Recent Advances in Bulk Heterojunction Polymer Solar Cells

Review pubs.acs.org/CR

Recent Advances in Bulk Heterojunction Polymer Solar Cells Luyao Lu, Tianyue Zheng, Qinghe Wu, Alexander M. Schneider, Donglin Zhao, and Luping Yu* Department of Chemistry and The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, United States 6.2. Third Component as Additional Acceptor in the P3HT System 6.3. Dye Molecules as the Third Component in the P3HT System 6.4. High-Performance Ternary Polymer Systems 7. Plasmonic Metal Nanostructures To Enhance Light Harvesting 7.1. Single-Metal NPs Positioned in Different Layers in PSCs 7.2. Cooperative Enhancement from Multiple Nanostructures 7.3. Other Complicated Metal Nanostructures Used in PSCs 8. Nonfullerene Polymer Acceptors for PSCs 8.1. Benzothiadiazole-Based Polymer Acceptors 8.2. Perylene Diimide-Based Polymer Acceptors 8.3. Naphthalene Diimide-Based Polymer Acceptors 9. Stability 10. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

CONTENTS 1. Introduction 2. New Materials and Design Rules for Efficient Light Harvesting 2.1. Donor Polymers Developed at Early Stage 2.2. Recent Advances in Polymer Designs 2.2.1. Effect of Polymer Backbone 2.2.2. Influence of Side Chains 2.2.3. Influence of the Substituents 2.2.4. Other Effects To Be Considered 3. Morphology Control in the Bulk Heterojunction Active Layer 3.1. Solvent Additives 3.2. Thermal Annealing 3.3. Choice of Host Solvents and Solvent Annealing 4. Charge Transport and the Effect of Charge Carriers’ Mobility 4.1. Charge-Transport Parameter Characterization Techniques 4.1.1. FET Mobility 4.1.2. SCLC Mobility 4.1.3. TOF Mobility 4.1.4. CELIV Mobility 4.2. OFET vs Solar Cell Device 4.3. Factors That Influence Mobility 5. Interfacial Engineering To Enhance PSC Performance 5.1. Requirements of Interfacial Layers 5.2. Anode Interlayer Material 5.2.1. Inorganic Materials as HSLs 5.2.2. Organic Polymers and Small Molecules as HSLs 5.3. Cathode Interlayer Materials 5.3.1. Inorganic Materials for Use as ESLs 5.3.2. Water/Alcohol-Soluble Polyelectrolytes as ESLs 5.3.3. Fullerene Derivatives as ESLs 6. Ternary Polymer Solar Cells 6.1. Third Component as Additional Donor for P3HT Ternary System © XXXX American Chemical Society

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1. INTRODUCTION It is clear that the current energy regimen, which relies on nonrenewable and polluting energy sources, has unsustainable consequences for societal, economical, geopolitical, and environmental issues. The only fully renewable source that has the capability to meet the world’s large and growing energy demand is solar energy. Solar cells, which directly convert sunlight into electricity, are one of the most promising and efficient technologies to harvest this energy. Currently, inorganic solar cells based on materials such as crystalline silicon, cadmium telluride, or copper indium germanium selenide (CIGS) are mature devices that exhibit a relatively high solar energy conversion efficiency of around 15−20%, and thus, dominate photovoltaic (PV) technologies available commercially. However, the high cost of inorganic solar cells and related environmental issues have partially impeded their pace to widespread deployment, which spurs research effort to

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Special Issue: Solar Energy Conversion Received: February 13, 2015

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DOI: 10.1021/acs.chemrev.5b00098 Chem. Rev. XXXX, XXX, XXX−XXX

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made in these efforts and point out the necessity to synergistically optimize solar cell performance. This review is organized as follows: In the first part, we focus on discussing new donor materials and polymer design rules in detail. In the second part, we introduce processing methods to control the morphology of the active layer. Then, we discuss the charge-transport process as characterized by the determining factors for hole mobility (μh) in PSCs as well as different interfacial engineering methods to improve charge transport and collection efficiency. Later, we briefly introduce the recent developments in alternative device architectures such as ternary PSCs with three materials combined in the active layer, the application of plasmonic nanostructures in PSCs to increase device performance, and all-polymer solar cells with both polymer donor and nonfullerene polymer acceptor. Next, we briefly discuss the current status of the environmental stability of PSCs. Finally, we conclude with a summary of the existing challenges and possible strategies for future progress in the field. Note that tandem solar cells will not be covered in this reivew. Readers who are interested in this topic are referred to several articles dedicated to them.10,11 Also, as several excellent and comprehensive reviews have appeared in the past such as articles by Sariciftci and co-workers12 and Fréchet and coworkers,13 this review will focus especially on more recent advances. The readers are encouraged to read these reviews for the basis of the current work discussed here. We would like to note that the solar cell parameters cited in the following sections may contain different significant figures quoted from the original articles.

explore alternative approaches given the growing demand for power worldwide. Thin film polymer solar cells (PSCs) hold promise for fabricating lightweight and flexible devices via the low-cost and high-throughput roll-to-roll production process.1,2 Due to these advantages, extensive research efforts around the world have been devoted to understanding and improving the performance of PSCs in the last two decades. The first successful organic photovoltaic device was reported by Tang in 1986.3 A power conversion efficiency (PCE) of about 1% with a high fill factor (FF) of 65% was achieved in a bilayer device composed of vacuum-evaporated p-type copper phthalocyanine and n-type bisbenzimidazo[2,1-a:2′,1′-a′] anthra[2,1,9-def: 6,5,10-d′e′f′] diisoquinoline-10,21-dione. Six years later, the Heeger and Wudl groups observed ultrafast electron transfer from the conjugated polymer poly[2-methoxy5-(2-ethylhexyloxy)]-1,4-phenylenevinylene (MEH-PPV) to fullerene (C60),4,5 which suggested the use of conjugated polymers as electron donors and fullerene derivatives as electron acceptors in PSCs. Concurrently, the concept of the bulk heterojunction (BHJ) was introduced to address the limited exciton (tightly bound electron hole pair) diffusion length in organic solar cells, which had been a problem for previous organic solar cells.6,7 In a BHJ structure, donor and acceptor materials are mixed together to form a bicontinuous interpenetrating network with large interfacial areas for efficient exciton dissociation. Heeger and co-workers introduced the BHJ structure for polymer:fullerene blends,7 while Friend and co-workers demonstrated the BHJ structure for all polymer blends with polymer donor and polymer acceptor for efficient photogeneration.6 Later, it was realized that domain sizes of donor and acceptor can be further optimized with additives.8,9 Since then, the BHJ structure has become the standard architecture for organic solar cells. The PCE of a solar cell is the product of open-circuit voltage (Voc), short-circuit current density (Jsc), and FF divided by input power (Figure 1). The currently accepted working

2. NEW MATERIALS AND DESIGN RULES FOR EFFICIENT LIGHT HARVESTING The sun gives out a tremendous amount of energy every second in the form of irradiation, which reaches the Earth at an energy density of about 1366 W/m2 just outside the atmosphere, although some energy will be lost after reflection and absorption by the atmosphere.14 Figure 2 shows the

Figure 1. Current density versus voltage plot characteristic of a solar cell device. Figure 2. Sun irradiance (red) and number of photons (black) as a function of wavelength. Reprinted with permission from ref 15. Copyright 2007 Elsevier B.V. All rights reserved.

mechanism of BHJ PSCs may be described in four key steps: (1) light absorption and generation of highly localized, tightly bound Frenkel excitons; (2) exciton diffusion to the donor− acceptor interface; (3) exciton dissociation at the interface, first creating charge transfer (CT) states or so-called polaron pairs; then CT states fully dissociate into free charge carriers; (4) charge transport and collection. Each of these processes is important and can be a bottleneck in determining the overall solar cell efficiency. Great efforts are devoted to developing materials, new device architectures, and methods to optimize each of these processes. This review will discuss recent progress

standard AM1.5 solar spectrum at the ground level,15 indicating the energy density and photon flux with respect to wavelength. This integrates to an energy density of about 1000 W/m2 with a photon flux of 4.31 × 1021 s−1 m−2, distributed across a wavelength range from 280 to 4000 nm.16 Most of the solar energy is concentrated in the visible and near-infrared (nearIR) region. Thus, to efficiently harvest solar energy, the absorption spectra of PSCs should have a large overlap with the B


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solar spectrum in this region. For example, a semiconducting polymer with a band gap of 1.1 eV (equivalent to photons with wavelength under 1100 nm) can absorb at most 77% of the solar energy incident upon it. If the band gap exceeds 2 eV (less than 620 nm), only about 30% of the solar energy can be absorbed at most.17 Thus, the first criteria in designing new polymers is high efficiency in absorbing solar energy in the whole solar spectrum. As mentioned above, the band gap determines the absorption limit for a particular polymer. As such, much effort in the research community has been devoted to improving the absorption features of PSCs by fine tuning absorption characteristics. With a low band gap and a broad absorption band, a polymer can absorb more photons, which will increase the Jsc, all else being equal. However, further narrowing the band gap of a polymer will lead to a decrease in the Voc, which is believed to correlate with the difference between the highest occupied molecular orbital (HOMO) of donor materials (e.g., donor polymers) and the lowest unoccupied molecular orbital (LUMO) of acceptor materials (e.g., fullerene derivatives). We would like to note here that comparison between the HOMO/ LUMO of active layer materials and the work functions of interfacial layers/electrodes discussed later is from their absolute values. Theoretical investigation by Shockley and Queisser indicated that the optimal band gap in light-harvesting materials is around 1.3 eV, which represents the theoretical best possible compromise between Voc and Jsc for an ideal solar cell.18 Brabec and co-workers showed that to achieve a PCE >10%, the band gap of donor polymers should be around 1.35− 1.65 eV.19 However, a suitable band gap alone does not ensure high efficiency. It is also necessary to design low-band-gap donor polymers with energy levels that match well with those of the electron acceptor materials, which are mostly fullerene compounds, such as [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) or [6,6]-phenyl C61 butyric acid methyl ester (PC61BM), so that the resulting BHJ solar cells will exhibit favored charge-separation and transport characteristics.16 Most importantly, the LUMO of the donor polymer should be at least 0.3 eV higher than the LUMO of the corresponding acceptor material to provide enough driving force to break the electron−hole (e−h) pair binding energy and lead to charge separation.20 Thus, with a certain acceptor material, the desired LUMO level of donor polymer has a limited range. In order to narrow the band gap of the donor polymer, one could raise the HOMO level of the donor polymer, which will sacrifice Voc as mentioned above. Therefore, it is important to compromise between narrowing the band gap and matching the HOMO/ LUMO energy levels. This section examines some of the representative examples of donor polymers developed over the years.

Figure 3. Structures of PPVs, P3HT, and PFDTBT polymers.

In 1995, Heeger and co-workers7 reported the first BHJ PSC by mixing MEH-PPV as the donor material and C60 or PC61BM as the acceptor materials directly into the active layer. After that, another PPV family polymer, MDMO-PPV, was developed with better processability, and the record PCE for MDMO-PPV cells was increased to over 3%.21 The different alkyl side chains of MDMO-PPV left the band gap and energy levels unchanged compared to MEH-PPV, but solubility and miscibility with PC61BM were enhanced. After careful optimization of processing conditions, especially solvents, Shaheen et al. found that the interactions between conjugated polymer and PCBM as well as polymer interchain interactions could be improved to form a better morphology in the blend composite, resulting in improved performance.22 As well as morphological control via modification of side chains and processing conditions, energy level control of the polymers was developed by introducing substituents on the polymer backbone, which can modify the energy levels of the polymer. Reynolds and co-workers23 reported the introduction of cyano groups (−CN) to the vinylene moiety of PPV polymers via a Knoevenagel polycondensation to form CN-PPV polymers (Figure 3). The electron-withdrawing CN group lowered both HOMO and LUMO levels by 0.5 eV without changing the band gap (2.1 eV). A LUMO of −3.8 eV was reported for CNPPV polymer, which matched the LUMO of PC61BM (−4.2 eV) very well. In the same report, they showed that replacement of the dialkoxybenzene unit to more electronrich dialkoxy thiophene units raised the HOMO with little change of the LUMO, resulting in narrowed band gaps. Although PPV-based polymers were the materials of focus at the beginning of PSC research, their large band gaps (>2 eV) and low photocurrent impeded further optimization. Research interest quickly shifted to polythiophenes, especially P3HT.24,25 The band gap of P3HT is about 1.9 eV and can be further reduced by increasing the quinoidal character in the polymer.26 P3HT solar cells generally exhibit high external quantum efficiency (EQE), which measures the number of charge carriers absorbed per photon incident on the device. This value can reach as high as 88%,27 and PCE reached >5% after modifications such as increasing regioregularity of the polymer and use of thermal or vapor annealing during device fabrication (discussed in the morphology section).27−30 Morphological optimization of the P3HT system plays a key role in achieving significant improvement in the performance of organic photovoltaic (OPV) solar cells, which was recently reviewed in detail by Wuest and co-workers.31 P3HT is still one of the benchmark materials for PSC studies, which helps to unravel structure−property relationships and device engineering methods for other high-performance polymers. However, as

2.1. Donor Polymers Developed at Early Stage

The development of donor polymers has gone through several phases of research. Three early classes of polymers are worthy of discussion: (1) poly(phenylenevinylene) (PPV) derivatives such as MEH-PPV and poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylenevinylene (MDMO-PPV); (2) poly(thiophene) derivatives, mainly poly(3-hexylthiophene) (P3HT); (3) polyfluorene derivatives like poly[2,7-(9-(2ethylhexyl)-9-hexyl-fluorene-co-5,5-(4′,7′-di-2-thienyl-2′,1′,3′benzothiadiazole)] (PFDTBT). Figure 3 shows the structures of these polymers. C


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with the PPV family of polymers, the performance of P3HT:PCBM solar cells is limited by the relatively large band gap and small energy difference between the LUMO energy level of PCBM and the HOMO energy level of P3HT, which lead to low Jsc and Voc (about 0.6 V).32 To address the low Voc, indene-C60 bisadduct (ICBA) was used as the acceptor. After solvent and thermal annealing, a high PCE of 6.5% was achieved with an impressive Voc of 0.84 V, which is due to the higher LUMO (by about 0.2 eV) of ICBA compared to PC61BM.30 The same group increased the PCE of P3HT:ICBA devices to 7.40% after optimization of device fabrication conditions.33 The third major family of polymers for PSCs has historically been polyfluorene. Polyfluorene contains fluorene and benzothiadiazole derivatives as repeating units in the polymer backbone. Typical features of this type of polymers included wide band gaps and low-lying HOMO levels, which resulted in relatively small Jsc but high Voc. Andersson and co-workers first reported PFDTBT polymers (Figure 3) composed of fluorene and dithienyl-benzothiadiazole (DTBT) moieties with a band gap of 1.9 eV and a HOMO of −5.7 eV. Though the Voc is very high (1.04 V), the solar cell prepared from PFDTBT:PC61BM showed a low PCE of 2.2% with a small Jsc of 4.66 mA/cm2 and a FF of 46%.34 Alkyl chain modification increased the Jsc to 7.7 mA/cm2 and FF to 54% with a PCE of 4.2% after optimization of device fabrication conditions.35

Figure 4. Molecular orbital theory explanation for band gap reduction in D−A copolymer.

donating or -accepting abilities as well as available sites for sidechain and functional group modifications and close to linear molecular structure. Although there are examples of D−A copolymers in the 1990s with band gaps as low as 1.1 eV,44 it was not until 2003 when a fluorene-based D−A polymer PFDTBT was used in BHJ PSCs.34 Now, D−A copolymers dominate the development of new OPV materials in the community. Currently, rational design of donor polymers is still one of the most important issues for further development of PSCs. When designing donor polymers for PSCs, careful selection of relative monomers is essential to making donor polymers with desired properties. However, selection of proper side chains, including position, size, and shape, can also significantly modify the properties of polymers. Several strategies for designing high-performance copolymers are discussed below. 2.2.1. Effect of Polymer Backbone. The backbone refers to the conjugated system along the polymer chain, and it is responsible for the fundamental optoelectronic properties of conjugated polymers, such as band gap, position of energy levels, charge carrier mobilities, and conductivity. As discussed above, when designing backbones for D−A-type polymers one can modify the donor and acceptor units separately to engineer polymers with desired properties, especially with regard to the band gap and energy levels of the conjugated polymers. 2.2.1.1. Exploring the Quinoidal Character of Polymers. An important consequence of the D−A repeating units in lowband-gap polymers is the existence of the quinoidal nature of the backbone structures, as shown in Figure 6. Although the existence of quinoidal characteristics in the backbone is not unique to D−A alternating copolymers, it is a common feature of them. Certain polymers naturally prefer to adopt the quinoidal structure in the conjugated backbone, which further lowers the band gap of the polymer because the quinoidal form increases the planarity of the polymer backbone and enables more efficient π delocalization along the polymer chain. The typical example is poly(benzo[c]thiophene) developed by Wudl and co-workers in 1984. Structurally, thiophene (lower resonance energy, 1.26 eV) is fused with benzene (higher resonance energy, 1.56 eV) to form this polymer. The thiophene backbone tends to adopt a quinoidal form in order to maintain the benzene ring in aromatic form due to the higher Eres of benzene (Figure 6). Overall, the quinoidal form is lower in energy and thus stabilized, resulting in the narrow band gap of polymer (1.1 eV).46 The same effect occurs in polythieno[3,4-b]pyrazine (Eg = 0.95 eV) and poly(thieno[3,4b]thiophene) (Eg = 0.8−0.9 eV), which have very narrow band gaps due to the quinoidal structures.26

2.2. Recent Advances in Polymer Designs

Major developments in designing solar cell polymers come from the breakthrough of donor−acceptor (D−A) (in some cases named as “push−pull”) copolymers, which lead to highly efficient BHJ PSCs.36−41 In these low-band-gap polymers, the energy levels and molecular structures can be optimized via molecular engineering. Unlike homopolymer P3HT, D−A copolymers incorporate one electron-rich moiety (donor) and one electron-deficient moiety (acceptor). Typical synthetic methods include the Stille polycondensation with thienyl repeating units or Suzuki coupling reactions for polymers with phenyl repeating units.42,43 This class of polymers often demonstrates a phenomenon known as the quinodal effect, where resonance between −D−A− and −D+−A−− increases the double-bond character of the single bonds in the polymer backbone. This results in a reduction of the bond length alternation and effectively modifies the energy levels and band gaps of the corresponding polymers.16,44 One unique feature of these polymers is that their HOMO and LUMO energy levels are largely determined by the HOMO energy level of the donor and the LUMO energy level of the acceptor, respectively.45 Therefore, the energy levels of polymers can be tuned by engineering the donor and acceptor units separately. Figure 4 shows a simplified molecular orbital theory explanation for the band-gap reduction in D−A repeating units of the copolymers, as well as the basis of the polymer HOMO and LUMO on the donor and acceptor units. Over the years, researchers have developed a variety of highperformance D−A polymers. Figure 5 summarizes the representative structure fragments of these polymers, such as fluorene, cyclopentadithiophene (CPDT), oligothiophene, benzodithiophene (BDT), indacenodithiophene (IDT), etc., for use as donor monomers and benzothiadiazole (BT), DTBT, dithienyl-diketopyrrolopyrrole (DTDPP), thienothiophene (TT), thienopyrroledione (TPD), isoindigo (IID), etc., for use as acceptor moieties. These units have proper electronD


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Figure 5. Structures of some widely used donor and acceptor fragments.

LUMO and high HOMO energy levels. After structural optimization, polymer PTTD:C with a composition of m:n = 1:3.7 showed a band gap of 1.45 eV and a PCE of 1.93% in polymer:PC 61 BM solar cells. Further structural tuning generated the regioregular copolymer PF (Figure 7) incorporating an oligothiophene unit with a PCE of 2.4% in the polymer:PC61BM device.48 A significant breakthrough was made when the PTB polymers, which consist of alternating electron-rich BDT units and electron-deficient TT units, were developed (Figure 8).49−51 The PTB family of polymers was designed based on Figure 6. Aromatic and quinoidal forms of poly(benzo[c]thiophene) and PTB polymers.

This property was explored by the Yu group in a systematic investigation on the synthesis and physical properties of TT copolymers.47 The first polymer PTTD (Figure 7) was

Figure 8. Structures of PTB polymers. Figure 7. Structures of PTTD and PF polymers.

the notion that the polymer must exhibit (1) a narrow band gap for efficient light absorption, (2) proper energy level alignment with the LUMO energy level of PCBM to facilitate charge separation and maximize Voc, (3) high hole mobility for efficient charge transport, and (4) excellent compatibility with PCBM to form a bicontinuous interpenetrating network to maximize exciton separation, migration, and charge transport. Under these considerations, six low-band-gap PTB family polymers (PTB1−6) were synthesized by using the Stille polycondensation approach.50 The incorporation of TT monomer unit ensured a low-energy band gap around 1.6− 1.8 eV because of its strong tendency to stabilize the quinoidal structure (Figure 6). Strong interchain π−π stacking was achieved in these systems by combining the highly planar

synthesized, in which the degree of quinoidal-form favorability was adjusted by controlling the ratio of TT to thiophene in the copolymer composition so that the energy levels and band gap of the copolymers could be fine tuned. The ester group was present to lower the HOMO energy level due to its electronwithdrawing ability. The stability against oxidation and the solubility of resulting polymers are also enhanced by the ester group. The polymer PTTD showed a broad absorption with a band gap of 1.2 eV. The space-charge-limited current (SCLC) hole mobility of PTTD was 1.5 × 10−4 cm2/(V s), comparable to that of P3HT (2.7 × 10−4 cm2/(V s), tested in the same condition) despite the presence of bulky side chains. However, only an inferior PCE of 0.73% was attained due to the low E


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quinoidal structure with the extended π system in the BDT unit, resulting in a high hole mobility for the polymer series. PTB4 showed the highest SCLC hole mobility of 7.7 × 10−4 cm2/(V s). Another important discovery from this work was that the introduction of a fluorine atom into the TT unit of PTB4 decreased both the HOMO energy level from −5.01 to −5.12 eV and the LUMO energy level from −3.24 to −3.31 eV compared to its structural analogue PTB5. As a result, increased Voc and Jsc were observed with the highest PCEs at 6.1% and 7.1% for PTB4:PC61BM and PTB4:PC71BM devices processed with diiodooctane (DIO) additive, respectively.50 These promising results encouraged further investigation in structural optimization of PTB polymers. A slight modification in the side chains of PTB4 by replacing the n-octyl group on the TT unit with a branched 2-ethylhexyl group resulted in polymer PTB7 with good solubility for solar cell fabrication (Figure 8). PTB7 shows a strong and broad absorption from 550 to 750 nm in its optical spectrum. In what was a record at that time, a PCE of 7.4% was achieved for the PTB7:PC71BM device with a Voc of 0.74 V, a Jsc of 14.50 mA/cm2, and a FF at 68.97%.40 Further device optimization based on PTB7 includes changing the device structure from conventional to inverted, which pushed the PCE of a PTB7 single-junction device to 9.2%.52 Gong and co-workers found that the PCE values of PTB7 devices may be increased to 8.5% by increasing the molecular weight (Mn) to 128 kg/mol and lowering the dispersity (Đ) to 1.12.53 This is attributed to enhanced light absorption and hole mobility of PTB7 at high Mn. Bhatta et al. computed the nanoconformational and electronic properties of PTB7 and P3HT with large-scale density functional theory (DFT) calculations.54 They found that PTB7 has a torsional potential almost independent of chain length in contrast to P3HT. This is due to electronic delocalization, hydrogen bonding, and the absence of adjacent side chains to interconjugation units in PTB7. In addition, PTB7 showed a more planar structure in its fully relaxed conformation (torsional angle of 25°) compared to P3HT (torsional angle of 47°), reflecting its more quinoidal nature. Following the success of PTB7, the Yu group further synthesized the PTB family polymers with enhanced 2-D orbital delocalization character by grafting aromatic groups like thienyl and phenyl groups to the BDT unit to improve the coplanar character of the polymer backbone, which would favor π-stacking and charge-transport properties. Several polymers with alternating FTT and BDT-Th are patented55 including PTB7-Th, initially developed in 2010, which showed a PCE of 4.6%. This design idea was also followed by some other research groups.56,58 PTB7-Th was revisited in 2013, when Chen and co-workers reported a PCE of 9.35% for inverted solar cell devices with PTB7-Th as the donor material.59 Very recently, Cao and co-workers achieved a 9.94% PCE in inverted PTB7-Th device.60 They found that domain purities of PC71BM are improved with increased PC71BM loading. This reduces the tail state density of PC71BM and enhances Voc for PTB7-Th:PC71BM devices. These low-band-gap PTB polymers have been a focus of interest in the research community and served as a benchmark material in the field, with the performance being improved. This system has been further explored by Solarmer Energy Co. based on the licensed patents from Yu’s group.50 Numerous structural parameters were adjusted to establish more accurate structure/property relationships.56 Due to these efforts, a series

of low-band-gap polymers similar to the PTB series with higher efficiency was disclosed.56 2.2.1.2. Effect of Donor and Acceptor Units on Solar Cell Properties. As shown previously in Figure 5, BT is a commercially available electron-deficient monomer. It is widely used to copolymerize with various donor monomers to tune the absorption properties. Janssen and co-workers first described the synthesis of BT-based low-band-gap polymers (PTPTB) for BHJ PSCs even though the degrees of polymerization were limited (Figure 9).61 The polymers showed low optical band

Figure 9. Structures of PTPTB, PCPDTBT, and PCDTBT polymers.

gaps below 1.60 eV. However, solar cell devices based on the polymer:PC61BM blend only exhibited a PCE of about 1%.62 Later, many conjugated polymers were synthesized with BT and different donor units. Among them, the most promising was the one containing CPDTs, which exhibit a high hole mobility due to its structural planarity and extended conjugation (Figure 9). PCPDTBT with two branched alkyl chains in the CPDT unit showed suitable HOMO and LUMO energy levels of −5.3 and −3.5 eV, respectively.63 A narrow optical band gap of about 1.4 eV is achieved, and the polymer:PC71BM solar cells yields a PCE of 3.5% with a Voc of 0.65 V and a Jsc of 11.8 mA/cm2. However, the overall PCE value was still low due to poor morphology and low EQE (max at 38%) and FF (47%). However, with the addition of a small amount of either alkanedithiol or diiodoalkane to the processing solvent as an additive, the performance could be improved to a PCE of 5.5% with a Voc of 0.62 V, a Jsc of 16.2 mA/cm2, and a FF of 55%.8 Further modifications lead to several other systems that exhibited a range of PCE values from 5% to 8%.64−66 A useful derivative of BT is DTBT, which is made by adding two flanking thienyl groups to the BT unit. When it was copolymerized with donor units like carbazole, a monomer structurally similar to fluorene, a series of polymers was synthesized by Leclerc and co-workers.67,68 PCDTBT (Figure 9) shows similar optical and electrical properties to PFDTBT (Figure 3), with a large band gap (1.9 eV) and deep HOMO level (−5.5 eV). The slightly higher HOMO may be due to the more electron-rich character of carbazole than fluorene. The overall PCE was 3.6%, which was later significantly improved by Heeger and co-workers by incorporating TiOx as optical spacer and hole blocker between the BHJ active layer and the metal electrode.69 TiOx acts as hole-blocking layer to enhance electron collection and also serves as an optical buffer to facilitate light absorption by redistributing the maximum light intensity of incident light closer to the center of the BHJ active layer. Although the Voc of the PCDTBT was lower due to the 0.2 eV higher HOMO energy level (−5.5 eV) than PFDTBT (−5.7 eV), Jsc and FF improved significantly, resulting in an F


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as high as 0.80 V and led to a PCE of 5.5% with a Jsc of 10.8 mA/cm2 and a FF of 65% in polymer:PC71BM solar cells.80 Another polymer, PDPP-BDT, incorporating BDT as the donor unit was reported by Yang and co-workers with a low band gap of 1.34 eV, similar to that of PBBTDPP-2. The solar cell of PDPP-BDT:PC71BM yielded a PCE of 4.5%.81 Following that, the same group did further modification on this polymer to synthesize PBDTT-DPP (Figure 10), which had an enhanced PCE of 6.5%.37 Later, McCulloch and coworkers reported a series of DPP-based copolymers with thiophene, selenophene, and tellurophene comonomers (Figure 10).82 The band gaps of the polymers are narrowed with increased size of the chalcogen atoms due to reduced aromaticity. The absorption of the three C3-DPPTT polymers is extended to near-IR region. C3-DPPTT-T-Se and C3DPPTT-T-Te show stronger intermolecular interactions compared to C3-DPPTT-T in the solid state. A high PCE of 8.8% is achieved in the inverted C3-DPPTT-T:PC71BM device with a Jsc of 23.5 mA/cm2. In addition, Nguyen and co-workers studied DPP-based small molecular organic solar cells (SMOSCs), which showed PCE values between 2% and 5%.83−85 Interestingly, they found that the performance of solar cells depended on the light intensity in some DPP-based SMOSCs. For example, compared to 0.1 unit of sun irradiation, the DPP(TBFu)2/PC61BM devices under 1 sun showed higher charge carrier density and a faster charge recombination rate, leading to inferior FF and decreased PCE.85,86 Their later investigation on nanoscale morphology, charge transport, and charge recombination of DPP-based SMOSCs is worth referring to for the study of PSCs.85,87,88 Another interesting acceptor monomer that has been widely investigated is TPD, which offers a site to attach a solubilizing alkyl side chain on the nitrogen of imide (Figure 5). Its strong electron-withdrawing ability can lower the energy levels of polymers, which is beneficial for high Voc. Also, its planar structure can promote electron delocalization and enhance intramolecular interactions between polymer chains. Copolymerization with various electron-rich monomers, especially BDT, leads to low-band-gap polymers with high efficiency. The PBDTTPD polymers (Figure 11), consisting of alternating BDT and TPD units are widely studied after Leclerc and coworkers first reported it with a PCE of 5.5%.89 In parallel, Jen and co-workers reported the same polymer but with an inferior PCE of 4.2%.90 Later, Fréchet and co-workers pushed the performance of this series of polymers to 6.8% with DIO additives.91 Recently, the best performance of PBDTTPD has

improved efficiency of 6.1%. It is noteworthy that the internal quantum efficiency (IQE) of the PCDTBT:PC71BM device reached ∼100% at around 450 nm and remained higher than 80% throughout the entire absorption spectrum (400−650 nm). Because of the advantages of the DTBT unit, several polymers have been developed which show high performance with PCEs over 7%.70,71 DPP (Figure 5) is a well-known building motif that was used in dye molecules as early as in 1987.72 Its diphenyl-substituted analogue was incorporated into conjugated polymers by Yu and co-workers in 1993 as photosensitizer for the photorefractive effect.73 The DPP moieties exhibit a planar conjugated bicyclic structure and strong electron-withdrawing ability due to the presence of two amide groups, typically resulting in small band gaps of DPP-based polymers. As an amide-functionalized arene, DPP is likely to engage in H bonding, which leads to the high backbone coplanarity as well.74,75 However, the high-lying HOMO energy level limits the performance of DPP-based polymers as in the case of PDTP-DTDPP (Figure 10).76

Figure 10. Structures of selected DPP polymers and DPP(TBFu)2.

Structural modification can improve performance by lowering the HOMO energy level of polymers while maintaining their low band gaps. Various DPP polymer systems have been reported for application in OPV solar cells,77 beginning with polymer PBBTDPP-2 (Figure 10) developed by Janssen and co-workers.78 PBBTDPP-2 showed a low band gap of 1.4 eV. The solar cell device prepared using the polymer:PC71BM (1:2 w/w ratio) and chloroform (CF)/dichlorobenzene (DCB) mixed solvent system achieved a PCE of 4% with a Voc of 0.61 V, a FF of 50%, and a Jsc of 11.3 mA/cm2. Polymerization of DPP and thiophene resulted in PDPP3T (Figure 10).79 Without long alkyl chains in the terthiophene unit, the PDPP3T could form a more planar structure compared to PBBTDPP-2. As a result, it showed a reduced band gap of 1.3 eV, a very large field effect transistor (FET) hole mobility of 0.05 cm2/(V s), and an improved solar cell performance with a PCE of 4.7%. When a phenyl ring was introduced, a new conjugated polymer PDPPTPT (Figure 10) showed a deep HOMO energy level of −5.35 eV. The Voc of the solar cell was

Figure 11. Structures of PBDTTPD polymers. G


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Figure 12. Structures of selected IID-based polymers.

from IID and the dithiophenecarbazole (DTC) monomer with more extended fusion.100 2.2.1.3. Effect of Conjugation in Repeating Units. A general issue is the effect of fused conjugated systems because they are frequently used in designing low-band-gap polymers, especially for donor monomers in D−A polymers. Proper selection of conjugation size can increase the thermal and chemical stability of polymers, facilitate π-electron delocalization and tune the band gaps and energy levels, enhance the charge carrier mobilities by preventing rotational disorder, and reduce reorganization energy of the covalently rigid conjugated units.26 However, when the conjugation is too large, problems may occur as well; not only will the quinoidal structures in the backbone be inhibited but also excessive intermolecular π−π stacking interaction may be introduced. Thus, the solubility, processability, and miscibility of the polymers with acceptor materials like PCBM will be harmed, leading to inferior performance. Therefore, more detailed studies on the effect of proper conjugation in donor/acceptor monomers are important. Son et al. synthesized a PTB7 analog photovoltaic polymer PTDBD (Figure 13) by extending the BDT with two fused thienyl units (DBD).101 The design of DBD was inspired by the research about the local dipole moment change of PTB

reached a PCE of 8.5% in a conventional device structure after side-chain engineering.37 They will be discussed in more detail in section 2.2.2. Several other polymer systems containing TPD also showed high performance.92−94 Recently, the isoindigo unit (IID) was used in preparing D− A-type low-band-gap polymers. IID is a symmetrical and planar structure consisting of two indolin-2-one units, which is the basis of many dye molecules. IID-based oligomers were first studied by Reynolds and co-workers.95 It was found that conjugated polymers based on IID and its derivatives exhibit high charge carrier mobilities comparable to amorphous silicon transistors for the application of organic field effect transistors (OFETs).96,97 Anderson and co-workers first synthesized the PTI-1 polymer (Figure 12) with a band gap of 1.6 eV from alternating IID and thiophene units.98 PTI-1 shows a very low HOMO of −5.85 eV which enables the high Voc around 0.90 V for the polymer:PC71BM device. Unfortunately, the Jsc is low (2% only when the numberaveraged molecular weight was larger than 10 kDa.148 The reduced performance was related to a reduced intermolecular ordering of the P3HT phase at low molecular weight, resulting in a low hole mobility for the donor polymers. Later, Heeger and co-workers studied the molecular weight effect on the solar cell performance of Si-PDTBT (Figure 37) polymer. SiPDTBT with a molecular weight of 38 kDa showed a FET mobility of 3.6 × 10−2 cm2/(V s), which is 700 times larger

Aεε0 d

where A is the rate of voltage increase and the other variables have been described before. If a light pulse is applied before the voltage, additional charge carriers will be generated and transported to the electrodes, which can be seen as an increased current ΔJ above J(0) (Figure 36).66 If the time that the current is maximized, tmax, can be observed, mobility can be calculated according to the following equation (when ΔJ < J(0)) μ=

2d 2

(

ΔJ

2 3Atmax 1 + 0.36 J(0)

)

Figure 36. CELIV curves of polymer:PCBM BHJs. Reprinted with permission from ref 66. Copyright 2013 Rights Managed by Nature Publishing Group.

4.2. OFET vs Solar Cell Device

Many literature papers measure the mobility parameters of the polymer in an OFET for new polymers and then attempt to use those values to explain the results of a PSC device fabricated with this polymer. Despite being somewhat prevalent, this situation is inadvisible because the FET mobility is almost always the mobility parallel to the device substrate, while SCLC mobility is that in the perpendicular direction. Most solar cells adopt the perpendicular direction for charge transport and collection. 4.3. Factors That Influence Mobility

Before the recent success of D−A copolymers, the most commonly studied systems were based on an active layer composed of MDMO-PPV or regioregular P3HT (RR-P3HT) with PCBM. The MDMO-PPV device typically showed a high Voc above 0.8 V and a Jsc of only 5−6 mA/cm2, while RR-P3HT was opposite, with a relatively higher Jsc of 8−9 mA/cm2 but a lower Voc of only ∼0.55 V.183 Pure PC61BM was known to have an electron mobility on the order of 10−3 cm2/(V s), which was decreased by an order of magnitude or more in actual devices, where the fullerene domains were “diluted” by polymer domains.195 MDMO-PPV was shown to have a hole mobility on the order of 10−6 cm2/(V s) by the TOF method.196

Figure 37. Chemical structure of Si-PDTBT. W


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the fluorene unit were changed from octyl to methyl, the resulting polymer shows the lowest hole mobility in the blend by about an order of magnitude lower than the others. However, in the pure polymer phase it has similar mobility to its counterparts. This suggests that sufficiently long alkyl chains are necessary to achieve favorable interaction with PCBM to enhance hole mobility in the blends. It must be pointed out that the critical requirement to optimizing efficiency is to achieve balanced carrier mobilities for both holes and electrons so that both charge carriers can be collected equally. Thus far, hole mobility is still one of the key limiting factors in the performance of PSCs. With a better understanding of the parameters that can be used to enhance mobility in OPV devices, as well as the emergence of new measurement techniques, there are great opportunities for further performance enhancement.

than the mobility obtained for Si-PDTBT with a molecular weight of 9 kDa.150 Morphology characterization indicates that this is the result of much stronger π−π interactions in high molecular weight samples. Chu et al. observed similar results for polymer PDTSTPD. When molecular weight increases from 10 to 18 and 31 kDa, hole mobility increases from 7 × 10−6 to 1 × 10−4 and 3.7 × 10−4 cm2/(V s), respectively.151 On the other hand, Li et al. controlled the molecular weight for polymer PBnDT-FTAZ to produce polymers of 10, 20, 40, and 60 kg/ mol and found that the polymer with a molecular weight of 40 kg/mol gives the highest hole mobility of 4.10 × 10−4 cm2/(V s), which is twice the value for polymer with a molecular weight of 10 kg/mol.200 Thus, we would like to remind the readers that the “optimal” molecular weight values vary for each polymer system. One recent trend in OPVs, started with the PTB family polymers, is to introduce fluorine into the polymer backbone to enhance device performance.40,49 As discussed in section 2, the primary effect of this has been to adjust the polymer band gap by lowering the energy levels and control blend morphology. In addition, the Yu group found that hole mobility increases from 2.7 × 10−4 cm2/(V s) in PTBF0 to 4.1 × 10−4 cm2/(V s) in PTBF1 upon fluorination of the TT unit. In contrast, mobility values decrease to 1.8 × 10−4 and 7.0 × 10−5 cm2/(V s) in PTBF2 and PTBF3, respectively, with fluorine substitution at other positions, following the trend in overall PCE.124 Similarly, Yang et al. found that fluorination uniformly increases mobility for two PNDT-DTBT polymers with different side chains due to changes in crystallinity and molecular packing inside the films.128 To solubilize low-band-gap polymers, it is necessary to introduce side-chain groups. It was found that side chains have a huge impact on the solar cell performance of donor polymers in many aspects, as discussed in section 2. Chen et al. showed the importance of branched chains by making a branched analogue of P3HT, poly(3-2-methylbutyl)thiophene (P3MBT).201 They found that the polymers with these branched chains spontaneously aggregate into rodlike structures and exhibit a SCLC hole mobility of 3.8 × 10−4 cm2/(V s) when mixed with PC71BM. In addition to P3HT-like systems, the side chains also have a dramatic influence on the performance in many other efficient PSC systems. For example, Reynolds and co-workers199 found that the polymers with straight alkyl chains on the DTS unit have either little change or decreased mobility as measured by SCLC in moving from pure film to blend, while the polymer with branched alkyl chains has increased mobility moving from pure polymer to blend. Notably, the polymer with the branched alkyl chain also has PCE values around twice that of the linear chain polymers, though the actual value for mobility is similar. This suggests that successful OPV polymers have a favorable self-assembly interaction with PCBM. Huo et al. demonstrated the importance of side-chain modification by changing the alkoxy groups on the BDT unit to alkylthienyl groups.202 This results in higher hole mobility than in the alkoxy version, with one polymer having a hole mobility of 0.27 cm2/(V s) and a correspondingly high PCE of 7.59%. The polymer:PCBM blends have higher mobility than the pure polymer, and this increase is greater in the case of alkylthienyl-substituted polymers than in the alkoxy-substituted polymers. Li et al. introduced flexible side chains on fluorene, thiophenyl, and benzothiadiazole moieties for a polymer series based on fluorene-TBT repeating units.198 When the alkyl chains on

5. INTERFACIAL ENGINEERING TO ENHANCE PSC PERFORMANCE From a materials design perspective, polymers with high mobilities necessary to efficiently extract and transport free charges to different electrodes are desired. However, device engineering must also be considered or resistance at the interface could negate the hard work spent on polymer design and processing. Interfacial layers between the electrodes and the active layer are introduced to facilitate this process, which include both a hole-selective layer (HSL) and an electronselective layer (ESL) in a single-junction PSC device. The multiple roles served by the interfacial layer include (1) tuning the work function of the electrode to promote Ohmic contact at the active layer and electrode interface, (2) determining the polarity of the device (conventional device or inverted device), (3) improving the selectivity toward holes or electrons while blocking the other and minimizing charge recombination in the interface, (4) enhancing light harvesting by introducing optical spacers, and (5) improving device stability. 5.1. Requirements of Interfacial Layers

In BHJ OPVs, Voc is determined by the difference between the LUMO energy level of the acceptor and the HOMO energy level of the donor provided that an Ohmic contact is formed between the active layer and both the cathode and the anode. According to the most widely used integer charge transfer (InCT) model, the Ohmic contact at both electrodes can be formed when choosing the anode electrode with work function higher than the InCT+ energy of the donor and the cathode electrode with the work function lower than the InCT− energy of the acceptor.203 In contrast, formation of a Schottky contact with either the cathode and/or the anode can severely reduce Voc, which will then be determined by the work function difference of the electrodes according to the metal−insulator− metal (MIM) model. Both conventional and inverted device structures are widely used for PSCs. In a conventional structure, holes are extracted by the bottom electrode, transparent indium tin oxide (ITO) in most cases, while electrons are extracted at the bottom electrode in the inverted structure. It is the interfacial layers which determine the polarity of the device. Therefore, by inserting interfacial layers with good charge selectivity, one can minimize the interfacial charge recombination and increase device performance. To extract holes to the anode, HSLs should have an appropriate energy alignment with polymer donors. It is equally important that the conduction band of X


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Figure 38. Molecular structures of organic anode interfacial materials for PSCs.

HSLs should be high enough to block the flow of electrons moving to the anode. Similar principles apply to the design of ESLs. The interfacial materials should also have larger band gaps than that of the donor and acceptor to prevent the excitons from recombining at the electrode. Finally, it is important to design interfacial materials with high conductivity to reduce the series resistance of the device.203,204 In the past few years, numerous interfacial materials and new cost-effective ways to make the buffer layers have been developed. Several recent review articles have extensively covered some of the past several years of developments.203−206 Therefore, in this section we mainly focus on summarizing the most recent developments in the area of interfacial materials for OPVs.

studied in OPVs. These metal oxides show high work functions which can facilitate Ohmic contact with active layers with minimal contact resistance. Their higher conduction band ensures efficient electron blocking, as well as providing the good optical transparency in the visible and near-IR regions that is necessary to allow photons to reach the active layer. In early studies, these metal oxides were prepared mostly through vacuum deposition processes. These are incompatible with the high-throughput roll-to-roll process and thus are not cost effective. Therefore, various solution-processed methods were developed to prepare metal oxide films that include NiO,210−213 V2O5,214,215 CuO,216 WO3,217,218 RuO2,219 CrOx,219 and NiAc.220 Due to space limitations, here we mainly discuss new solution-processing methods that have been developed to prepare molybdenum oxide (MoO3) thin films, as these have proven most useful for OPVs. However, similar solutionprocessing methods have been developed to prepare other metal oxide films mentioned above. As the most extensively investigated inorganic HSL for OPVs, solution-deposited MoO3 films have drawn much attention and been widely used. Murase et al. reported lowtemperature solution-processed smooth MoO3 thin films by preheating precursor ammonium heptamolybdate (NH4)6Mo7O24·4H2O (AHM) in deionized water followed by annealing in air.221 Later, several other approaches to make smooth MoO3 films from AHM were developed: mixing AHM with PEDOT:PSS,222 use of a combustion reaction,223 and modifying with hydrogen peroxide.224 Jasieniak et al. prepared MoOx thin films from (MoO2(acat)2) precursor with a lowtemperature treatment of 70 °C. The performance of the corresponding P3HT:PC61BM PSCs utilizing such MoOxmodified anodes shows strong dependence on the density of Mo(V) species in the MoOx film.225 Device performance comparable to devices with PEDOT:PSS as the ESL is achieved when the density of Mo(V) is minimized.225,226 In addition, MoOx227 films made from molybdenum tricarbonyl trispropio-

5.2. Anode Interlayer Material

In a conventional device structure, the most commonly used transparent electrode is ITO. However, the work function of ITO is around −4.7 eV, which cannot form an Ohmic contact with most donors due to the deep HOMO energy levels of these materials, which necessitates the use of an anode interlayer. Poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) is widely used for this purpose to promote the formation of an Ohmic contact for efficient hole collection. In addition to its proper energy alignment, two strong advantages it has are high optical transparency and solution processability. However, PEDOT:PSS exhibits several disadvantages: its acidic nature etches the ITO, and diffusion of indium into the PEDOT:PSS layer can occur, which is detrimental to device stability; the structural and electrical inhomogeneity caused by the insulating PSS component limits hole collection efficiency; finally, the hydrophilic nature of the material leads it to absorb moisture from air, which limits device stability.207−209 Therefore, great effort has been made to develop anodic interlayer materials to replace PEDOT:PSS. 5.2.1. Inorganic Materials as HSLs. p-Type transition metal oxides such as NiO and V2O5 as HSLs have been widely Y


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is used as the HSL, which results from suitable energy levels, high transparency, and low film roughness. Ginger and co-workers introduced a self-assembled monolayer of phosphonic acid on the ITO surface to replace PEDOT:PSS in PSCs.241 A linear dependence of Voc on modified ITO work function is observed. Devices with higher Voc showed longer carrier lifetimes, which is attributed to the suppression of surface bimolecular recombination of charge carriers at the HSL/BHJ interface. Changes in internal electrical field induced by variations in band bending at the interface were proposed to account for the beneficial effect. Several types of modified conjugated polymers were found to be effective as efficient HSLs in PSCs. Heeger and co-workers reported a conjugated polyelectrolyte (CPE-K) (Figure 38) with a HOMO energy level of −4.9 eV as an efficient HSL in PTB7 PSCs.242 Compared with PEDOT:PSS, CPE-K shows intrinsic superiority in homogeneous conductivity as well as pH neutrality. PSC devices based on PTB7:PC71BM with CPE-K as the HSL show slightly increased PCE (8.2%) compared to PEDOT:PSS (7.9%). The HSL layer reduced series resistance (Rs) and improved Jsc and FF values, accounting for higher PCEs. Meng et al. later reported phosphonate-functionalized PCDTBT (PCDTBT-Pho) as the HSL for PCDTBT:PC71BM devices. Due to the structural similarity and proper surface energy, more PCDTBT was found to accumulate on the PCDTBT-Pho surface than on the PEDOT:PSS surface. This results in a PCDTBT-rich anode, which is very beneficial for charge collection. As a result, PCE increases from 5.61% in the control device to 6.03% in the PDCTBT-Pho device.243 Cross-linkable charge-transporting materials have been widely used as the interfacial materials in PSCs due to their excellent solvent resistance during the subsequent processing of the BHJ active layer. These types of materials can be synthesized by thermal treatment or UV irradiation of the cross-linker which is introduced on the active components as a pendant group. The insertion of a pure cross-linked donor polymer layer between the PEDOT:PSS layer and the active layer is also an effective method to reduce charge carrier recombination by providing an additional donor/acceptor interface which allows holes to directly transport to the anode and blocks electrons.244 Ma and co-workers reported a novel lithium perchloratedoped conjugated microporous polymer (CMP) HSL prepared via electrochemical polymerization of precursor molecule TPTCz composed of a tetrahedral tetraphenylmethane core and four peripheral carbazole groups.245 The thickness and doping level of the HSL film can be precisely controlled by regulating the oxidative potential and/or the number of scan cycles. The moderately doped film (47%) shows a work function of 5.25 eV and an electric conductivity of 46 S m−1, while PEDOT:PSS exhibits a work function of 5.05 eV and a conductivity of 27 S m−1 when measured under the same conditions. The contact between the CMP films and the active layer as well as the hole transporting ability are enhanced due to the inherently 3D nanostructures with the uniform micropores 1 nm in diameter. A PCE of 8.42% is obtained for PTB7:PC71BM devices with CMP as HSL and PFN as ESL.

nitrile [Mo(CO)3(EtCN)3] have been reported. Xie et al. prepared low-temperature solution-processed hydrogen molybdenum bronze (HxMoO3) by molebdenum oxide reduction in ethanol, resulting in a HSL which shows good charge-transport properties in efficient OPV devices. The oxygen vacancies in HxMoO3 play a crucial role in its electronic properties, and the metal oxide with its excess oxygen is detrimental to the device performance when used as the anode buffer layer.228 Chen et al.229 reported a water-free and solution-processed HxMoO3 HSL in inverted OPV devices, and the performance can be further improved by the introduction of silver nanoparticles to produce nanoparticle−molybdenum oxide (Ag NP−MoOx) composite HSLs. In parallel, Argitis and co-workers demonstrated the incorporation of vapor-deposited oxygen-deficient HxMoO3 with well-defined stoichiometry and hydrogen content as the HSL for PSCs. PCE increases from 0.96% to 4.34% in P3HT:PC71BM devices by controlling hydrogen conent.230 The same group further synthesized hydrogen molybdenum bronzes by a sol−gel method. The key step is partial hydrogenation using an alcohol solvent. A moderate degree of oxide reduction (HxMoO2.75) gave high work functions and good charge-transport properties, resulting in a PCE of 4.6% for P3HT:PC71BM devices, which is larger than 2.9% from PEDOT:PSS.231 Solution-processed graphene oxide (GO) is another alternative as an efficient HSL. Chhowalla and co-workers used 2 nm GO as the HSL for P3HT:PCBM devices which achieve comparable PCE to PEDOT:PSS HSL devices.232 Murray et al. reported an enhanced lifetime for PTB7 PSCs under both thermal (5 times) and humid ambient (20 times) conditions by replacing PEDOT:PSS with GO flakes.233 The work function of GO can be tuned by chlorination. Due to a better energy level matching with the donor polymer PCDTBT, PCE increases from 5.49% for the PEDOT:PSS device and 5.59% for the GO device to 6.56% in the GO-Cl device.234 5.2.2. Organic Polymers and Small Molecules as HSLs. In addition to inorganic metal oxides, many organic compounds are used as efficient HSLs in PSCs. Figure 38 summarizes the structures of some of the representative organic materials which will be discussed below. Polyanilines are a type of classical conducting polymers that can exist in several forms and be easily doped to obtain high conductivity. For example, by acid self-doping, these materials can change from salt form (undoped) to conducting form (doped).235 They are widely studied for applications in PSCs and light-emitting diodes due to high conductivity, ease of synthesis, and good environmental stability.236−238 For example, poly(styrene sulfonic acid) (PSSA) doped graft copolymer PSSA-g-PANI has been used as the HSL for P3HT:PC61BM devices. Due to its high transparency over the P3HT absorption range and its high conductivity, a ∼20% enhancement in PCE can be achieved compared to PEDOT:PSS.237 Jen and co-workers used self-doped sulfonic acid polyaniline (SPAN) as the HSL for inverted P3HT:PC61BM devices.239 Later, Zhao et al. applied watersoluble hydrochloric acid-doped polyanilines (HAPAN) as an ultrathin HSL for highly efficient PSCs.240 HAPAN exhibits a work function of 5.12 eV, which matches well with PTB family polymers. The transparency over the whole absorption range for ultrathin HAPAN film (5.0 nm) is much higher than that of the PEDOT:PSS film (35 nm). A high PCE of ∼9% was achieved for the PBDTTT-EFT:PC71BM device when HAPAN

5.3. Cathode Interlayer Materials

In conventional PSC devices, the cathode is always thermally evaporated Al, with Ca or LiF as ESLs for efficient hole blocking and electron collection. However, low work function metals are sensitive to oxidation, which leads to an unstable Z


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ZnO NPs films. For example, metal doping is used to enhance the electron transport properties.269,270 In addition to ZnO NPs, other structures such as ZnO nanoflakes, nanorods, and nanowalls are also reported as ESLs for PSCs.271,272 Caution should be exercised during the preparation of ZnO ESLs. Absorption of oxygen at the surface of ZnO nanoparticles can result in the formation of barriers for electron extraction at the interface.273,274 Passivation of ZnO nanoparticles is an effective approach to reduce the defects at the ZnO surface and further enhance the performance of the corresponding OPV devices. Olson and co-workers showed modification of the ZnO surface through spin casting a layer of bipolar benzyl phosphonic acid. In this way, the work function of ZnO could be modulated to control Voc.275 Sun and co-workers passivated a ZnO NP surface with a network of in situ cross-linked three-dimensional poly(zinc diacrylate). They showed that after surface passivation, a dense and homogeneous ZnO interlayer with less defects is achieved, resulting in better FF and Jsc in both P3HT and PTB7 devices.276 Stubhan et al. modified an Aldoped ZnO surface with a self-assembled monolayer of phosphonic acid-functionalized fullerenes. After modification, parallel resistance increased while series resistance decreased, leading to a PCE of 3.32% for P3HT devices.277 Later, Liao et al. functionalized a ZnO surface with a hydroxyl-containing fullerene derivative. A PCBM-rich surface was formed on the modified ZnO ESL, which increased electron conductivity and enhanced electron collection efficiency for PTB7 and PTB7-Th devices.59 Small et al. reported PCEs higher than 8% for PDTGTPD:PC71BM inverted devices with a ZnO−polyvinylpyrrolidone (PVP) nanocomposite as the ESL.94 They found that ZnO nanocluster sizes and concentrations may be tuned by controlling the Zn2+ and PVP ratio. In addition, a uniform distribution of ZnO nanoclusters could be formed in PVP. Use of a polar solvent (2-methoxyethanol or ethanolamine) treatment on the ZnO surface was reported to reduce the barriers between the conduction band of the ZnO and the LUMO energy level of the acceptor (Figure 39), and thus, enhancement of PCE from 6.71% to 8.69% occurs in PTB7:PC71BM solar cells.278 Finally, Marks and co-workers inserted a copper hexadecafluorophthlocyanine (F16CuPc) layer between ZnO and ITO to improve charge-transport properties. Due to the parallel π−π stacking alignment of F16CuPc with photocurrent flow and increased interfacial contact area, enhanced solar cell performances are observed for both P3HT- and PTB7-based devices.279 In addition to MOs, GO can be used as an ESL to improve the PCE and stability of the corresponding PSCs. Wang et al. applied stretchable GO as an ESL in PCDTBT:PCBM devices via a stamping transfer process.280 An 18% enhancement of PCE is attained with GO compared to the corresponding device without an interlayer. The GO device exhibits enhanced stability with a 3% decay in PCE after 30 days in air, while the reference device showed a 56% decay. Kymakis and co-workers inserted lithium-neutralized GO (GO-Li) between TiOx and the PCDTBT:PCBM active layer.281 As a result, PCE increases from 5.5% to 6.3% due to better Ohmic contact at the interface and increased electric field amplitude in the active layer. GO-Li also serves as an internal shield against humidity, which helps to increase device stability as well. 5.3.2. Water/Alcohol-Soluble Polyelectrolytes as ESLs. Water- and alcohol-soluble and inexpensive nonconjugated polyelectrolytes (NPEs) are promising cathode modifiers which can reduce the work function of the electrode by the formation

cathode contact. In addition, vacuum deposition is not compatible with the large-scale roll-to-toll manufacturing. Therefore, various solution-processed methods have been developed with the aim toward preparing stable cathode buffer layers, such as an inorganic cesium coumpound,246−249 tungsten polyoxometalate,250 titanium oxide,251,252 titanium chelate,253−255 zirconium acetylacetonate,256 and conjugated polyelectrolytes (CPEs).257 Below we will separately discuss both inorganic and organic ESLs. 5.3.1. Inorganic Materials for Use as ESLs. Besides functioning as HSLs, inorganic metal oxides can also work as efficient ESLs. Among them, ZnO is the most widely used inorganic ESL in OPVs due to its environmental friendliness, ambient stability, low cost, high transparency, high conductivity, and good hole-blocking properties. Here we focus on discussing recent developments in ZnO films as the ESL. Solution-cast ZnO ESL via the sol−gel method requires hightemperature thermal annealing to increase the crystallinity of the ZnO film and maintain high carrier mobility.258,259 Recent studies focus on preparing the ZnO buffer layer by solution processing at low temperature (6% over a range of fulleropyrrolidine interlayer thickness from 5 to 55 nm. Ma et al.307 demonstrated the use of the amine-based fullerene derivative PCBDAN in inverted P3HT:PC61BM devices. PCBDAN was codissolved in the active layer solution (Figure 45). They found that vertical phase separations occurred, with PCBDAN aggregating at the ITO surface, which results in surface energy differences in the P3HT:PC61BM:PCBDAN blend compared to the base devices and reduces the ITO work function. A PCE of 3.7% is achieved in this way. The Jen group demonstrated that the application of fulleropyrrolidinium iodide (FPI) as the ESL significantly improves device performance and stability, which is due to its high conductivity

6. TERNARY POLYMER SOLAR CELLS Currently, the PCE for single-junction PSCs has reached more than 10% due to synergistic effects through the optimization of AF


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Table 1. Summary of Device Characteristics of Representative Ternary Blend PSCs Discussed in Section 6 composition of ternary blend

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

ref

P3HT:PCBM:SQ P3HT:PC61BM:DTDCTB P3HT:PCBM:TCTA P3HT:PC61BM:Si-PCPDTBT P3HT:ICBA:PBDTTT-C P3HTT-DPP-10%: PC61BM: P3HT75-co-EHT25 P3HT:PCBM:ICBA PDPP2TBP:PCBM:ICBA P3HT:PCBM:SiPc P3HT:PCBM:ZnPc P3HT:PCBM:SiPc:SiNc P3HT:PCBM:ZnFc P3HT:PCBM:tert-butyl SiNc PCPDTBT:PC61BM:P3HT Si-PCPDTBT:PC61BM:P3HT DTffBT:DTPyT:PCBM PCDTBT:PC71BM:PFDTBT PTB7:PC71BM:PID2 PBDTTPD-HT:PCBM: BDT-3T-CNCOO PTB7:PC71BM:ICBA PTBT:PC61BM:PC71BM

0.60 0.69 0.66 0.6 0.79 0.603 0.804 0.83 0.58 0.48 0.57 0.60 0.62 0.61 0.626 0.87 0.87 0.72 0.969 0.720 0.90

11.6 9.67 9.48 12 9.41 15.05 8.18 9.69 7.9 6.2 10.9 10.7 14.2 10.2 13.8 13.7 14.32 16.8 12.17 16.32 13.20

64.8 61 66.7 55.8 60 61 60 60 59 37 69 46 64 54.3 47.9 58.9 54.7 68.7 71.23 69.2 59

4.51 4.07 4.14 4.0 4.38 5.51 3.91 4.86 2.7 1.1 4.3 3.0 4.93 3.4 4.13 7.02 6.81 8.22 8.40 8.24 7.00

313 315 316 318 321 322 323 327 328 328 329 333 335 58 336 337 338 339 340 342 344

in P3HT:PC61BM devices.313 The SQ dye molecule shows an absorption peak at 647 nm, which not only covers a different region of the spectrum compared to P3HT but also overlaps strongly with the photoluminescence of P3HT, enabling efficient Förster resonance energy transfer between them (Figure 48). The transfer efficiency was calculated to be 77%

energy levels, donor polymer band gaps, mobilities, and device morphology. To further push PCE beyond 12% or even 15%, one needs to enhance light absorption without sacrificing Voc and FF. To address this issue, tandem PSCs consisting of two or more single cells with complementary absorption wavelength ranges are stacked together and show impressively improved efficiency.10,11,66 However, the fabrication of the tandem architecture is more complex, which leads to increased costs. The absorption benefits of tandem cells can also be achieved by combining two complementary donor materials directly into a single active layer, known as a ternary blend cell, which retains a simple device structure. Recently, ternary blend PSCs with two or more materials in the active layer are emerging as a competitive alternative to address the insufficient light absorption issues in many polymer systems (Figure 47). In ternary PSCs, the third component may be a donor polymer, a different fullerene derivative, or, in some cases, small molecules and dye molecules. In addition to having better light absorption, ternary PSCs might also exhibit improved chargeseparation and -transport efficiencies from the cascaded energy level alignments and in some cases even higher Vocs than in the host blends. You et al.311 and Brabec et al.312 published two excellent reviews on the working mechanisms of ternary PSCs including energy transfer, cascade charge transfer, and parallellike cell formation between the additional material and the binary blend. This section is divided into four parts by the role of the third component and the materials used in the ternary system. We will give special emphasis on some of the latest successful ternary PSC systems. Table 1 summarizes device characteristics of representative ternary blend PSCs discussed below.

Figure 48. Energy level diagram of the ternary blend solar cell with energy transfer from P3HT to SQ highlighted for charge carrier generation. Reprinted with permission from ref 313. Copyright 2013 Rights Managed by Nature Publishing Group.

or 96% at 1 or 5 wt % SQ loading, respectively. Further morphology studies revealed that SQ molecules prefer to stay at the P3HT/PCBM interface and facilitate more ordered packing of P3HT into crystalline fibrils. Both strong energy transfer and better morphology contribute to the much enhanced PCE from 3.27% in the reference cell to 4.51% in the device with 1 wt % SQ content. Later, Tang et al. reported very similar results by adding SQ molecules into the P3HT:PCBM binary blend. PCE increased from 3.05% to 3.72% with 1.2 wt % SQ incorporation through efficient energy

6.1. Third Component as Additional Donor for P3HT Ternary System

Similar to the optimization of binary PSCs, initial studies of ternary PSCs begin with the incorporation of a third material into the P3HT:PC61BM system to enhance Jsc. Huang et al. used 2,4-bis[4-(N,N-diisobutylamino)-2,6dihydroxyphenyl] squaraine (SQ) dye as the additional donor AG


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Figure 49. Chemical structures of DTDCTB, PID2, PBDTTPD-HT, Si-PCPDTBT, BDT-3T-CNCOO, PBDTTT-C, PDPP2TBP, PTBT, DPPDTT, and PDVT-10.

transfer from P3HT to SQ and extended optical absorption provided by the SQ molecule.314 Xu and co-workers demonstrated ∼25% PCE enhancement in the P3HT:PC61BM system using a low-band-gap small molecule DTDCTB (Figure 49) as the additional donor at 20 wt %.315 DTDCTB shows an absorption peak at 690 nm, which is more red shifted than that of P3HT. In addition, the energy levels of DTDCTB are positioned in between P3HT and PC61BM. As a result, Jsc is enhanced due to the combination of complementary absorption and energetically favored charge

transfer at the interfaces of the three materials. Photoinduced absorption spectra of these devices are shown in Figure 50. After photoexcitation of DTDCTB, a significant population of positive polarons exist in the P3HT phase, confirming charge transfer between the two donor materials. Meanwhile, the majority of bimolecular recombination in the ternary system still happens at the P3HT:PC61BM interfaces. As well as using the third material to enhance light harvesting, Chang and co-workers used a hole-preferential small molecule 4,4′,4″-tris(N-carbazolyl)-triphenylamine AH


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Figure 50. Photoinduced absorption spectra of P3HT/DTDCTB/ PC61BM (0.8:0.2:1) (solid squares) and DTDCTB/PC61BM (1:1) (●) excited with a pump wavelength of 780 nm and P3HT/PC61BM (1:1) (▲) excited with a pump wavelength of 532 nm as a reference. Reprinted with permission from ref 315. Copyright 2014 American Chemical Society.

Figure 51. EQE spectra of P3HT:Si-PCPDTBT:PCBM ternary devices. Reprinted with permission from ref 318. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

(TCTA) to enhance Voc in P3HT:PCBM devices.316 In addition to having complementary absorption to P3HT, the deep HOMO energy level of TCTA increases Voc from 0.6 V in the control device to 0.89 V in device with 2.5 wt % TCTA content. Morphological studies show that TCTA penetrate into the space between different P3HT side chains and enhances the crystallinity of P3HT, thereby facilitating exciton dissociation. As a result, PCE increases from 3.54% in the control device to 4.14% in the device with 0.8 wt % TCTA incorporation. In addition to small molecule donors, Brabec et al. studied use of the polymer PCPDTBT and PCPDTBT with silicon incorporated into the repeating unit (Si-PCPDTBT) (Figure 49) as near-IR sensitizers to enhance light absorption in P3HT:PC61BM devices.317,318 PCPDTBT slightly improved PCE from 2.5% in the P3HT:PC61BM reference device to 2.8% in the optimized ternary device. This comes from two steps: first, optical absorption is enhanced in the ternary devices at longer wavelengths due to the presence of PCPDTBT. Second, in the ternary blend, positive charges formed in the PCPDTBT phases are transferred to P3HT after charge separation due to the cascaded energy levels, leading to a better charge collection efficiency than in the PCPDTBT:PC61BM device. Introducing Si-PCPDTBT as a sensitizer into the P3HT:PC61BM system leads to a larger PCE enhancement from 3.1% in the reference device to 4.0% in the optimized ternary device.318 Increasing SiPCPDTBT content up to 40% extends device absorption to 800 nm while not causing any significant morphological changes in the ternary blends compared with the reference cell (Figure 51). Similar to PCPDTBT, charge carrier dynamics of the P3HT:Si-PCPDTBT:PC61BM ternary system revealed that after photoexcitation, holes on Si-PCPDTBT were transferred to P3HT within hundreds of picoseconds.319 Through intensity dependence measurements, it was further found that this process competes with charge carrier recombination between holes in Si-PCPDTBT phases and electrons in PC61BM phases (Figure 52). At 1 sun illumination, the probability of hole transfer was predicted to exceed 90%, explaining the higher Jsc attained in the ternary device compared to that in P3HT:PC61BM binary reference device. The authors further studied morphological changes in these two systems and concluded that the interfacial surface energy

Figure 52. Energy level diagram of the ternary blend with photophysical processes after Si-PCPDTBT excitation indicated by arrows. Reprinted with permission from ref 319. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

plays an important role in determining the position of the third component in the ternary blend.320 In the ternary system made from P3HT:PC61BM/ICBA and either Si-PCPDTBT or PCPDTBT as the third component, Si-PCPDTBT was found to be located at amorphous interfaces and P3HT crystallites, while PCPDTBT tended to accumulate at amorphous interfaces and agglomerated domains of PC61BM or ICBA. As a result, a high content of PCPDTBT mainly influences the aggregation of PCBM or ICBA and reduces electron mobility. Similarly, Si-PCPDTBT mainly affects P3HT ordering and decreases hole mobility. However, as loadings of Si-PCPDTBT up to 40% actually increased device performance compared to the P3HT:ICBA reference cell, the disruption of fullerene domains was suggested to be more detrimental than the disruption of the polymer domains in the P3HT ternary system. Teng et al. used PBDTTT-C (Figure 49) as the additional component to form a cascade energy level system within bulk P3HT:ICBA binary blend.321 They found that by introducing only 3% of PBDTTT-C, the device performance increases from 3.32% to 4.38%. This is attributed to the better exciton dissociation provided by the cascaded energy levels, as well as improved charge carrier transport and collection due to the reduced surface roughness shown by AFM. AI


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a ternary structure.323 A similar linear dependence of Voc on ICBA compositions is attained. On the other hand, Jsc decreases at higher ICBA concentrations as a result of the weaker absorption coefficient of ICBA than PCBM. Due to the decreased Jsc and intermediate Voc, the PCE of the ternary system does not surpass that of the P3HT:ICBA binary devices. Further studies from the same group were taken to determine the origin of the linear dependence of Voc on ternary P3HT PSCs composed of either two polythiophene donors and a fullerene acceptor or a polythiophene donor and two fullerene acceptors.324 Through precise measurements of device EQE as a function of photon energy, they measured the photocurrent spectral response of the ternary PSCs to elucidate the electronic state changes in these systems. In this system, the HOMO and LUMO levels of the two donors or two acceptors change continuously with different compositions, explaining the consistently changing Voc, similar to the formation of an organic alloy. On the other hand, optical absorption of the excited states still retains the features of the two individual materials across the blend composition. It is likely that only polymers with similar surface energies as in the Thompson system can facilitate polymer mixing and the formation of an organic alloy, while dramatically different surface energies instead favor polymer demixing, preventing the formation of an alloy.325,326 However, Janssen and co-workers studied charge transfer characteristics in ternary PSCs based on a biphenyldithienyldiketopyrrolopyrrole copolymer PDPP2TBP (Figure 49) and two fullerene acceptors PCBM and ICBA.327 The EQE of these devices was measured and fitted with both the alloyed CT state model and the two individual CT states model. Although the Voc of PDPP2TBP:PCBM:ICBA ternary devices shows a similar sublinear relationship with different ratios between PCBM and ICBA, the modeling results suggest that only the two individual CT state model fits the experimental results, while the alloy CT state model does not. We would like to note that the origin of Voc dependence on the fraction of the third component is different for different systems and should be carefully addressed on a case by case basis.

Thompson et al. reported ternary PSCs based on two P3HT analogues: a high-band-gap poly(3-hexylthiophene-co-3-(2ethylhexyl)thiophene) (P3HT75-co-EHT25) and a low-bandgap poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) (P3HTT-DPP-10%) with PC61BM as acceptor. Jsc improves due to complementary absorption from the two polymers.322 Unlike in previous reports with an additional polymer donor, where Voc follows the smallest Voc of the corresponding binary device, in this ternary system a linear dependence of Voc on different polymer compositions is observed when device fabrication conditions at each ratio are optimized (Figure 53). As a result, the PCE is enhanced from 5.07% in the reference device to a maximum of 5.51% in the ternary cells.

6.3. Dye Molecules as the Third Component in the P3HT System

The previous sections focused on incorporation of either donor or acceptor materials in the P3HT system, and though some of these modifications used materials with complementary spectra and thus could have increased absorption, the main modifications were found to be due to enhancing charge separation and transport via cascading energy levels. Separately, many groups have studied the effects of incorporating organic dye molecules as sensitizers in P3HT ternary PSCs to improve device performance. Among all ternary PSCs with dye molecules as the third component, the most widely investigated system is based on phthalocyanines (Pcs) and their metal complexes. Honda et al. introduced the near-IR bis(tri-n-hexylsilyl oxide) silicon phthalocyanine (SiPc) dye molecule to improve the PCE of the P3HT:PC61BM system.328 The PCEs of annealed P3HT devices increase ∼20% from 2.2% to 2.7% after SiPc loading. The EQE spectra of the two devices reveal that the presence of SiPc not only enhances EQE at its own absorption region but also significantly improves EQE in the P3HT absorption region. Combined with photoluminescence results, this suggested that the additional absorption contribution from SiPc molecules is limited due to the narrow absorption band of

Figure 53. Voc (black ■, left axis) and Jsc (red ●, right axis) for optimized ternary blend solar cells with different P3HT75-co-EHT25 concentration. (b) Voc for optimized ternary blend solar cells (□) and devices with overall polymer:PC61BM ratio fixed at 1:1.1 (blue ★) and 1:1.0 (green ▲). Reprinted with permission from ref 322. Copyright 2012 American Chemical Society.

6.2. Third Component as Additional Acceptor in the P3HT System

In addition to increasing the performance of the P3HT:PCBM system with additional donor materials through complementary light absorption, cascaded charge transfer or energy transfer, and controlled morphology, additional acceptor components may be used as well. This is typically accomplished through the use of modified fullerene derivatives and represents another type of ternary PSCs. As discussed above, a linear dependence of Voc on polymer compositions is observed in the ternary system composed of two P3HT analogues. The same group reported mixing two fullerene acceptors, namely, PC61BM and ICBA, with P3HT in AJ


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SiPc. The key contribution is instead due to SiPc molecules promoting the charge separation of P3HT excitons at the P3HT:PC61BM interface due to the cascaded energy levels and efficient energy transfer from P3HT to SiPc (Figure 54). They

Figure 54. Schematic illustration of the interface in the P3HT:PCBM:SiPc device after annealing. Reprinted with permission from ref 328. Copyright 2009 American Chemical Society.

later introduced silicon naphthalocyanine bis(trihexylsilyloxide) (SiNc) dye molecule into the P3HT:PC61BM:SiPc ternary system and further pushed the PCE to 4.3%, benefiting from the wide light-harvesting range up to 800 nm.329 The ∼20% increased Jsc in these two-additive solar cells is doubled from ∼10% in the two individual ternary solar cells with only one additive. Detailed surface energy and absorption characterization of th e selective lo calizations of dye m olecules at P3HT:PC61BM:SiPc interfaces suggests that more SiPc is located in disordered P3HT domains at the P3HT:PC61BM interfaces than in PC61BM phases or P3HT crystal domains.330 The local concentrations of SiPc in the device fabricated with RRa-P3HT:PC61BM was estimated to be ∼14 wt %, and in the device fabricated with RR-P3HT:PC61BM, the local concentration was estimated to be ∼20 wt % before annealing and as high as ∼25 wt % after annealing. All are much higher than the total content of SiPc (3.4 wt %). These results suggest two driving forces are involved in the localization of SiPc molecules at the interface. The first one is the surface energies of these materials, and the second one occurs during crystallization of P3HT, which evicts dye molecules from P3HT domains to the interface. The light-harvesting mechanism of the P3HT:PC61BM:SiPc device was studied in detail with TA spectroscopy.331 In thermal-annealed P3HT:PC61BM:SiPc film, upon excitation of SiPc, excitons in SiPc are rapidly quenched by P3HT to form P3HT+/ SiPc− charge pairs with a time constant of 2 ps, followed by efficient charge transfer from SiPc anions to PCBM with a time constant of 50 ps. In contrast to the device made with thermal annealing, in the solventannealed film, P3HT+/PCBM− pairs are rapidly generated after SiPc excitation with a time constant of 2 ps, while the SiPc anion signal is not observed. The difference is due to lower PC61BM concentrations at the disordered interface due to thermal annealing (Figure 55). Moreover, energy transfer from P3HT to SiPc quenches 34% of P3HT excitons in thermalannealed film and 21% P3HT excitons in solvent-annealed film. The electronic properties of metal Pcs can be tuned by controlling the macrocyclic ligands and using different central metals.332 Satoshi et al. synthesized zinc fluorenocyanines (ZnFcs) composed of porphyrazine rings fused with fluorene

Figure 55. Schematic illustrations of phase separations in P3HT:PCBM:SiPc films. (a) Phase separations in of P3HT:PCBM:SiPc film. (b) Enlarged local area of a. Reprinted with permission from ref 331. Copyright 2011 American Chemical Society.

rings.333 Compared with ZnPcs, both HOMO and LUMO energy levels are decreased. The ZnFc compound with eight alkyl chains improves the performance of P3HT:PCBM devices from 2.6% to 3.0% due to complementary absorption and enhanced charge carrier mobility. Lessard and co-workers demonstrated the effects of heteroatom substitution and side chain modification by synthesizing a series of different phthalocyanine additives to improve the performance of P3HT:PCBM PSCs.334 Despite the similar HOMO and LUMO energy levels, replacing silicon with boron and germanium leads to decreased device performance compared to the silicon version. In addition, they substituted tri-nhexylsilyl solubilizing groups with pentadecyl phenoxy groups. Although device performance increased slightly (3.02%) at low dye loading due to increased Jsc as compared to the P3HT reference device (2.74%), it is still lower than the case with SiPc (3.29%). The results are attributed to a larger driving force for crystallization of SiPc when it moves to P3HT:PCBM interfaces. Lim et al. incorporated a tert-butyl-functionalized silicon 2,3-naphthalocyanine bis(trihexylsilyloxide) (tert-butyl SiNc) dye molecule as the third component into P3HT:PCBM ternary PSCs.335 The dye molecule showed a strong absorption from 700 to 900 nm, which covers a different region than P3HT. tert-Butyl functionalization enabled an increased volume fraction (20 wt %) of the dye molecule to be incorporated AK


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Figure 56. EQE spectra of thermally annealed (a) and unannealed (b) P3HT:PC60BM:tert-butyl SiNc devices. Reprinted with permission from ref 335. Copyright 2014 American Chemical Society.

range from 375 to 575 nm in the ternary cells. More importantly, a small amount of P3HT was shown to increase the crystallinity of Si-PCPDTBT, as with the PCPDTBT system. After adding 2 wt % P3HT, Si-PCPDTBT crystallite size increased from 15.2 nm in the reference device to 17.8 nm in the ternary film. Addition of 2 wt % P3HT also caused a 25% reduction of PL emission intensity with significantly decreased PL decay lifetime for Si-PCPDTBT, indicating better charge separation at the polymer:fullerene interfaces. However, compared to uniformly distributed P3HT under 1 wt % loading, the ternary system with 2 wt % P3HT content exhibited vertical phase compositions with higher P3HT concentrations near ITO surface, leading to decreased Jsc compared to 1 wt % loading. As a result, average PCE increased from 3.75% in the reference device to 4.13% in a device with 1 wt % P3HT. Wei and co-workers demonstrated parallel-like BHJ ternary PSCs with two donor polymers and PCBM as the acceptor.337 Four polymers with different band gaps and absorption features were synthesized for two different ternary PSCs. It was found that the absorption spectra of the two ternary PSCs were exactly the linear combination of the two subcells (Figure 57a and 57b). Similar observations were attained for the EQE spectra of the two ternary PSCs (Figure 57c and 57d). In addition, the polymer with a large band gap and high mobility serves to create additional charge-transport channels which enhances EQE in the lower band-gap polymer absorption region. Voc of the ternary solar cells was found to be in between that of the two individual subcells, while Jsc combined those of each subcell (Figure 57e and 57f). Finally, a PCE of 7.02% is achieved in the device with DTffBT:DTPyT in equal amounts. Moreover, PFDTBT quantum dots (QDs) with absorption between 350 and 450 nm and emission between 600 and 720 nm were applied as an efficient third component to enhance the performance of the PCDTBT:PC71BM device in an inverted structure.338 By adding only 0.058 wt % PFDTBT QDs into the host blend, PCE increases significantly from 5.50% to 6.81% due to dramatically improved Jsc and FF. The better Jsc is attributed to the combined effects of the high mobility of PFDTBT, which improves charge carrier transport characteristics, and better absorption through the downconversion process where the fluorescence of the PFDTBT QDs is absorbed by the PCDTBT:PCBM host in the ternary devices. Lu et al. reported enhancement in performance of the stateof-the-art PTB7:PC71BM PSC in a ternary structure by incorporating an additional polymer PID2 (Figure 49).339 PID2 shows a maximum absorption around 610 nm, which is different from that of PTB7 at 680 nm. Besides the absorption

compared to the unfunctionalized dye (9 wt %) before PCE of the ternary devices starts to decrease. In addition, tert-butyl groups enabled the formation of dimeric or otherwise aggregated phases of tert-butyl SiNc molecules at concentrations higher than 8 wt % in the ternary device after annealing, resulting in a red-shifted absorption (∼30 nm) in the EQE spectra from the tert-butyl SiNc contribution to the blend, as shown in Figure 56. Overall, increased absorption from expanded spectral response in the ternary devices compensated for decreased FF at high dye concentration and ultimately results in 32% increased Jsc (14.2 mA/cm2) and 19% enhanced PCE (4.92%) compared to the reference device. 6.4. High-Performance Ternary Polymer Systems

The previous sections have all focused on additives to the P3HT:PCBM system, as it has been the benchmark system for which device physics are most well understood. However, as discussed in section 2, advances in donor polymer design have moved research beyond polythiophene homopolymer systems to alternating copolymer systems such as the PTB series. Recent work on ternary devices has likewise been following this trend. In contrast to P3HT-based systems, in which the bulk polymer is a wide-band-gap material which necessitates a lowband-gap material as a co-donor, these systems are often based on low-band-gap materials and require the use of a wider bandgap material to obtain complementary absorption. In a reversal from the system made using PCPDTBT and SiPCPDTBT as the third component to improve PCE for P3HT:PCBM devices, P3HT has also been used as the additional component to enhance the performance of both PCPDTBT:PC61BM and Si-PCPDTBT:PC71BM solar cells. For example, Chang et al. utilized P3HT as an effective morphology control agent for efficient PCPDTBT:PC61BM solar cells.58 P3HT enhances the performance of devices processed in both pure CB and CB with DIO. For the film fabricated with DIO, PCE increases from 2.9% to 3.4% after addition of 1 wt % P3HT. In addition to the improved absorption from P3HT, morphology studies revealed the appearance of longer and larger domains together with increased π−π stacking of PCPDTBT as well as aggregation of PC61BM after P3HT incorporation. Improved morphology in the ternary device facilitates charge separation and transport, resulting in enhanced hole mobility and improved EQE over the whole spectrum range. Similar studies were made by Rui and co-workers, who used P3HT as the additional component to enhance performance in the Si-PCPDTBT:PC71BM host blend.336 Much like the PCPDTBT:PC61BM system, addition of P3HT enhanced the spectral response over its absorption AL


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impressive considering the low efficiency of PID2 alone (Figure 58a). In addition to the absorption benefits from PID2, the bulk morphology of the active layer is optimized, including increased nanofibrillar structures in the ternary devices compared to the reference device. Also, both donor and acceptor domain sizes are reduced in the ternary devices, as revealed from RSoXS measurements. The improved hole transport, complementary absorption, and optimized morphology result in larger EQE over the whole wavelength for the best condition ternary devices compared with the reference cell (Figure 58b). Later, Wei and co-workers incorporated a small conjugated molecule BDT-3T-CNCOO as the third component into PBDTTPD-HT:PC71BM binary blend (Figure 49).340 A maximum PCE of 8.4% is achieved at the optimal weight ratio (PBDTTPD-HT:BDT-3T-CNCOO:PCBM 60:40:100), which is higher than that of either PBDTTPD-HT:PCBM (6.85%) or BDT-3T-CNCOO:PCBM (7.48%) binary devices alone. BDT-3T-CNCOO shows much better crystallinity than PBDTTPD-HT. As a result, BDT-3T-CNCOO promotes the crystallization and phase separation in the ternary device, and the morphology of the ternary device is optimized compared to PBDTTPD-HT:PCBM reference cell. This could enhance charge generation and collection in ternary solar cells, which is similar to the morphology improvement in the ternary devices from Lu et al.339 More recently, Yan and co-workers introduced high-mobility polymers as the third component into PTB7/PTB7Th:PC71BM devices.341 For the PTB7:PC71BM device, polymer DPP-DTT is used, while for the PTB7-Th:PC71BM device, polymer PDVT-10 is used. Chemical structures of the two polymers are shown in Figure 49. Both DPP-DTT and PDVT10 show high hole mobilities of around 10 cm2/(V s). The HOMO energy level of DPP-DTT (PDVT-10) is 5.20 eV (5.28 eV), cascaded between that of PTB7 (PTB7-Th) and PC71BM. As a result, charge recombination is suppressed and charge carrier lifetime is increased in the ternary devices compared to the references. Due to these benefits, the PCE of the PTB7:PC71BM system increases from 7.58% to 8.33% with 1.0 wt % of DPP-DTT incorporation, and for the PTB7Th:PC71BM system, a PCE of 10.08% is achieved by adding 0.5 wt % of PDVT-10 with Jsc increases from 17.11 to 18.73 mA/ cm2.

Figure 57. Absorption spectra of ternary devices together with two subcells based on (a) TAZ/DTBT and (b) DTffBT/DTPyT. EQE curves of ternary devices together with two subcells based on (c) TAZ/DTBT and (d) DTffBT/DTPyT. J−V characteristic of ternary devices together with two subcells based on (e) TAZ/DTBT and (f) DTffBT/DTPyT. Reprinted with permission from ref 337. Copyright 2012 American Chemical Society.

enhancement from PID2 in the ternary system, both the HOMO and the LUMO energy levels of PID2 are cascaded between that of PTB7 and PCBM, facilitating charge transfer between the main donor PTB7 and acceptor PCBM. Although the binary device made with PID2 only exhibits a poor PCE of 2.01%, by adding only 10% of PID2 into the host binary blend, PCE increases from 7.25% to 8.22%. Device PCE remains almost unchanged with up to 50% of PID2 loading, which is

Figure 58. (a) J−V characteristics of PTB7:PID2:PC71BM ternary solar cells. (b) EQE curves of PTB7:PID2:PC71BM ternary solar cells. Reprinted with permission from ref 339. Copyright 2014 Rights Managed by Nature Publishing Group. AM


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Table 2. Summary of Device Characteristics of Representative Plasmonic PSCs Discussed in Section 7 composition of plasmonic device

position of NPs

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

ref

P3HT:PCBM:Au NPs P3HT:PC71BM:Au NPs PCDTBT:PC71BM:Au NPs Si-PCDTBT:PC71BM:Au NPs PCDTBT:PC71BM:Ag NPs PTB7:PC71BM:Au NPCs P3HT:PC61BM:Au NPs PTB7:PC71BM:Au NPs PTB7:PC71BM:Ag NPs PTB7:PC71BM:Au + Ag NPs PTB7:PC71BM:Au nanorods PCDTBT:PC71BM:Au + Al NPs P3HT:PCBM: Ag NPs + Ag nanoprisms PIDT-PhanQ:PC71BM:Au NPs PIDTT-DFBT:PC71BM:Ag nanoprisms P3HT:PCBM:Au NPs PBDTTT-C-T: PC71BM:Au NPs + Ag nanograting electrode PBDTTT-C:PCBM:Ag NPs PCDTBT:PC71BM:Ag NPs PTB7:PCBM:Au doped MCNTs + Au NPs PTB7:PC71BM:Ag@SiO2 NPs PTB7:PC71BM:Au@Ag NCs PTB7:PC71BM:Au@PS NPs P3HT:PC61BM:Au-Cu NPs PBDTTT-C-T:PC71BM:Pt-Ni MPs PTB7:PC71BM:Ag P3HT:PC71BM:Ag

PEDOT:PSS active layer active layer active layer active layer active layer PEDOT:PSS TiO2 TiO2 PEDOT:PSS PEDOT:PSS active layer active layer PEDOT:PSS + C70-bis PEDOT:PSS + C60-bis PEDOT:PSS + active layer PEDOT:PSS + electrode TiO2 MoO3 PEDOT:PSS + active layer between PEDOT:PSS and active layer PEDOT:PSS PEDOT:PSS between ITO and PEDOT:PSS active layer back grating electrode back grating electrode

0.63 0.63 0.89 0.57 0.89 0.73 0.59 0.71 0.71 0.71 0.71 0.86 0.64 0.87 0.96 0.61 0.76 0.71 0.88 0.743 0.76 0.75 0.75 0.58 0.78 0.720 0.64

8.94 11.18 11.16 13.13 11.61 17.75 11.36 18.07 16.80 17.7 17.2 12.71 10.61 12.90 14.36 9.74 18.39 16.46 10.58 18.50 70 17.50 16.19 9.37 16.23 15.50 9.03

62 61 65 61 69 73.3 43 68.1 63 69.0 69.8 56.00 63.33 67 63 65.00 62.87 59.19 65 72.61 68 70 68 61 66.1 69.08 63.57

3.51 4.36 6.45 4.54 7.1 9.48 2.88 8.74 7.52 8.67 8.52 6.12 4.30 7.50 8.75 3.85 8.79 6.92 6.07 9.98 8.92 9.19 8.27 3.35 8.48 7.73 3.68

345 347 347 347 348 349 353 354 355 356 356 357 385 359 360 361 362 363 364 367 368 370 371 372 373 374 375

than the binary devices. In addition, PC71BM disrupts interchain ordering and the crystalline morphology of PTBT. Thus, fine tuning of the composition is necessary to take advantage of better absorption from PC71BM while minimizing the disruption of the interchain packing of the donor polymer. Although significant progress has been achieved with ternary PSCs, at this stage, the tandem cell architecture is still much more effective than the ternary cell structure to enhance efficiency. One of the reason is that in most of the results for ternary PSCs reported so far the host device is based on P3HT:PCBM, which has already been surpassed by much better donor materials. More attention should be paid to other high-performance donor polymer systems in the near future to fully exploit the potential of this concept. Because there are still no general design rules for efficient ternary cells, successful third components in P3HT cases may simply not apply to other high-performance polymer systems. Future optimization of the third component should aim at materials that are compatible with different polymer systems, especially those which have complementary optical properties to the low-band-gap polymers which currently hold PCE records in the field. In addition, multiple mechanisms such as complementary absorption, energy transfer, cascaded charge transfer, parallellike charge transfer, and morphology optimization coexist in these ternary cells, making it significantly more complicated than for tandem cells. While these intermingling mechanisms are a challenge to the synthetic chemist, they should prove to be a rich area to explore for those who are active in the area. A combination of comprehensive understanding and material optimization is needed to drive ternary OPVs toward higher efficiencies.

In addition to incorporating a second donor into highperformance binary PSCs, Zhan and co-workers342 incorporated ICBA as an additional acceptor into highly efficient PTB7:PC71BM binary host cells. It is known that ICBA outperforms PCBM when mixed with P3HT due to a larger Voc as a result of the higher LUMO energy level of ICBA compared to PCBM. V oc in PTB7:PCBM:ICBA films increased continuously with increased ICBA content. By adding 15% of ICBA into PTB7:PCBM, a maximum PCE of 8.24% is achieved. The enhancement mainly comes from significantly enhanced Jsc and slightly improved Voc. The morphology is almost unchanged with low ICBA loading (e.g., 15% in fullerene). The authors claim that the cascaded LUMO energy level of ICBA in between that of PTB7 and PCBM facilitates charge separation and transport inside the ternary devices and contributes to the enhanced Jsc. We would like to remind the readers that ICBA should be used with caution because the Jenekhe group found that compared to PC71BM or PC61BM, polymer:ICBA blends showed decreased driving force for photoinduced hole transfer from ICBA excitons to the donor polymer, even as electron transfer from donor polymer to ICBA remains active, which leads to lower efficiencies in devices with ICBA.343 Ko et al. explored the use of PC61BM and PC71BM as mixed acceptors to enhance the performance of PTBT polymer (Figure 49).344 The PTBT:PC71BM binary device shows improved Jsc due to better absorption from PC71BM compared to the PTBT:PC61BM device. Unfortunately, FF decreases, resulting in a similar PCE for the two devices. By mixing PTBT:PC61BM:PC71BM in a 1.0:0.8:1.2 weight ratio, a peak PCE of 7% is achieved with a Jsc value higher than both binary devices and a FF value in between the two binary devices. The ternary devices show smoother surfaces AN


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7. PLASMONIC METAL NANOSTRUCTURES TO ENHANCE LIGHT HARVESTING Similar to the use of the ternary structure to achieve extended absorption, better charge separation, and improved charge transport, the application of the plasmonic effect through the use of noble metal NPs is another valuable route to improve light harvesting within the absorption range of a given PSC device. However, because device fabrication and physics are different, this section will cover NPs and their effects separately from ternary devices above. Localized surface plasmonic resonance (LSPR) from noble metal NPs and nanostructures enhances the electromagnetic field and thus facilitates light absorption, generating excess excitons in the active layer. In addition, metal NPs or nanostructures can scatter incident photons to generate longer propagation pathways in the active layer, especially when the sizes of the metal nanostructures are large. Indeed, metal nanostructures have received great attention due to their extraordinary optical properties. In particular, Ag and Au NPs with different sizes, shapes, surface modifications, and concentrations are widely integrated into different layers of PSCs to achieve enhanced absorption. Below we will discuss some of the most recent representative work using different plasmonic metal nanostructures to enhance PSC performance. Table 2 summarizes device characteristics of representative plasmonic PSCs discussed in below.

30, 40, and 60 nm on the performance of PCDTBT:PC71BM devices.348 The absorption range of the NPs gradually red shifts with increased sizes. Under the optimized conditions (1 wt % of 40 nm sized NPs Ag-based clusters), PCE increases from 6.3% to 7.1% due to enhanced absorption and reduced resistance. Park et al. introduced Au NPCs at the bottom of the PTB7:PC71BM active layer to enhance light trapping.349 The NPCs are composed of small Au NPs with diameters around 21 nm. Due to the strong near-field coupling among the closely packed Au NPs, Au NPCs give rise to a much broader LSPR absorption and better electric field not only between the gaps of NPs but also at the outer surface compared to conventional Au NPs. The absorption of PTB7 devices is improved over the spectral region from 400 to 700 nm with the help of the Au NPCs. In addition, from a time-resolved PL study, they showed that the strong local electromagnetic field around Au NPCs facilitates exciton dissociation of the surrounding polymers due to plasmon−exciton coupling, which results in shorter exciton lifetimes. This was not observed in the device with Au NPs. The PCE of PTB7:PC71BM devices increases from 8.29% in the reference cell to 9.48% in Au NPC cell owing to significantly enhanced Jsc. Ginger and co-workers found that plasmon-resonant Ag nanoprisms lead to three times more charge generation in P3HT:PC61BM films.350 In a later study, they demonstrated that electron transfer from PCBM to Ag nanoprisms occurs, which results in a new feature in the photoinduced absorption spectrum of P3HT:PCBM located around the LSPR peak of Ag nanoprisms.351 By inserting a thin dielectric layer between the active layer and Ag nanoprisms or by using nanoprisms with a Ag core surrounded by a silica shell structure they could completely suppress this electron transfer process without sacrificing plasmonic enhancement (Figure 59). The same

7.1. Single-Metal NPs Positioned in Different Layers in PSCs

The use of one type of metal NPs such as nanospheres or nanoprisms in the active layer, HSL, or ESL is the most widely studied method to obtain plasmonic PSCs because of simplicity of the fabrication process. In addition, any physical insights into the mechanism of action of these plasmonic PSCs will form a basis for more complicated architectures incorporating multiple metals, shapes, or locations of modification. He and co-workers studied the optical and electrical effects of incorporating polyethylene glycol (PEG) modified Au NPs into the PEDOT:PSS HSL for P3HT:PCBM devices.345 They found that the absorption enhancement from LSPR of Au NPs is insignificant due to the lateral distribution of the enhanced field along the PEDOT:PSS layer. On the other hand, series resistance and exciton quenching are reduced, and the surface roughness of PEDOT:PSS is enhanced after Au NP incorporation. They suggested that the electrical properties contributed to enhanced Jsc and FF. Later, the same group incorporated Au NPs into the active layer of PFSDCN:PC71BM devices.346 When the concentration of NPs was low, both Voc and Jsc are improved. Similar to the results discussed above, they showed that LSPR-enhanced field is laterally distributed along the active layer, and this contributes to enhanced light absorption. In addition, hole mobility improves dramatically with Au NPs at all different conditions, while exciton dissociation probability gradually decreases at high Au NPs concentrations due to changed morphology and reduced Voc. Heeger and co-workers incorporated 70 nm Au NPs into the active layer of three polymer:PC71BM systems, namely, P3HT, PCDTBT, and Si-PCPDTBT.347 In addition to the enhanced absorption from LSPR of Au NPs, series resistance is reduced and the Au NPs facilitate charge transfer from P3HT to the ITO electrode due to the cascaded energy levels. Later, the same group investigated the influence of Ag NP clusters (NPCs) composed of Ag NPs with different sizes ranging from

Figure 59. Photoinduced absorption spectra of P3HT:PCBM on glass (black line) and on silver nanoprisms with (blue line) and without silica shell (red line). (Inset) TEM image of a silica-coated silver nanoprism. Reprinted with permission from ref 351. Copyright 2012 American Chemical Society.

group found that the position of NPs in the PSC devices is critical to performance enhancement.352 They showed that when the Ag nanoprisms were placed in a 77 nm thick active layer at a distance of ∼15 nm from PEDOT:PSS, over 30% enhancement can be attained in the PIDT-PhanQ:PC71BM device. Placing the Ag nanoprisms just on top of the PEDOT:PSS or on the Ag back contact leads to less enhancement. They also suggested that the concentration of NPs needs to be carefully controlled against potential losses AO


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P3HT devices measured under monochromatic light of 532 nm show S-shaped J−V curves with and without Au NPs. However, the S shape is eliminated under monochromatic light illumination of 600 nm in the device with Au NPs. This is attributed to the fact that plasmonically generated (excited at 560−600 nm) hot carriers from Au NPs fill the trap states in TiO2 ESL and lowered the extraction barrier. A PCE of 8.74% is achieved with PTB7 devices. On the other hand, Xu et al. found that LSPR from Au or Ag NPs in the TiO2 ESL can enhance light absorption in PTB7 devices. 355 Exciton dissociation is facilitated with NPs as well due to the strong plasmon−exciton coupling.

due to charge recombination at the surface of NPs in the active layer. Kozanoglu et al. studied the effects of Au NPs morphology on solar cell performance.353 Three Au NPs with star, rod, and sphere shapes were incorporated into the PEDOT:PSS layer of P3HT:PC61BM PSCs. PCEs for the three devices are 2.88%, 2.54%, and 2.47%, respectively. All are higher than 2.23% as in the reference cell. The authors suggested that the strong LSPR, large size, and branched structure of nanostars led to the largest increase in Jsc and highest PCE. Cao and co-workers incorporated Au NPs into the TiO2 ESL of inverted P3HT and PTB7 PSCs.354 As shown in Figure 60,

7.2. Cooperative Enhancement from Multiple Nanostructures

Although the LSPR region can be easily tuned for a single metal NP or nanostructure, it will only enhance the light absorption of active layer materials around the resonance wavelength region. To expand the enhancement region, one facile approach is to simultaneously integrate multiple NPs or nanostructures with multiple plasmonic resonances at different wavelength regions together into solar cell devices. Lu et al. demonstrated the concept of a cooperative plasmonic effect by combining Au and Ag NPs with sizes between 40 and 50 nm together into the PEDOT:PSS layer of PTB7:PC71BM devices.356 Compared to a single type of NPs, dual NPs further improved PCE to 8.67% in the devices (Figure 61a). Detailed EQE studies revealed that Ag NPs mainly improve light absorption of the PTB7:PC71BM device over the 450−600 nm region, unlike Au NPs which improve light absorption throughout the region between 550 and 700 nm, while in the dual-NPs device, both absorption enhance-

Figure 60. J−V characteristics of P3HT:PC61BM PSCs measured under monochromatic light of 532 and 600 nm with pristine TiO2 and Au NP-TiO2 composite as electron transport layers. Reprinted with permission from ref 354. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 61. (a) J−V characteristics of PTB7:PC71BM solar cells with and without NPs. (b) EQE spectra of PTB7:PC71BM devices with and without NPs. (c) UV−vis absorption spectra of PTB7:PC71BM devices without NPs, with Ag NPs, and with dual NPs. (Inset) UV−vis absorption spectrum of NPs in water. (d) Photocurrent density (Jph) versus effective voltage (Veff) characteristics of the reference device and dual-NPs device. Reprinted with permission from ref 356. Copyright 2012 American Chemical Society. AP


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Figure 62. (a), Energy level diagram of active layer materials. (b) UV−vis absorption spectra of PTB7 devices with NCNT, BCNT, Au:NCNT, and Au:BCNT. (c and d) Time-resolved PL measurements and calculated exciton lifetimes of PTB7 solutions with Au:NCNT (c) and Au:BCNT (d). (e and f) Steady-state PL spectra of PTB7 solutions with NCNT and Au:NCNT (e) and BCNT and Au:BCNT (f). Reprinted with permission from ref 367. Copyright 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

to achieve a broader light absorption in the device.358 Ag NPs show an absorption enhancement peak at 590 nm, while with Ag nanoprisms, the maximum enhancement came from 670 to 800 nm. The high-order resonances from the nanoprism at 670 nm contributed to the boosted absorption in the devices. As a result, average Jsc increases from 8.99 mA/cm2 in the reference cell to 9.80 mA/cm2 for the Ag NPs device, 9.93 mA/cm2 for the Ag nanoprisms device, and finally 10.61 mA/cm2 for the mixed device. Jen and co-workers doped Au NPs with different sizes into both the front and the rear interfacial layers to improve light harvesting in PIDT-PhanQ:PC71BM devices.359 By incorporating Au NPs (50 nm) into a C70-bis rear ESL, the PCE increases from 6.65% to 6.99% due to enlarged Jsc. Further incorporating Au NPs (70 nm) into a PEDOT:PSS front HSL pushes PCE to 7.50%. The similar absorption range from the two Au NPs limits the overall enhancement of device performance. Later, the same group embedded Ag nanoprisms with different sizes into both the ESL and the HSL for several

ment regions are active and enhancement from 450 to 700 nm is attained (Figure 61b and 61c). Exciton dissociation probabilities are also enhanced in the dual-NPs device compared to the reference cell (Figure 61d). Further studies show that cooperative enhancement can be achieved by using Au nanorods with two plasmonic resonances due to the asymmetric structure. A PCE of 8.41% is attained for devices with Au nanorods, which is comparable to the performance of dual NPs. Following this idea, Kakavelakis et al. incorporated small diameter Au NPs and large diameter Al NPs into the PCDTBT:PC71BM active layer.357 The dual-NPs device shows superior absorption enhancement compared to the single ones. This is attributed to a synergetic effect of LSPR from Au NPs and scattering from Al NPs. The PCE of the modified PCDTBT:PC71BM devices increases from 5.33% in the reference cell to 5.76% with Au NPs, 5.84% with Al NPs, and 6.12% with dual NPs. In parallel, Li et al. mixed Ag NPs and Ag nanoprisms together into the P3HT:PCBM active layer AQ


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polymer systems.360 Ag nanoprisms show stronger local field enhancement compared to the previous study with Au NPs. The two Ag nanoprisms exhibit absorption maxima at 450 and 535 nm, respectively. As a result, the dual nanoprisms improve the performance of PIDTT-DFBT:PC71BM devices from 7.66% in the reference cell to 9.02% in the cell with both Ag nanoprisms. In addition to a broader absorption enhancement from the cooperative LSPR, the two NPs can improve device performance through a combination of improved optical and electrical properties. Xie and co-workers incorporated Au NPs with different sizes into the PEDOT:PSS HSL and the P3HT:PCBM active layer.361 Au NPs in the PEDOT:PSS layer improved electrical properties such as hole collection, while Au NPs in the active layer increased both light harvesting from LSPR and charge carrier mobilities. As a result, PCE increased from 3.16% in the reference cell to 3.85% in Au NPs device.

absorption of PTB7 devices, while further Au doping greatly enhances absorption from 450 to 600 nm. Time-resolved PL spectroscopy in Figure 62c and 62d reveals that the exciton lifetime decreases significantly in PTB7 solutions after the addition of Au:CNTs, indicating facilitated exciton dissociation and transfer from PTB7 to Au:CNTs. Enhanced exciton generation is confirmed by steady-state PL results in Figure 62e and 62f. Although CNTs quench PL intensity of PTB7 solutions, Au doping increases the PL intensity through enhanced light absorption. A high PCE of 9.98% is achieved by embedding Au NPs in the PEDOT:PSS layer and Au:BMCNTs in PTB7 active layer. Choi et al. coated Ag NPs with a silica shell and studied the effect of the positions of Ag@SiO2 NPs on device performance.368 As discussed above in Ginger’s work,351 the SiO2 shell helps to eliminate direct contact between Ag cores and active layer materials, which is known to create trapping states in the device. Reasonably, they found that inserting the NPs between the PEDOT:PSS layer and the PTB7:PC71BM active layer gives better performance than placing the NPs between the ITO electrode and the PEDOT:PSS layer due to the closer distance to the active layer materials. A high PCE of 8.92% is achieved in this way. Later, the same group introduced carbon-dotsupported silver nanoparticles (CD-Ag nanoparticles) into PTB7:PC71BM devices and achieved an IQE of 99% for 460 nm light.369 Lee and co-workers introduced Au@Ag core−shell nanocubes (NCs) into the PEDOT:PSS HSL to enhance light trapping in PCDTBT:PC71BM devices.370 Core−shell NCs show stronger scattering abilities in the long-wavelength region than Au NPs. As a result, the PCDTBT:PC71BM device with Au@Ag core−shell NCs shows over two times enhanced absorption compared to the device with Au NPs. In addition to inorganic shells, organic materials can be used to construct core−shell structures for metal NPs as well. For example, Kim et al. synthesized monodispersed, cross-linked polystyrenecoated Au core−shell NPs (Au@PS NPs).371 The PCE of PTB7:PC71BM devices increases from 7.59% in the reference cell to 8.27% in the Au@PS NPs device due to the LSPR effect from Au NPs. In addition, the core−shell structure can serve as a template for producing the PEDOT:PSS layer. In this manner, the amount of PEDOT:PSS is reduced in the HSL, which improves the long-term stability of the PSCs. Heo et al. first reported utilizing the LSPR effect from Au− Cu alloy NPs to enhance exciton generation in P3HT:PC61BM devices.372 Au−Cu alloy NPs show an absorption maximum at 580 nm, which is intermediate between Au (540 nm) and Cu (600 nm) NPs. The PCE increases from 2.90% in the reference cell to 2.98% and 3.22% when the Au and Cu NPs were incorporated, respectively. For the device with Au−Cu alloy NPs, PCE improves further to 3.35% due to a slightly higher FF. Later, Chou and co-workers reported the synthesis of 3D urchin-like Pt−Ni alloy multipods (MPs) and embedded the MPs into the active layer composed of PBDTTT-CT:PC71BM.373 Under optimized conditions, PCE improves, enhanced from 7.38% in the reference device to 8.48% in the device with 5% MPs. In addition to the scattering effect from the metal alloy, the LUMO energy level of MPs is in between that of PBDTTT-C-T and PC71BM, facilitating electron transport in the device and reducing bimolecular recombination due to the cascade effect. Also, hole mobility increases by one order of magnitude after incorporation of 5% of Pt−Ni MPs. This is attributed to finer interpenetrating domains in the active layer and better conductivity of MPs.

7.3. Other Complicated Metal Nanostructures Used in PSCs

In addition to the varied nature and locations described previously, other complex noble metal structures have been explored as well. This has included more complicated metal− MO/carbon nanotube composites, core−shell structures, metal alloys, and metal grating back electrodes which have all been incorporated recently into PSCs to enhance device performance. Li et al. combined Au metal NPs and Ag nanopatterns in a PTB-family polymer device and achieved a high average PCE of 8.79% in an inverted device structure.362 This results from the combined effects of hybridized LSPRs from Au NPs and surface plasmonic resonances (SPRs) from the Ag nanograting electrode with waveguide modes and light diffractions. Yin and co-workers synthesized Ag−TiO2 nanorod composites, where each TiO2 nanorod contains a single Ag nanoparticle.363 They used these composites as the ESL component for inverted PBDTTT-C:PCBM devices and achieved a PCE at 6.92% due to increased Jsc and FF. Jung et al. reported using MoO3-covered Ag NPs through nanobump assembly (NBA) as an efficient HSL for PCDTBT:PC71BM PSCs.364 The upper half surface of the Ag NPs was covered with 20 nm thick MoO3 of a protruding shape. Compared with embedding Ag NPs into a flat PEDOT:PSS or MoO3 layer or directly into the active layer, the NBA structure showed prominent enhancement due to the LSPR effect. As a result, PCE increases from 5.16% in the reference cell to 6.07% in the NBA-constructed device. It has been reported that incorporating B-doped or N-doped multiwalled carbon nanotubes (B-MCNTs, N-MCNTs) can enhance the performance of P3HT:PC61BM devices via improved charge-transport properties.365 Later, Lu et al. demonstrated that N-MCNTs can be used to improve the PCE of PTB7:PC71BM solar cells through enhanced light absorption, better charge separation, improved transport, and optimized morphology.366 On the basis of these results, the Kim group further investigated incorporating Au NP-decorated B- or N-MCNTs into PTB7:PCBM devices to achieve synergistic effects through LSPR-improved charge generation and dissociation, facilitated charge transport, and better ordering of active layer materials from the inclusion of heteroatom-doped CNTs.367 Figure 62a shows energy level diagrams for the active layer materials. It is clear in Figure 62b that incorporation of B- or N-MCNTs slightly increases light AR


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Figure 63. Scheme of PDMS patterned polymer solar cells: (a) spin coating ZnO NPs as ESL, (b) spin coating active layer on top of ZnO NPs, (c) imprinting PDMS pattern on active layer, (d) lift-off PDMS pattern, (e) deposited MoO3/Ag on active layer to form metal grating. Reprinted with permission from ref 374. Copyright 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

required acceptor material physical properties and design a material which will form a similar BHJ when combined with available donor materials. However, compared with fullerene acceptors, polymer acceptors have certain advantages, such as potentially lower cost and easier modification of optoelectronic properties through the same design rules discussed for donor polymers in section 2.376 At this stage, the development of allpolymer solar cells, which consist of both polymer donor and polymer acceptor, is still far behind that of polymer:PCBM systems. In this section, some of the recent examples of polymeric alternatives to fullerene acceptors are discussed. Chemical structures of all of the donor and acceptor polymers discussed in this section are summarized in Figures 64 and 65. For polymer acceptors to be useful and competitive against fullerenes, the following features are desirable: (1) easy solution processing, (2) well-aligned HOMO and LUMO energy levels for efficient charge transfer with donor polymers, (3) strong and broad absorption in the visible range, (4) high electron mobility for efficient charge transport, and (5) favorable molecular interactions, including miscibility with donor polymers to form a phase-separated BHJ structure.

In addition to embedding metal nanostructures into different layers of the device, several groups have used a metal grating back electrode to enhance device performance through surface plasmon and scattering effects. You et al. used polydimethylsiloxane (PDMS) as the template to pattern the Ag electrode (Figure 63).374 The PCE of PTB7 solar cells increases from 7.20% to 7.73% due to enhanced Jsc. In parallel, Choy and coworkers explored the same technique for the P3HT:PCBM system.375 As well as enhancing Jsc from the surface plasmon and scattering effects, FF also increases dramatically. This is attributed to a larger interfacial area after nanoimprinting. However, we would like to note that FF slightly decreases in You’s work.374 They further showed that the surface plasmon polariton modes are divided into two plasmonic band edge states with different resonance frequencies by the periodic metal grating nanostructures. As discussed above, the use of plasmonic nanostructures offers a great opportunity to maximize solar energy harvesting in the absorption range of a PSC without increasing device thickness. The optical properties of the NPs can be well controlled through structural design and material selection. In addition, the electrical properties and concentrations of the NPs should be carefully considered to avoid the NPs serving as charge-trapping centers in these PSCs, which will lead to decreased Jsc and Voc. The PCE of PSCs with plasmonic nanostructures has already surpassed 9%. Further improvement should come from improvements in both optical absorption and electrical properties such as enhanced charge separation and transport.

8.1. Benzothiadiazole-Based Polymer Acceptors

The benzothiadiazole (BT) unit exhibits the low-lying energy levels that are necessary to be utilized as a building block for acceptor polymers. Huch and co-workers reported all-polymer solar cells based on the P3HT:PF8TBT system with a PCE of 1.9%. They were able to control nanostructure morphology of the active layer through a double-nanoimprinting process.377 The domain sizes can be as small as 25 nm. Later, Yu et al. also demonstrated the ability to control the morphology of the P3HT:PF8TBT system by forming P3HT crystalline nanowires which achieved a very high Voc of 1.35 V and an overall PCE of 1.87%.378 Miyake and co-workers studied the effects of acceptor polymer molecular weight on the solar cell performance of P3HT:PF12TBT devices.379 When the molecular weight of PF12TBT increases from 8.5 to 78 kg/mol, PCE improves from 1.8% to 2.7%. The enhancement was attributed to a more optimal blend morphology in the high molecular weight device

8. NONFULLERENE POLYMER ACCEPTORS FOR PSCS Fullerene derivatives, especially PC61BM and PC71BM, are the most common acceptor materials for use in PSCs, and most of the design rules for donor polymers assume that one of them will be used as a partner in the BHJ blend. As such, most currently used donor polymers are uniquely compatible with the properties of PCBM, such as energy levels, carrier mobilities, and morphology. This makes acceptor material design a unique challenge in that one must address both AS


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Figure 64. Chemical structures of all donor polymers used in all-polymer solar cells discussed in this section.

structure was replaced by bilayer to reduce bimolecular recombination which had resulted from the formation of large, but impure, domains. NPE-PEIE was used to modify the ZnO interface to increase Voc, and a cosolvent system was used to control the diffusion of P3HT into PIDSe-DFBT. The results illustrated the importance of optimizing processing conditions for all-polymer solar cell device performance.

after thermal annealing, which provides efficient avenues for charge generation and transport (Figure 66). The Pei group replaced thiophenyl groups in the traditional DTBT unit with thiazole units to produce DTABT.380 The LUMO energy levels of the resulting polymer DTABT-IDT decreased from −3.21 to −3.45 eV compared to the base polymer DTBT-IDT. This change to the energy level is responsible for an enhancement of the electron mobility by 2 orders of magnitude. In addition, better miscibility with P3HT is observed for the DTABT-IDT polymer compared to DTBTIDT, resulting in a PCE of 1.18% for the P3HT:DTABT-IDT system compared to 0.58% for the P3HT:DTBT-IDT system. Yao et al. demonstrated a P3HT:PIDSe-DFBT bilayerstructured device with a PCE of 2.5% by optimizing each component in the all-polymer solar cell.381 In this case, the BHJ

8.2. Perylene Diimide-Based Polymer Acceptors

Perylene diimide (PDI) and its derivatives represent one of the most promising classes of electron acceptors because of their outstanding chemical and physical properties, including high electron mobility, strong intermolecular π−π interactions, and high absorption coefficients.382 In addition, the PDI molecule offers two positions for functionalization: one via substitution AT


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Figure 65. Chemical structures of all of the acceptor polymers used in all-polymer solar cells discussed in this section.

of the β positions of the central perylene ring and another with substitution at the imide nitrogens. Roy and co-workers reported a copolymer with perylene bisimide moieties as acceptor and phenylenevinylene as donor units attached to the imide position.383 The corresponding polymer shows a high SCLC electron mobility of 8.5 × 10−3 cm2/(V s) with HOMO and LUMO energy levels of −5.75 and −3.95 eV, respectively. Solar cells fabricated from this accepting polymer and P3HT as

the donor material exhibited a PCE of 2.32% after the films are thermally annealed. An electron-transporting polymer based on PDI and the dithienothiophene unit (PDI-DTT) was reported by Zhan et al., which shows a high FET mobility of 1.3 × 10−2 cm2/(V s).384 PDI-DTT shows a broad absorption in the visible range with an extension to the near-IR region. Solar cells fabricated from PT2 and PDI-DTT give a PCE of 1.48%. The same group investigated the application of PDI-DTT acceptor in large-area AU


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Figure 66. Schematics of performance of P3HT:PF12TBT devices with different Mw. Reprinted with permission from ref 379. Copyright 2012 American Chemical Society.

8.3. Naphthalene Diimide-Based Polymer Acceptors

devices with roll-to-roll processing and PSBTBT as the electron donor. However, a poor PCE of only 0.2% resulted.385 A series of polymers containing β-substituted PDI with various donor units such as thiophene, DTP, and carbazole were developed by Zhou et al.386 Two donor polymers, P3HT and PT1, were used to test the solar cell performance of the PDI-based polymers. Compared to P3HT, PT1 exhibits a lower HOMO energy level and better film morphology in the blends. Among all the devices, the combination between PT1 and the carbazole-based PDI polymer (PC-PDI) gives the best performance with a Voc of 0.70 V, a Jsc of 6.35 mA/cm2, and a FF of 50%, ultimately resulting in a PCE of 2.23%. The Yu group recently developed electron-deficient TPTI387 and CN monomers and synthesized a series of alternating acceptor polymers containing different monomer combinations.388 They found that the LUMO energy levels of the polymers are strongly dependent on the more electrondeficient monomers, while the HOMO energy levels are largely determined by the less electron-deficient monomers. Fluorescence quantum yield was found to be closely related to the photovoltaic properties, indicating that internal polarization plays a role in determining the photovoltaic properties. Among all the acceptor polymers investigated, polymer PNPDI gives the best performance with a PCE of 1.03% when PTB7 is used as donor material. PDI-2DTT, a 3-unit small molecule fragment of the polymer PDI-DTT, was used as a processing additive to improve the performance of PBDTTT-CT:PDI-DTT solar cells.389 The use of PDI-2DTT smooths the polymer domains and enhances donor/acceptor mixing for more efficient charge transfer, leading to an improvement of average PCEs from 1.16% to 1.43%. In addition, DIO facilitated the aggregation and crystallization of PBDTTT-CT, leading to improved average PCEs of 2.92%. By combining the two additives together, PCE can be further pushed to 3.45%. To control phase separation, a novel strategy of introducing a small percentage of PS side chain to the donor polymer was applied.390 A series of isoindigo-containing polymers was studied as donor polymers and a perylene tetracarboxlic diimide (PTCDI) based acceptor polymer (P(TP)) was used. Among the donor/acceptor combinations, Pil-2T donor shows the highest PCE at 3.48%. After introducing 5 mol % PS side chain, the efficiency is further increased to 4.21%, mainly due to the increase in Jsc from the decreased phase separation due to smaller domain length scales.

The polymer N2200 (or in some cases named P(NDI2ODT2)) is the most thoroughly studied polymer acceptor so far. Composed of NDI and bithiophene moieties, N2200 was first reported for use in OFETs.391 A high electron mobility of 0.45−0.85 cm2/(V s) was demonstrated for N2200 under ambient conditions in combination with Au contacts and various polymeric dielectrics. Later, it was introduced as an acceptor material for all-polymer photovoltaic devices. At the initial efforts by Moore et al.,392 despite the high electron mobility, deep LUMO energy level, and a complementary absorption with P3HT, the efficiency of the P3HT:N2200 device was only 0.2% with very poor Jsc and FF. They found that fast geminate recombination within 200 ps of excitation contributed to the low Jsc. This is due to a poor morphology with widely varied and overly large domain sizes up to 1 μm in the blend. However, Fabiano et al. found that due to balanced electron and hole mobility, high FF approaching 70% can be achieved in P3HT:N2200 devices.393 Unfortunately, due to the poor morphology, no improvement in PCE was attained. Later, Neher and co-workers solved the strong tendency for N2200 to aggregate by using suitable solvents with large and highly polarizable aromatic cores.394 The preaggregation could be completely suppressed, and intermixing between P3HT and N2200 increased. Further tuning of donor/acceptor ratios, spin-casting conditions, and additives pushed efficiency to 1.4% due to improved morphology, which led to improved Jsc. Fabiano et al. further showed that by using different solvents, the film morphology of P3HT:N2200 can be controlled, resulting in changes to the hierarchical structure, polymer aggregations, and phase separations.395 Both solar cell devices and FET devices were fabricated and studied. By using xylene:CN cosolvents, laterally phase-separated blends were obtained, leading to a high solar cell performance with a PCE of 1.31%. DCB was also used as the processing solvent, which resulted in large and balanced ambipolar FET mobility, but unfortunately solar cell performance diminished. To address the low photocurrent generation, Schubert et al. studied the processes that controlled free charge carrier generation for P3HT:N2200 devices.396 They correlated the amount of photocurrent produced to changes in polymer crystallite orientations. EQE results indicated that both donor and acceptor polymers contribute to current generation; however, the acceptor only contributes from one-half to oneAV


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Figure 67. (a) J−V characteristics and (b) EQE spectra of all-polymer solar cells from PSEHTT:PNDIT, PSEHTT:PNDIS, and PSEHTT:PNDISHD blends. Reprinted with permission from ref 405. Copyright 2013 American Chemical Society.

occurs in the optimized devices as observed by X-ray data. The processing additive DIO is crucial to device performance by increasing the crystalline character of N2200 domains together with the appearance of highly ordered polymer organizations with face-on geometry, which enhanced electron mobility and Jsc in the devices. In parallel, Ito and co-workers achieved a PCE of 5.73% for PTB7-Th:N2200 devices with a maximum EQE ∼60%.402 The authors ascribed the success to large charge generation and collection efficiency (both over 80%), which were comparable to those in efficient polymer:PCBM devices. Yan and co-workers mixed N2200 with the donor polymer NT and achieved a large PCE of 5.0% with a high Jsc of 11.5 mA/cm2.403 The high Jsc is attributed to the low band gap of the donor polymer and favorable morphology in the blend films. Detailed studies indicated that donor polymer NT maintains its crystallinity with a face-on orientation in the BHJ blends, resulting in a high hole mobility that is balanced with the electron mobility of N2200. Recently, McNeill and co-workers achieved >4% PCE for allpolymer solar cells based on a BFS4:N2200 blend.404 Voc of the best device reaches 0.9 V, which is the highest value for N2200 devices. The blends show a coarse phase-separated morphology with domains of a semicrystalline nature. In addition, the top surface of the blends is composed of 100% BFS4, while the bottom surface shows a mixed composition roughly correlated to the overall blend ratio, indicating vertical phase separations. TA spectra revealed that device efficiency is limited by incomplete exciton separation and high geminate recombination. In addition to N2200, another polymer containing NDI and selenophene units shows great potential toward application in all-polymer solar cells.405 Jenekhe and co-workers reported using a NDI-selenophene copolymer (PNDIS-HD) as the acceptor and a thiazolothiazole copolymer (PSEHTT) as the donor, leading to a PCE of 3.3% (Figure 67). Selenophenecontaining polymers show higher electron mobility than their thiophene counterparts, which is ascribed to better orbital overlapping from the larger π orbitals in the selenium atom. The crystalline morphology of PNDIS-HD also helps to achieve a balanced charge mobilities in the blends. Later, the same group enhanced the performance of PSEHTT:PNDISHD devices by using CB/DCB cosolvents at a 9:1 volume ratio.406 Use of cosolvents reduced polymer domain sizes (to 20−40 nm) on the surface of the active layer compared to films processed from CB (>100 nm) and suppressed bimolecular recombination in the devices. In addition, in the cosolvent

third as much as the donor. After the addition of CN additive, N2200 domains become smaller and P3HT domains become more pure. By increasing the amount of CN, N2200 stacking changes from face on to edge on. They indicated that face-toface stacking of the donor and acceptor polymer crystals is necessary to generate free charges, while misoriented chains inevitably cause geminate recombination losses of excitons. Schmitt-Mende et al. introduced a comb-like bilayer structure for P3HT:N2200 devices.397 The bilayer structure was achieved via photo-cross-linking of the N2200 network followed by solution deposition of P3HT. They found that when the interfacial area increased by this process, Jsc increased due to enhanced exciton separation, while Voc slightly decreased due to increased bimolecular recombination. They proposed that the ideal morphology should consider not only D/A domain sizes but also the spatial arrangement. In addition to using P3HT as the donor polymer, several other donor polymer systems have been studied with N2200 as acceptor. Ito and co-workers used PTQ1 as the donor polymer and achieved a large PCE of 4.1%.398 When the donor/acceptor weight ratio is varied from 20%:80% to 70%:30%, PCE increases. Note this is quite different from polymer/fullerene blends, which normally require an equal or a larger weight ratio of fullerene. Tang et al. reported all-polymer BHJ PSCs with PTB7 as the donor and N2200 as acceptor.399 The two polymers exhibit complementary absorption spectra. Both electron transfer from PTB7 to N2200 and hole transfer from N2200 to PTB7 are observed in the EQE spectra. Morphology studies revealed that crystalline N2200 domains dispersed in amorphous PTB7, and a N2200-rich top layer is formed in the device. A modest PCE of 1.1% results. Later, Marks and co-workers tuned the morphology of PTB7:N2200 devices via carefully selecting processing solvents.400 Morphology, charge-transport properties, and solar cell performance of the corresponding devices are found to be sensitive to the processing solvents. Xyleneprocessed films exhibit significantly more ordered π−π stacking for the two polymers with higher mobilities for both electrons and holes. A PCE of 2.66% is realized when xylene was used as the solvent, which was much larger than devices fabricated with CB (1.35%) and CF (1.78%). Kim and co-workers achieved over 4.5% PCE by using PTB7-Th and N2200 blends as the active layer materials.401 Morphology studies showed that the blend has highly intermixed domains in the BHJ due to low interfacial tension. Also, face-on π−π stacking of PTB7-Th and N2200 domains AW


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Figure 68. EQE plots of all-polymer solar cell based on PTB7:PNDTI-BT-DT spin coated by CF with corresponding absorption spectra of the two polymers. Reprinted with permission from ref 409. Copyright 2014 American Chemical Society.

However, molecular engineering of the morphological aspect of all-polymer devices has proven more difficult as compared to polymer:fullerene devices. For example, phase-separated domain sizes and intermixing between donor and acceptor are hard to control. Even studying the morphology differences is difficult due to poor spectral contrast in the composing elements. However, these challenges are being overcome, with the highest PCE for all-polymer solar cells recently reaching 6.47% by Polyera Corp.376 This demonstrates the great potential for these types of devices. To further improve the performance of all-polymer solar cells, future research should aim at material and device engineering to achieve complementary absorption, realize long-lived charge-separated states, and, most importantly, tune blend morphology to reduce geminate and bimolecular recombination.

system, carrier mobilities were found to be even more balanced. A PCE of 4.8% resulted with a high Jsc of 10.5 mA/cm2. The performance of PSEHTT:PNDIS-HD could also be improved through side-chain engineering of the PNDIS acceptor. By tuning the ratios of 2-decyltetradecyl (DT) to 2-butyloctyl (BO) side chains on the backbone, crystallinity and electron mobility can be optimized, resulting in a PCE of 4.4%.407 The crystallinity of PNDIS-HD is further tailored by synthesizing random copolymers xPDIs (Figure 65) based on NDIselenophene and PDI-selenophene monomers.408 The average crystalline domain size decreases from 10.22 nm in PNDIS-HD to 9.47 nm in 10PDI, 5.11 nm in 30PDI, and 3.62 nm in 50PDI. As a result, compatibility with donor PBDTTT-CT is greatly improved for 30PDI (domain sizes ≈ 20 nm in the mixture) compared to PNDIS-HD (domain sizes ≈ 200 nm in the mixture). The PBDTTT-CT:30PDI mixture exhibits balanced electron and hole transport as well, resulting in a PCE of 6.3% in the PBDTTT-CT:30PDI device. Finally, Zhou et al. reported all-polymer solar cells based on PTB7 as donor polymer and a NDI-family copolymer based on naphthodithiophene diimide and bithiophene to produce the polymer (PNDTI-BT-DT) as an acceptor material.409 PNDTIBT-DT shows strong absorption in the near-IR region (Figure 68). A PCE of 2.56% is attained with CF as the solvent. In addition to incorporating commonly used BTD, PDI, and NDI moieties, another strategy to develop acceptor polymer is to modify donor polymer moieties to make them more electron deficient. For example, the Janssen group synthesized an acceptor polymer PDPP2TzT which was based on DPP units by replacing the two thiophene units in the high-performance donor polymer PDPP3T 410 with two thiazole units. PDPP2TzT exhibits deep HOMO and LUMO energy levels at −5.63 and −4.00 eV, respectively, high electron mobility, and a broad absorption range up to 850 nm. By mixing PDPP2TzT with the structurally similar donor polymer PDPP5T and carefully optimizing processing conditions, a PCE of 2.9% is accomplished.411 Research on acceptor polymers has been advancing quickly in recent years. In order to be viable, they will need to exhibit similar features to PCBM both electrochemically and morphologically in the BHJ active layer when combined with donor polymers. The electrochemical features have thus far proved relatively easy to properly engineer, with promising properties such as complementary absorption with common donors, low-lying energy levels, and tunable dipole moments.

9. STABILITY Underlying all these important design parameters described throughout this review for pursuing high PCEs, device longterm stability is another crucial issue that needs to be addressed before commercial application of PSCs can become widespread. Although the degradation process is still not fully understood, several mechanisms have been identified. For example, in a conventional structure, electrodes such as Ca and Al are easily oxidized when exposed to air due to the formation of oxides. Internally, PEDOT:PSS induces possible damage to both the ITO and the active layer due to its acidic and hygroscopic nature. Thermal and photo-oxidation of active layers under illumination in ambient conditions also occur. In addition, in most systems, blend morphology is not thermodynamically stable and phase separations deteriorate with time, and these thermal changes will decrease device performance. In this case, both polymer and PCBM contribute to the photothermal degradation of the photoactive layer morphology.412 Those interested in detailed mechanisms for OPV degradation may be referred to the review by Krebs and co-workers.413 In this section we briefly introduce methods to enhance the thermal and photostability of the active layers and the current status of the stability of PSCs. Side-chain functionalization is an efficient way to enhance the thermal stability of the photoactive layer. By inserting ester or alcohol moieties on PCPDTBT side chains, a better interface between the active layer and electrodes forms, leading to increased thermal stability.414 Kesters et al. produced a series of P3HT derivatives with either alcohol or ester groups AX


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accelerated testing methods, and their reliability in reflecting the real lifetime of different solar cell systems may be questioned. A more straightforward experiment was conducted by Krebs and co-workers.429 The stability of a fully printed and ITO-free P3HT:PCBM solar cell was evaluated under testing protocols from the International Summit on OPV stability. Ninety-five percent of the initial performance was preserved after 1 year of outdoor operation and storage in The Netherlands, Denmark, and India. Compared to the fast development in increasing the PCE of PSCs, relatively less attention is given to improving device longterm stability in the community. Each layer inside PSCs may deteriorate in its performance under different conditions. A better understanding of the degradation mechanisms will help develop methods to improve the stability of PSCs via a combination of stabilization of the active layer, engineering of more benign interfacial materials, and physical encapsulation techniques. A standard test protocol for lifetime measurement is desired to compare results published by different groups in the future.

introduced into a small portion of P3HT side chains (5−15%) and achieved dramatically improved thermal stability without sacrificing initial PCEs of these devices compared to P3HT.415 Increasing polymer purity can also improve the long-term stability of solar cell devices. Mateker et al. showed that removing low Mn species from high Mn PBDTTPD via sizeexclusion chromatography led to only a 6% decrease in Jsc even after 111 days of storage in a glovebox and in the dark. In comparison, Jsc dropped by 20% in 7 days for the unpurified sample.416 Similarly, for polymer PCDTBT, removing median and low Mn portions resulted in dramatically decreased burn-in loss. The corresponding high Mn PCDTBT device retained 82% of its initial PCE under continuous illumination for 1400 h.417 Introducing compatibilizers helps to restrain the movement of PCBM molecules and increase the morphological stability of the resulting PSC devices. Fréchet and co-workers synthesized a diblock copolymer compatibilizer containing pendent P3HT repeating units and pendent fullerene derivatives. Adding a small amount of this copolymer into the P3HT:PCBM system altered the interfacial energies and limited phase separations for PCBM.418 Lee et al. introduced a C60-end-capped P3HT as compatibilizer for P3HT:PCBM film.419 The PCE in the reference device decreased to less than 1% after annealing for 140 h, while a device with 2.5 wt % compatibilizer remained nearly unchanged at a PCE of 2.5% at the same condition. Freezing bulk morphology through intermolecular crosslinking of either polymer or fullerene represents another fashion of improving the thermal stability of PSCs. Miyanishi et al. first mixed cross-linkable donor poly(3-(5-hexenyl)thiophene) with PCBM.420 Upon thermal treatment, crosslinking occurs between the vinyl groups of the side chains and limits the formation of large PCBM aggregations. Derue et al. synthesized a bis-azide cross-linker for multiple donor:PCBM systems.421 Specific cross-linking with fullerenes is achieved and fullerene diffusion is prevented with the formation of small crystallites. A small molecule bis-azide 1,6-diazidohexane crosslinker may be applied to thermally stabilize the P3HT:PCBM morphology. The cross-linker is initiated by a UV-light curing process and suppresses both fullerene aggregation in the bulk blend and polymer aggregation at the cathode interface.422 PCBM aggregations can be suppressed by porphyrins due to the supramolecular interactions between PCBM and porphyrins.423 For the P3HT:PCBM deivce, PCE drops only by 10.5% after 48 h of annealing when 8 wt % of porhyrins are incorporated, while the reference device shows a 71.5% decrease in PCE after only 3 h of annealing. As discussed in section 5, replacing Al with high work function metals like Au or Ag and PEDOT:PSS with MOs or GOs helps to improve the stability of electrodes and interfacial layers, two of the most commonly attacked components. Diffusion of water into the active layer is prevented when using hydrophobic interfacial layers.424 However, for long-term stability, encapsulation is desired, which would avoid direct exposure of the device to oxygen and moisture.425 To this end, several groups have reported encouraging results for long-term stability for different PSCs. For example, McGehee and coworkers reported a ∼7 year lifetime for encapsulated PCDTBT devices.426 Similarly, Sapkota et al. reported less than a 10% PCE drop for P3HT devices after 12 000 h of illumination.427 A lifetime of over 18 000 h was achieved for PCDTBT cells with glass−glass sealing and TiOx/aluminum cathodes.428 However, it should be noted that these results are estimations from

10. SUMMARY AND OUTLOOK Tremendous progress has been made recently in almost every aspect of PSCs. Many empirical design rules have been extracted from these results. In this review, we examined a wide range of polymer materials to provide useful guidelines to help elucidate the complex interplay of various types of structures (chemical, morphological, and in device fabrication) and their effects on various key solar cell properties. Going from a microto a macrolevel, this begins with controlling the molecular structure via the choice of different donor−acceptor copolymer backbones, side-chain engineering, and heteroatom substitution in the polymer backbone. We summarized the effects that arise from these structures in terms of both active layer morphology and ultimately the physical properties which emerge when energy levels and domain sizes are optimized in harmony. On the most macroscale, we described in detail the modification of device structure via the composition of interfacial HSLs and ESLs fabricated from materials such as metal oxides and CPEs, as well as the emergence of ternary blend solar cells. Next, both the incorporation of dopants such as noble metal nanoparticles to improve absorption and maximize solar cell properties via the plasmonic effect and all-polymer solar cells made from both polymer donor and acceptor were examined. Finally, we briefly introduced recent studies and current status of stability for PSCs. Recently, several groups predicted the thermodynamic PCE limit of PSCs to be >20% in single-junction devices.430,431 To achieve this goal, an EQE of 90% over a broad range and a FF of 80% are necessary. At this stage, encouraging EQE of ∼80%60,71 and FF of approximately 80%33,71,175,176,432 have already been reported in different polymer systems. Several recent studies reveal that blend nanomorphology plays an important role in determining device FF. For example, utilizing 1-chloronaphthalene additive to optimize the interpenetrating network for P3HT:ICBA devices results in a FF of 75%.33 Guo et al. showed that FF of 76−80% may be realized from highly ordered molecular packing and optimal vertical phase separations.175 Yan and co-workers controlled the phase separations by temperature-dependent aggregation behavior of donor polymers and achieved a FF of 77%.71 Li et al. achieved a FF of 74% for a 300 nm thick diketopyrrolopyrrolebased polymer device with interconnecting and crossing AY


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crystalline fibrous structures for high charge carrier mobilities.432 Still, at this stage, there is no polymer that exhibits a PCE value more than twice that of P3HT, and all device PCEs that are worthy of mention are within 8 ± 2%. The current challenge then, is to achieve a high Jsc while maintaining a large Voc in PSC devices. From the discussion in this review, several promising routes can be proposed. (a) Material engineering to achieve broad absorption and high hole mobility in donor polymers. These two are the main challenges in materials design for PSCs. The absorption spectra of the active layer materials need to be extended to the near-IR region while maintaining a high absorption coefficient throughout the absorption range. Hole mobilities should be high enough to efficiently transport holes and balance charge transport. In addition, the HOMO and LUMO energy levels of polymers can be engineered to achieve better performance for PSCs.

Biographies

Luyao Lu received his B.S. degree in Chemistry from Nanjing University in China in 2010. He obtained his Ph.D. degree from the University of Chicago in 2015 under the direction of Professor Luping Yu. His Ph.D. research was focused on understanding the structure− property relationship of conjugated polymers and developing highly efficient polymer solar cells. He is currently a postdoctoral fellow in the lab of Prof. John A. Rogers at the University of Illinois at Urbana− Champaign.

(b) Combining intramolecular charge separation as well as optimized morphology to facilitate charge separation. The key is to minimize hole and electron binding energy. A high dielectric constant in the polymer blend will be desirable. (c) Interfacial engineering to facilitate charge transport and collection in high-performance PSCs through decreased resistance, as well as improving device stability. Supramolecular chemistry might be a powerful approach to address these issues. (d) Mixing multiple donors or acceptors in a high-performance ternary structure to obtain complementary light absorption, increase Jsc, as well as provide better charge transport and improve Voc.

Tianyue Zheng received his B.S. degree in Chemistry from Nanjing University, People’s Republic of China, in 2010. Then he moved to the University of Chicago. He is currently a Ph.D. candidate under the direction of Professor Luping Yu. His current research focuses on the rational design of conjugated polymers for high-efficiency polymer solar cells.

(e) Utilizing metal nanostructures to achieve improved optical and electrical properties in PSC devices such as increased absorption and improved charge transport. Although the design guidelines provided here are limited in detail and scope, we hope that the readers have gained useful insights into material synthesis, device engineering, and physics that may help to drive them toward the development of more efficient PSC materials and devices. An encouraging conclusion to be drawn from this review ought to be that the general trend of development in PSCs is exciting and the future seems to be bright; yet, there is much science to be done before these OPV devices can have a significant impact on our renewable energy landscape and our society.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

Qinghe Wu received his B.S. degree in Chemistry from Henan University in 2007 and Ph.D. degree in Organic Chemistry from the Shanghai Institute of Organic Chemistry, Chinese Academy of

Notes

The authors declare no competing financial interest. AZ


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on polymer chemistry, molecular electronics, and solar energy conversion.

Sciences, in 2013. He is currently a Post-Doctoral Scholar in Prof. Luping Yu’s group at the University of Chicago. His current research interests focus on the design and synthesis of acceptor materials for nonfullerene solar cells.

ACKNOWLEDGMENTS This review would not have been possible without support from U.S. National Science Foundation grants, the Air Force Office of Scientific Research, and DOE via the ANSER Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DE-SC0001059 and the NIST ChiMad project. ABBREVIATIONS PV photovoltaic PSCs polymer solar cells PCE power conversion efficiency FF fill factor BHJ bulk heterojunction Voc open-circuit voltage Jsc short-circuit current density CT charge transfer μh hole mobility near-IR near-infrared HOMO highest occupied molecular orbital LUMO lowest unoccupied molecular orbital e−h electron−hole EQE external quantum efficiency OPV organic photovoltaic D−A donor−acceptor IQE internal quantum efficiency FET field effect transistor SMOSCs small molecular organic solar cells OFETs organic field effect transistors Mn molecular weight Đ dispersity index DFT density functional theory SCLC space-charge-limited current GIXS grazing incidence X-ray scattering GIWAXS grazing incidence wide-angle X-ray scattering RSoXS resonant soft X-ray scattering AFM atomic force microscopy TEM transmission electron microscopy EF-TEM energy-filtered TEM μe electron mobility TOF time-of-flight CELIV carrier extraction by linearly increasing voltage DI dark injection AS admittance spectroscopy IS impedance spectroscopy TRMC time-resolved microwave conductivity HSL hole-selective layer ESL electron-selective layer InCT integer charge transfer MIM metal−insulator−metal Rs series resistance NP nanoparticle NPEs nonconjugated polyelectrolytes PNs precursor polymers QDs quantum dots LSPR localized surface plasmonic resonance NPCs NP clusters SPRs surface plasmonic resonances

Alex Schneider received his B.S. degree in Chemistry from Purdue University in West Lafayette, IN. He currently works as a Ph.D. candidate under Prof. Luping Yu. His interests lie in the synthesis of new donor materials, especially those for ternary solar cell systems.

Donglin Zhao received her B.S. degree in Chemistry from Jilin University, People’s Republic of China, in 2010. She is currently a Ph.D. candidate under the direction of Professor Luping Yu. Her current research focuses on the design and synthesis of nonfullerene acceptor materials for solar cell device optimization.

Luping Yu was born in Zhejiang Province, People’s Republic of China. He received his B.S. (1982) and M.S. (1984) degrees in Polymer Chemistry from Zhejiang University and his Ph.D. degree (1989) from the University of Southern California. He is currently a Professor of Chemistry at the University of Chicago. His present research focuses BA


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MCNTs multiwall carbon nanotubes NCs nanocubes MPs multipods

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Recent Advances in Bulk Heterojunction Polymer Solar Cells

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