Crystalline Medium-Bandgap Light-Harvesting Donor Material Based

Sep 7, 2017 - It is worth noting that lower energy loss is obtained in fullerene-free-based PSCs, which is essential to overcome the trade-off between...
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Crystalline Medium-Bandgap Light-Harvesting Donor Material Based on β-Naphthalene Asymmetric-Modified Benzodithiophene Moiety toward Efficient Polymer Solar Cells Yonghai Li,† Deyu Liu,†,∥ Junyi Wang,†,∥ Zhi-Guo Zhang,§ Yongfang Li,§ Yanfang Liu,† Tingting Zhu,‡ Xichang Bao,*,† Mingliang Sun,‡ and Renqiang Yang*,† †

CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China ‡ Institute of Material Science and Engineering, Ocean University of China, Qingdao 266100, China § Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: In this paper, we reported a crystalline p-type medium-bandgap conjugated D−A polymer asy-PBDBTN based on a symmetry-breakingmodified BDT moiety to combine the advantages of both one-dimension (1D) and two-dimension (2D) symmetric BDTs. Polymer asy-PBDBTN is a highly efficient light-harvesting donor material. Single BHJ PSCs exhibit PCE of 8.88% with PC71BM as acceptor. Also, PCE values of 10.50% are achieved with the use of ITIC as an acceptor to couple asy-PBDBTN with VOC of 0.942 V, JSC of 16.81 mA cm−2, and FF of 0.663. It is worth noting that lower energy loss is obtained in fullerene-free-based PSCs, which is essential to overcome the tradeoff between VOC and JSC and boost these two parameters simultaneously for high photovoltaic performance. The combination process of additive and thermal annealing is critical to enhance and retain the π−π stacking behavior of donor and fullerene-free acceptor; as a result, the trap-assisted recombination was greatly suppressed. This work demonstrates a great prospect for the construction of the symmetry-breaking BDT-based D− A conjugated polymers toward high-performance PSCs, especially with fullerene-free acceptor material.

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separation to overcome binding energy.11 This larger driving force would cause an increasing photon energy loss (Eloss), which is defined as Eloss= Eg − eVOC, where Eg is the lowest optical bandgap between donor and acceptor materials and VOC represents the open-circuit voltage of solar cell.12,13 The increasing Eloss will make output of a high short-circuit current density (JSC) and open-circuit voltage (VOC) difficult. To overcome the shortcomings of fullerene acceptors and discover a new approach to reduce energy loss, n-type imide-derivative semiconductors including perylenediimide and naphthalenediimide derivatives (for example N2200) are well-investigated, as fullerene-free acceptor materials with PCE exceeding 9% have already been recorded.14−24 Moreover, Prof. Zhan first reported a kind of n-type fused-ring electron acceptor ITIC in 2015 which greatly expands the outlook and imagination of fullerene-free solar cells.25 Efficiency over 12% based on single BHJ PSCs has been reached in a short span of two years.26,27 Very recently, Hou

INTRODUCTION

Great progress has been achieved for bulk heterojunction polymer solar cells (BHJ PSCs) over the past decades, driven by optimization of high-quality semiconductors, development of interfacial materials, and revolutionary device constructions.1−3 A substantial number of efficient electron-donating materials have been intensively studied for their facility to chemically tailor and adjust material absorption spectra and molecular energy levels. However, the variety of electron-accepting materials is pale by comparison. Fullerene-based [6,6]-phenyl-(C61 or C71)butyric acid methyl ester (PC61BM or PC71BM) derivatives have been proven to be highly efficient for the excellent properties of three-dimensional electron transport and have good compatibility with most donor materials. To date, power conversion efficiencies (PCEs) of single BHJ PSCs based on fullerene acceptors have reached 10%.4−10 Hoewever, fullerene acceptors usually exhibit a very low ability for light-harvesting and demand narrow-bandgap donor materials to cover a larger fraction of the solar spectra. Besides, it is commonly accepted that the gaps of frontier energy levels (ΔHOMO and ΔLUMO) between donor and acceptor materials should be higher than 0.3 eV for exciton © 2017 American Chemical Society

Received: June 16, 2017 Revised: September 6, 2017 Published: September 7, 2017 8249

DOI: 10.1021/acs.chemmater.7b02495 Chem. Mater. 2017, 29, 8249−8257

Chemistry of Materials

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Article

RESULTS AND DISCUSSION Material Synthesis and Characterization. The molecular structure of polymer asy-PBDBTN is shown in Figure 1a, and the

and co-workers presented an inspiring work with a record high efficiency of 13.1%, suggesting the promise of fullerene-free PSCs in practical applications.28 Different from most conventional fullerene-based PSCs, the results show that a smaller ΔHOMO value is allowable and reduced Eloss would be obtained in fullerene-free-based PSCs.29−31 This would achieve the optimal compromise between photocurrent and photovoltage, making it feasible to enhance JSC and VOC simultaneously and finally give a high efficiency. Currently, most of the donor materials employed in fullerenefree PSCs are donor−acceptor (D−A) type conjugated polymers based on a symmetric-modified one-dimension (1D) or twodimension (2D) benzodithiophene (BDT) moiety as the electron-donating building block.22−26,32−34 The 1D-BDTbased polymer usually shows good solubility, closed stacking distance, and ordered film microstructure, which would be beneficial to charge transport. While a 2D-BDT-based polymer can possess a low-lying HOMO energy level for high VOC through adjusting the ionization potential of 2D substitution, our previous work reveals a symmetry-breaking strategy to combine the advantages of both 1D and 2D symmetric BDTs and construct high-performance photovoltaic materials.35 Recently, we investigated two forms of naphthalene groups bonded via the α- and β-position linkage (α- and β-naphthalene) as substitutions of an asymmetric BDT unit and found the βnaphthalene group can provide an optimal π−π stacking distance and produce a balance between JSC and VOC. Polymer based on βnaphthalene-modified asymmetric BDT and strong electronwithdrawing unit DTff BT(5,6-difluoro-2,1,3-benzothiadiazole) exhibits excellent photovoltaic performance with PC71BM as acceptor but relatively low efficiency (63°). The distortion of the naphthyl group would help enhance polymer solubility and could also partly reduce the delocalization of the electron cloud, resulting in a slightly decreased HOMO energy level. Besides, the twisted side group can enlarge the distance of π−π stacking in the film state, which could also lower the energy levels to some extent. GIWAXS results of the as-cast film reveal an intense lamellar stacking (100) and π−π stacking (010) shown in Figure 1c. The (100) diffraction peak at qz = 0.37 Å−1 corresponds to an interchain distance of 16.76 Å. The (010) diffraction peak at qz = 1.69 Å−1 corresponds to a distance of 3.71 Å, indicating a strong tendency to adopt a favorable face-on orientation. The ordered molecular packing would probably facilitate charge transport in the vertical direction of PSC devices. Photophysical Properties and Frontier Energy Levels. The UV−vis absorption spectra of asy-PBDBTN in dilute solution and thin film are shown in Figure S3, and the data were collected in Table S2. Similar spectral profiles of the dilute solution and film are observed, with mainly two absorption bands. The minor bands at 404 nm probably originate from π−π* transitions. The long-wavelength absorption bands (about 580 nm, A0−1) with well-resolved vibronic peaks (about 620 nm, A0−0) in the range 500−750 nm are attributed to intramolecular charge transfer (ICT) between the donor and acceptor moieties of asy-PBDBTN in the ground state.44 The film of asy-PBDBTN possesses a high light-harvesting ability with a molar extinction coefficient at 621 nm reaching nearly 1 × 105 cm−1, which is essential for high-performance solar cells to capture more solar photons. The strong vibronic peaks indicate a raised intermolecular interaction even in hot o-DCB solutions (70 °C) obtained from temperature-dependent absorption spectra (Figure S4).45,46 The optical bandgap of asy-PBDBTN was calculated to be 1.83 eV from the onset of film absorption spectra. Compared to the analogue polymer PBDTβNPFBT based on asy-BDT and DTf f BT, polymer asy-PBDBTN shows a hypsochromic shift induced by the weakened ICT effect between the donor and acceptor units. Combined with the cyclic voltammetry (CV) measurement as shown in Figure S5, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels are confirmed to be −5.41 and −3.58 eV, respectively. Moreover, asy-PBDBTN has decreased HOMO/LUMO energy levels as compared to those of the symmetric analogue PBDB-T reported by Prof. Hou,47,48 which should be attributed to the lower electron density of the naphthalene group than the thiophene group substituted onto the BDT core. The deeper HOMO level of asy-PBDBTN would tend to afford a raised VOC from the polymer-based PSCs. Figure 2a presents the molecular structures of materials employed in this work. Polymer asy-PBDBTN was used as a donor material. PC71BM and ITIC were chosen as acceptor materials to couple medium-bandgap asy-PBDBTN. PDINO is adopted as a cathode buffer layer.49 As shown in Figure 2b, the film absorption spectra of donor/acceptor materials are wellmatched for a fullerene blend film (asy-PBDBTN/PC71BM) and a fullerene-free blend film (asy-PBDBTN/ITIC). This is important for PSCs to harvest sufficient photons. From Figure 2c we can see that the gaps of HOMO and LUMO levels between asy-PBDBTN and PC71BM are larger than 0.3 eV, which would help to yield an ample driving force for exciton separation considered from the point of view of the donor/acceptor materials. However, the HOMO difference between asyPBDBTN and ITIC is determined to be 0.19 eV.

Figure 2. (a) Chemical structures of materials used in PSCs; (b) film absorption spectra of asy-PBDBTN, PC71BM, and ITIC, and spectra under the illumination of 1.5 AM 100 mW cm−2; (c) energy level diagrams of the materials in the PSC device.

In order to investigate the exciton separation and charge transfer behavior in the blend films and with consideration of different ΔHOMO values in fullerene-based and fullerene-freebased devices, we examined the photoluminescence (PL) spectra of asy-PBDBTN and different donor/acceptor blend films. Figure 3a shows that the PL emission of asy-PBDBTN excited at

Figure 3. (a) PL spectra of asy-PBDBTN and blend films of asyPBDBTN/PC71BM (w/w = 1:1) and asy-PBDBTN/ITIC (w/w = 1:1); (b) PL spectra of ITIC and ITIC/asy-PBDBTN blend films (w/w = 1:1) without or with optimizing process (TA = thermal annealing).

570 nm is in the range 650−750 nm, peaking at 665 nm, whereas for the blend films, the PL emission of asy-PBDBTN is largely quenched by PC71BM and ITIC. Figure 3b shows that the PL emission of ITIC excited at 700 nm is in the range 760−825 nm, peaking at 790 nm, while for the blend film of asy-PBDBTN/ ITIC, the PL emission of ITIC is almost completely quenched by asy-PBDBTN. This indicates that the exciton separation and charge transfer are both highly effective for asy-PBDBTN/ PC71BM and asy-PBDBTN/ITIC blend films even if the latter possess a very small ΔHOMO value. Photovoltaic Performance. PSC devices were fabricated with the configuration ITO/PEDOT:PSS/asyPBDBTN:PC71BM or ITIC/PDINO/Al (PEDOT:PSS = poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) under the illumination of AM 1.5 G (100 mW cm−2). The devices were optimized by different conditions (D/A weight ratios, additive, thermal annealing) for fullerene- and fullerene-freebased PSCs with o-DCB and CB as the processing solvent, respectively. The results from Table S3 show that the best weight ratios for both types of PSCs are confirmed to be 1:1. On the basis of the optimal weight ratios, the photovoltaic data of two types of PSCs under different processing conditions are collected in Table 1, and the respective J−V curves are shown in Figure 4a. 8251

DOI: 10.1021/acs.chemmater.7b02495 Chem. Mater. 2017, 29, 8249−8257

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Table 1. Optimal Device Parameters of PSCs with Different Acceptors under Different Optimizing Conditions (w/w = 1:1) and Mobilities Based on the SCLC Model DIO

VOCb

JSCb

PCEb

JSCEQE

Rsc

Rshc

μh −4

Deviceasy-PBDBTN:PC71BM

(v/v) 0 1.5%

(V)

(mA cm−2)

0.930 (0.921 ± 0.006) 0.893 (0.884 ± 0.007)

12.65 (12.38 ± 0.23) 13.98 (13.77 ± 0.19)

(mA cm−2) 12.15 13.28

FFb

(%)

0.692 (0.681 ± 0.013) 0.711 (0.702 ± 0.009)

8.14 (7.90 ± 0.19) 8.88 (8.59 ± 0.27)

μe 10−4 cm2 V−1 s−1

(Ω cm−2)

(Ω cm−2)

10 cm2 V−1 s−1

9.73

2140

1.68

3.12

7.08

2843

1.85

4.06

asy-PBDBTN:ITIC

0

0.951 (0.940 ± 0.011)

15.69 (15.44 ± 0.26)

14.86

0.640 (0.633 ± 0.006)

9.55 (9.28 ± 0.22)

6.06

761

0.70

1.43

asy-PBDBTN:ITICa

0.5%

0.942 (0.930 ± 0.009)

16.81 (16.48 ± 0.30)

16.09

0.663 (0.654 ± 0.011)

10.50 (10.22 ± 0.20)

5.53

1025

2.91

2.67

PBDB-T:ITICd

0.5%

0.898

16.90

0.654

9.93 (9.72 ± 0.16)

The blend films were thermally annealed at 150 °C for 10 min. bValues were provided in optimal (statistical) results based on 34 devices for each case. cThe data are obtained from J−V curves of optimal devices. The thicknesses of all blend films are 115 ± 10 nm. dAs reported in ref 48, the conventional device structure was with PEDOT:PSS as anode buffer layer; the average value was obtained from 10 individual devices.

a

Figure 4. (a) J−V curves of polymer solar cells under different optimizing conditions (w/w = 1:1); (b) EQE curves of the corresponding solar cells; (c, d) JSC-ILP and VOC-ILP curves of two types of BHJ devices under different optimizing conditions. The solid lines represent the corresponding fitted curves (TA = thermal annealing).

As for fullerene-based PSCs, the optimal device was finally obtained using o-DCB as a processing solvent with 1.5% (v/v) additive of 1,8-diiodooctane (DIO). The best as-cast device exhibits a VOC of 0.930 V, JSC of 12.65 mA cm−2, FF of 0.692, and resulting PCE of 8.14%. After the addition of DIO, the JSC was improved from 12.38 ± 0.23 to 13.77 ± 0.19 mA cm−2, and the solar cell performance was enhanced, showing a maximum PCE of 8.88% with VOC of 0.893 V, JSC of 13.98 mA cm−2, and FF of 0.711. Meanwhile, the average PCEs increased from 7.90% to 8.59% based on 34 devices for each case, and the statistical histograms are shown in Figure S6.

Highly efficient photovoltaic performance is achieved when ITIC is used as the acceptor material. For these fullerene-freebased PSCs, the best as-cast device exhibits a higher VOC of 0.951 V, JSC of 15.69 mA cm−2, and a slightly decreased FF of 0.640, compared to fullerene-based PSCs. The improved VOC and JSC should be attributed to the decreased HOMO energy level and extended absorption range of ITIC, respectively. When additive DIO (0.5%) and thermal annealing are combined to process a asy-PBDBTN/ITIC blend film, the photovoltaic performance is greatly enhanced. The optimal fullerene-free-based PSC reveals the remarkable PCE of 10.50%, with a favorable VOC of 0.942 V, enhanced J SC of 16.81 mA cm −2 , and FF of 0.663. 8252

DOI: 10.1021/acs.chemmater.7b02495 Chem. Mater. 2017, 29, 8249−8257

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boost the two parameters simultaneously for high photovoltaic performance. Charge Separation and Recombination Studies. In order to obtain a further in-depth investigation about the charge separation and recombination process in PSCs, we performed the measurement of photocurrent generation and dependence of photovoltaic performance on incident light power (ILP). Figure 4c depicts the log−log plot of JSC as a function of the light intensity. Generally, JSC has a power-law dependence on incident light power (JSC ∝ ILPS), where S is a power-law scaling exponent and should be unity when the bimolecular recombination of charge carries is negligible.53,54 As shown in Figure 4c, relatively low S values were fitted to be 0.90 and 0.88 for asyPBDBTN:PC71BM and asy-PBDBTN:ITIC as-cast devices, respectively, suggesting relatively serious bimolecular recombination under short-circuit conditions. However, perfect S values (1.00 and 0.99) were obtained for optimized fullerene- and fullerene-free-based devices, which indicates that bimolecular recombination was significantly suppressed after suitable optimization and should make a great contribution to the enhanced FF and JSC. Figure 4d presents the semilogarithmic plot of VOC as a function of the light intensity which can help us to determine the degree of trap-assisted recombination. For free-carrier bimolecular recombination in BHJ solar cells, the semilogarithmic plot of VOC as a function of the light intensity should show a linear relationship with a slope of 1 × kT/q, where k is the Boltzmann constant, T is temperature, and q is the elementary charge. In contrast, a slope of 2 × kT/q implies that the trap-assisted or Shockley−Read−Hall recombination is the dominating mechanism.55,56 As shown in Figure 4d, the slopes of asyPBDBTN:PC71BM-based devices without and with DIO process were calculated to be 1.49 kT/q and 1.27 kT/q, suggesting considerable trap-assisted recombination involved in the as-cast device under open-circuit conditions and partially suppressed after the optimizing process. A decreased slope of 1.12 kT/q was observed for the asy-PBDBTN:ITIC as-cast device, indicating that bimolecular recombination is the dominating mechanism and trap-assisted recombination is minor. Notably, after processing by DIO and thermal annealing to the fullerene-free device, the trap-assisted recombination was almost completely suppressed with the slope of 1.01 × kT/q. The low recombination of free carriers should partially contribute to the low VOC loss and energy loss. Molecular Packing and Morphology in Blends. The detailed nanoscale morphology of the blend films was examined by AFM and TEM. As shown in Figure 5a, the as-cast asyPBDBTN:PC71BM blend film exhibits relatively serious selfaggregation with some large separated domains which would limit the probability for exciton dissociation and suffer serious trap-assisted recombination consistent with the charge separation and recombination studies.57 More favorable morphology was obtained after optimizing by DIO additive without changing the root-mean-square (RMS) roughness of the film (Figure 5b). As for the asy-PBDBTN:ITIC blend film shown in Figure 5d,e, an increased domain size was observed after DIO and thermal annealing accompanying RMS increased from 1.24 to 1.71 nm, indicating slightly enhanced aggregation which would benefit the charge transport.58 Meanwhile, as shown in Figure 5c,f and Figures S11 and S12, the blend films of TEM images also exhibited a similar tendency with AFM studies to evolve into more uniform and well-distributed interpenetrating nanoscale networks after a suitable optimizing process, supporting the

Simultaneously, the average PCEs increased from 9.28% to 10.22% based on 34 devices for each case, and the statistical histograms are shown in Figure S7. Furthermore, it can be found that the three parameters (VOC, JSC, and FF) and the overall performance based on asy-PBDBTN/ITIC are simultaneously improved compared to the PBDTβNPFBT/ITIC-based device reported in our previous work, which should be attributed to the deeper HOMO energy level, more complementary absorption spectra, and better polymer solubility. As shown in Table 1, asyPBDBTN/ITIC devices also reveal comparable high efficiencies as the PBDB-T:ITIC device with a conventional structure, mainly induced by the lower HOMO value.48 The optimal photovoltaic performance in this work is one of the best in single binary BHJ PSCs, suggesting that the symmetry-breaking BDT moiety is a promising electron-donating building block in constructing efficient light-harvesting D−A polymers for fullerene-free PSCs. The series resistance (Rs) and shunt resistance (Rsh) of PSCs are measured to elucidate the effects of additive and thermal annealing from their respective J−V curves under illumination. As shown in Table 1, for two types of PSCs with different acceptors, the optimal devices present a relatively lower Rs and higher Rsh in comparison to their corresponding as-cast devices, suggesting better diode characteristics after the optimizing process.50 Furthermore, the charge carrier mobilities of the PSCs were measured by space-charge-limited current (SCLC) model. The hole and electron mobilities of optimal devices are summarized in Table 1, and the plots of the current density versus voltage of devices based on SCLC are shown in Figures S8 and S9. The results from SCLC measurement reveal that both hole and electron moblities are simultaneously increased for two types of PSCs after the optimizing process, and for optimal asyPBDBTN/ITIC, μh/μe = 1.1. The higher and more balanced mobilties should be beneficial for a higher FF and PCE of devices. Figure 4b shows the external quantum efficiencies (EQEs) of the respective PSC devices. As for fullerene-based devices, the addition of DIO improves the EQE values between 450 and 650 nm with the maximum reaching 84% at 500 nm, accounting for the enhancement of JSC. The fullerene-free devices exhibit an extended photoresponse to 800 nm in comparison with fullerene-based PSCs. Also, the combination of DIO and thermal annealing process further significantly increased the EQE response between 430−500 and 560−800 nm with the maximum value up to 76% at 600 nm (Figure 4b); however, the EQE values between 300 and 430 nm were decreased. As shown in Figure 2b, solar spectra exhibit stronger radiation energy at the above two bands other than 300−430 nm; therefore, the EQE response after the optimizing process will definitely contribute to the enhancement of the JSC of the devices. As shown in Table 1 and Figure S10, the integrated current densities (JSCEQE) from the EQE spectra are slightly smaller than the JSC obtained from the J−V measurements, with a small deviation less than 6%. It is noteworthy that fullerene-free devices exhibit a high VOC of 0.94−0.95 V, indicating decreased Eloss (Eloss= Eg − eVOC) to 0.62−0.63 eV, where Eg is calculated to be 1.57 eV from the onset absorption of the ITIC film at 790 nm (see Figure 2b). Similar low Eloss values were also reported in ultra-narrow-bandgap donor materials and fullerene acceptor-based devices.51,52 Even so, the Eloss value in this work is smaller than those of most PSCs and approaches the empirically low threshold of 0.6 eV, which is important to overcome the trade-off between VOC and JSC and 8253

DOI: 10.1021/acs.chemmater.7b02495 Chem. Mater. 2017, 29, 8249−8257

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Chemistry of Materials

π−π stacking, respectively. As for the as-cast blend film of asyPBDBTN:PC71BM (Figure 6b,h), long-range ordered laminar packing with the (200) diffraction peak appeared, while after optimization with DIO (Figure 6c), a weak (010) diffraction peak at 1.74 Å−1 in the out of plane direction of the blend film was detected, implying an emerged π−π stacking. For asy-PBDBTN/ ITIC blend films without and with the optimizing process, more pronounced and different changes occurred. As indicated from Figure 6d,e and Figure 6i,j, after the addition of DIO, the blend film of asy-PBDBTN:ITIC exhibits a sharper and stronger (010) diffraction peak, indicating more fractions of face-on π−π stacking which will facilitate the charge transport in the BHJ blend films. Then, a combination process of additive DIO and thermal annealing was further carried out. As Figure 6f,i,j reveal, two pronounced and separated π−π diffraction peaks (010) and (010′) in the out of plane direction were found. Also, the peaks at 0.43 Å−1 (100′) in the in plane direction and 1.57 Å−1 (010′) in the out of plane direction should be attributed to the enhanced ordering of ITIC. This reveals that the combination process of DIO and thermal annealing can retain their own π−π stacking for asy-PBDBTN and ITIC in the blend film, which would facilitate the charge transport and minimize charge recombination consistent with the results of charge recombination studies.59 Finally, the concurrently enhanced preferable π−π face-on orientation of both donor and acceptor materials will significantly facilitate the charge transport of PSCs devices in the vertical direction and thus afford a high current density and photovoltaic performance.

Figure 5. (a,b,d,e) AFM topography images (2.5 μm × 2.5 μm) and (a and f) TEM images of asy-PBDBTN:PC71BM and asy-PBDBTN:ITIC blend films (w/w = 1:1) under different conditions (TA = thermal annealing).

enhanced and more efficient charge transport of the corresponding PSC devices. The microstructural features of asy-PBDBTN neat film and blend films with different acceptors were further investigated by GIWAXS to explore the effect of additive and thermal annealing processes. The detailed structure data obtained from GIWAXS results were collected in Table S4. As demonstrated in Figures 1c and 6a, the neat film of asy-PBDBTN shows for (100) and (010) two diffraction peaks located at qz = 0.37 Å−1 and qz = 1.69 Å−1 in the out of plane direction, ascribed to the laminar packing and

Figure 6. GIWAXS images of (a) asy-PBDBTN neat film, (b, c) blend films of asy-PBDBTN:PC71BM, and (d−f) asy-PBDBTN:ITIC under different optimizing conditions. Line cuts of GIWAXS (g, i) in plane and (h, j) out of plane of the blend films (TA = thermal annealing). 8254

DOI: 10.1021/acs.chemmater.7b02495 Chem. Mater. 2017, 29, 8249−8257

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CONCLUSION

In conclusion, we reported an efficient p-type medium-bandgap light-harvesting polymer asy-PBDBTN based on symmetrybreaking-modified BDT to combine the advantages of both 1D and 2D symmetric BDTs, including good solubility of polymers, enhanced light-harvesting ability, and easy-tuned molecular energy levels. The low-lying HOMO energy level of polymer is beneficial for a high VOC of the resulting solar cells. Single BHJ PSCs exhibit PCE of 8.88% with PC71BM as acceptor. Also, highly efficient PCE values of 10.50% are achieved when ITIC was used as an acceptor to couple asy-PBDBTN with VOC of 0.942 V, JSC of 16.81 mA cm−2, and FF of 0.663. The relatively low-energy loss in fullerene-free-based PSCs is important to overcome the trade-off of VOC and JSC and boost these two parameters simultaneously for high photovoltaic performance. We provide some insight as to how the processing of DIO and annealing impact the structural morphology of blend films. The combination process of additive and thermal annealing is helpful to enhance and retain their own π−π stacking of donor and fullerene-free acceptor, leading to a greatly suppressed trapassisted recombination. This work demonstrates the great prospect to construct symmetry-breaking BDT-based D−A conjugated polymers toward high-performance PSCs and should help guide the future development of new photovoltaic materials. Moreover, we are largely attracted by the perfectly complementary absorption of PC71BM (300−550 nm), mediumbandgap donor asy-PBDBTN (550−650), and narrow-bandgap ITIC (650−800 nm) along with well-aligned cascade energy levels, and work for more efficient panchromatic response PSCs through reasonable fabrication of ternary polymer solar cells is under way.

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ACKNOWLEDGMENTS

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REFERENCES

The authors are deeply grateful to the National Natural Science Foundation of China (21502205, 51573205) and China Postdoctoral Science Foundation (2016T90656) for financial support. X.B. thanks the Youth Innovation Promotion Association CAS (2016194) for financial support. The authors acknowledge beamline BL16B1 (Shanghai Synchrotron Radiation Facility) for providing beam time. We greatly appreciate the kind help from Prof. Zhenggang Lan for computational calculations by the ωB97XD method with tuning of different ω values.

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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.7b02495. Experimental details of device fabrication and characterizations, TGA analysis, absorption spectra and electrochemical measurements, SCLC data, and structure data from GIWAXS measurements (PDF)

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Article

AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected] (X.B.). *E-mail: [email protected] (R.Y.). ORCID

Zhi-Guo Zhang: 0000-0003-4341-7773 Yongfang Li: 0000-0002-2565-2748 Xichang Bao: 0000-0001-7325-7550 Mingliang Sun: 0000-0002-6245-3844 Renqiang Yang: 0000-0001-6794-7416 Author Contributions

Yonghai Li and Deyu Liu contributed equally to this paper. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 8255

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