Selenium-Containing Medium Bandgap Copolymer for Bulk

May 10, 2017 - In this work, a new D–A copolymer based on m-alkoxyphenyl substituted benzodithiophene (BDT-m-OP) as donor unit and benzo[1,2-c:4 ...
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Selenium-Containing Medium Bandgap Copolymer for Bulk Heterojunction Polymer Solar Cells with High Efficiency of 9.8% Zhuo Xu,†,∥ Qunping Fan,†,∥ Xiangyi Meng,‡ Xia Guo,*,† Wenyan Su,† Wei Ma,‡ Maojie Zhang,*,† and Yongfang Li†,§ †

State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Laboratory of Advanced Optoelectronic Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People’s Republic of China ‡ State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China § Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: In this work, a new D−A copolymer based on m-alkoxyphenyl substituted benzodithiophene (BDT-m-OP) as donor unit and benzo[1,2c:4,5-c′]dithiophene-4,8-dione (BDD) as acceptor unit was designed and synthesized, in which selenophene unit as π-conjugated spacer was incorporated into the polymer backbone to broaden the absorption spectrum, enhance the charge transport properties, and even improve the photovoltaic properties. Compared with PBPD-Th with thiophene as πconjugated spacer, PBPD-Se exhibits an evidently extended absorption spectrum and an enhanced hole mobility with a slightly raised HOMO energy level. The PBPD-Se:PC71BM-based PSCs exhibits a significantly improved PCE of 9.8% with an enhanced Jsc of 14.9 mA cm−2 and a slightly lower Voc of 0.90 V in comparison with a PCE of 8.4% with a Voc of 0.95 V and a Jsc of 12.4 mA cm−2 for PBPD-Th:PC71BM-based devices. These results indicate that the rational selection of π-conjugated spacer in the D−A copolymer backbone is very essential to achieve high efficiency PSCs.

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Moreover, the relatively larger polarizable radius of selenium atom leads to a higher degree of rigidity and stronger intermolecular interactions, which is beneficial for improving the mobility.37−39,47 For instance, Yang et al. found that when changing the π-conjugated spacers from thiophene to selenophene units in the PBDTT-DPP system,37 the polymer PBDTT-SeDPP with selenophene spacers displayed an obviously smaller bandgap of 1.38 eV and slightly higher hole mobility of 6.9 × 10−4 cm2/(V s) compared with that of PBDTT-DPP (1.46 eV and 2.5 × 10−4 cm2/(V s)) with thiophene spacers, resulting in the increased short circuit current density (Jsc) from 13.7 to 16.8 mA/cm2 and improved PCE from 6.5% to 7.2%. However, although selenophene as πconjugated spacer into D−A polymers is beneficial to broaden the absorption spectrum and achieve higher Jsc in PSCs, it is worth noting that most selenophene-containing polymers showed significantly increased HOMO level resulting in a large sacrifice of the open circuit voltage (Voc) of less than 0.8 V in PSCs.37−39,43−46

n the past decades, bulk heterojunction (BHJ) polymer solar cells (PSCs), incorporating conjugated polymers as donor and fullerene derivatives as acceptor, have attracted much attention in both academia and industry research, owning to their advantages of low cost, lightweight, easy fabrication, and potential application for use in flexible devices.1−12 As the key component of the active layer, efficient conjugated polymer donors should possess broad absorption for harvesting more sunlight, suitable energy levels, and higher hole mobility.13−25 Until now, the donor−acceptor (D−A) alternating conjugated polymers have been one of the most successful material systems because of their effectively tuning optical absorption band, energy levels, and carrier mobility by rational selection of D and A units.26−36 The power conversion efficiency (PCE) over 10% has been obtained in PSCs based on D−A polymers.13−19 Recently, introducing selenophene as a π-conjugated spacer into the D−A-conjugated polymers has become an effective strategy to broaden the absorption spectrum, enhance the charge transport properties, and even improve the photovoltaic properties.37−48 Compared with the widely used thiophene units, the lower aromaticity of selenophene can increase the quinoidal resonance form in the ground state of its resulting polymers, leading to significantly improved planarity, increased effective conjugation length, and lower optical bandgap.41−46,48 © 2017 American Chemical Society

Received: February 21, 2017 Revised: May 9, 2017 Published: May 10, 2017 4811

DOI: 10.1021/acs.chemmater.7b00729 Chem. Mater. 2017, 29, 4811−4818

Article

Chemistry of Materials Scheme 1. Synthetic Route and Molecular Structure of the D−A Copolymers PBPD-Th and PBPD-Se

In our previous work, we have synthezied a series of twodimension (2D)-conjugated polymers based on m-alkoxyphenyl substituted benzodithiophene unit (BDT-m-OP).49,50 Compared with the alkylthienyl substituted benzodithiophene unit (BDT-T)-based polymers,51 the BDT-m-OP-based polymers showed obviously deeper HOMO levels leading to higher Voc and PCE. However, the blue shifts in the absorption spectra to some extent limited the extended application of the (BDT-mOP)-based polymers in the construction of conjugated polymer donors. Considering the above-mentioned effect, introducing the BDT-m-OP as donor unit and selenophene as π-conjugated spacer to the D−A polymer backbone could be an effective strategy to further improve the performance of PSCs. Hence, in this work, we employed the selenophene unit as a π-conjugated spacer and synthesized a new D−A copolymer, named, PBPD-Se, based on BDT-m-OP as donor unit and 5,7bis(2-ethylhexyl)-benzo[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) as acceptor unit. In order to investigate the effect of selenophone as spacer on material properties and photovoltaic performance, the analogue polymer with thiophene unit as πconjugated spacer, named, PBPD-Th, was also synthesized. The polymer PBPD-Se exhibits a smaller optical bandgap of 1.77 eV, an enhanced hole mobility of 1.12 × 10−3 cm2/(V s), and a slightly higher HOMO level of −5.35 eV, compared with those of PBPD-Th (1.90 eV, 7.56 × 10−4 cm2/(V s) and −5.42 eV, respectively). Consequently, the PBPD-Se:PC71BM-based PSCs exhibits a significantly improved PCE of 9.8% with an enhanced Jsc of 14.9 mA cm−2 and a slightly lower Voc of 0.90 V in comparison with a PCE of 8.4% with a Voc of 0.95 V and a Jsc of 12.4 mA cm−2 for PBPD-Th:PC71BM-based devices. Meanwhile, the PCE of 9.8% is the highest value reported in the literature for the PSCs based on selenium-containing polymers. These results indicate that the rational selection of πconjugated spacer in the D−A copolymer backbone is very essential to achieve high efficiency PSCs. The two polymers, PBPD-Th and PBPD-Se, were synthesized by Stille-coupling polymerization as shown in Scheme 1. PBPD-Se shows a slightly higher number-average molecular weight of 48 kg/mol and a slightly smaller weight-average molecular weight of 92 kg/mol than PBPD-Th (42 and 107 kg/ mol). These polymers display good solubility in commonly used halogenated solvents, such as chloroform and odichlorobenzene (o-DCB). Moreover, the two polymers all show a high decomposition temperature over 440 °C with 5% weight loss in the thermogravimetric analysis (see Supporting Information Figure S1). No obvious endotherms or exotherms were observed from 50 to 278 °C in the differential scanning calorimetry measurement of these polymers (see Figure S2). Figure 1a shows UV−vis absorption spectra of the two polymers in chloroform solution and in thin film. In solution, PBPD-Se shows a significantly red-shifted and enhanced

Figure 1. (a) UV−vis absorption spectra of the polymers in chloroform solution and in thin films. (b) Molecular energy level diagrams of the polymers.

shoulder peak at 632 nm compared with PBPD-Th (591 nm), which means that PBPD-Se can form stronger aggregation in solution. In solid films, compared to PBPD-Th, PBPD-Se displays an obviously red-shifted absorption spectrum and significantly enhanced vibronic absorption peak, which is consistent with those reported in the literature about the polymers with selenophene as π-conjugated spacer.37−48 Moreover, the extinction coefficient of 0.84 × 105 cm−1 at 637 nm for PBPD-Se film is slightly higher than that of PBPDTh film (0.70 × 105 cm−1 at 556 nm) (see Figure S3). The absorption edge of the polymer films are located at 652 and 701 nm corresponding to the optical bandgap of 1.90 and 1.77 eV for PBPD-Th and PBPD-Se, respectively. The smaller bandgap of PBPD-Se will facilitate the absorption of the sunlight and hence the improvement of Jsc. The molecular energy levels of the polymers were measured by electrochemical cyclic voltammetry (see Figure S4), the corresponding energy level diagrams are shown in Figure 1b. The onset oxidation/reduction potentials (Eox/Ered) are 0.71/− 1.35 V and 0.64/−1.40 V versus Ag/Ag+ for PBPD-Th and PBPD-Se, respectively. The corresponding HOMO/LUMO levels are estimated to be −5.42/−3.36 eV and −5.35/−3.31 eV for PBPD-Th and PBPD-Se, respectively, according to the following equations:52−54 EHOMO = −e(Eox + 4.71) (eV) and ELUMO = −e(Ered + 4.71) (eV), respectively. Obviously, compared to PBPD-Th, PBPD-Se possesses a slightly higher HOMO level, which is consistent with the tendency of the reported selenium-containing polymers.37−48 In addition, these two polymers all demonstrate lower HOMO energy level in comparison with that of the analogue PBDTBDD (−5.23 eV) based on BDT-T and thiophene as π-conjugated spacer,51 resulting from the introduction of BDT-m-OP unit, which is beneficial for the higher Voc in the devices. Notably, the polymers PBPD-Th and PBPD-Se showed similar electrochemical bandgaps of about 2.06 and 2.04 eV, respectively, 4812

DOI: 10.1021/acs.chemmater.7b00729 Chem. Mater. 2017, 29, 4811−4818

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Figure 2. (a) J−V curves of the polymer:PC71BM-based PSCs without or with 1% DIO under the AM 1.5G illumination (100 mW cm−2). (b) EQE curves of the related PSCs. (c) Photocurrent density (Jph) versus effective voltage (Veff) characteristics. (d) Jph versus light intensity of the polymer:PC71BM-based PSCs with 1% DIO.

to the PBPD-Th-based blends.57 The negative effects of the reduction of Voc were totally canceled out by the enhancement of Jsc and FF, and the final improved PCE was achieved. Figure 2a shows the current density−voltage (J−V) curves of the devices, and the corresponding device parameters are listed in Table 1. To the best of our knowledge, the PCE of 9.8% is the

while PBPD-Th possessed a smaller optical bandgap of 1.90 eV in comparison with PBPD-Se (1.77 eV). This discrepancy between the optical and electrochemical band gaps might be induced by the presence of a different energy barrier at the different interface between these polymer films and the electrode surface.55 To investigate the photovoltaic properties of the polymers, PSCs were fabricated with a simple device structure of ITO/ PEDOT:PSS/polymer:PC 71BM/zirconium acetylacetonate (ZrAcac)/Al, where the commercially available ZrAcac was used as the cathode interfacial layer because of its high performance in PSCs.56 The effect of the donor/acceptor (D/ A, polymer:PC71BM) weight ratios (w/w) in the active layer on device performance was preliminarily studied in the range of D/A ratios from 1:1 to 1:2 for the blends (see Figure S5 and Table S1). The optimum D/A weight ratio was found to be 1:1.5. The PBPD-Se:PC71BM-based PSCs showed a PCE of 7.0% with Voc of 0.92 V compared with PBPD-Th:PC71BMbased PSCs with a PCE of 5.2% and a Voc of 1.00 V. It is notable that, compared with the analogue PBDTBDD,51 both PBPD-Th and PBPD-Se show higher Voc in the device, which is identified with variation of the HOMO levels. Subsequently, the commonly used processing additive with high boiling point, 1,8-diiodooctane (DIO), was used to optimize the morphology of the active layer for obtaining more efficient PSCs (see Figure S6 and Table S2). It was found that adding 1% (v/v) DIO into the o-DCB blend solution can significantly improve the photovoltaic performance of the device due to the promotion of Jsc and FF. As a result, the champion device for PBPD-Se:PC71BM showed a maximum PCE of 9.8% with Voc of 0.90 V, Jsc of 14.9 mA cm−2, and FF of 73%, while the device based on PBPD-Th:PC71BM showed a PCE of 8.4% with Voc of 0.95 V, Jsc of 12.4 mA cm−2, and FF of 71%. With the use of 1% DIO, the PBPD-Th-based PSCs show a slightly larger Voc loss of 0.05 V compared to the PBPD-Sebased PSCs with a Voc loss of 0.02 V, which may be attributed to the higher crystallinity of PBPD-Se-based blends compared

Table 1. Photovoltaic Performances of the PSCs Based on Polymer:PC71BM (1:1.5, w/w) without or with 1% DIO under Illumination of AM 1.5G, 100 mW/cm2

a

polymer

Voc (V)

Jsc (mA cm−2)

FF (%)

PBPD-Th PBPD-Tha PBPD-Se PBPD-Sea

1.00 0.95 0.92 0.90

9.7 12.4 12.0 14.9

53 71 64 73

PCEmax (PCEave)b (%) 5.2 8.4 7.0 9.8

(5.1) (8.2) (6.9) (9.6)

With 1% DIO. bThe average PCE is obtained from over 30 devices.

highest value reported in the literature for the PSCs based on polymers with selenophene as π-conjugated spacers (see Figure S7 and Table 2). The external quantum efficiency (EQE) spectra were measured to confirm the Jsc values of the PSCs, as shown in Figure 2b. Compared to the PBDP-Th-based devices, the PBDP-Se-based devices show a broadening response over 40 nm in the long wavelength region, which is consistent with UV−vis absorption spectra of the blend films (see Figure S8). Moreover, compared to the devices without DIO, the devices with DIO show higher EQE photoresponse in the whole absorption range from ca. 300 to 730 nm, which is consistent with the increase of the Jsc of the devices after DIO treatment. The EQE values higher than 70% were observed at 410−660 nm with a maximum value of 75% at 582 nm for the PBDP-Sebased devices and 420−580 nm with a maximum value of 72% at 460 nm for the PBDP-Th-based devices, respectively. Higher EQE value and wider EQE spectrum indicate that the PBDPSe-based device has more efficient photon harvesting and 4813

DOI: 10.1021/acs.chemmater.7b00729 Chem. Mater. 2017, 29, 4811−4818

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Table 2. Photovoltaic Performances of the PSCs Based on the Reported Representative Polymers with Selenophene as πConjugated Spacer polymer

Voc (V)

Jsc (mA cm−2)

FF (%)

PCEmax (%)

ref

PBDTT-SeDPP C3-DPPTT-Se P(FBT-alt-Se2Th2) PSeB2 PBnDT-SeFTAZ PBDTTSBO PBPD-Se

0.69 0.56 0.70 0.64 0.779 0.79 0.90

16.8 21.5 15.8 16.8 13.44 11.9 14.9

62 63 66.4 64 54.6 50 73

7.2 7.6 7.34 6.87 5.72 4.7 9.8

37 38 47 40 43 44 this work

Figure 3. (a) GIXD 2D profiles of pure and blend films. (b) Corresponding in-plane (dashed lines) and out-of-plane (solid lines) line cuts.

competing with the carrier extraction as carriers slow down due to the reduced electric field. However, the Jph of the PBDP-Thbased and PBDP-Se-based PSC devices are still as high as 10.8 and 13.2 mA cm−2 at the maximum power point, and the corresponding exciton dissociation probabilities (Pdiss) were calculated as 83% and 86% under the short circuit condition, respectively, according to the following equation: Pdiss = Jph/Jsat, which indicates that the device based on PBDP-Se has a higher exciton dissociation efficiency. We also measured Jsc under different light intensities (P) to study the charge recombination under the short circuit condition of the devices. The relationship between Jsc and P can be defined as Jsc ∝ PS. If all free carriers are swept out and collected at the electrodes prior to recombination, S should be equal to 1, while S < 1 indicates the existence of bimolecular recombination.60 As shown in Figure 2d, the values of S for the PBDP-Th-based and PBDP-Se-based devices are 0.989 and 0.995, respectively. The higher S value for the PBDP-Se:PC71BM-based device implies much less bimolecular recombination in the device, which could also account for its higher Jsc and FF. The influence of the different π-conjugated spacers in the D−A copolymers on the hole/electron mobilities (uh/ue) of active layers in PSCs was investigated by the space charge limited current (SCLC) method.59,61−63 As shown in Figure S10, in blend films, the devices based on PBDP-Se:PC71BM show higher uh/ue of (6.22/6.41) × 10−4 cm2/(V s) with a uh/ ue ratio of 0.97 compared to the PBDP-Th:PC71BM-based devices ((5.76/4.71) × 10−4 cm2/(V s) with a uh/ue ratio of 1.22). The higher and more balanced uh/ue of the PBDP-Sebased active layer should be conducive to obtaining higher FF and PCE values for the corresponding PSCs. Structural order and crystallinity of the two polymers in the pure and optimized blend films were studied by grazing

charge collection. According to the solar irradiation spectrum and the EQE curves, the differences between the integral Jsc value and the measured Jsc value are below 5%. Photoluminescence (PL) spectra of the pure polymers and their blend films with PC71BM were measured to study the photoinduced electron transfer performance between polymer and PC71BM.58 As shown in Figure S9, the blend films with DIO showed higher PL quenching efficiency over 95% in comparison with that of the blend films without DIO (ca. 90%). High PL quenching efficiency means that the devices with DIO additive treatment have an effective photoinduced charge transfer between polymer and PC71BM, which is consistent with the higher EQE and Jsc values of the related devices. To further study the influence of the different π-conjugated spacers in the D−A copolymers on the charge generation and extraction behavior for the PSCs, the photocurrent density (Jph, defined as JL − JD, where JL and JD are the current densities under illumination and in the dark) versus effective voltage (Veff, defined as V0 − Vappl, where V0 is the voltage at which Jph = 0 and Vappl is the applied voltage) of the PSCs was measured.59 As shown in Figure 2c, Jph reaches saturation (13.1 and 15.3 mA cm−2 for the PBDP-Th-based and PBDP-Se-based PSCs, respectively) at Veff ≥ 2 V, which indicates that all photogenerated charge carriers can be extracted by the electrodes. Under short circuit condition of the PSC devices, Jph are 12.4 and 14.9 mA cm−2 for the PBDP-Th-based and PBDP-Se-based PSCs, respectively, which are comparable to that at high Veff. Moreover, under short circuit condition of the PSC devices for the exciton dissociation probabilities (Pdiss) test, the values of Veff are 0.79 and 0.76 V for the PBDP-Thbased and PBDP-Se-based PSC devices, respectively. Near the maximum power output point, recombination will be strongly 4814

DOI: 10.1021/acs.chemmater.7b00729 Chem. Mater. 2017, 29, 4811−4818

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Chemistry of Materials incidence X-ray diffraction (GIXD).4,64−69 The diffraction images and the related in-plane (IP) and out-of-plane (OOP) line-cut profiles are displayed in Figure 3. For the pure polymer films, both of the two polymers show a predominant “face-on” orientation, as evidenced by the strong π−π stacking in the OOP direction. In the OOP profiles, PBDP-Se shows a stronger (100) diffraction peak compared to PBDP-Th. Moreover, for (010) diffraction peak in this direction, PBDPSe shows a smaller π−π stacking distance of 3.78 Å (peak located at 1.66 Å−1) compared to PBDP-Th (3.80 Å, peak located at 1.65 Å−1). The corresponding crystal coherence length (CCL) of (010) peak in this direction was 16.2 and 18.9 Å for PBDP-Th and PBDP-Se, respectively. In the IP direction, PBDP-Se also exhibited a smaller lamellar distance of 19.6 Å (peak located at 0.32 Å−1) than that of PBDP-Th (20.3 Å, peak located at 0.31 Å−1), which may be due to the relatively larger polarizable radius of selenium atom leading to a higher degree of rigidity and stronger intermolecular interactions.37−39,47 The corresponding CCLs were 61.2 and 75.9 Å for PBDP-Th and PBDP-Se, respectively. The above results show that the crystallinity, the π−π stacking, and lamellar packing of PBDPSe are enhanced compared to PBDP-Th, which is consistent with the higher hole mobility (1.12 × 10−3 cm2/(V s)) of PBDP-Se compared to that of PBDP-Th (7.56 × 10−4 cm2/(V s)) (see Figure S10). In addition, for all the polymer:PC71BM blend films with DIO additive treatment, a weak π−π stacking peak of the polymers in the OOP profile is still observed, which means that these blends still maintain a predominantly face-on orientation. The peaks at 1.36 and 1.96 Å−1 are the characteristic diffraction peaks of PC71BM. In the OOP profiles, PBDP-Se also shows a stronger (100) diffraction peak compared to PBDP-Th. Moreover, compared with the (010) diffraction peak in the OOP direction of the pure films, the PBDP-Se:PC71BM blend shows the same π−π stacking distance and almost unchanged CCL of 18.6 Å, while the PBDP-Th:PC71BM blend shows the same π−π stacking distance and significantly reduced CCL of 15.4 Å, which means that PBDP-Se still has a higher crystallinity than PBDP-Th, and the crystallinity in the OOP (010) direction of PBDP-Se will not be affected after blending with PC71BM. Compared with the pure films, in the IP direction, both of the two polymers show the same lamellar distance (peak at 0.33 Å−1) and increased crystallinity in the blends (CCL of 77.2 Å for PBDP-Th and 131 Å for PBDP-Se), while PBDP-Se still shows a higher crystallinity than PBDP-Th. The high crystallinity is good for the charge transport and reduces the probability of bimolecular recombination, thus yielding a high FF, which is consistent with the hole/electron mobilities (see Figure S10) and the exciton dissociation probabilities (see Figure 2c) of the devices. The surface and bulk morphologies of the blend films in PSCs were surveyed with atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements. Figure 4 shows the TEM images of the polymer:PC71BM blends without and with 1% DIO additive treatment. Without DIO processing, the PBDP-Th showed no obvious fibrous features, while the PBDP-Se showed a clear but small fiber structure in the active layer. Upon mixing of 1% DIO to the blend solution, the blend films showed more obvious phase separation. Notably, the PBDP-Se:PC71BM film showed a bicontinuous D/A interpenetrating network, well-developed and uniform fibrillar morphology with more suitable fiber size compared to the blend film without DIO, which would be

Figure 4. TEM images of the polymer:PC71BM blend films without or with 1% DIO additive treatment.

beneficial to get more efficient exciton dissociation and charge transport for the PSCs, and thus higher FF and PCE values can be obtained.52 For the AFM images (see Figure S11), without or with DIO processing, the PBDP-Se:PC71BM films showed higher root-mean-square (RMS) roughness values compared to the PBDP-Th:PC71BM films. Moreover, upon mixing of 1% DIO to the blend solution, the PBDP-Se:PC71BM film showed a more suitable fiber structure and a more obvious phase separation compared to the PBDP-Th:PC71BM film (Figure S11, panel i versus panel j), which was consistent with the TEM test. In conclusion, two new conjugated polymers, PBPD-Th and PBPD-Se, based on BDT-m-OP as donor and BDD as acceptor units with different π-conjugated spacers of thiophene and selenophene, respectively, were synthesized and developed for photovoltaic applications. Compared with PBPD-Th, PBPD-Se exhibited a broadened absorption spectrum with higher extinction coefficient, enhanced hole mobility, and smaller lamellar stacking and π−π stacking spacing. As a result, the device based on PBPD-Se:PC71BM showed a significantly enhanced PCE of 9.8% with an enhanced Jsc of 14.9 mA cm−2 and a slightly lower Voc of 0.90 V in comparison with a PCE of 8.4% with a Voc of 0.95 V and a Jsc of 12.4 mA cm−2 for PBPDTh:PC71BM-based devices. Furthermore, the PCE of 9.8% is the highest value reported in the literature for the PSCs based on selenium-containing polymers. These results indicate that the simultaneous introduction of BDT-m-OP as donor unit and selenophene as π-conjugated spacer to the D−A copolymer backbone is an effective strategy for achieving high efficiency PSCs.

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EXPERIMENTAL SECTION

Materials. All chemicals and solvents were reagent grade and purchased from Alfa Aesar and TCI. The monomer BDT-m-OP,49 BDD-Th, and BDD-Se53 were synthesized according to the reported literature. PBPD-Th. In a dry 50 mL flask, tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4, 18.5 mg) and 1 mL of DMF were added to a solution of BDT-m-OP (184 mg, 0.200 mmol) and BDD-Th (152 mg, 0.200 mmol) in 10 mL of degassed toluene under nitrogen and stirred vigorously at 110 °C for 10 h until the reaction mixture became viscous. Then the mixture was poured into methanol (100 mL) to form a precipitate. The polymer was dissolved in chloroform, and the solution was filtered through a silica gel column. The collected chloroform solution was concentrated and precipitated with methanol to provide a dark solid (180 mg, 75%). Anal. Calcd for C72H84O4S6 (%): C, 71.72; H, 7.02. Found (%): C, 70.13; H, 7.16. PBPD-Se. PBPD-Se was prepared according to the synthetic procedure of PBPD-Th by the reaction of BDT-m-OP (256 mg, 0.276 mmol), BDD-Se (238 mg, 0.276 mmol), Pd(PPh3)4 (18.5 mg), and 1 mL of DMF in 10 mL of dry toluene. The reaction was stirred for 11 h until the reaction mixture became viscous to provide a dark solid (240 mg, 67%). Anal. Calcd for C72H84O4S4Se2 (%): C, 66.54; H, 6.51. Found: C, 65.33; H, 6.62. Measurements and Instruments. GPC was carried out on an Agilent Technologies PL-GPC-220 instrument at 160 °C, with 1,2,44815

DOI: 10.1021/acs.chemmater.7b00729 Chem. Mater. 2017, 29, 4811−4818

Article

Chemistry of Materials tricholorobenzene as the eluent and polystyrene as the standard. Elemental analysis was performed on a flash EA1112 analyzer. TGA was measured on a PerkinElmer TGA-7 apparatus at a heating rate of 10 °C/min under inert atmosphere. DSC was taken on a TA DSC Q200 instrument at a scan rate of 2 °C/min under inert atmosphere. UV−vis absorption spectrum was recorded on a UV−vis−near-IR spectrophotometer of Agilent Technologies Cary Series, in which the extinction coefficient was defined by the absorption intensity of the active layer with a thickness of 1 cm. The electrochemical CV was taken on a electrochemical workstation of Zahner Ennium IM6 in a acetonitrile solution with 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6), with a glassy carbon disk, Ag/Ag+ electrode, and Pt wire as working electrode, reference electrode, and counter electrode, respectively. PL spectrum was acquired on an Edinburgh Instrument FLS-980. In the PL quenching studies, the maximum absorption wavelength (λmax) of the UV−vis absorption spectrum of the polymer film is first used as the excitation wavelength to measure the PL spectrum of the polymer film, and then the maximum emission peak wavelength was obtained. Finally, the corresponding maximum emission wavelength is used as the excitation wavelength to accurately measure the PL spectrum of the polymer films. AFM was taken on a Veeco Dimension-3100 atomic force microscope using the tapping mode. TEM was recorded on a Tecnai G2 F20 S-TWIN instrument under 200 kV accelerating voltage, where the polymer:PC71BM films were prepared by the following processing techniques for the TEM measurement: The polymer:PC71BM films were spin-cast on the ITO/ PEDOT:PSS substrates, and then the resulting substrates with the polymer:PC71BM films were submerged in deionized water to make these polymer:PC71BM films float onto the water/air interface, and finally the floated polymer:PC71BM films were picked up on unsupported 200 mesh copper grids. GIXD tests were recorded by beamline 7.3.3 at the Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA. Samples were prepared using self-same blend solutions as those used in the devices on a precoated Si substrate. The 104 eV X-ray beam was incident at a grazing angle of 0.14°, in which the scattering intensity was maximized from the samples. The scattered intensity was detected using a Pilatus detector. The crystal coherence length (CCL) was defined as CCL = 0.9 × (2π/ fwhm) (Å), where fwhm is the full width at half-maximum of the corresponding diffraction peak. Devices Fabrication and Characterization. The PSC devices structure was ITO/PEDOT:PSS/polymer:PC71BM/ZrAcac/Al. In an ultrasonic bath, the ITO-coated glass (10 Ω/sq) was cleaned with deionized water, acetone, and isopropanol, respectively. After oxygen plasma cleaning for 10 min, a 30 nm thick poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS; Bayer Baytron 4083) anode buffer layer was spin-cast onto the ITO substrate and then dried by baking in an oven at 150 °C for 15 min. The active layer was then deposited on top of the PEDOT:PSS layer by spincoating from o-DCB solution (10 mg/mL of polymers) of polymer:PC71BM. In the case of the devices using a processing additive, DIO (1% by volume) was added to the solutions before use. The methanol solution of ZrAcac at a concentration of 0.5 mg/mL was deposited on the active layer at 3000 rpm for 30 s. The thickness of the active layers was controlled by adjusting the spin speed during the spin-coating process and measured by a KLA Tencor D-100 profilometer. Finally, 100 nm of Al was successively deposited on the photosensitive layer under vacuum at a pressure of ca. 4 × 10−4 Pa. The overlapping area between the cathode and anode defined a pixel size of 0.1 cm2. Except for the deposition of the PEDOT:PSS layers, all the fabrication processes were carried out inside a controlled atmosphere of nitrogen drybox containing less than 5 ppm oxygen and moisture. The PCE values of the PSCs were measured under a illumination of AM 1.5G (100 mW/cm2) using a SS-F5-3A solar simulator (AAA grade, 50 × 50 mm2 photobeam size) of Enli Technology CO., Ltd. A 2 × 2 cm2 monocrystalline silicon reference cell (SRC-00019) was purchased from Enli Technology Co., Ltd.. PCE statistics were obtained using 30 individual devices fabricated under the same conditions. The EQE was measured by a solar cell spectral response measurement system QE-R3011 of Enli Technology Co.,

Ltd. The light intensity at each wavelength was calibrated with a standard single crystal Si photovoltaic cell.

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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.7b00729. TGA and DSC curves, UV−vis adsorption spectra, cyclic voltammograms, J−V characteristics of SCLC, photoluminescence spectra, AFM images, and other relevant device data (PDF)

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AUTHOR INFORMATION

ORCID

Maojie Zhang: 0000-0002-6102-5856 Yongfang Li: 0000-0002-2565-2748 Author Contributions ∥

Z.X. and Q.F. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC; Grant Nos. 51422306, 51573120, 51503135, 21504066, and 21534003), Ministry of Science and Technology (Grant No. 2016YFA0200700), the Priority Academic Program Development of Jiangsu Higher Education Institutions, Jiangsu Provincial Natural Science Foundation (Grant No. BK20150332), and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 15KJB430027). X-ray data were acquired at beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. We thank Chenhui Zhu at beamline 7.3.3 and Cheng Wang at beamline 11.0.1.2 for assistance with data acquisition.

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