Molecular Engineering on Conjugated Side Chain ... - ACS Publications

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Molecular Engineering on Conjugated Side Chain for Polymer Solar Cells with Improved Efficiency and Accessibility Wei Huang,† Meilin Li,† Luozheng Zhang,† Tingbin Yang,*,† Zhi Zhang,† Hao Zeng,† Xing Zhang,† Li Dang,‡ and Yongye Liang*,† †

Department of Materials Science and Engineering, South University of Science and Technology of China, Shenzhen, 518055, P. R. China ‡ Department of Chemistry, South University of Science and Technology of China, Shenzhen, 518055, P. R. China S Supporting Information *

ABSTRACT: Recent advances in the power conversion efficiency (PCE) of polymer solar cells (PSCs) have demonstrated their potential for alternative photovoltaic technology. Besides photovoltaic performance, material cost and device fabrication process are among the key factors for practical applications. Herein, we report a facile molecular engineering approach with 4-methoxy modified thiophene as the conjugated side chain for polymer donors, which afforded lower highest occupied molecular orbital (HOMO) energy level and enhanced interchain interactions. Therefore, the corresponding PSCs exhibit enhanced open-circuit voltage and PCE compared with the counterparts with thiophene as the conjugated side chain. When fabricated from a nonhalogenated solvent without any additives, over 8% efficiency could be achieved in a representative polymer, PMOT5. Both the synthetic simplicity of materials and high performance from environmentally friendly solvent improve the accessibility of PSCs.

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INTRODUCTION The bulk-heterojunction (BHJ) polymer solar cell (PSC) is an emerging photovoltaic technology, which possesses the potential for a variety of applications in modern life, such as semitransparent solar windows and flexible electronic devices.1,2 The active layer materials of PSCs can be solution processed on various substrates to afford inexpensive fabrication and the possibility for flexibility.3,4 Also, the high absorption coefficient of organic semiconductors allows lightweight devices in thin film.5 Besides, the optoelectronic properties of active layers are easily tunable owing to the diversity of organic constructing units, so the PSC devices can be colorful and semitransparent.5−8 These unique advantages render PSCs with potentials for novel applications where inorganic materials are difficult to be applied.2,5,9 The power conversion efficiency (PCE) of PSCs has exceeded 10% in recent years, which is comparable to that of thin film silicon solar cells.5,10−14 BHJ of PSCs is usually formed by blending an electron donor material with an electron acceptor material, in order to achieve efficient photon harvesting and photogenerated charge separation.1 Semiconducting polymers as electron donors have played a critical role in the development of BHJ PSCs, though the very recent advances of small molecule acceptors demonstrate alternative opportunities for high-performance photovoltaics.15−23 The PCE of PSCs has boosted from 2% to 3% for MDMO-PPV,1,24 3−5% for P3HT25,26 to over 10% for the state-of-the-art PTB7, PTB7-Th,10−13 and PffBT4T,14 as a result of the development © 2016 American Chemical Society

of a vast variety of donor polymers. In addition, some important working mechanisms involved in PSCs have been gradually elucidated with elaborate molecular engineering, such as the influence of regioregularity,25 preferential face-on packing model,27 and self-aggregation properties.28 Benzo[1,2-b:4,5-b′]dithiophene (BDT) appears to be a versatile constructing unit for high-performance photovoltaic polymers.29−35 The fused ring structure of the BDT unit enables the resulting polymers with rigid backbones and good planarity, which benefit charge delocalization and interchain interactions, so as to improve the charge mobility of polymers.36,37 Besides, polymers with suitable energy level and solubility can be readily synthesized through side chain modifications on the BDT unit.29,30,33,34 A variety of side chain modifications of the BDT unit have been reported for highperformance photovoltaic polymers, ranging from alkoxyl,31,38 alkyl,32,34 and alkylthiol39 to two-dimensional conjugated side chains,30,40 such as alkyl,30,41 alkoxyl,42,43 or alkylthiol42,43 substituted thiophene, benzene,44,45 and alkyl substituted thieno[3,2-b]thiophene.46 Recently, 4-fluorinated thienyl substituted BDT (denoted as BDT-F)-based polymers were reported with an open-circuit voltage (Voc) enhancement of 0.18 and 0.12 V compared with the thiophene counterparts when copolymerized with thieno[3,4-b]thiophene (TT) and Received: June 13, 2016 Revised: July 29, 2016 Published: July 31, 2016 5887

DOI: 10.1021/acs.chemmater.6b02381 Chem. Mater. 2016, 28, 5887−5895

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Chemistry of Materials

also noticed that the two oxygen atoms on EDOT could induce different electronic effects to the conjugated backbone: the oxygen atom adjacent to the BDT backbone was electrondonating, whereas the oxygen atom away from the BDT backbone was electron-withdrawing. Actually, meta-substituted alkoxybenzene was reported to lower the highest occupied molecular orbital (HOMO) energy level in PTTQx-m56 and PBT-OP.44 As a result, we propose that 4-methoxy modified thiophene (denoted as MOT) could modify the energy levels of BDT (denoted as BDT-MOT) as an electron-withdrawing side chain (Scheme 1a), which works in a similar way as BDTF.47,48 It should be noted that BDT-MOT is easier to synthesize than BDT-F. We thus synthesized two BDTMOT-based polymers by copolymerizing with two kinds of acceptor units, BDD48,57 and 2,3-diphenyl-5,8-di(thiophen-2yl)quinoxaline (DTQx-2F),58 which are denoted as PMOT5 and PMOT2, respectively. Interestingly, both PMOT5 and PMOT2 exhibited lower HOMO energy level (around 0.1 eV) and higher hole mobility compared with their thiophene counterparts. PMOT5 exhibited a Voc of 0.96 V and a PCE up to 9.25% from halogenated solvents and a PCE over 8% from oxylene without any additives. The high performance from both halogenated and nonhalogenated solvents as well as simplified synthetic procedures may render BDT-MOT-based polymers as promising candidates for low cost organic photovoltaics (OPVs).

1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c]dithiophene-4,8-dione (BDD), respectively.47,48 Besides photovoltaic performance, cost of materials and device fabrication process should also be considered for PSCs to become a viable solar harvesting technology.9,49 However, some high-performance donor polymers require complex synthetic procedures.47,50 The cost of active layer materials increases linearly with the number of synthetic steps of the photoactive compound in theoretical estimations.9 Thus, easily accessible donor polymers with high performance are favored. On the other hand, most donor polymers require halogenated solvents for processing, such as chloroform (CF), chlorobenzene (CB), and o-dichlorobenzene (DCB). These solvents prove to be environmental and health hazards, thus not suitable for large-scale fabrication.14,51,52 However, most of the highperformance donor materials processed from nonhalogenated solvents exhibit much inferior photovoltaic performance to those from halogenated solvents.52−54 Very recently, Yan et al. reported a PCE over 11% when processing from hot 1,2,4trimethylbenzene (TMB) with 1-phenylnaphthalene (PN) as the additive.14 PTB7-Th and PffBT4T-2OD could also achieve PCEs of 8.5% and 9.5% from o-xylene with anisaldehyde as the additive, respectively.51 Nevertheless, these high-performance PSCs processed from nonhalogenated solvents suffered from either a complexity of high temperature processing or strong demand for additives.14,51−53 Herein, we present a feasible and efficient molecular engineering approach (Scheme 1a) for a high-performance

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RESULTS AND DISCUSSION The chemical structures of the investigated polymers in this work are depicted in Scheme 1b. The BDT-MOT monomer was synthesized in three steps starting from the commercially available 3-methoxythiophene (Scheme S1). Lithiation of 3methoxythiophene and reacting with 2-ethylhexyl bromide afforded the major product of 2-(2-ethylhexyl)-3-methoxythiophene. After purification, it was treated with n-butyllithium, followed by reacting with benzo[1,2-b:4,5-b′]dithiophene-4,8dione and stannous chloride, to yield 4,8-bis(5-(2-ethylhexyl)4-methoxythiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene. The BDT-MOT distannous monomer was obtained straightforward in 3 steps, which is more easily accessible than BDT-F monomer requiring 6 steps. It was then copolymerized with the acceptor units BDD and DTQx-2F by Pd(PPh3)4 catalyzed Stille coupling reactions to afford PMOT5 and PMOT2, respectively. Thiophene substituted BDT (denoted as BDT-T)based polymers, PBDTBDD57 and PBQ-358 were also synthesized for comparison (Scheme 1b). The synthetic details of the BDT-MOT monomer and the polymers are described in the Supporting Information. The resulting four polymers exhibited good solubility in CF, CB, DCB, and o-xylene at room temperature. The number-average molecular weight (Mn) and polydispersity index (PDI) were estimated by high temperature gel permeation chromatography (GPC). The Mn and PDI were 45.9 kDa and 2.45 for PMOT5, 30.0 kDa and 2.52 for PBDTBDD, 27.0 kDa and 1.95 for PMOT2, and 23.5 kDa and 2.04 for PBQ-3, respectively (Table S1).All polymers exhibited excellent thermal stability under a N2 atmosphere with decomposition temperature exceeding 400 °C (Figure S1). Preliminarily density functional theory (DFT) calculations were carried out to estimate the energy level of frontier orbitals of the polymers with three repeating units based on B3LYP with a basis set of 6-31G (Table S2, Figures S2 and S3, in the Supporting Information). It should be pointed out that the simple B3LYP functional tends to over-delocalize wave

Scheme 1. (a) Schematic Illustration of the Molecular Engineering Strategy. (b) Chemical Structures of the Related Polymers in This Work

polymer solar cell with enhanced Voc fabricated from either halogenated solvents or nonhalogenated solvents. Recently, we reported a 3,4-ethylenedioxythiophene (EDOT) modified BDT unit to construct semiconducting polymers for highly sensitive photodetectors.55 It is interesting to find that such EDOT modified polymers exhibited similar Voc compared with the thiophene counterparts, though it is generally thought that EDOT is a more electron-donating unit than thiophene. Apart from the larger steric hindrance of the bulkier EDOT unit, we 5888


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in both solution and solid state compared with the control thiophene polymers. PMOT2 and PMOT5 exhibited different absorption patterns at the π−π* band region from 300 to 500 nm when comparing with PBQ-3 and PBDTBDD, respectively, suggesting different charge distributions on the BDT backbone with the introduction of the 4-methoxy group. All polymers exhibited a noticeable absorption vibronic peak at longer wavelength in solution (Figure 1). It might be due to the polymer aggregation by interchain stacking or intramolecular folding in solutions at ambient temperature.61 We observed that the vibronic peaks became weaker at lower concentration from 0.01 to 0.0025 mg/mL, suggesting that the interchain stacking might be the dominant cause. The polymer aggregation in solution was further confirmed by temperature-dependent absorptions in DCB (Figure 2). With the rise of solution temperature, all polymers showed decreasing intensity of vibronic shoulder peaks. At 85 °C, the vibronic shoulder peaks in MOT polymers (PMOT5 and PMOT2) almost disappeared while the thiophene polymers (PBDTBDD and PBQ-3) still retained a distinct vibronic shoulder peak. We reasoned that the pendant methoxy groups on MOT could help the dissolution of PMOT5 and PMOT2 in solution at high temperature. When cast in film, the absorption peaks and onsets of band edge of four polymers exhibited red shifts of about 10−20 nm compared with those in solution. PMOT2 showed a more intense vibronic peak than that of PBQ-3, probably because the 4-methoxy thiophene could enhance the interchain interactions by noncovalent bonding.62 All the photophysical properties are summarized in Table S1. Interestingly, both PMOT5 and PMOT2 exhibited higher extinction coefficients on the peak absorption when comparing with the thiophene polymer counterparts in both solution and film, which was possibly due to the enhancement of interchain interactions. The optical band gaps of four polymers were determined from the absorption edge in the solid state with 1.83 eV,1.78 eV for PMOT5, PMOT2 and 1.82 eV, 1.73 eV for PBDTBDD, PBQ-3, respectively. Clearly, BDT-MOT-based polymers demonstrated slightly larger band gaps than those of BDT-T-based polymers, which were similar to those of BDT-Fbased polymers.48,58 Cyclic voltammetry (CV) was used to evaluate the ionization potential (IP) and electron affinity (EA) values of the polymers by using ferrocene as reference (Figure S3).63 The energy level diagram is depicted in Figure 1b. The IP/EA values are 5.22 eV/3.21 eV and 5.10 eV/3.15 eV for PMOT2 and PBQ-3, 5.28 eV/3.22 eV and 5.16 eV/3.11 eV for PMOT5 and PBDTBDD, respectively. Both PMOT5 and PMOT2 show a lower HOMO level than the thiophene counterparts.63 For all the polymers, the energy offset of the EA values between polymers and PC71BM are larger than 0.3 eV,55 which indicates that charge transfer between the polymer donor and the fullerene acceptor can be efficient.64 In order to investigate the photovoltaic properties of the polymers, conventional structure devices were fabricated with the architecture of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/donor polymer:PC71BM/perylene diimides functionalized with amino oxide (PDINO)65/Al (Figure 3a). Typical current density− voltage (J−V) curves of PSCs are displayed in Figure 3b,d, and the corresponding device parameters are summarized in Table 1 and Table S3. For PMOT5, when processing from DCB with 1% DIO as additive, a Voc of 0.96 V, Jsc of 13.45 mA cm−2, FF of 71%, and

functions in large conjugated backbones due to the inaccurate evaluation of torsion potentials and dihedral angles,59,60 though approximate frontier energy levels can be obtained. The calculations showed that the 4-methoxy group on thiophene indeed lowered the HOMO and LUMO simultaneously when comparing PMOT5 and PMOT2 with PBDTBDD and PBQ-3, respectively (Table S2), which agreed with our assumption. PMOT5 (Figure S2b) seemed to show more delocalized distribution on HOMO than PBDTBDD (Figure S2d) due to the contribution of the 4-methoxy group of the thiophene unit, while a negligible difference was observed on the lowest unoccupied molecular orbital (LUMO) of the two polymers. The comparison between PMOT2 and PBQ-3 also revealed such a difference (Figure S3b,d). More accurate calculations employed optimally tuned long-range corrected functionals to get more reliable computational results are underway in our lab.60 The normalized absorption spectra of the polymers in CB solution and in film are shown in Figure 1. The BDT-MOTbased polymers showed slight blue shifts of the absorption edge

Figure 1. (a) UV−vis absorption spectra of the polymers in chlorobenzene solution and in film. (b) Schematic energy levels of the polymers and PC71BM measured by cyclic voltammetry. 5889


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Figure 2. Temperature-dependent absorption spectra of the polymers: (a) PMOT2, (b) PBQ-3, (c) PMOT5, and (d) PBDTBDD in dilute odichlorobenzene solutions at a temperature interval of 10 °C.

Figure 3. (a) The device structure used in this work. (b) The J−V characteristics of the polymer:PC71BM solar cells fabricated from chlorinated solvents with DIO or DPE as additives under AM 1.5G solar radiation. (c) EQE spectra of the corresponding devices in (b). (d) The J−V characteristics of the polymer:PC71BM solar cells fabricated from o-xylene with or without additives under AM 1.5G solar radiation (100 mW cm−2).

higher performance than PBQ-3, with a Voc of 0.88 V vs 0.81 V and PCE of 7.73% vs 6.48%, respectively (Figure 3b and Table 1). The superior performance of PMOT5 and PMOT2 resulted from larger Voc and slightly higher FF, which could be attributed to lower HOMO levels and enhanced interchain

PCE up to 9.25% were obtained in the champion device with the optimal D/A ratio of 1:1.2 (Table 1). In contrast, the thiophene counterpart PBDTBDD exhibited an inferior performance, with a Voc of 0.84 V, Jsc of 12.45 mA cm−2, FF of 65%, and PCE of 6.77% (Table 1). PMOT2 also showed a 5890


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Table 1. Summary of Device Parameters of the Polymer Solar Cells Processed from Chlorinated or Nonchlorinated Solvents under AM 1.5G Solar Radiation (100 mW cm−2) PCE (%) donor PMOT5

PBDTBDD PMOT2 PBQ-3

solvent

Voc (V)

Jsc (mA cm−2)

JscEQEa (mA cm−2)

FF

best

DCB + 1.0% DIO o-xylene + 1.0% DIO o-xylene DCB + 1.0% DIO CB + 3% DPE o-xylene + 3% DPE CB + 3% DPE

0.96 0.95 0.98 0.84 0.88 0.88 0.81

13.45 13.36 13.41 12.45 12.20 13.18 11.73

13.11 13.08 13.17 12.11 11.86 12.82 11.46

0.71 0.69 0.61 0.65 0.72 0.69 0.68

9.25 8.77 8.02 6.77 7.73 8.01 6.48

averageb 9.11 8.63 7.87 6.65 7.62 7.91 6.32

± ± ± ± ± ± ±

0.14 0.23 0.18 0.28 0.16 0.15 0.21

a

The JscEQE was calculated by integrating the corresponding EQE spectra. bAverage PCEs and standard deviations were obtained from more than 10 devices.

interactions in PMOT5 and PMOT2. It should be pointed out that the controls PBDTBDD and PBQ-3 showed comparable performance to that reported in the literature.57,58 The corresponding external quantum efficiency (EQE) spectra of the respective champion devices were measured and are depicted in Figure 3c. It is clear that the polymers are very efficient to convert the photons into electrons in the 400−700 nm range. The integrated Jsc from the EQE spectra correlated well with the respective Jsc measured from the J−V curve within 5% mismatch (Table 1). Most of the high-performance donor materials could yield inferior performance when processed from nonhalogenated solvents.51−54,66 It is possibly due to the lower solubility in nonhalogenated solvents, which affords nonoptimal BHJ morphology.14,51−53 Longer or branching alkyl chains could improve the solubility of the polymers, but hinder the interchain interactions and make the photovoltaic performance improvement in nonhalogenated solvents difficult.28,67 It was intriguing to find that BDT-MOT-based polymers exhibited enhanced interchain interactions in the solid state while better dissolution ability than the thiophene counterparts at elevated temperatures. We also fabricated PSC devices by employing oxylene as processing solvent. For PMOT5, when processed from o-xylene with 1% DIO, a Voc of 0.95 V, Jsc of 13.36 mA cm−2, FF of 69%, and PCE up to 8.77% were obtained in the champion device (Figure 3d and Table 1). Similarly, PMOT2 could also achieve a PCE exceeding 8% with o-xylene and 3% diphenyl ether (DPE) as the processing solvents (Figure 3d and Table 1). The thiophene-based polymers also exhibited inferior performance compared with MOT-based polymers from oxylene (Table S3). Compared with the devices processed from DCB, the devices from o-xylene just showed a slight decrease in FF. Even without any additives, the PSC of PMOT5 still exhibited a PCE exceeding 8% when processed from o-xylene (Figure 3d and Table 1). To the best of our knowledge, PMOT5 is one of the few examples that could be processed from o-xylene without any additives showing over 8% efficiency at room temperature, which may be suitable for printing electronics fabrication. The EQE spectra of the corresponding devices are included in Figure S4. In order to investigate the charge transport properties of the polymers, hole only diodes (ITO/PEDOT:PSS/polymer/ MoOx/Ag) were fabricated, and the corresponding current− voltage plots are presented in Figure 4. On the basis of the space-charge-limited current (SCLC) model, the intrinsic hole mobilities of the neat polymer films were determined to be 1.3 × 10−4, 6.8 × 10−5 cm2 V−1 s−1 for PMOT5, PMOT2 and 2.5 × 10−5, 1.7 × 10−5 cm2 V−1 s−1 for PBDTBDD, PBQ-3,

Figure 4. J−V plots of the polymer films from experimental results (black dot) with the diode structure of ITO/PEDOT:PSS/polymer/ MoOx/Ag and the SCLC model simulation (red line) of four polymers in pristine films, (a) PMOT2, (b) PBQ-3, (c) PMOT5, and (d) PBDTBDD.

respectively. Clearly, BDT-MOT-based polymers showed significant enhancement of hole mobility compared with the thiophene control polymers, which agreed well with the enhanced interactions between polymers chains in BDTMOT polymers as discussed above. Finally, the bulk and surface morphologies of the active layers were investigated by transmission electron microscopy (TEM) (Figure 5) and atomic force microscopy (AFM) (Figure 6). When processed from DCB with DIO additive, both PMOT5:PC71BM and PBDTBDD:PC71BM blends exhibited fine structures with nanofiber morphology, while PMOT5 demonstrated more prominent nanofibrous networks than that of PBDTBDD (Figure 5a,b). DIO proved to be crucial for the formation of fiber networks (Figure 5c,d). The interpenetrating nanowires in BHJ solar cells are advantageous for high performance of PSCs.45,68,69 When the processing solvent changed from DCB to o-xylene with DIO as additive, PMOT5: PC71BM blends showed less obvious nanofiber networks (Figure 5a vs 5e). The nanofiber network morphology disappeared in the PMOT5:PC71BM blend processed from oxylene without DIO (Figure 5f). Such morphology changes might account for the decrease of FF in devices processed from o-xylene. As shown in Figure 6, AFM topography images showed smooth features with small root-mean-square (RMS) roughness for all the blends of PMOT5:PC71BM, but increases 5891


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side chain. The 4-methoxy group monomer, BDT-MOT, is as easily accessible as the thiophene counterparts. Larger absorption coefficient, deeper HOMO levels, and stronger interchain interactions were observed for BDT-MOT polymers compared with thiophene control polymers. When fabricated from halogenated solvent, PSC of PMOT5 exhibited a Voc of 0.96 V and PCE up to 9.25%, superior to the thiophene counterpart PBDTBDD. More interestingly, PMOT5 devices still showed PCE exceeding 8% when fabricated from o-xylene at room temperature without any additives. These results suggest that the 4-methyloxy modified conjugated side chain is effective to enhance the Voc and PCE of polymer solar cells. The high photovoltaic performance, along with simple preparation and suitability for nonhalogenated solvents processing, may render the derived polymers as promising candidates for organic photovoltaics.

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EXPERIMENTAL SECTION

Materials. The synthesis of the conjugated polymers is available in the Supporting Information. [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM, ≥99.5%) was purchased from American Dye Source. Chlorobenzene, 1,2-dichlorobenzene, o-xylene, and 1,8-diiodoctance were purchased from Sigma-Aldrich. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Clevios P VP AI4083) was obtained from NCM International. Diphenyl ether was purchased from Aladdin. Perylene diimides functionalized with amino N-oxide (PDINO) was synthesized in our laboratory according to the literature.65 Fabrication of Polymer Solar Cells. The device structure was ITO/PEDOT:PSS/active layer/PDINO/Al. The indium tin oxide (ITO) glass substrates were sequentially ultrasonically washed in detergent, deionized water, acetone, and isopropanol. Then, the ITO substrate was dried in an oven at 80 °C and treated with UV-Ozone for 15 min before depositing PEDOT:PSS. The PEDOT:PSS solution was spin-coated at 2500 rpm for 30 s and thermally annealed at 150 °C for 10 min in air. The thickness of the PEDOT:PSS layer was about 40 nm. Subsequently, the substrates were transferred into a nitrogen-filled glovebox system (MBraun) for the preparation of the active layer. PMOT2 and PBQ-3 were blended with PC71BM (1:1.5, by weight) and dissolved in CB/DPE (97:3, v/v) or xylene with 8 mg/mL for polymer. The thickness of active layers was varied by changing the spin-coating speed, and the optimized thickness was about 100 nm. PMOT5 and PBDTBDD were blended with PC71BM (1:1.2, by weight) and dissolved in DCB/DIO (99:1, v/v) or xylene with 10 mg/ mL for polymer. The optimized thickness of the PMOT5 or PBDTBDD active layer was about 110 nm. The PDINO was dissolved in methanol at a concentration of 1.5 mg mL−1 and spincoated on top of the active layer at 3000 rpm for 30 s. The thickness of the PDINO thin film was about 10 nm. Finally, the Al electrode was deposited in vacuum at a pressure of ∼3 × 10−6 mbar. The active area of the device was 4.5 mm2. Average PCEs and standard deviations were obtained from more than 10 devices. Characterization. The current−voltage characteristics of the PSCs were measured on a computer-controlled Keithley 2400 source− measure unit. Light source was obtained from an Oriel Sol3A Class AAA solar simulator (Newport) with a 450 W xenon lamp and an air mass (AM) 1.5G filter. The light intensity was calibrated to 100 mW cm−2 by a silicon reference cell. The EQE spectrum was measured with a QE-R system (Enli Technology). UV−vis absorption spectra of the polymers were measured by a UV−vis−NIR spectrophotometer (UV3600, Shimadzu). The thickness of the film was determined by a profilometer (D120, Tencor). To study the charge transporting properties, hole-only devices were fabricated on the ITO coated glass with a layer of PEDOT:PSS as bottom contact and a MoOx/Ag as top contact. The hole mobility was determined by taking current−voltage and fitting the curves to a space-charge-limited form. The SCLC is described by

Figure 5. TEM images of (a) PMOT5:PC71BM with 1% DIO, (b) PBDTBDD:PC71BM with 1% DIO, (c) PMOT5:PC71BM without DIO, (d) PBDTBDD:PC71BM without DIO blend films processed from DCB and PMOT5:PC71BM blend films processing from o-xylene (e) with 1% DIO and (f) without additive, respectively. The scale bar is 200 nm.

Figure 6. AFM topography images of PMOT5:PC71BM blend films processed from (a) DCB with 1% DIO, RMS roughness: 2.36 nm, (b) DCB without DIO, RMS roughness: 1.20 nm, (c) o-xylene with 1% DIO, RMS roughness: 2.51 nm, (d) o-xylene without DIO, RMS roughness: 1.43 nm. The size of the images is 5 μm × 5 μm.

of roughness were observed when DIO was added for the films cast from both o-DCB and o-xylene. It was consistent with TEM results that the nanofiber network formed in DIO added samples.

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CONCLUSION In summary, we reported a feasible way to improve the photovoltaic performance of BDT-based polymers by introducing a 4-methoxy group modified thiophene as the conjugated 5892


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Chemistry of Materials J = (9/8)εrε0μ h (V 2/L3)

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where ε0 is the permittivity of free space, εr is the dielectric constant of the polymer, μh is the hole mobility, V is the voltage drop across the device, and L is the polymer thickness. The dielectric constant εr is assumed to be 3, which is a typical value for conjugated polymers. The thickness of the films was measured with a profilometer (Tenco D120).

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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.6b02381. Synthesis and characterization of polymers, computational calculations, cyclic voltammetry, additional PSC performance data, Figures S1−S5, Scheme S1, and Tables S1−S3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (T.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the South University of Science and Technology of China, “The Recruitment Program of Global Youth Experts of China”, the National Science Foundation of China (51503095), and the Shenzhen fundamental research programs (Nos. JCYJ20150630145302226, JCYJ20150630145302236), Shenzhen Key Lab funding (ZDSYS201505291525382), and Peacock Plan (KQTD20140630110339343).

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REFERENCES

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