A Rational Design and Synthesis of Cross-Conjugated Small Molecule

Article Cite This: Chem. Mater. 2018, 30, 4331−4342

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A Rational Design and Synthesis of Cross-Conjugated Small Molecule Acceptors Approaching High-Performance Fullerene-Free Polymer Solar Cells Yan Liu, Gongchu Liu, Ruihao Xie, Zhenfeng Wang, Wenkai Zhong, Yuan Li,* Fei Huang,* and Yong Cao

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Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China S Supporting Information *

ABSTRACT: Considering the combination of the advantages of both the acceptor−donor−acceptor and perylene diimide type acceptors, three new cross-conjugated small molecule non-fullerene acceptors were rationally designed and synthesized. Their thermal, optical, electrochemical, and photovoltaic properties were systematically investigated. These acceptors exhibit highly extended absorption spectra ranging from 300 to 700 nm as well as appropriate lowest unoccupied molecular orbital levels compared with those of the general acceptors based on perylene diimide. Non-fullerene polymer solar cells based on the resultant acceptors were fabricated in the ITO/PEDOT:PSS/PTB7-Th:acceptor/PFN-Br/Ag configuration. Benefiting from efficient exciton dissociation, reduced bimolecular recombination, and enhanced and balanced electron mobility, the device-based PDIBDT-IT exhibited a power conversion efficiency (PCE) of 6.06% with a high short-circuit current density (JSC) of 13.60 mA/cm2, a fill factor (FF) of 60.45%, and an open-circuit voltage (VOC) of 0.74 V, which was higher than those of PDIBDT-RDN and PDIBDT-ITF. The underlying mechanism was carefully studied and discussed. The extended absorption of PDIBDT-IT contributes to its relatively high PCE; however, the performance will be potentially improved by ehnancing the electron mobility of these acceptors. Our results provide an efficient cross-conjugated approach and great flexibility in fine-tuning physicochemical properties, including absorption spectra and energy levels of the acceptors toward high-performance solar cells.

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INTRODUCTION Polymer solar cells (PSCs) with conjugated polymers as the electron donor applied in bulk heterojunction (BHJ) devices have attracted a great deal of attention in terms of the development of new materials and new energies considering their advantages of low cost, light weight, semitransparency, flexibility, and readily large-area fabrication through potential roll-to-roll solution processing,1−8 which has proven to be efficient for both fullerene and non-fullerene systems. Endowed with superior electron mobility, isotropic charge transporting properties, and the formation of appropriate phase separation,9,10 fullerene and its derivatives (such as PC61BM and PC71BM) are still applied as electron acceptors in efficient PSCs. In generally reviewing the whole field, we found the development of fullerene derivatives is relatively more mature than that of non-fullerenes, and the PCE of fullerene-based binary single-junction PSCs has surpassed 11%.11−16 However, fullerene derivative materials still exhibit many intrinsic shortcomings, such as high production cost, narrow and weak absorption in the visible region, poor environmental stability, difficulty in functionalization and energy level © 2018 American Chemical Society

tenability, and inferior morphology stability that resulted from their ready aggregation.17−20 In contrast, during the journey to achieve the breakthrough of efficient fullerene-free OSCs, non-fullerene acceptors have attracted intense interest because of their advantages of wide and strong absorption in the visible region, convenient synthesis, finely tuned energy levels through various chemical modification, and combinations to match the donor materials.21 In particular, an acceptor−donor−acceptor (A−D−A) type of relatively planar molecule,22 which is composed of an electron-rich core and end-capped with different electronwithdrawing moieties like 2-(3-oxo-2,3-dihydroinden-1ylidene)malononitrile (IC),23,24 its fluorine- and alkyl-substituted products (e.g., ICF, ICFF, and ICM),25−27 3ethylrhodanine (RDN)28−34 and barbituric acid (BBT),35 has proven to be a highly effective strategy for high-performance PSC materials. At present, state-of-the-art single-junction PSCs Received: April 12, 2018 Revised: June 4, 2018 Published: June 8, 2018 4331

DOI: 10.1021/acs.chemmater.8b01491 Chem. Mater. 2018, 30, 4331−4342

Article

Chemistry of Materials

Figure 1. Chemical structures of the three acceptors and the electron-donating polymer PTB7-Th.

based on A−D−A non-fullerene acceptors have already presented a remarkable power conversion efficiency (PCEs) of >14%.36−39 In addition, it is noteworthy that the PCEs of the ternary cells based on A−D−A non-fullerene acceptors have also significantly improved close to 15%.40−42 Moreover, the small molecule A−D−A structure is also a good candidate for application as the key building block for non-fullerene polymer acceptors. For instance, Li et al. reported a new lowband gap polymer acceptor PZ1 featuring A−D−A structured IDIC-C16 as a building block and thiophenes as the linking units. The all-PSCs of PZ1 as the acceptor material and a twodimensionally conjugated polymer PBDB-T as the donor material exhibited a high PCE of 9.19%.43 It means that there is still much room for further development of the A−D−A structured materials in OSC applications. During the past few years, many electron-deficient building blocks for constructing non-fullerene acceptors have been widely studied and reported, including diketopyrrolopyrrole (DPP), thienopyrrolodione (TPD), naphthobisthiadiazole (NT), benzothiadiazole (BT), benzotriazole (BTA), isoindigo, naphthalene diimide (NDI), and perylene diimide (PDI). However, many opportunities to explore new electrondeficient fragments to achieve high performance remain. In the work presented here, we chose the PDI as the building block of the acceptor because of its appealing properties, such as strong electron-withdrawing ability, high electron mobility, high photochemical stability, and convenient preparation and functionalization.44,45 So far, a considerable PCE of >9% has been achieved for the PDI-based small molecule acceptor in PSCs.46 Our research will focus on the understanding of the chemical structure−property−performance relationship in solar cell devices. Compiling an overview of the three kinds of acceptors mentioned above, we compared them with each other and reached a brief conclusion about their advantages and disadvantages. First, for the fullerene acceptor, its advantages include high electron mobility and isotropic charge transport and its disadvantages are as follows. (1) Absorption is mainly in the ultraviolet region, weak absorption capacity in the visible region, and almost no absorption in the near-infrared region. (2) The chemical structure and energy level cannot be readily modified, thereby limiting the enhancement of the open-circuit

voltage (VOC). (3) The purification process is difficult (multiple substitution byproducts and isomers), the cost of synthesis is high. (4) It exhibits poor light stability and thermal stability (easy to dimerize under light and crystallize during film annealing). Second. A−D−A non-fullerene small molecule receptors have the following advantages: (1) an intramolecular charge transfer effect giving the molecule a strong transition dipole and broad and strong visible or even near-infrared absorption, (2) easy modification of the molecule structure and the ability to conveniently adjust the energy levels and absorption spectra, (3) simple synthesis, higher yields, and easy purification, (4) good light/thermal stability, and (5) generally good solubility and processability in common solvents at room temperature. Its disadvantage is anisotropy charge transport. Finally, the advantages of NDI/PDI type acceptors are as follows: (1) high electron affinity, (2) large π-conjugated plane, (3) high electron mobility, (4) strong chemical modification (ortho, bay, and imide positions can be readily modified), and (5) weak photobleaching and high thermal/ chemical stability. Disadvantages include the following: (1) absorption mainly between 400 and 600 nm, relatively narrow, and limited short-circuit current and (2) intensive aggregation in solution caused by the strong stacking effect. Cross-conjugation was developed as an efficient strategy for the design of both the donors and acceptors for OSCs in our previous work.47,48 Polymeric donors PFDCN and PFPDT featured an electron-rich conjugated backbone, and the crossconjugated side chain exhibited photovoltaic properties with a PCE as high as 4.74% using PCBM as the acceptor in a conventional device.47 More recently, our group reported a family of NDI-based cross-conjugated polymer acceptors consisting of donor−acceptor main chain with pendant acceptor groups at the end of the side chains. The device fabricated with these NDI-based acceptors and the polymer PTB7-Th as the electron donor exhibited a best power conversion efficiency of 5.55%,48 which showed the feasibility and great potential of the strategy for designing highperformance non-fullerene acceptors. To combine the advantages of both the acceptor−donor− acceptor and PDI type acceptors, we designed and synthesized a series of novel cross-conjugated small molecule acceptors for non-fullerene polymer solar cells. Herein, we chose the donor unit, benzo[1,2-b:4,5-b′]dithiophene (BDT), as the building 4332


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Chemistry of Materials Scheme 1. Synthetic Routes for PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF

Figure 2. Ultraviolet−visible absorption spectra of all the acceptors (a) in a chloroform solution and (b) as a thin film. (c) Cyclic voltammograms of the acceptors. (d) Energy level diagrams of the materials in the photovoltaic device. 4333


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Chemistry of Materials Table 1. Ultraviolet−Visible Absorption and Electrochemical Properties of PDIBDT Series Acceptors acceptor

λabsSol (nm)

λabsFilm (nm)

λonset (nm)

Egopt (eV)a

Eox (V)

Ered (V)

EHOMO (eV)b

ELUMO (eV)c

EgCV (eV)d

PDIBDT-RDN PDIBDT-IT PDIBDT-ITF

330, 497, 529 307, 499, 534 308, 499, 534

344, 508 427, 550 427, 552

758 735 740

1.64 1.69 1.68

1.23 1.31 1.28

−0.83 −0.66 −0.64

−5.86 −5.95 −5.92

−3.81 −3.97 −3.99

2.05 1.98 1.92

Calculated from the onset of ultraviolet−visible absorption as pristine thin films. bEHOMO = −e(Eox + 4.64) (eV). cELUMO = −e(Ered + 4.64). dEgcv = Eox − Ered. a

shira coupling between compound 7 and 5-bromo-2,9di(undecan-6-yl)anthra[2,1,9-def:6,5,10-d′e′f ′]diisoquinoline1,3,8,10(2H,9H)-tetraone (9) with Pd(PPh3)2Cl2/CuI as the catalyst and chlorobenzene/diisopropylamine (DIPA) as the solvent afforded the precursor molecule PDIBDT-CHO (10). Finally, target compounds PDIBDT-RDN (11), PDIBDT-IT (12), and PDIBDT-ITF (13) were prepared by the Knoevenagel condensation of dialdehyde compound PDIBDT-CHO with 3-ethylrhodanine (RDN), 2-(3-oxo-2,3dihydro-1H-inden-1-ylidene)-malononitrile (IT), and 2-(6fluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile (ITF) in yields of 85, 83, and 85%, respectively. The chemical structures of all three target molecules were fully characterized by nuclear magnetic resonance (NMR) spectroscopy (1H and 13 C) and matrix-assisted laser desorption ionization time-offlight mass spectrometry (MALDI-TOF MS). All the nonfullerene acceptors exhibit good solubility in common solvents, such as dichloromethane (DCM), tetrahydrofuran (THF), chloroform (CF), and chlorobenzene (CB). Thermal Properties. The thermal properties of three PDIBDT-A series molecules were evaluated by thermogravimetric analysis (TGA) and differential scanning chromatography (DSC). The TGA and DSC curves are shown in Figure S2. The Td (decomposition temperature, 5% weight loss) values of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were recorded as 388.3, 359.3, and 353.7 °C, respectively, which indicated that the thermal stability of the compounds was promising considering their practical application in PSCs. No distinct thermal transition feature was observed in the scanning range of 30−300 °C in the DSC measurements. This result is also consistent with the excellent solubility detected for the three molecules in organic solvents. Optical and Electrochemical Properties. The ultraviolet−visible (UV−vis) absorption spectra of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were measured in a dilute (∼1 × 10−5 M) CHCl3 solution and as thin films spun cast on quartz plates, and the corresponding data are summarized in Table 1. The three small molecules exhibited broad absorption bands between 300 and 700 nm, which can be attributed to their unique cross-conjugated molecular structure. As shown in Figure S3 and panels a and b of Figure 2, the positions of absorption bands in the three new cross-conjugated small molecule acceptors are seriously affected by introducing different end groups of RDN, IT, and ITF; compared to that of PDIBDT-RDN, the PDIBDT-IT and PDIBDT-ITF acceptors show much stronger absorption. For better attribution of all peaks, the UV−vis absorptions of the intermediate compound BDTTIPS-CHO (6) and compound PDIBDTCHO (10) both in chloroform solutions (10−5 mol/L) and as thin films were measured (see Figure S3a,b). It can be clearly observed that the solution spectrum of PDIBDT-CHO exhibits four main vibronic peaks between 350 and 570 nm, and compared to the spectrum of BDTTIPS-CHO, a strong 0−0 (I00) absorption peak and a weak 0−1 (I01) transition appear in

block of our small molecule acceptors, which has been proven to be one of the most promising donor units for highperformance OSC materials, because of its advantages, including its rigid and planar conjugated structure, regioregularity, ready modification, and the high mobility of its related polymers and small molecules.49,50 However, most of the works based on the BDT have focused on the construction of polymer/small molecule donors,50 and there are several examples with BDT as the building block in the construction of acceptors.51,52 In this work, molecular acceptors with electron-rich benzo[1,2-b:4,5-b′]dithiophene (BDT) as the core flanked by two different kinds of electron-deficient moieties (PDI and RDN, IC, ICF) in four directions, named PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF, are presented in Figure 1. Combining the PDI unit and the A−D−A structure, we can easily tune the absorption spectra and energy levels by changing the end groups as well as increase the carrier mobility by introducing the perylene imide unit. The synthesis routes of the monomers are shown in Scheme 1, and further experiments indicated that these acceptors exhibit strong and broad absorption in the range from 300 to 700 nm as well as appropriate lowest unoccupied molecular orbital (LUMO) levels compared to those of the general acceptors based on perylene diimide.53−58 The absorption spectra of these acceptors matched well with that of PTB7-Th (Figure 2b), and a high PCE of 6.06% with an outstanding JSC of 13.60 mA/ cm2 can be achieved for the device based on PDIBDT-IT. The detailed synthesis, characterization, including their optical and electrochemical properties, and photovoltaic performances as acceptors in OPVs were carefully investigated and discussed.

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RESULTS AND DISCUSSION Synthesis and Characterization. The chemical structure and the synthetic route of PDIBDT series SMs are shown in Scheme 1, and the detailed procedures are provided in the Supporting Information. 4,8-Bis[(triisopropylsilyl)ethynyl]benzo[1,2-b:4,5-b′]dithiophene (3) was synthesized by consulting similar literature procedures.59,60 Compound 3 was subsequently lithiated with n-butyllithium (n-BuLi) at a low temperature, and then the reaction precursor was quenched with trimethyltin chloride to yield {[2,6-bis(trimethylstannyl)benzo[1,2-b:4,5-b′]dithiophene-4,8-diyl]bis(ethyne-2,1-diyl)}bis(triisopropylsilane) (4). The Stille coupling reaction between compound 4 and 5-bromo-4-(2-ethylhexyl)thiophene-2-carbaldehyde (5) produced 5,5′-{4,8-bis[(triisopropylsilyl)ethynyl]benzo[1,2-b:4,5-b′]dithiophene-2,6diyl}bis[4-(2-ethylhexyl)thiophene-2-carbaldehyde] (6). Then compound 6 was treated with a few milliliters of a potassium hydroxide solution and methanol in a THF solution at 60 °C for 4 h to give 5,5′-(4,8-diethynylbenzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis[4-(2-ethylhexyl)thiophene-2-carbaldehyde] (7) with a high yield of 91%. It is worth noting that compound 7 must be freshly prepared because of the photochemical instability of the alkyne. Subsequent Sonoga4334


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Figure 3. (a) J−V characteristics and (b) EQE spectra of the all-PSCs.

electrode, and a platinum (Pt) wire as the counter electrode. The cyclic voltammograms are shown in Figure 2c, and the corresponding results are listed in Table 1. The onset reduction (Ered) and oxidation (Eox) potentials of PDIBDTRDN, PDIBDT-IT, and PDIBDT-ITF were −0.825 and 1.230 eV, −0.664 and 1.314 eV, and −0.641 and 1.282 eV, respectively. The lowest unoccupied molecular orbital (E LUMO ) and the highest occupied molecular orbital (EHOMO) energy levels of the molecules can be calculated from the equations ELUMO = −e[Ered + (4.80 − EFc/Fc+)] eV and EHOMO = −e[Eox + (4.80 − EFc/Fc+)] eV, respectively, assuming the absolute energy level of the ferrocene/ferrocenium redox couple (Fc/Fc+) to be 4.8 eV below the vacuum. Therefore, the ELUMO and EHOMO of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were estimated to be −3.81 and −5.86 eV, −3.97 and −5.95 eV, and −3.99 and −5.92 eV, respectively. It is clear that all the molecules exhibit very similar HOMO energy levels (−5.86 to −5.95 eV), which is predominantly determined by their similar molecular structures. However, the end group of PDIBDT-RDN possessed an electron-withdrawing ability that was weaker than those of PDIBDT-IT and PDIBDT-ITF, which results in a higher ELUMO for PDIBDTRDN (−3.81 eV) and may lead to a higher open-circuit voltage (VOC) of PSCs with PTB7-Th as the donor material. It was found that PDIBDT-IT and PDIBDT-ITF exhibited similar LUMO and HOMO energy levels, indicating that the incorporation of the fluorine atom does not significantly disturb molecular orbital levels. The electrochemical energy gaps of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were thus determined to be 2.05, 1.98, and 1.92 eV, respectively, which are larger than their corresponding optical band gaps in their thin film states. Photovoltaic Properties. To understand the effect of the cross-conjugated structure on the photovoltaic performance of the n-OS acceptors, we fabricated polymer solar cell devices with ITO/PEDOT:PSS/active layer/PFN-Br/Ag structures, where PFN-Br was used to effectively alleviate the interfacial energy barrier.62−66 In this work, we used Ag as the anode instead of Al to fabricate the solar cell devices because of the stable physicochemical property that still can exhibit excellent antioxidant capacity when vapor deposited in the case of high temperature and vacuum. Also, Ag exhibits a lower electron injection barrier resulting from its work function being lower than that of Al (−4.6 eV for Ag and −4.3 eV for Al),65,67 which is beneficial for electron injection and extraction, thus yielding efficient photoelectric devices. Furthermore, Ag might possess

that of the PDIBDT-CHO, which is mainly caused by the introduction of the PDI unit. The solution absorptions of PDIBDT-IT and PDIBDT-ITF exhibit 0−0 (I00) absorption peaks stronger than the 0−1 (I01) transition, while PDIBDTRDN exhibits a 0−1 (I01) absorption peak stronger than the 0−0 (I00) transition. The red-shift of the highest absorption peak based on PDIBDT-IT and PDIBDT-ITF should be related to their strong intermolecular interaction in the solid state, leading to the formation of an excimer in a very dilute solution (10−5 mol/L).61 Similar spectra were recorded for PDIBDT-IT and PDIBDT-ITF, and both acceptors showed strong optical absorption at 450−650 nm in the diluted CHCl3 solution and as solid thin films. The absorption peaks of the PDIBDT-IT and PDIBDT-ITF films are the same, 534 nm, with large molar extinction coefficients of 1.49 × 105 and 1.55 × 105 M−1 cm−1, respectively. PDIBDT-RDN exhibited an absorption band at 400−550 nm with a molecular absorption coefficient of 1.42 × 105 M−1 cm−1 at a peak wavelength of 529 nm. In thin films (see Figure 2b), the maximum absorption peaks of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF are redshifted by 11, 16, and 18 nm, respectively, compared with those of their solutions. Moreover, the levels of intramolecular charge transfer (ICT) between the donor and acceptor moieties and π−π stacking between molecular backbones of three acceptors are increased with the enhancement of the electron-withdrawing ability of the end groups. The PDIBDTIT and PDIBDT-ITF film exhibits a red-shift of ∼42 nm relative to the PDIBDT-RDN film because of their stronger electron-withdrawing acceptor groups, which is beneficial for the harvesting of solar photons to achieve a high short-circuit current density (JSC). It is also worth noting that the film absorption ranges of these resulting non-fullerene acceptors (450−700 nm) and polymer donor PTB7-Th (550−770 nm) are complementary and favor solar energy harvesting. The optical band gaps of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF are calculated to be 1.64, 1.69, and 1.68 eV, respectively, from the absorption edge of their thin films [according to the equation Egopt = (1240/λonsetfilm) eV]. The electrochemical properties and energy levels of PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were investigated by cyclic voltammetry (CV) with ferrocene as an internal reference, a tetra-n-butylammonium hexafluorophosphate (n-Bu4NPF6)/acetonitrile solution (0.1 M) as the supporting electrolyte, a glassy carbon electrode as the working electrode, a saturated calomel electrode (SCE) as the reference 4335


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Table 2. Photovoltaic Performance of the PSCs Based on PTB7-Th and Acceptors [1:1 (w/w)] under Simulated AM 1.5 G, 100 mW cm−2 Illuminationa μe (cm2 V−1 s−1) active layer (PTB7Th:A) PDIBDT-RDN PDIBDT-IT PDIBDT-ITF

VOC (V) 0.76 (0.759 ± 0.002) 0.74 (0.739 ± 0.002) 0.71 (0.710 ± 0.002)

JSC (mA cm−2)

μh (cm2 V−1 s−1)

FF (%)

PCE (%)

pure

blend

blend

9.28 (9.18 ± 0.07)

47.33 (47.4 ± 0.3)

3.34 (3.30 ± 0.03)

5.25 × 10−6

2.87 × 10−5

1.52 × 10−3

13.60 (12.97 ± 0.34) 12.78 (12.43 ± 0.38)

60.45 (61.3 ± 0.9)

6.06 (5.87 ± 0.09)

7.39 × 10−6

3.51 × 10−5

8.96 × 10−4

51.45 (51.5 ± 0.9)

4.67 (4.54 ± 0.11)

6.19 × 10−6

2.55 × 10−5

1.90 × 10−3

a

The values are the average and best of eight devices.

Figure 4. (a) PL spectra of a PTB7-Th neat film and blend films, excited at 550 nm. PL spectra of (b) a PDIBDT-RDN neat film, (c) a PDIBDTIT neat film, or (d) a PDIBDT-ITF neat film and a blend film, excited at 550 nm.

particular, the PSCs based on PDIBDT-RDN show a VOC that is 0.76 V higher than those of PDIBDT-IT and PDIBDT-ITF, which is primarily due to its high LUMO energy level (VOC is determined by the gap between the LUMO of the acceptor and the HOMO of the donor). However, the low JSC and FF seriously limit the performance of the PDIBDT-RDN-based device. It was realized that PDIBDT-ITF shows a short-circuit current and a fill factor slightly lower than those of PDIBDTIT, which is not consistent with its relatively high absorption coefficient as revealed by UV−vis spectra. We suspect that this may be correlated to its more unbalanced charge transport, exciton recombination, or unfavorable nanoscale phase separation in the photoactive layer. Here, we also used polymer PBDB-T as a donor to fabricate PSC devices. As shown in Tables S2−S5, a device based on PTB7-Th and acceptors exhibited a JSC higher than that of a device based on PBDB-T and acceptors, which is consistent with the

better conductive properties because its electrical resistivity is lower than that of Al. The photoactive layer was spin-coated from a CB solution; the optimized donor:acceptor weight ratio was determined to be 1:1, and the optimum thermal annealing temperature was 80 °C. All the devices were measured under AM 1.5 G, 100 mW cm−2 illumination. The current density− voltage (J−V) characteristics are shown in Figure 3a, and the corresponding photovoltaic parameters (VOC, JSC, FF, and PCE) are summarized in Table 2. The device based on PTB7Th and PDIBDT-IT exhibited a best photovoltaic performance of 6.06% with a VOC of 0.74 V, a JSC of 13.60 mA cm−2, and a FF of 60.45. While the device based on PTB7-Th and PDIBDT-ITF exhibited a moderate photovoltaic performance of 4.67% with a VOC of 0.71 V, a JSC of 12.78 mA cm−2, and a FF of 51.45, the device based on PTB7-Th and PDIBDT-RDN exhibited a low photovoltaic performance of 3.34% with a VOC of 0.76 V, a JSC of 9.28 mA cm−2, and a FF of 47.33. In 4336


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Figure 5. J1/2−V characteristics of (a) hole-only blend films, (b) electron-only blend films, and (c) electron-only neat films. (d) JSC, (e) VOC, and (f) FF vs light intensity (Plight) for PSC devices.

complementary absorption between the PTB7-Th donor and PDIBDT-IT acceptor (Figure S3c). More information about device performances under different conditions can be found in TableS S2−S5. The external quantum efficiency (EQE) spectra of the optimized devices are shown in Figure 3b. The PSCs based on these three PDIBDT-based acceptors showed a broad photocurrent response extending from 310 to 750 nm because of the effect of the donor−acceptor complementary absorption strategy. The integrated current densities were 9.20, 13.00, and 12.52 mA cm−2 for the devices based on PTB7-Th and PDIBDT-RDN, PTB7-Th and PDIBDT-IT, and PTB7-Th and PDIBDT-ITF, respectively, which agrees well with the J−V measurements. Although the PDIBDT-ITF-based device, in the ranges of 310−410 and 480−680 nm, showed responses that were stronger than that of PDIBDT-IT and a maximum value of 70% recorded at 540 nm, a generally stronger and broader photocurrent response in the range of 410−750 nm was achieved in the PDIBDT-IT-based device, resulting in an increased JSC. Photoluminescence Quenching Study. To investigate the exciton dissociation and charge transfer behavior in the blend of the polymer donor (PTB7-Th) and the small molecule acceptor (PDIBDT-RDN, PDIBDT-IT, or PDIBDT-ITF), we measured the photoluminescence (PL) spectra of the pure donor, the pure acceptor, and the blend films with the optimized weight ratio. The PL spectra are shown in Figure 4a−d. As we can see from the absorption spectra (Figure 2b), the absorption band was mainly distributed in the range of 500−750 nm for the film of polymer donor PTB7-Th and 450−700 nm for the PDIBDT-A series small molecules. Thus, we chose the light with a

wavelength at 550 nm to excite both the donor and the acceptor materials. We found that the PL of the polymer donor PTB7-Th of the three blend films was efficiently quenched by 95.8% for PDIBDT-RDN, 95.2% for PDIBDT-IT, and 95.3% for PDIBDT-ITF, indicating that effective electron transfer occurred from PTB7-Th to the acceptors in the blend films. Compared with the neat film of the small molecules, the PL of PDIBDT-IT and PDIBDT-ITF in the blend films with PTB7Th was dramatically quenched by 96.9 and 97.5%, respectively; however, PDIBDT-RDN was quenched by only 93.1%, which means more effective hole transfer from the acceptor PDIBDTITF and PDIBDT-ITF to the polymer donor68 PTB7-Th than that of PDIBDT-RDN. It is can be understood that the lower quenching efficiency of PDIBDT-RDN stems from the smaller ΔEHOMO value (0.23 eV),69,70 resulting in less effective exciton dissociation and charge transfer, which is in good agreement with the JSC values obtained from PSC devices. Charge Transfer and Recombination. The hole-only and electron-only mobilities of PTB7-Th/acceptor blend films and the acceptor neat films were measured by using the space charge limited current (SCLC) method, with the corresponding J1/2−V characteristics shown in Figure 5a−c, which is described by the equation J=

9 V2 ε0εrμe 3 8 L

where ε0 is the permittivity of free space, εr is the relative permittivity of the organic material, μe is the electron mobility, V is the effective applied voltage, and L is the thickness of the active layer. The hole and electron mobilities were measured using the hole-only devices with the ITO/PEDOT:PSS/blend film (or 4337


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Figure 6. AFM height images (5 μm × 5 μm) and TEM images for PTB7-Th and PDIBDT-RDN, PTB7-Th and PDIBDT-IT, and PTB7-Th and PDIBDT-ITF.

neat film)/MoO3/Ag structure and the electron-only devices with the ITO/ZnO/blend film (or neat film)/Ca/Al structure, respectively. It was found that both the blend film and the neat film based on PDIBDT-IT exhibited highest electron mobilities of 3.51 × 10−5 and 7.39 × 10−6 cm2 V−1 s−1, respectively, which are much higher than the values of the other two molecules, and these results are favorable for obtaining high JSC values for PSC devices.71−73 However, these three acceptors exhibited twist chemical structure (see Figure S1), and their relatively low electron mobility limited the further enhancement of their performances. In addition, the absorption ranging from 500 to 700 nm is not as intense as those of other efficient A−D−A acceptors. All of these drawbacks of our materials might be resolved in future work that aims to optimize the performance of OSCs. To understand the dominant charge recombination mechanism of the polymer solar cell devices based on PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF, we investigated JSC, VOC, and FF versus the light intensity (Plight, which varies from 5 to 100 mW cm−2) characteristics for PSC devices, and the relevant characteristics are plotted in Figure 5d−f. It has been reported that the relationship between JSC and light intensity (Plight) can be expressed by JSC ∝ Plightα, where α should be an exponential factor (when α = 1, there is no charge recombination, and when α < 1, there is a certain degree of charge recombination). It is noted that the JSC of the studied device increases linearly with light intensity. The α value of the device based on PTB7-Th and PDIBDT-IT (0.907) is slightly higher than that of the device based on PTB7-Th and PDIBDT-ITF (0.899), and the α value of the device based on PTB7-Th and PDIBDT- RDN (0.890) is the lowest, indicating that bimolecular recombination is significantly suppressed when PDIBDT-IT is used as the electron acceptor, which can lead to an enhanced JSC and FF.71,74 The slope (n; n = m kT/q) of the plot of VOC versus the natural logarithm of the light intensity (Plight) reflects the property of charge recombination under open-circuit conditions, where k

is Boltzmann’s constant, T is the temperature, and q is the elementary charge. When m = 1, there is only bimolecular recombination, and when m = 2, monomolecular recombination or trap-assisted recombination is dominant.75 As shown in Figure. 5e, the PTB7-Th/PDIBDT-RDN, PTB7-Th/PDIBDTIT, and PTB7-Th/PDIBDT-ITF devices exhibit slopes of 1.19, 1.17, and 1.19 kT/q, respectively. The higher slope value of the PDIBDT-RDN- and PDIBDT-ITF-based device indicates that more pronounced monomolecular and/or trap-assisted recombination is involved, thus resulting in a lower JSC and FF, which may be due to the suboptimal morphology of the PTB7-Th/PDIBDT-RDN and PTB7-Th/PDIBDT-ITF blend films. As shown in Figure. 5f, with an increase in light intensity, the FF first increases distinctly and then decreases slowly for the PDIBDT-RDN-, PDIBDT-IT-, and PDIBDT-ITF-based devices. Furthermore, the device based on PDIBDT-IT exhibited a FF higher than those of the other devices over the whole Plight region, implying that the PDIBDT-IT-based device could better suppress the charge recombination and prevent loss of FF. Film Morphology. The morphology of the blend films was investigated by tapping mode atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM height images (5 μm × 5 μm) and TEM images of blend films of PTB7-Th and acceptors are shown in Figure 6. It can clearly be observed that the PTB7-Th/PDIBDT-IT blend film exhibits a slightly rough surface morphology with a rootmean-square (RMS) value of 1.06 nm, while the PTB7-Th/ PDIBDT-RDN and PTB7-Th/PDIBDT-ITF blend films exhibit RMS values of 1.29 and 1.31 nm, respectively. The PTB7-Th/PDIBDT-IT blend film shows a nanoscale interpenetrating structure with a proper uniform microphase separation, leading to an efficient charge transfer between PTB7-Th and PDIBDT-IT as seen in PL spectra. The PTB7Th/PDIBDT-RDN and PTB7-Th/PDIBDT-ITF blend films exhibit large scale phase-separated structures, which is detrimental for exciton diffusion and dissociation and, thereby, 4338


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Chemistry of Materials the poor JSC and FF values of their resulting devices compared with those of PTB7-Th/PDIBDT-IT-based devices. Grazing incidence wide-angle X-ray scattering (GIWAXS) was performed to access the ordering structures inside blends.76 Figure 7 depicts the two-dimensional (2D) GIWAXS

levels, leading to a remarkable enhancement of efficiency. In contrast to the general fluorination strategy for efficient PSCs, the PDIBDT-ITF-based device exhibited a PCE lower than that of PDIBDT-IT-based devices, resulting from the comparatively large roughness and phase separation. Overall, the results of this work indicate that the novel molecular design strategy for the cross-conjugation concept for the development of fullerene-free acceptors is promising for efficient nonfullerene PSCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b01491. Details of materials (PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF) synthesis, TGA and DSC thermograms of neat PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF, solution and thin film UV−vis absorption spectra of neat BDTTIPS-CHO (compound 6), PDIBDT-CHO, PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF, thin film absorption spectra of blend films, OPV fabrication, device characterization data based on PDIBDT-IT/ PBDB-T, J1/2−V characteristics of the devices, 1H NMR and 13C NMR spectra, and MALDI-TOF MS spectra (PDF)

Figure 7. (a−c) Two-dimensional GIWAXS patterns of blend films and (d) corresponding line-cut profiles in out-of-plane (solid lines) and in-plane (dotted lines) directions.

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AUTHOR INFORMATION

Corresponding Authors

patterns and the corresponding line cuts of the PTB7-Th/ PDIBDT-RDN, PTB7-Th/PDIBDT-IT, and PTB7-Th/ PDIBDT-ITF blend films. The 2D patterns of blends combined with the scattering features arose from both components with a broad (100) peak at 0.27 Å−1 with no azimuthal dependence, corresponding to a lamellar spacing of 23.3 Å. However, a (010) peak located at 1.66 Å−1 in the outof-plane direction was seen, suggesting molecular π−π stacking with a distance of 3.78 Å assumed face-on orientation relative to the substrate, suitable for vertical charge transportation. The similar characteristics of the crystallites in blends can be related to the highly distorted skeleton of the three cross-conjugated small molecule acceptors (see Figure S1). This result points out that the crystallinity of acceptors might be helpful for the design of higher-performance acceptors based on crossconjugated PDI for OPVs.

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yuan Li: 0000-0002-7931-9879 Fei Huang: 0000-0001-9665-6642 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the Natural Science Foundation of China (21490573 and 91633301) and the Ministry of Science and Technology (2014CB643501), the Foundation of Guangzhou Science and Technology Project (201707020019), and the Science and Technology Planning Project of Guangdong Province, China (2017A050503002).

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CONCLUSIONS In summary, the rational design and synthesis of a new series of cross-conjugated non-fullerene acceptors PDIBDT-RDN, PDIBDT-IT, and PDIBDT-ITF were reported. Our novel cross-conjugated structure endowed these molecules with very broad absorption band in the range of 300−700 nm with extinction coefficients up to 5 orders of magnitude compared to the values of those general molecules based only perylene diimide. Importantly, the optical properties and energy levels of these acceptors can be easily tuned using different electronwithdrawing end groups. The fullerene-free OSCs fabricated by these three acceptors and PTB7-Th as the donor exhibited the best photovoltaic performance with a PCE of 6.06%. Among them, PDIBDT-IT-based PSC exhibited an excellent JSC of 13.60 mA/cm2 and a FF of 60.45% that benefited from the better complementary absorption and suitable energy

REFERENCES

(1) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Polymer photovoltaic cells: Enhanced efficiencies via a network of internal donor-acceptor heterojunction. Science 1995, 270, 1789− 1791. (2) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Plastic Solar Cells. Adv. Funct. Mater. 2001, 11, 15−26. (3) Coakley, K. M.; McGehee, M. D. Conjugated Polymer Photovoltaic Cells. Chem. Mater. 2004, 16, 4533−4542. (4) Thompson, B. C.; Frechet, J. M. J. Polymer-Fullerene Composite Solar Cells. Angew. Chem., Int. Ed. 2008, 47, 58−77. (5) Kaltenbrunner, M.; White, M. S.; Głowacki, E. D.; Sekitani, T.; Someya, T.; Sariciftci, N. S.; Bauer, S. Ultrathin and lightweight organic solar cells with high flexibility. Nat. Commun. 2012, 3, 770− 776. (6) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107, 1324−1338.

4339


Article

Chemistry of Materials (7) Helgesen, M.; Søndergaard, R.; Krebs, F. C. Advanced materials and processes for polymer solar cell devices. J. Mater. Chem. 2010, 20, 36−60. (8) Li, N.; Baran, D.; Spyropoulos, G. D.; Zhang, H.; Berny, S.; Turbiez, M.; Ameri, T.; Krebs, F. C.; Brabec, C. J. Environmentally Printing Effi cient Organic Tandem Solar Cells with High Fill Factors: A Guideline Towards 20% Power Conversion Efficiency. Adv. Energy Mater. 2014, 4, 1400084−1400090. (9) Li, G.; Zhu, R.; Yang, Y. Polymer solar cells. Nat. Photonics 2012, 6, 153−161. (10) Bin, H. J.; Zhang, Z. G.; Gao, L.; Chen, S. S.; Zhong, L.; Xue, L. W.; Yang, C.; Li, Y. F. Non-fullerene polymer solar cells based on alkylthio and fluorine substituted 2d-conjugated polymers reach 9.5% efficiency. J. Am. Chem. Soc. 2016, 138 (13), 4657−4664. (11) Zhao, J. B.; Li, Y. K.; Yang, G. F.; Jiang, K.; Lin, H. R.; Ade, H.; Ma, W.; Yan, H. Efficient organic solar cells processed from hydrocarbon solvents. Nat. Energy. 2016, 1, 15027−15033. (12) Li, W.; Cai, J. L.; Cai, F. L.; Yan, Y.; Yi, H. N.; Gurney, R. S.; Liu, D.; Iraqi, A.; Wang, T. Achieving over 11% power conversion efficiency in PffBT4T-2OD-based ternary polymer solar cells with enhanced open-circuit-voltage and suppressed charge recombination. Nano Energy 2018, 44, 155−163. (13) Nian, L.; Gao, K.; Liu, F.; Kan, Y. Y.; Jiang, X. F.; Liu, L. L.; Xie, Z. Q.; Peng, X. B.; Russell, T. P.; Ma, Y. G. 11% Efficient Ternary Organic Solar Cells with High Composition Tolerance via Integrated Near-IR Sensitization and Interface Engineering. Adv. Mater. 2016, 28, 8184−8190. (14) Zhou, H. Q.; Zhang, Y.; Mai, C. K.; Collins, S. D.; Bazan, G. C.; Nguyen, T. Q.; Heeger, A. J. Polymer Homo-Tandem Solar Cells with Best Efficiency of 11.3%. Adv. Mater. 2015, 27, 1767−1773. (15) Kumari, T.; Lee, S. M.; Kang, S. H.; Chen, S. S.; Yang, C. Ternary solar cells with a mixed face-on and edge-on orientation enable an unprecedented efficiency of 12.1%. Energy Environ. Sci. 2017, 10, 258−265. (16) Zhao, W. C.; Li, S. S.; Zhang, S. Q.; Liu, X. Y.; Hou, J. H. Ternary Polymer Solar Cells based on Two Acceptors and One Donor for Achieving 12.2% Efficiency. Adv. Mater. 2017, 29, 1604059−1604065. (17) Chen, W. Q.; Zhang, Q. C. Recent progress in non-fullerene small molecule acceptors in organic solar cells (OSCs). J. Mater. Chem. C 2017, 5, 1275−1302. (18) Li, S. X.; Liu, W. Q.; Li, C. Z.; Shi, M. M.; Chen, H. Z. Efficient Organic Solar Cells with Non-Fullerene Acceptors. Small 2017, 13, 1701120−1701134. (19) Xu, X. F.; Li, Z. J.; Zhang, W.; Meng, X. Y.; Zou, X. S.; Di Carlo Rasi, D.; Ma, W.; Yartsev, A.; Andersson, M. R.; Janssen, René. A. J.; Wang, E. G. 8.0% Efficient All-Polymer Solar Cells with High Photovoltage of 1.1 V and Internal Quantum Efficiency near Unity. Adv. Energy Mater. 2018, 8, 1700908−1700918. (20) Anctil, A.; Babbitt, C. W.; Raffaelle, R. P.; Landi, B. J. Material and Energy Intensity of Fullerene Production. Environ. Sci. Technol. 2011, 45, 2353−2359. (21) Yan, C. Q.; Barlow, S.; Wang, Z. H.; Yan, H.; Jen, A. K. Y.; Marder, S. R.; Zhan, X. W. Non-fullerene acceptors for organic solar cells. Nat. Rev. Mater. 2018, 3, 18003−18021. (22) Lin, Y. Z.; Zhan, X. W. Oligomer Molecules for Efficient Organic Photovoltaics. Acc. Chem. Res. 2016, 49, 175−183. (23) Lin, Y. Z.; Wang, J. Y.; Zhang, Z. G.; Bai, H. T.; Li, Y. F.; Zhu, D. B.; Zhan, X. W. An Electron Acceptor Challenging Fullerenes for Efficient Polymer Solar Cells. Adv. Mater. 2015, 27, 1170−1174. (24) Hou, R.; Li, M.; Feng, S. Y.; Liu, Y. H.; Wu, L. L.; Bi, Z. Z.; Xu, X. J.; Ma, W.; Bo, Z. S. Fused pentacyclic electron acceptors with four cisarranged alkyl side chains for efficient polymer solar cells. J. Mater. Chem. A 2018, 6, 3724−3729. (25) Dai, S. X.; Zhao, F. W.; Zhang, Q. Q.; Lau, T. K.; Li, T. F.; Liu, K.; Ling, Q. D.; Wang, C. R.; Lu, X. H.; You, W.; Zhan, X. W. Fused Nonacyclic Electron Acceptors for Efficient Polymer Solar Cells. J. Am. Chem. Soc. 2017, 139, 1336−1343.

(26) Zhao, F. W.; Dai, S. X.; Wu, Y.; Zhang, Q. Q.; Wang, J. Y.; Jiang, L.; Ling, Q. D.; Wei, Z. X.; Ma, W.; You, W.; Wang, C. R.; Zhan, X. W. Single-Junction Binary-Blend Nonfullerene Polymer Solar Cells with 12.1% Efficiency. Adv. Mater. 2017, 29, 1700144− 1700150. (27) Li, S. S.; Ye, L.; Zhao, W. C.; Zhang, S. Q.; Mukherjee, S.; Ade, H.; Hou, J. H. Energy-Level Modulation of Small-Molecule Electron Acceptors to Achieve over 12% Efficiency in Polymer Solar Cells. Adv. Mater. 2016, 28, 9423−9429. (28) Suman; Bagui, A.; Datt, R.; Gupta, V.; Singh, S. P. A simple fluorene core-based non-fullerene acceptor for high performance organic solar cells. Chem. Commun. 2017, 53, 12790−12793. (29) Baran, D.; Kirchartz, T.; Wheeler, S.; Dimitrov, S.; Abdelsamie, M.; Gorman, J.; Ashraf, R. S.; Holliday, S.; Wadsworth, A.; Gasparini, N.; Kaienburg, P.; Yan, H.; Amassian, A.; Brabec, C. J.; Durrant, J. R.; McCulloch, I. Reduced voltage losses yield 10% efficient fullerene free organic solar cells with > 1 V open circuit voltages. Energy Environ. Sci. 2016, 9, 3783−3793. (30) Wu, Y.; Bai, H. T.; Wang, Z. Y.; Cheng, P.; Zhu, S. Y.; Wang, Y. F.; Ma, W.; Zhan, X. W. A planar electron acceptor for efficient polymer solar cells. Energy Environ. Sci. 2015, 8, 3215−3221. (31) Badgujar, S.; Song, C. E.; Oh, S.; Shin, W. S.; Moon, S. J.; Lee, J. C.; Jung, I. H.; Lee, S. K. Highly efficient and thermally stable fullerene-free organic solar cells based on a small molecule donor and acceptor. J. Mater. Chem. A 2016, 4, 16335−16340. (32) Holliday, S.; Ashraf, R. S.; Wadsworth, A.; Baran, D.; Yousaf, S. A.; Nielsen, C. B.; Tan, C.-H.; Dimitrov, S. D.; Shang, Z.; Gasparini, N.; Alamoudi, M.; Laquai, F.; Brabec, C. J.; Salleo, A.; Durrant, J. R.; McCulloch, I. High-efficiency and air-stable P3HT-based polymer solar cells with a new non-fullerene acceptor. Nat. Commun. 2016, 7, 11585−11595. (33) Holliday, S.; Ashraf, R. S.; Nielsen, C. B.; Kirkus, M.; Rohr, J. A.; Tan, C. H.; Collado-Fregoso, E.; Knall, A. C.; Durrant, J. R.; Nelson, J.; McCulloch, I. Rhodanine Flanked Nonfullerene Acceptor for Solution-Processed Organic Photovoltaics. J. Am. Chem. Soc. 2015, 137, 898−904. (34) Kim, Y.; Song, C. E.; Moon, S. J.; Lim, E. Effect of dye end groups in non-fullerene fluoreneand carbazole-based small molecule acceptors on photovoltaic performance. RSC Adv. 2015, 5, 62739− 62746. (35) Feng, H. R.; Li, M. M.; Ni, W.; Kan, B.; Wang, Y. C.; Zhang, Y. M.; Zhang, H. T.; Wan, X. J.; Chen, Y. S. A series of dithienobenzodithiophene based small molecules for highly efficient organic solar cells. Sci. China: Chem. 2017, 60, 552−560. (36) Zhang, S. Q.; Qin, Y. P.; Zhu, J.; Hou, J. H. Over 14% Efficiency in Polymer Solar Cells Enabled by a Chlorinated Polymer Donor. Adv. Mater. 2018, 30, 1800868−1800874. (37) Xiao, Z.; Jia, X.; Ding, L. M. Ternary organic solar cells offer 14% power conversion efficiency. Sci. Bull. 2017, 62, 1562−1564. (38) Li, H.; Xiao, Z.; Ding, L. M.; Wang, J. Z. Thermostable singlejunction organic solar cells with a power conversion efficiency of 14.62%. Sci. Bull. 2018, 63, 340−342. (39) Li, S. S.; Ye, L.; Zhao, W. C.; Yan, H. P.; Yang, B.; Liu, D. L.; Li, W. N.; Ade, H.; Hou, J. H. A Wide Band-Gap Polymer with a Deep HOMO Level Enables 14.2% Efficiency in Polymer Solar Cells. J. Am. Chem. Soc. 2018, 140, 7159−7167, DOI: 10.1021/jacs.8b02695. (40) Zhang, X. A Tandem Polymer Solar Cell Based on Nonfullerene-acceptors Yields an Efficiency Approaching 15%. Gaofenzi Xuebao 2018, 2, 129−131. (41) Cui, Y.; Yao, H. F.; Yang, C. Y.; Zhang, S. Q.; Hou, J. H. Organic Solar Cells with an Efficiency Approaching 15%. Gaofenzi Xuebao 2017, 2, 223−230. (42) Che, X. Z.; Li, Y. X.; Qu, Y.; Forrest, S. R. High fabrication yield organic tandem photovoltaics combining vacuum- and solutionprocessed subcells with 15% efficiency. Nat. Energy 2018, 3, 422−427. (43) Li, Y. F.; Zhang, Z. G.; Yang, Y. K.; Yao, J.; Xue, L. W.; Chen, S. S.; Li, X. J.; Morrison, W.; Yang, C. A New Strategy to Construct Low Bandgap Polymer Acceptor for High Performance All-Polymer Solar Cells. Angew. Chem., Int. Ed. 2017, 56, 13503−13507. 4340


Article

Chemistry of Materials (44) Zhan, X. W.; Tan, Z. A.; Domercq, B.; An, Z. S.; Zhang, X.; Barlow, S.; Li, Y. F.; Zhu, D. B.; Kippelen, B.; Marder, S. R. A HighMobility Electron-Transport Polymer with Broad Absorption and Its Use in Field-Effect Transistors and All-Polymer Solar Cells. J. Am. Chem. Soc. 2007, 129, 7246−7247. (45) Lin, Y. Z.; Wang, Y. F.; Wang, J. Y.; Hou, J. H.; Li, Y. F.; Zhu, D. B.; Zhan, X. W. A Star-Shaped Perylene Diimide Electron Acceptor for High-Performance Organic Solar Cells. Adv. Mater. 2014, 26, 5137−5142. (46) Meng, D.; Fu, H. T.; Xiao, C. Y.; Meng, X. Y.; Winands, T.; Ma, W.; Wei, W.; Fan, B. B.; Huo, L. J.; Doltsinis, N. L.; Li, Y.; Sun, Y. M.; Wang, Z. H. Three-Bladed Rylene Propellers with Three-Dimensional Network Assembly for Organic Electronics. J. Am. Chem. Soc. 2016, 138, 10184−10190. (47) Huang, F.; Chen, K.-S.; Yip, H.-L.; Hau, S. K.; Acton, O.; Zhang, Y.; Luo, J. D.; Jen, A. K.-Y. Development of New Conjugated Polymers with Donor-π-Bridge-Acceptor Side Chains for High Performance Solar Cells. J. Am. Chem. Soc. 2009, 131, 13886−13887. (48) Xie, R. H.; Chen, Z. M.; Liu, Y.; Wang, Z. F.; Chen, Z. X.; Ying, L.; Huang, F.; Cao, Y. Cross-conjugated n-type polymer acceptors for efficient all-polymer solar cells. Chem. Commun. 2018, 54, 2204− 2207. (49) Hou, J. H.; Park, M. H.; Zhang, S. Q.; Yao, Y.; Chen, L. M.; Li, J. H.; Yang, Y. Bandgap and Molecular Energy Level Control of Conjugated Polymer Photovoltaic Materials Based on Benzo[1,2b:4,5-b’]dithiophene. Macromolecules 2008, 41, 6012−6018. (50) Yao, H. F.; Ye, L.; Zhang, H.; Li, S. S.; Zhang, S. Q.; Hou, J. H. Molecular Design of Benzodithiophene-Based Organic Photovoltaic Materials. Chem. Rev. 2016, 116, 7397−7457. (51) Ye, L.; Jiao, X. C.; Zhang, H.; Li, S. S.; Yao, H. F.; Ade, H.; Hou, J. H. 2D-Conjugated Benzodithiophene-Based Polymer Acceptor: Design, Synthesis, Nanomorphology, and Photovoltaic Performance. Macromolecules 2015, 48, 7156−7163. (52) Wu, Q. H.; Zhao, D. L.; Schneider, A. M.; Chen, W.; Yu, L. P. Covalently Bound Clusters of Alpha-Substituted PDI-Rival Electron Acceptors to Fullerene for Organic Solar Cells. J. Am. Chem. Soc. 2016, 138, 7248−7251. (53) Jiang, B.; Zhang, X.; Zhan, C. L.; Lu, Z. H.; Huang, J. H.; Ding, X. L.; He, S. G.; Yao, J. N. Benzodithiophene bridged dimeric perylene diimide amphiphiles as efficient solution-processed nonfullerene small molecules. Polym. Chem. 2013, 4, 4631−4638. (54) Sharenko, A.; Proctor, C. M.; Van Der Poll, T. S.; Henson, Z. B.; Nguyen, T. Q.; Bazan, G. C. A High-Performing SolutionProcessed Small Molecule:Perylene Diimide Bulk Heterojunction Solar Cell. Adv. Mater. 2013, 25, 4403−4406. (55) Zhao, D. L.; Wu, Q. H.; Cai, Z. X.; Zheng, T. Y.; Chen, W.; Lu, J.; Yu, L. P. Electron Acceptors Based on α-Substituted Perylene Diimide (PDI) for Organic Solar Cells. Chem. Mater. 2016, 28, 1139− 1146. (56) Zhong, Y.; Trinh, M. T.; Chen, R. S.; Wang, W.; Khlyabich, P. P.; Kumar, B.; Xu, Q. Z.; Nam, C. Y.; Sfeir, M. Y.; Black, C.; Steigerwald, M. L.; Loo, Y.-L.; Xiao, S. X.; Ng, F.; Zhu, X.-Y.; Nuckolls, C. Efficient Organic Solar Cells with Helical Perylene Diimide Electron Acceptors. J. Am. Chem. Soc. 2014, 136, 15215− 15221. (57) Hadmojo, W. T.; Nam, S. Y.; Shin, T. J.; Yoon, S. C.; Jang, S. Y.; Jung, I. H. Geometrically controlled organic small molecule acceptors for efficient fullerene-free organic photovoltaic devices. J. Mater. Chem. A 2016, 4, 12308−12318. (58) Kamm, V.; Battagliarin, G.; Howard, I. A.; Pisula, W.; Mavrinskiy, A.; Li, C.; Müllen, K.; Laquai, F. Polythiophene:Perylene Diimide Solar Cells − the Impact of Alkyl-Substitution on the Photovoltaic Performance. Adv. Energy Mater. 2011, 1, 297−302. (59) Lim, N.; Cho, N.; Paek, S.; Kim, C.; Lee, J. K.; Ko, J. HighPerformance Organic Solar Cells with Efficient Semiconducting Small Molecules Containing an Electron-Rich Benzodithiophene Derivative. Chem. Mater. 2014, 26, 2283−2288. (60) Kim, J. H.; Song, C. E.; Kim, H. U.; Grimsdale, A. C.; Moon, S. J.; Shin, W. S.; Choi, S. K.; Hwang, D. H. High Open Cir cuit Voltage

Solution-Processed Tandem Organic Photovoltaic Cells Employing a Bottom Cell Using a New Medium Band Gap Semiconducting Polymer. Chem. Mater. 2013, 25, 2722−2732. (61) Wu, Q. H.; Zhao, D. L.; Yang, J. H.; Sharapov, V.; Cai, Z. X.; Li, L. W.; Zhang, N.; Neshchadin, A.; Chen, W.; Yu, L. P. PropellerShaped Acceptors for High-Performance Non-Fullerene Solar Cells: Importance of the Rigidity of Molecular Geometry. Chem. Mater. 2017, 29, 1127−1133. (62) Huang, F.; Wu, H. B.; Wang, D. L.; Yang, W.; Cao, Y. Novel Electroluminescent Conjugated Polyelectrolytes Based on Polyfluorene. Chem. Mater. 2004, 16, 708−716. (63) Zhang, K.; Huang, F.; Cao, Y. Water/Alcohol Soluble Conjugated Polymer Interlayer Materials and Their Application in Solution Processed Multilayer Organic Optoelectronic Devices. Gaofenzi Xuebao 2017, 9, 1400−1414. (64) Hu, Z. C.; Zhang, K.; Huang, F.; Cao, Y. Water/alcohol soluble conjugated polymers for the interface engineering of highly efficient polymer light-emitting diodes and polymer solar cells. Chem. Commun. 2015, 51, 5572−5585. (65) Jia, T.; Zheng, N. N.; Cai, W. Q.; Zhang, J.; Ying, L.; Huang, F.; Cao, Y. Microwave-assisted One-pot Three-component Polymerization of Alkynes, Aldehydes and Amines toward Amino-functionalized Optoelectronic Polymers. Chin. J. Polym. Sci. 2017, 35, 269− 281. (66) Duan, C. H.; Zhang, K.; Zhong, C. M.; Huang, F.; Cao, Y. Recent advances in water/alcohol-soluble p-conjugated materials: new materials and growing applications in solar cells. Chem. Soc. Rev. 2013, 42, 9071−9104. (67) Fan, B. B.; Ying, L.; Zhu, P.; Pan, F. L.; Liu, F.; Chen, J. W.; Huang, F.; Cao, Y. All-Polymer Solar Cells Based on a Conjugated Polymer Containing Siloxane-Functionalized Side Chains with Efficiency over 10%. Adv. Mater. 2017, 29, 1703906−1703912. (68) Jung, J. W.; Jo, J. W.; Chueh, C.-C.; Liu, F.; Jo, W. H.; Russell, T. P.; Jen, A. K.-Y. Fluoro-Substituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%. Adv. Mater. 2015, 27, 3310−3317. (69) Bin, H. J.; Zhong, L. A.; Yang, Y. K.; Gao, L.; Huang, H.; Sun, C. K.; Li, X. J.; Xue, L. W.; Zhang, Z. G.; Zhang, Z. J.; Li, Y. F. Medium Bandgap Polymer Donor Based on Bi(trialkylsilylthienylbenzo[1,2-b:4,5-b′]-difuran) for HighPerformance Nonfullerene Polymer Solar Cells. Adv. Energy Mater. 2017, 7, 1700746−1700752. (70) Li, Y. X.; Zhong, L.; Gautam, B.; Bin, H. J.; Lin, J. D.; Wu, F. P.; Zhang, Z. J.; Jiang, Z. Q.; Zhang, Z. G.; Gundogdu, K.; Li, Y. F.; Liao, L. S. A near-infrared non-fullerene electron acceptor for high performance polymer solar cells. Energy Environ. Sci. 2017, 10, 1610−1620. (71) Gao, L.; Zhang, Z. G.; Xue, L. W.; Min, J.; Zhang, J. Q.; Wei, Z. X.; Li, Y. F. All-Polymer Solar Cells Based on AbsorptionComplementary Polymer Donor and Acceptor with High Power Conversion Efficiency of 8.27%. Adv. Mater. 2016, 28, 1884−1890. (72) Min, J.; Zhang, Z. G.; Zhang, S. Y.; Li, Y. F. Conjugated SideChain-Isolated D−A Copolymers Based on Benzo[1,2-b:4,5-b’]dithiophene-alt-dithienylbenzotriazole: Synthesis and Photovoltaic Properties. Chem. Mater. 2012, 24, 3247−3254. (73) Li, W. T.; Albrecht, S.; Yang, L. Q.; Roland, S.; Tumbleston, J. R.; Mcafee, T.; Yan, L.; Kelly, M. A.; Ade, H.; Neher, D.; You, W. Mobility-Controlled Performance of Thick Solar Cells Based on Fluorinated Copolymers. J. Am. Chem. Soc. 2014, 136, 15566−15576. (74) Bartesaghi, D.; Perez, I. D. C.; Kniepert, J.; Roland, S.; Turbiez, M.; Neher, D.; Koster, L. J. A. Competition between recombination and extraction of free charges determines the fill factor of organic solar cells. Nat. Commun. 2015, 6, 7083−7093. (75) Lu, L. Y.; Chen, W.; Xu, T.; Yu, L. P. High-performance ternary blend polymer solar cells involving both energy transfer and hole relay processes. Nat. Commun. 2015, 6, 7327−7333. (76) Hexemer, A.; Bras, W.; Glossinger, J.; Schaible, E.; Gann, E.; Kirian, R.; MacDowell, A.; Church, M.; Rude, B.; Padmore, H. A 4341


Article

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