film. Conversely, the twisted structure of P293 resulted in its amorphous character. Photovoltaic devices fabricated by P292 and P293 achieved a PCE of 6.74% and 4.44%, respectively. It should be noted that the PSC device based on P292 exhibited good stability in air (i.e., retaining 95% of the initial PCE after being stored for over 1000 h without encapsulation). Chu and co-workers prepared a series of copolymers, P294− P296, based on DPP and DTBDT, and different substitutions, alkoxy, thienyl, and TIPS, were introduced on the DTBDT to tune the optical absorption, molecular energy levels, and charge transporting properties of the polymers.314 The optical bandgaps of P294−P296 ranged from 1.45 to 1.48 eV, and their HOMO levels changed from −5.13 to −5.25 eV. A best PCE of 6.39% with a VOC of 0.75 V, a JSC of 13.9 mA/cm2, and an FF of 61.3% was obtained for the P296-based PSC device. Additionally, Son and co-workers prepared the P297 copolymer based on alkysubstituted DTBDT and DPP, demonstrating a hole mobility as high as 2.7 cm2 V−1 s−1.315 Then, they fabricated the PSC device based on P297, which achieved a PCE of 4.75%.316 In addition to the copolymers discussed above, researchers also designed several other DTBDT-based polymers by applying different acceptor units. For example, Yang and co-workers prepared the P298 copolymer, which achieved a maximum PCE of 4.43% in the PSC device.317 P299, which was developed by Cao et al., achieved a PCE of 7.52% for an inverted PSC device.318 Park et al. used the highly polarized group,

Sun et al. introduced the benzene ring on the BDF unit and synthesized the P286 copolymer.233 P286 had a large optical bandgap of 1.83 eV and a HOMO level of −5.43 eV. Furthermore, its hole mobility reached 0.014 cm2 V−1 s−1. The P286-based PSC device achieved an impressive PCE of 9.43% with a VOC of 0.92 V, a JSC of 13.28 mA/cm2, and an FF of 77.4%, which was the highest PCE value obtained in BDF-containing polymers. Figure 30 summarizes the molecular structures, and their basic optical and electronic properties as well as photovoltaic performances are listed in Table 22. To extend the π-conjugated system of BDT, DTBDT was designed and applied by researchers when preparing photovoltaic polymers. In 2012, Hou and co-workers first synthesized the P287 copolymer based on alkoxy-substituted DTBDT and TT.284 P287 had an optical bandgap of 1.59 eV and a HOMO level of −5.15 eV, which was similar to its BDT-based counterpart. The P287-based PSC device achieved a PCE of 3.64% (VOC = 0.68 V, JSC = 9.85 mA/cm2, and FF = 54%). Yu et al. synthesized the alkyl-substituted DTBDT and prepared three P288 copolymers by copolymerizing with the fluorinated TT unit.311 The P288-based PSC device exhibited a PCE of 7.6% (VOC = 0.89 V, JSC = 13 mA/cm2, and FF = 65.3%). Afterward, they further extended the π-conjugated system and prepared the P290 and P291 copolymers using a ladder-type structure.312 As the conjugated area increased, the optical bandgaps increased to 1.66, 1.78, and 1.84 eV for P289, P290, and P291, respectively. Furthermore, the HOMO levels of these copolymers were located at −5.03, − 5.14, and −5.12 eV for P289, P290, and P291, respectively. The P289-based PSC device achieved a PCE of 5.0%, whereas the P291-based PSC device achieved a PCE of 5.5%. Unexpectedly, the PSC device fabricated P290 achieving 7436


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as solubility, molecular stacking, energy levels, absorption spectrum, and morphology. To investigate the effects of the alkyl side chains on the photovoltaic performance of small molecule donors, a series of BDT-based small molecules, S2,331 S3,332 and S4,327 were synthesized by Chen et al. These three small molecular donors shared the same backbone structure and end-capped groups but different substitutions on the BDT. By replacing the branched alkoxy with octyl on the BDT, a deeper HOMO level and a higher VOC value were obtained for S3 due to the weaker electron donating ability of alkyl.332 After a two-step annealing (TSA) approach involving thermal annealing and solvent vapor annealing to optimize the film morphology, a best PCE of 8.26% was acquired for the S3-based SMSC device. The alkylthiol side chain was used to modify the BDT unit to further optimize the optoelectronic properties and molecular packing for S4.327 A remarkable PCE as high as 9.95% was achieved using a TSA-processed S4-based device (a VOC = 0.92 V, JSC = 14.61 mA/cm2, and FF = 74%), which was one of the highest PCE for single junction OPVs. 2D-BDT units were frequently used to further improve the photovoltaic performance of BDT-based SMSCs. Li et al. introduced thiophene substitution on BDT and synthesized S6 and S8. As compared to the alkoxy substituted counterparts S5 and S7, S6, and S8 showed enhanced and extended absorption properties.333 Benefiting from their improved absorption properties, the JSC of the SMSC devices based on S6 and S8 increased to 11.05 and 11.07 mA/cm2 from 8.58 and 9.47 mA/ cm2 for S5 and S7, respectively. Chen et al. constructed a range of 2D conjugated small molecules, S9−S11, using varied 2D conjugated side chains.334,335 Compared with S2, which used the alkoxy-substituted BDT, the introduction of conjugated side chains on BDT resulted in a red-shifted absorption spectrum. An initial PCE of 8.12% was obtained for the S9-based SMSC device.334 After solvent annealing the S9/PC71BM blend using carbon disulfide, a high PCE of 9.58% was achieved for the S9based SMSC device.335 When a longer alkyl side chain 2hexyldocyl (HD) was attached to the thienyl-BDT, the resulting S10 showed an improved VOC of 0.96 V, which may be caused by the larger steric hindrance of HD. However, the JSC and FF values of the S10-based SMSC device were all lower than that of the S9based device, thus limiting its PCE. The insertion of bithiophene increased the electron-donating ability and the delocalization of the π electron, thus slightly lifting the HOMO level of S11. Recently, Chen et al. introduced an alkyl- and alkylthiosubstituted thieno[3,2-b]thiophene (TT) unit on BDT and synthesized the S12 and S13.336 The alkylthio-containing molecule S13 demonstrated an increased dihedral angle between the thiophene and the BDT, thus slightly decreasing its intermolecular interactions. The SMSC device based on S12 achieved a PCE of 8.70% with a notable FF of 72%. Yang et al. synthesized S14 and fabricated the SMSC device, and they determined that using polydimethylsiloxane (PDMS) as an additive in the blend solution could improve the PCE from 7.2% to 8.1%.326 With the use of a homotandem device structure, a record PCE of 10.1% (VOC = 1.82 V, JSC = 7.70 mA/cm2, and FF = 72%) was achieved. Li et al. introduced alkylthio-substituted thiophene on the BDT and synthesized the S15 molecule.337 The introduction of alkylthio-thienyl downshifted the HOMO level, slightly red-shifted the absorption spectrum, and enhanced the hole mobility of S15, thus leading to an overall improvement in the photovoltaic properties for the S15-based device. Surprisingly, a maximum PCE value of 9.20% was obtained for the S15based SMSC device processed without any additives and

dithieno[3,2-b:2′,3′-d]phosphole oxide, as the acceptor unit to construct the P300 copolymer, which achieved a PCE of 6.81% in the PSC device.319 Recently, Sun et al. prepared the P301 copolymer, which possessed a wide optical bandgap of 1.85 eV and a low-lying HOMO level of −5.36 eV.320 The PSC device achieved a high PCE of 9.74% with an impressive FF of 75%, which was the highest PCE value for the PSC devices constructed with DTBDT-based polymers, thus suggesting that DTBDT had significant potential in designing highly efficient conjugated molecules for solar cells.

5. BDT-BASED PHOTOVOLTAIC SMALL MOLECULES In recent years, small molecule solar cells (SMSCs) emerged as challenging rivals to PSCs due to their advantages of defined structure and thus less batch-to-batch variation.321−325 The gap between the PCEs of SMSCs and PSCs is further diminished in the wake of state-of-the-art SMSCs achieving PCEs over 10%.326−328 In the early stages, several triphenylamine (TPA)based molecules and hyperbranched molecules were used as the photovoltaic active layer in the SMSC device and achieved great progress. Over the past few years, following the considerable success of the BDT unit in designing highly efficient photovoltaic polymers, researchers incorporated the BDT units in small molecules and the SMSC devices achieved good photovoltaic performance. Before BDT, thiophene core-based small molecules achieved the highest PCE of approximately 5% for solar cell devices.329 Compared to the thiophene unit, BDT possesses a large and rigid conjugated structure, which is in favor of electron delocalization and coplanar π−π stacking, thus benefiting the charge transport and resulting in a higher FF value. Additionally, as a relative weak electron donor, BDT would lower the HOMO level of the resulting molecule, thus, the corresponding device possessed a high VOC value. In this section, we summarize the BDT-containing small molecules and discuss their application in SMSCs. For the structures of the BDT-containing molecules, various alkyl side chains, π-bridges, and electron-withdrawing units have been widely used to modify the optical, electronical properties of the materials and thus the photovoltaic performance of the SMSCs. Furthermore, a few other small molecules based on the building blocks derived from BDT will be presented. 5.1. Side Chain Engineering of BDT-Based Photovoltaic Small Molecules

In 2011, Chen et al. first introduced BDT units as the core building block in SMSCs to improve the film quality and the charge mobility of small molecules.330 They designed and synthesized S1 using the unsubstituted BDT unit as the central unit and the electron-withdrawing alkyl cyanoacetate group as the end-capped group (Figure 31 and Table 23). S1 had an optical bandgap of 1.83 eV and a HOMO level of −5.11 eV. Furthermore, its absorption spectrum showed a shoulder peak at a long wavelength, suggesting that a rigid coplanarization of the conjugated systems existed in the S1 film. Additionally, the XRD measurement suggested that S1 possessed a highly crystallized character, which was beneficial for the charge transport. The optimized device based on the S1/PC61BM blend achieved a PCE of 5.44% (VOC = 0.93 V, JSC = 9.77 mA/cm2, and FF = 59.9%) without being subjected to any special treatments. Afterward, side chain engineering was performed by researchers to modulate the properties of the molecules, such 7437


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Figure 32. Different building blocks in the backbone of S29−S61. 7438


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Table 24. HOMO Energy Levels, Optical Bandgap, and Photovoltaic Parameters of S29−S61 molecules

Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

S29 S30 S31 S32 S33 S34 S35 S36 S37 S38 S39 S40 S41 S42 S43 S44 S45 S46 S47 S48 S49 S50 S51 S52 S53 S54 S55 S56 S57 S58 S59 S60 S61

1.84 1.72 1.75 1.84 1.76 1.85 1.93 1.96 1.90 1.80 1.77 1.77 1.78 1.85 1.75 1.71 1.55 1.60 1.65 1.59 1.51 1.51 − 1.66 1.69 1.69 1.58 1.53 1.55 1.85 − 1.82 1.68

−5.04 −5.45 −5.40 −5.08 −5.18 −5.11 −5.27 −5.33 −5.44 −5.13 −5.17 −5.11 −5.20 −5.37 −5.5 −5.30 −5.14 −5.20 −5.23 −5.36 −5.08 −5.13 − −5.15 −5.15 −5.15 −5.16 −5.17 −5.13 −5.10 −5.20 −5.20 −5.38

0.95 0.91 0.90 0.89 0.92 0.93 0.92 0.89 0.89 0.89 0.89 0.82 0.85 0.97 0.85 0.76 0.67 0.80 0.84 0.76 0.62 0.67 0.82 0.74 0.73 0.64 0.78 0.78 0.64 0.89 0.83 0.975 0.92

8.00 5.17 9.08 9.98 10.2 6.1 4.7 2.7 7.94 2.07 9.33 4.74 10.48 11.48 8.7 5.22 4.12 3.49 11.97 12.2 15.64 8.35 7.6 9.32 4.62 0.45 4.22 3.44 9.66 8.83 4.60 10.08 11.68

60.0 46.0 66.0 72.0 68.0 53.0 54.4 55.2 40.0 29.0 54.5 40.5 66.0 70.0 51.0 55.0 58.3 52.5 57.6 62.0 59.4 57.0 60.0 54.6 54.9 42.4 27.0 57.0 46.0 66.2 43.0 51.3 62.0

4.56 2.13 5.42 6.4 6.4 3.0 2.40 1.33 2.83 0.54 4.53 1.58 5.88 8.10 3.8 2.19 1.62 1.46 5.79 5.9 5.77 3.20 3.74 3.76 1.83 0.12 0.91 1.52 2.85 5.20 1.62 5.05 6.66

331 348 348 349 349 349 350 350 351 352 353 353 354 355 356 358 357 357 359 362 363 363 372 364 364 364 365 365 365 366 361 368 369

designed molecules S24 and S25 with different lengths of their alkyl side chains attached to the π-bridge and end-capped group.344 S25, which had a shortened alkyl side chain, exhibited more dense packing and a higher degree of crystallinity than S24, which was also consistent with the DSC and GIXRD measurements. The SMSC device based on S25 achieved a PCE of 5.64% with an enhanced JSC of 9.94 mA/cm2, which was due to the purer crystalline domains of the donor material. S26, which was synthesized by Jones and co-workers and based on dialkyl-substituted thienyl-BDT, exhibited a nematic liquid crystalline behavior, which was seldom observed in previous reports.345 The GIWAXS pattern indicated that S26 possessed two different molecular arrangements (including both edge-on and face-on) in the pristine film. Benefiting from its high hole mobility, S26 exhibited excellent photovoltaic performance. A best PCE of 9.3% with a remarkable FF of 77% was obtained for the SMSC device. Additionally, when the thickness of the active layer increased to 400 nm, an FF exceeding 70% with ca. 8% PCE was achieved for the SMSC device. These inspiring results proved the application potential of S26 in the large-area printing technology field. Furthermore, S27 and S28 were developed by researchers and depicted similar results.346,347

annealing treatments. Except for the thiophene unit, several other 2D conjugated side chains were developed by researchers to modify the BDT. For example, Kim et al. designed and synthesized S16338 and S17339 using alkylthienyl and alkylselenophenyl-substituted BDT units, respectively. From S16 to S17, the optical bandgaps shrank substantially from 1.97 to 1.86 eV, which played a key role in improving the JSC of the SMSC devices. Lastly, a PCE of 4.37% with a JSC of 10.5 mA/cm2 was achieved for the S17-based SMSC device. Yang et al. attached the alkoxyphenyl substituent on the BDT unit as a weak electron-donating conjugated side chain and synthesized the S18 molecule, which achieved a PCE of 4.51%.340 Li et al. designed and synthesized S19 and S20 using alkoxyphenyl-substituted BDT and meta-fluorinated-alkoxyphenyl-substituted BDT as the core, respectively.341 They determined that the introduction of F atoms on BDT significantly lowered the HOMO level of S20, thus resulting in an increase of ca. 60 mV in VOC for the corresponding SMSC device. Using the triisopropylsilylethynyl (TIPS) substitution, Ko et al. synthesized S21 and S22.342 The best PCEs of 5.84% and 5.03% were obtained for devices fabricated from the S21 and S22/PC61BM blend with a small amount of a high boiling point additive, respectively. Wong et al. used TVT as a side chain to synthesize S23, which achieved a PCE of 4.0%.343 To investigate the effects of the alkyl side chains on the photovoltaic performances of small molecules, Wei et al.

5.2. Electron-Withdrawing Units used in the Backbone of BDT-Based Photovoltaic Small Molecules

The electron-withdrawing moiety plays an important role in tuning the absorption spectra molecular energy level and charge 7439


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behavior and film morphology, the SMSC device based on S42 exhibited a PCE of 8.10%. This result offered a new strategy in the structural design of small molecule donors. Marks et al.356 and Nguyen et al.357 independently synthesized molecule S43 based on BDT and DPP. S43 possessed a relatively low optical bandgap of approximately 1.7 eV. The S34-based SMSC device achieved a PCE of 3.8% with a VOC of 0.85 V. Furthermore, Tu and co-workers reported similar results using S44.358 Nguyen et al. focused on the effect of end-capped groups on the molecular energy levels and absorption spectra and reported the molecules S45 and S46 end-capped with thiophene and benzofuran, respectively.357 Zhan et al.359 and Yao et al.360 independently used thienyl-substituted BDT and DPP to synthesize a 2D-counjugated S47, which showed a low optical bandgap of 1.64 eV. The SMSC device based on S47 gave a PCE of 5.29% by optimizing the blend film morphology. Subsequently, Yao et al. also utilized S43 and S47 to fabricate nonfullerene solar cell devices by blending them with a perylene diimide dimer as the acceptor, and a PCE of 2.01% was obtained.361 By changing the end-capped groups of S47, a series of molecules S48−S50 were synthesized by researchers, and the corresponding SMSC devices achieved PCEs of approximately 3%−6%.362,363 Yao et al. synthesized S52−S54 to explore the effects of anchoring terminals on the photovoltaic performance.364 They determined that S53 with a COOCH3 side chain demonstrated a stronger molecular interaction and smaller π−π stacking distance than the others. The SMSC device based on S53 achieved a maximum PCE of 3.76% with an increased JSC of 9.32 mA/cm2. They also synthesized S55−S57 to investigate the influence of end-capped groups on the photovoltaic performance.365 Among them, S56 with the CNR terminal demonstrated a low bandgap of 1.53 eV. The morphological characterization suggested that the end-capped groups had a significant influence on the phase separation of the blend film. In addition to the end-capped groups, the π-bridges also played critical roles in tuning the photovoltaic performance for small molecules. Yang et al. synthesized S58 using thieno[3,2b]thiophene as the π-bridge.366 The introduction of thieno[3,2b]thiophene prolonged the conjugation length of the main chain and enhanced the π−π stacking, which facilitated the charge carrier transport. The S58-based SMSC device achieved a PCE of 5.20%. Yao designed a new small molecule S59 by increasing the number of thiophene π-bridges.361 A theoretical calculation demonstrated that the introduction of the extra thiophene in the main chain caused a more twisted backbone structure. Due to its poor blend morphology, the S59-based SMSC device achieved a relatively low PCE of 1.62%. Then, they synthesized a series of small molecules based on the BDT core and methyl-dioxocyanopyridine (MDP) terminal to systemically investigate the effects of the oligothiophene π-bridges on the photovoltaic performance.367 Chen and co-workers used dithieno[3,2-b:2′,3′-d]silole (DTS) as the π-bridge to synthesize the S60 molecule.368 Compared with S9, which used thiophene as the π-bridge, S60 showed a narrowed absorption spectrum with a band gap of 1.82 eV. Meanwhile, the deeper HOMO level resulted in a high VOC of 0.98 V for the S60-based SMSC device. However, only a moderate PCE of ca. 5% was attained due to the lower JSC and FF values. Palomares et al. used a cyclopentadithiophene moiety as the π-bridge to synthesize the S61 molecule.369 After performing a TSA approach, the devices achieved a PCE of 6.66%.

carriers mobility of small molecules. In addition to the endcapped groups, such as rhodamine and indenedione, several other electron-withdrawing units, such as BT, DPP, and BT, were developed by researchers to construct small molecules for solar cells. The rational selection of end-capped groups is of great significance in improving the photovoltaic performances of the SMSCs. Additionally, during the preparation of S2, Chen et al. synthesized S29 end-capped with octyl cyanoacetate, which showed a red-shifted absorption spectrum of approximately 40 nm compared to S2.331 The SMSCs based on S29 achieved a moderate PCE of 4.56% with a high VOC of 0.95 V. Chu et al. also designed two molecules, S30 and S31, that were end-capped with dicyanovinyl (CN) or cyanoacetate (CNR) units, respectively.348 The XRD pattern indicated that S31, using the CNR terminal group, showed enhanced crystalline property. The SMSC device based on S31 exhibited a PCE of 5.42%, whereas its counterpart fabricated from S30 only obtained a PCE of 2.13% due to the dramatically reduced JSC value. Wei et al. synthesized S32−S34 with three different endcapped groups [i.e., octyl-2-cyanoacetate, 3-oxoundecanenitrile, and 2-(octylsulfonyl) acetonitrile].349 The authors determined that S32 depicted the strongest molecular interaction, whereas S34 demonstrated the weakest. As the electron-withdrawing capability of acceptors increased, the molecular HOMO levels were downshifted while their absorption spectra were red-shifted to a certain extent. Lastly, the best PCE of 6.4% was achieved from the SMSC device based on S33. Figure 32 summarizes the different building blocks in the backbones, and their energy levels, band gaps, as well as photovoltaic performances are listed in Table 24. Park et al. incorporated TPD as the electron-withdrawing unit to synthesize the S35 and S36 molecules, which achieved a relatively low PCE of approximately 1%−2%.350 Lee and coworkers synthesized S37 using BT and TPA as the terminal groups.351 Benefiting from a highly ordered π-stacked structure upon thermal annealing, the corresponding device obtained a PCE of 2.83%. Yang et al. synthesized the S38 molecule using the BT unit as the electron-withdrawing unit.352 The OFET device based on S38 showed a remarkable hole mobility as high as 0.016 cm2 V−1 s−1 due to its uniform film morphology, which was one of the highest values for solution processing small-molecule donors in organic solar cells at that time. However, the SMSC device based on S38 achieved a PCE of 0.54% with a low FF of 0.29. Furthermore, Li and co-workers synthesized two BT-based small molecules, S39 and S40, to investigate the impact of the moiety sequence in the backbone structure.353 S40 showed a redshifted absorption spectrum than S39 in the solution state. Due to the low-lying HOMO level of S39 and its enhanced crystalline property, the device based on S39 achieved a considerably higher PCE of 4.53% whereas the PCE of the device fabricated by S40 was only 1.58%. Li et al. introduced monofluorinated BT to synthesize S41, which exhibited a PCE of 5.88% at a relative low fullerene acceptor content of 25%.354 Generally, small molecules have a better crystallinity than their polymer counterparts, thus forming purer domains. Wei et al. reported a novel D−A−D−A− D-structured small molecule S42 with thiophene-substituted BDT as donor and BT as acceptor.355 Compared to its polymeric counterpart (i.e., the copolymer based on BDT and BT), S42 had a shorter conjugated length and a larger optical band gap (1.83 eV). However, the HOMO energy levels of the two compounds were similar, thus resulting in close VOC values in the photovoltaic devices. Benefiting from the better crystalline 7440


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Figure 33. Small molecules based on building blocks derived from BDT.

Table 25. HOMO Energy Levels, Optical Bandgap, and Photovoltaic Parameters of S62−S78 molecules

Egopt (eV)

HOMO (eV)

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

S62 S63 S64 S65 S66 S67 S68 S69

1.75 1.72 1.88 1.72 1.70 2.16 1.96 1.71

−5.48 −5.41 −5.61 −5.40 −5.11 −5.16 −5.34 −5.19

0.88 0.86 0.85 0.84 0.755 0.93 0.95 0.766

6.32 9.94 10.6 11.2 11.7 3.79 6.18 11.4

53.6 59.1 56.0 42.7 50.1 28.0 37.0 63.2

2.98 5.05 4.98 4.0 4.4 0.98 2.20 5.53

370 370 371 374 374 375 375 376

5.3. Photovoltaic Small Molecules Based On BDT Derivatives

similar absorption spectrum but a higher HOMO level and enhanced hole mobility. The SMSC device based on S66 achieved a PCE of 4.4%. Additionally, Lee et al. synthesized S67 and S68 using an unsubstituted NDT unit, and the SMSC device based on S68 achieved a PCE of 2.2%.375 Marks et al. introduced benzo[1,2-b:6,5-b′] dithiophene when designing photovoltaic small molecule and synthesized S69, which achieved a PCE of 5.53% in the SMSC device.376 In conclusion, various excellent small molecule donors based on BDT units have been synthesized and applied to SMSCs in recent years, and a PCE over 10% was achieved. BDT units have been approved as successful building blocks for constructing photovoltaic small molecules, and the absorption spectra, molecular energy levels, molecular stacking, and blend film morphologies can be finely tuned through comprehensive side chain engineering and rational selection of other electronwithdrawing building blocks in the backbone.

In addition to the BDT-based small molecules, there were several photovoltaic small molecules constructed by building blocks derived from BDT, such as naphtho[1,2-b:5,6-b′]-dithiophene (NDT) and DTBDT. For example, Chu et al. synthesized S62 and S63 using benzotrithiophene as the core to investigate the effects of the length of the π-bridge in the backbone.370 S63 with more thiophene units achieved a better PCE of 5.05%. Kim et al. synthesized S64 based on DTBDT and TPD, which achieved a PCE of 4.98% for the S63-based SMSC device.371 Inganäs et al. replaced one of the thiophene with furan on the BDT unit and synthesized the thieno[2,3-f ]benzofuran unit, and the S51-based SMSC device achieved a PCE of 3.74%.372 Figure 33 summarizes the small molecules based on different building blocks derived from BDT, and their energy levels, band gaps, as well as photovoltaic performances are listed in Table 25. In 2011, Marks and co-workers synthesized S65 by incorporating NDT as the core.373 Due to its broad absorption and ordered molecular packing, the SMSC device based on S65/ PC61BM achieved a PCE of 4.0% with a VOC of 0.84 V. Furthermore, they synthesized the NDT analogue, naphtho[1,2b:5,6-b′]dithiophene.374 As compared to S65, S66 exhibited a

6. APPLICATION OF BDT-BASED POLYMER DONORS IN FULLERENE-FREE PSCS Fullerene derivatives (PC61BM and PC71BM) are the most common and successful acceptor materials used in highly 7441


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Figure 34. Highly efficient BDT-containing polymer donors used in fullerene-free PSC device.

NF1, as acceptor and P90 as donor only gave a PCE of 0.13%. Therefore, twisted PDIs were designed to avoid the large aggregation domains, by means of PDI dimers linked at the imide positions or bay positions. When blended with the same polymer donor P90, a thiophene-linked bis-PDI molecule, NF2, showed significant reduction of aggregation size in the blend film, and thus leading to an outstanding PCE of 4.03% for the PSC device. Similarly, PDI dimers were linked together through different methods. For instance, Wang et al. synthesized NF3 by directly linking the bay positions of two PDI units and introduced branched alkyl chains on the imide positions.405 By using P90 as polymer donor material, PCE of 3.63% was obtained in the fullerene-free PSC device. Jen and co-workers replaced P90 by P79 as donor material in the NF3-based cell device, and PCE was boosted to 5.9% after the optimization of device engineering.384 Recently, Jen et al. synthesized a series of fused PDI derivatives (NF5−NF7) bridged by furan, thiophene, and selenophene to alleviate the overstrong aggregation problem.388 In comparison with the unfused PDI molecule (NF4), the reduced reorganization energy and extended effective π-conjugation of NF5-NF7 facilitated the exciton diffusion and charge transport, which might contribute to the improved photovoltaic performance. By selecting P79 as the donor material, PCE of 6.48% was recorded in the NF6-based PSC device. Wang et al. introduced sulfur and selenium atoms into the cores of PDIs and synthesized NF8386 and NF9,383 respectively. By incorporating a wide bandgap polymer PDBT-T1 as donor material, the NF8- and NF9-based fullerene-free PSC devices yielded very impressive PCE of 6.90% and 8.23%, respectively. In addition to the bay-linked PDIs discussed above, linking two PDI units via the imide positions is another effective method to disrupt the planarity of PDIs, and thereby PDI dimers and PDI trimers were designed and synthesized. In 2013, Narayan and coworkers fabricated the PSC device by employing the NF10 as acceptor and P90 as donor, and PCE of 3.2% was obtained.406 By changing the alkyl chain of NF10 and selecting P57 as donor in the device, Hou et al. improved the PCE of the NF11:P57-based PSC device to 5.4%.407 Jen and co-workers systematically investigated the impact of the molecular geometry of the polymer donors (P9 and P79) and the PDI dimer acceptors (NF3 and

efficient BHJ PSCs. However, the fullerene-based PSCs suffer relatively large energy loss (generally over 0.6 eV) and thus limit its further improvement.267,377 What is more, fullerene-based materials have some other drawbacks such as poor absorption properties and costly preparation.378,379 To address these problems, more and more efforts have been devoted to designing and synthesizing nonfullerene acceptor materials, and the PCE of fullerene-free PSCs has been improved to over 11%, almost approaching the best results of its fullerene counterparts.203,380−383 Over the past several years, many types of polymers and small molecules acceptors (as shown in Figure 35) containing perylene diimides (PDIs),384−391 naphthalene diimides (NDIs), 3 8 0 , 3 9 2 − 3 9 7 or indacenodithiophenes (IDTs)398−402 units as core structures379,403 were developed and applied in PSCs, and some of them achieved very impressive results. The rapid development of fullerene-free PSCs has opened a new avenue for the fundamental study of organic photovoltaics. Even though a lot of polymer donors exhibited superior photovoltaic properties in fullerene-based PSC systems, selecting proper donors with suitable energy levels, complementary absorption spectra, as well as favorable blend morphology for nonfullerene acceptors is of sigificance to obtain highly efficient fullerene-free PSCs. As continued research focuses on the fullerene-free PSCs, BDT-based polymers donors, especially the 2D-BDT-based polymers, have come to the fore in fabricating PSC devices due to their superior properties. BDT-based polymers like PBDT-TT, PBDT-BT, and PBDT-BDD families (Figure 34) were widely used as donor materials in the highly efficient fullerene-free PSC devices. In this section, we present a short review and have a discussion about the application of BDTbased polymers donors in fullerene-free PSCs. PDI-based molecules (polymers and small molecules, as shown in Figure 35 and Table 26) are promising nonfullerene acceptors due to their high electron mobility, strong absorption ability, and high environmental/thermal stability.386 Traditional PDIs often have overstrong aggregation in the active layer, which limits the exciton diffusion and separation process, leading to poor photovoltaic performance of the devices.404 For example, in 2013, the PSC device fabricated from monomeric PDI molecule, 7442


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Figure 35. Molecular structures of some representative nonfullerene acceptors.

also could find that polymer with 2D conjugated side chains always showed better photovoltaic performance than their 1D counterparts. Jenekhe et al. synthesized NF16409 and NF17,382 which showed PCE of 3% and 6.7%, respectively, in the PSC devices blended with P79 as donor material. Over the past two years, small acceptor molecules based on IDT units have been explored and showed great potential in achieving outstanding photovoltaic performance. In 2015, Zhan et al. reported the synthesis of NF18, which showed a low bandgap of 1.57 eV and appropriated LUMO level of −3.82 eV.400 PSC device based on NF18:P79 showed high PCE of

NF11) on the BHJ morphology of the blend films.387 Their results suggested polymer with 2D conjugated side chains could enhance the miscibility with PDIs and thus lead to better photovoltaic performance. Furthermore, the N−N linked PDI trimer, NF12, exhibited impressive PCE of 7.25% when blended with P57 as donor material.408 Helical PDIs were also developed as acceptors for PSC devices and achieved some very impressive results. For example, NF13− NF15 were synthesized by Nuckolls and co-workers and showed PCE ranging from 5.94% to 8.3%, when blended with P79 as donor material in the PSC devices.381,385 From their results, we 7443


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Polymer acceptors containing PDIs 389,411−413 and NDIs380,392−394,414,415 were also developed and applied in fullerene-free PSC devices, and some of them achieved very good photovoltaic performance. For instances, polymer NF22 composed of NDI and thiophene was synthesized by Kim and co-workers, which exhibited a PCE of 5.96% by incorporating P79 as donor material in the PSC device.394 Jenekhe et al. replaced the thiophene of NF22 with selenophene and synthesized NF23, and a high PCE of 7.73% was recorded in the PSC device by using the same donor.415 N24 was also a widely studied polymer acceptor, and some impressive results were obtained by N24 and BDT-based polymer donors. In 2014, via incorporating P79 as donor, a PCE of 5.73% was obtained by Ito et al. in the N24-based PSC device.396 Li and co-workers further improved the PCE of N24-based PSC device by using P164 as donor material.380 Hou et al. conducted a deep research about the influence of the donor/acceptor molecular interactions and orientation on the PSC device performance, and their results suggested that BDT-containing polymer donors with 2D conjugated side chains were more likely to obtain better photovoltaic performance than their 1D counterparts.393 Jen et al. synthesized a fluorinated nonfullerene acceptor, N25, and showed a maximum PCE of 6.71% when blended with P79 as donor in the PSC device.392 In addition, PDI-based polymers like NF26389 and NF27411 were also developed and applied in fabricating PSC devices, and moderate PCE were recorded in the PSC devices. As discussed above, the best PCE of fullerene-free PSC device have reached 11.21%, approaching the fullerene-based PSCs. As more efforts continue to develop highly efficient nonfullerene acceptors and suitable donors, further improvement of PCE is expected in the near future, and we believe BDT-containing polymers will still be the most promising donors for the fullerenefree PSCs.

Table 26. Photovoltica Paremeters of Fullerene-Free PSC Devices acceptor

donor

VOC (V)

JSC (mA/cm2)

FF (%)

PCE (%)

ref

NF1 NF2 NF3 NF3 NF3 NF3 NF4 NF5 NF6 NF7 NF8 NF9 NF10 NF11 NF11 NF11 NF12 NF13 NF13 NF14 NF14 NF15 NF15 NF16 NF17 NF18 NF19 NF19 NF19 NF20 NF21 NF22 NF23 NF24 NF24 NF24 NF25 NF26 NF27

P90 P90 P90 P9 P79 P79 P79 P79 P79 P79 P301 P301 P90 P9 P79 P57 P57 P9 P79 P9 P79 P9 P79 P79 P79 P79 P79 P165 P12 P301 P301 P79 P79 P79 P164 P12 P79 P57 P90

0.97 0.85 0.73 0.78 0.79 0.80 0.88 0.92 0.93 0.92 0.90 0.947 0.77 0.79 0.79 0.82 0.732 0.789 0.796 0.77 0.81 0.79 0.80 0.96 0.95 0.97 0.81 0.89 0.899 0.88 0.89 0.79 0.81 0.794 0.83 0.87 0.81 0.77 0.752

0.33 8.86 10.58 10.51 12.86 11.98 9.74 8.71 11.95 11.19 11.65 12.48 9.0 3.66 13.12 12.51 16.52 11.0 13.5 13.2 14.5 12.9 15.2 9.02 13.99 13.55 14.21 17.43 16.81 16.24 15.05 13.46 18.80 13.0 14.18 11.7 13.53 13.74 8.55

41.8 54.1 46.80 58 54 59 41 40 58 55 65.5 69.7 46 51 60 53 60.03 59 55 63 67 64 68 35 51 48 59.1 61.48 74.2 67.1 65 56 51 55.6 70.24 57.5 62 53.22 51.5

0.13 4.03 3.63 4.77 5.45 5.90 3.54 3.20 6.48 5.59 6.90 8.23 3.20 1.47 6.19 5.40 7.25 5.14 5.94 6.4 7.9 6.5 8.3 3.00 6.70 6.31 6.80 9.53 11.21 9.6 8.71 5.96 7.73 5.73 8.27 5.8 6.71 5.63 3.31

404 404 405 387 387 384 388 388 388 388 386 383 406 387 387 407 408 385 385 381 381 381 381 409 382 400 399 203 410 402 401 394 415 396 380 393 392 389 411

7. SUMMARY AND OUTLOOK Organic semiconductors based on BDT units are promising photovoltaic materials for flexible and low-cost solar cells. There have been rapid advances in the PCEs of these materials over the past several years. In this article, we offered an overview of the organic photovoltaic materials based on BDT and its equivalents from the aspects of backbones, functional groups, alkyl chains, and device performance. The impressive results indicate that BDT and its analogs are versatile and useful building blocks for the design of high-performance photovoltaic materials. Although the current record efficiency of OPVs based on BDT-polymers is ∼11%, which is approaching the requirement for industrialization, certain fundamental challenges and interesting research topics still need to be addressed. For example, devices based on BDT materials still have comparatively low VOC values and thus cause considerable energy loss (>0.6 eV) during the photoelectric conversion process. Minimizing the energy loss will be a key to realize high VOC. The geminate and bimolecular recombination in OPV devices still needs to be reduced, thus improving the JSC and FF values. Recent studies revealed that BDT-based polymer materials could also be used as good electron acceptors,413,416 and systematic studies on this topic still need to be performed. Furthermore, BDT-based polymers can be used as donors in fullerene-free OPV devices,380−383,386,388,389,393,399,400,407 and our team recently demonstrated that a single junction OPV device using a BDT-polymer donor and nonfullerene acceptor could achieve a PCE of 11.2%.410 We believe that BDT-polymers will be good

6.31%. Then they designed and synthesized a series of nonfullerene acceptors containing indacenodithieno[3,2-b]thiophene with different side chains to further optimize their photovoltaic performance.399,401,402 NF19 with phenyl sidechains was developed in 2015 and exhibited initial PCE of 6.8% when P79 was used as donor material.399 By applying novel polymer donor materials based on benzodithiophene and fluorobenzotriazole, further improvement of PCE was obtained for NF19-based PSCs by Li and co-workers.203 For instance, a high PCE of 9.53% was obtained when P165 was used as donor in the PSC device. Recently, a breakthrough PCE of 11.21% with excellent thermal stability was recorded by Hou and co-workers by using P12 as donor and NF19 as acceptor in the PSC device.410 Furthermore, they also certified a high PCE of 10.78% for the PSC device with an area of 1 cm2. When the phenyl of NF19 was replaced by alkyl401 or thienyl402 as side chains, PCEs of 8.71% and 9.6% were obtained, respectively, in the PSC devices by selecting P12 as donor material. 7444


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REFERENCES

candidates to fully explore the potential of fullerene-free OPVs. The stability of OPVs based on BDT materials has seldom been studied. Therefore, testing the water and oxygen/air stability and long-term lifetime of high-performance materials as well as screening good candidate materials for practical applications is required. We anticipate that more exciting BDT-based photovoltaic materials and devices will be developed in the near future through synergistic efforts in material synthesis/design, device processing/optimization, and interface/architecture design.

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Huifeng Yao has been a Ph.D. candidate at Institute of Chemistry, Chinese Academy of Sciences (ICCAS) under the supervision of Prof. Jianhui Hou since 2012. His research interests focus on designing and synthesizing high-performance organic photovoltaic materials. Long Ye completed his Ph.D. research (Adviser: Prof. Jianhui Hou) in Polymer Physics and Chemistry at Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in July 2015. Currently, he is a postdoctoral research associate at Prof. Harald Ade’s group in North Carolina State University. His research interests are understanding the thermodynamics of polymer photovoltaics and revealing the critical factors related to the morphology/performance of organic electronic devices using soft X-ray scattering/microscopy. Hao Zhang has been pursuing a Ph.D. degree in polymer physics and chemistry at Institute of Chemistry, Chinese Academy of Sciences (ICCAS) under the mentorship of Prof. Jianhui Hou since 2013. His research interests are centered on the molecule design and application of photovoltaic polymers for solar cells. Sunsun Li obtained her B.S. degree in chemistry from University of Science and Technology Beijing, in 2013. She is currently a Ph.D. candidate majoring in material sciences, under the direction of Professor Jianhui Hou. Her current research focuses on the design and synthesis of novel organic semiconducting materials for nonfullerene solar cells. Shaoqing Zhang has been a joint Ph.D. student at Institute of Chemistry, Chinese Academy of Sciences (ICCAS) and University of Science and Technology Beijing (USTB) under the supervision of Prof. Jianhui Hou since 2012. Her research interests are the molecular design and preparation of high-efficiency photovoltaic polymers. Jianhui Hou received his Ph.D. degree in physical chemistry from ICCAS in 2006. During 2006−2008, he worked as a postdoctoral researcher at the University of California, Los Angeles (UCLA). Before he joined ICCAS as a full professor in 2010, he worked as a team leader of research department of Solarmer Energy Inc. His recent research focuses on the design, synthesis, and application of the organic/polymer photovoltaic materials.

ACKNOWLEDGMENTS This work was supported by the National Basic Research Program 973 (2014CB643501), NSFC (Grants 21325419, 91333204 and 51261160496), and the Chinese Academy of Science (Grant XDB12030200). 7445


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Molecular Design of Benzodithiophene-Based Organic Photovoltaic

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