Improving Active Layer Morphology of All-Polymer Solar Cells by

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Improving Active Layer Morphology of All-Polymer Solar Cells by Dissolving the Two Polymers Individually Ning Wang,†,‡ Xiaojing Long,† Zicheng Ding,*,† Jirui Feng,§ Baojun Lin,§ Wei Ma,§ Chuandong Dou,† Jun Liu,*,† and Lixiang Wang† †

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State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Science and Technology of China, Hefei 230026, P. R. China § State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China S Supporting Information *

ABSTRACT: All-polymer solar cells (all-PSCs) use a blend of polymer donor and polymer acceptor as the active layer. The active layer morphology in all-PSCs cannot be effectively tuned by conventional methods. Here, we report a simple method to improve the active layer morphology of all-PSCs, i.e., dissolving polymer donor and polymer acceptor individually and then blending them immediately before spin-coating (DI method). Compared with the regular method of dissolving two polymers together (DT method), the DI method leads to enhanced polymer ordering and higher domain purity due to the partially preserved polymer aggregates from solution. With poly[4-(5-(4,8-bis(5-(dodecylthio)thiophen-2-yl)-6-methylbenzo[1,2-b:4,5b′]dithiophen-2-yl)thiophen-2-yl)-5,6-difluoro-2-(2-hexyldecyl)-7-(5-methylthiophen-2-yl)-2H-benzo[d]1,2,3-triazole] (J61) as donor and poly[5,10-bis(2-decyltetradecyl)-2-(5-(2,5-difluoro-4-(5-methylthiophen-2-yl)phenyl)thiophen-2-yl)-4,4,9,9-tetrafluoro-7-methyl-4,5,9,10-tetrahydro-3a,5,8a,10-tetraaza-4,9-diborapyrene-3a,8a-diium-5,11-diuide] (PBN-10) as acceptor, the power conversion efficiency of all-PSC device increases from 5.36% with the DT method to 7.09% with the DI method at a high open-circuit voltage of 1.24 V. This DI method is also applicable for other polymer donor/polymer acceptor systems and is expected to promote the development of all-PSCs.

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INTRODUCTION Organic solar cells (OSCs), which use a blend of organic/ polymer electron donor and organic/polymer electron acceptor as active layer, are a promising photovoltaic technology due to their advantages of low-cost solution processing and flexibility.1−7 The photocurrent generation process is sensitive to the phase separation morphology of the active layer, and researchers have to carefully optimize the active layer morphology to obtain excellent photovoltaic performance in OSCs.8−11 Several methods have been used to manipulate the active layer morphology, such as thermal annealing,12,13 solvent annealing,14−16 solvent vapor annealing,17−20 and addition of high-boiling-point solvent additive.21−23 These methods contribute greatly to the dramatic enhancement of OSC device efficiency during the past two decades. When both polymer donor and polymer acceptor are used in OSCs, the resulting devices are regarded as all-polymer solar cells (all-PSCs).24−26 Compared with the typical OSCs containing organic small molecules, all-PSCs have received particular attention because of their merits of superior thermal stability and excellent mechanical properties. The power conversion efficiency (PCE) of all-PSCs has increased from lower than 2% to exceeding 10% recently.27−38 However, the © XXXX American Chemical Society

manipulation of active layer morphology in all-PSCs is different from that in typical OSCs containing small organic molecules.39−41 On the one hand, conventional methods, e.g., thermal annealing and solvent annealing, do not work well in all-PSCs. There are very few reports on controlling the active layer morphology of all-PSCs through thermal annealing or solvent annealing.34,42 On the other hand, as observed by Huang et al., the active layer morphology and device performance of all-PSCs are strongly affected by the aggregation properties of the conjugated polymer in solution.43,44 It is may due to the much lower diffusion coefficient of the long polymer chains in solution than that of the small organic molecules. During the rapid film-forming process in spin-coating, as polymer chains diffuse slowly, the nonaggregated or stacked states of polymer chains in solution can be partially preserved in the resulting thin film. Moreover, it is difficult for the polymer chains to move for further phase separation in the solid film because of the large size and possible entanglement of polymer chains. Therefore, to Received: January 10, 2019 Revised: February 26, 2019

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DOI: 10.1021/acs.macromol.9b00057 Macromolecules XXXX, XXX, XXX−XXX

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method. The LUMO/HOMO energy levels were determined to be −3.08/−5.32 eV for J61 and −3.42/−5.81 eV for PBN10, respectively (Figure S1). Thus, both the LUMO and HOMO energy level offsets between the polymer donor and the polymer acceptor are >0.3 eV, which ensures the efficient photoinduced hole/electron transfer and enable operation of all-PSCs.55,56 Aggregation Behaviors in Solution. To investigate the aggregation behaviors of J61 and PBN-10 in solution, we measured their UV−vis absorption spectra in dilute chlorobenzene solution at various temperature. As shown in Figure 1b, J61 shows two absorption peaks at 551 and 587 nm in solution at 20 °C. The absorption peak at long wavelength is assigned to the aggregation of J61 chains.45 As the temperature increases, the long wavelength absorption peak is blue-shifted and becomes slightly less intensive. This indicates strong aggregation of J61 chains in the solution irrespective of the temperature. The absorption spectrum of PBN-10 in dilute chlorobenzene solution at 20 °C is dominated by the two peaks at 567 and 623 nm (Figure 1c). With the temperature increased from 20 to 90 °C, the intensity of the long wavelength absorption peak at 623 nm decreases dramatically. The temperature-dependent absorption spectra indicate that PBN-10 chains aggregate at 20 °C and tend to deaggregate at high temperature.57 The active layers of all-PSC devices are spin-coated with the concentrated solution of the two polymers at room temperature. On the basis of the temperature-dependent absorption spectra, we believe that both J61 and PBN-10 form aggregates in the solution used for the all-PSCs device fabrication. Figure 1d shows the absorption spectra of the two polymers in thin film. J61 and PBN-10 exhibit the overlapped absorption spectra. The optical bandgaps of J61 and PBN-10 are 1.94 and 1.90 eV, respectively. Both J61 and PBN-10 show red-shift absorption spectra and enhanced long wavelength absorption peak in thin film compared to in solution, suggesting close stacking of polymer chains in thin film. The blend films show the overlapping spectra of J61 and PBN-10 with the strong long wavelength absorption peak at 606 nm, indicating the strong aggregation of the polymer chains in blend films. The blend film from DI method exhibits much higher absorption coefficient than that from the DT method (Figure 1e), which benefits the light absorption of the active layer in the allPSC device. Photovoltaic Performance. To study the effect of solution preparation methods on the all-PSC device performance, we fabricated the device with the structure of ITO (indium tin oxide)/PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate))/active layer/Ca/Al. The active layer was deposited by spin-coating the J61:PBN10 blend solution, which was prepared from the DT or DI method. For the DT method, J61 and PBN-10 were dissolved together in chloroform (CF) with a total concentration of 8 mg mL−1, and the blending solution was stirred at room temperature for 12 h before spin-coating. For the DI method, J61 and PBN-10 were individually dissolved in CF with a concentration of 8 mg mL−1 for each polymer, and both solutions were stirred at room temperature for 12 h. Before spin-coating, the two solutions were blended and slightly shaken for several minutes. The current density−voltage (J−V) plots of the all-PSC devices are shown in Figure 2a, and the corresponding photovoltaic parameters are summarized in Table 1. The device with regular DT method shows an open-

improve the device performance, researchers need to develop new methods to manipulate the active layer morphology of allPSCs. In this article, we improve the active layer morphology and enhance the photovoltaic performance of all-PSCs by controlling the solution preparation methods for device fabrication. The regular method is to dissolve polymer donor and polymer acceptor together to get a well-intermixed blend solution (referred to as the DT method, Scheme 1a). Our new Scheme 1. Schematic Procedures of (a) DT and (b) DI Methods To Prepare the Active Layers for All-PSCs

method is to dissolve the two polymers individually and then mix the two solutions immediately before spin-coating (referred to as the DI method, Scheme 1b). In this case, both polymers have formed aggregates in their individual solutions. When mixed together, because of the low diffusion coefficient of polymer chains, these aggregates of polymer chains have not been disturbed completely and can be partially preserved in the thin film after spin-coating. It leads to improved domain purity and enhanced polymer ordering in the active layer, resulting in superior photovoltaic performance of all-PSCs. We select poly[4-(5-(4,8-bis(5-(dodecylthio)thiophen-2-yl)6-methylbenzo[1,2-b:4,5-b′]dithiophen-2-yl)thiophen-2-yl)5,6-difluoro-2-(2-hexyldecyl)-7-(5-methylthiophen-2-yl)-2Hbenzo[d]1,2,3-triazole] (J61)45 as the polymer donor and poly[5,10-bis(2-decyltetradecyl)-2-(5-(2,5-difluoro-4-(5methylthiophen-2-yl)phenyl)thiophen-2-yl)-4,4,9,9-tetrafluoro-7-methyl-4,5,9,10-tetrahydro-3a,5,8a,10-tetraaza-4,9-diborapyrene-3a,8a-diium-5,11-diuide] (PBN-10)46 as the polymer acceptor to fabricate all-PSCs. While the device based on the regular DT method shows the PCE of 5.36%, the all-PSC device with the DI method exhibits an enhanced PCE of 7.09% at a high open-circuit voltage (VOC) of 1.24 V. It is worthy to mention that this PCE is the highest for OSCs with VOC higher than 1.2 V.47−54 This work provides a simple and effective method to manipulate active layer morphology and improve photovoltaic performance of all-PSCs.

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RESULTS AND DISCUSSION Materials. The commercially obtained J61 with a numberaverage molecular weight (M n ) of 24.5 kDa and a polydispersity (PDI) of 2.37 was used. PBN-10 (Mn = 46.1 kDa, PDI = 1.70) was synthesized in our laboratory according to the previously reported method.46 The lowest unoccupied molecular orbital (LUMO) energy levels and the highest occupied molecular orbital (HOMO) energy levels of the two polymers were estimated by using the cyclic voltammetry B

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Figure 1. (a) Chemical structures of J61 and PBN-10 and the temperature-dependent UV−vis absorption spectra of (b) J61 and (c) PBN-10 in dilute solution. UV−vis absorption spectra of (d) pure J61 and PBN-10 films and (e) J61:PBN-10 blend films with different solution preparing methods.

of the all-PSCs based on J61:PBN-10 blends with different solution preparation methods were studied to explain their distinct difference in device performance. The hole and electron mobilities were estimated by the space-charge limited current (SCLC) method. The dark J−V plots of the hole-only and electron-only devices and the fitted curves are shown in Figure S2, and the mobilities are summarized in Table S1. The J61:PBN-10 blend with the DT method shows a hole mobility (μh) of 6.05 × 10−5 cm2 V−1 s−1 and an electron mobility (μe) of 3.16 × 10−4 cm2 V−1 s−1, corresponding to a μh/μe value of 0.19. In comparison, the μh and μe are improved to 1.83 × 10−4 and 5.88 × 10−4 cm2 V−1 s−1, and the μh/μe value increases to 0.31 for the J61:PBN-10 blend with the DI method. The higher and more balanced hole/electron mobilities in the active layer with the DI method can efficiently improve the charge sweep-out and suppress the charge recombination and are consistent with the high FF in the related all-PSC device. The photocurrent density (Jph) versus effective voltage (Veff) plots of the two all-PSC devices were measured to evaluate the charge generation and collection efficiency (Figure 3a).58,59 The Jph grows as the Veff increases, but it is not fully saturated at high voltage for both the all-PSC devices. The device with the DI method shows a Jph,SC/Jph,sat value of 85.4% (Jph,SC is the

circuit voltage (VOC) of 1.24 V, a short-circuit current density (JSC) of 8.63 mA cm−2, and a fill factor (FF) of 50.1%, corresponding to a PCE of 5.36%. This photovoltaic performance is in accordance with our previous reports.46 In contrast, the device with the DI method shows a VOC of 1.24 V, a JSC of 10.12 mA cm−2, and a FF of 56.5%, leading to a PCE as high as 7.09%. This PCE value is 1.3 times higher than that of the device with the DT method. Figure 2b shows the external quantum efficiency (EQE) curves of the two devices. Compared with the device with DT method, the device with the DI method displays higher EQE values across the range from 400 to 600 nm and exhibits the maximum EQE value of 0.62 at 552 nm. These results suggest that the solution preparation methods play an important role in the all-PSC device performance of the J61:PBN-10 blend. In addition, we would like to emphasize that the PCE of over 7% is among the highest values reported for OSCs with the VOC higher than 1.2 V.49,50 The all-PSC device of J61:PBN-10 blend only utilizes the sunlight in the 300−670 nm region and does not absorb the near-infrared light, so this device should be very suitable for the front cell in tandem solar cells. Charge Generation, Transport, and Recombination. The charge generation, transport, and recombination behaviors C

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Figure 3. (a) Jph−Veff plots and (b) light-intensity-dependent JSC of the all-PSC devices based on J61:PBN-10 blend with different solution preparing methods.

Figure 2. (a) J−V plots and (b) EQE curves of the all-PSCs based on J61:PBN-10 blend with different solution preparing methods.

Table 1. Photovoltaic Performance of the All-PSCs Based on the J61:PBN-10 Blend with Different Solution Preparing Methods methods

VOC (V)

JSC (mA cm−2)

FF (%)

PCEmax (PCEav)a (%)

DT DI

1.24 1.24

8.63 10.12

50.1 56.5

5.36 (5.28) 7.09 (7.02)

with the DI method. However, compared with the highly efficient PSCs, the unsaturated Jph and the relatively low Jph,SC/ Jph,sat value suggest the inefficient charge collection for the allPSCs based on the J61:PBN-10 blend, which results in the low FF. Active Layer Morphology. To uncover the reasons behind the device performance enhancement with the DI method, we studied the phase separation morphology of the J61:PBN-10 active layers using several methods, including twodimensional grazing incidence wide-angle X-ray scattering (2D-GIWAXS), resonant soft X-ray scattering (R-SoXS), photoluminescence quenching, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Figure 4 shows the 2D-GIWAXS patterns of the pure J61 film, the pure PBN-10 film, and the J61:PBN-10 blend films with the DT or DI method. The corresponding data of the GIWAXS patterns are summarized in Table 2. The pure J61 film shows strong (100) reflection in the in-plane direction and strong (010) reflection in the out-of-plane direction, indicating the face-on orientation of polymer backbone relative to the substrate.45 The pure PBN-10 film displays strong (100), (200), and (300) lamellar reflections and moderate (010) π−π stacking reflection, indicating the high crystallinity of PBN-10. The lamellar reflections and the π−π stacking reflection mainly locate in the out-of-plane direction with an angular distribution, indicating the relative random molecular orientation of PBN-10 polymer backbone.46,62 The blend

a

The average PCE values were obtained from 16 devices.

Jph under short-circuit condition, and the Jph,sat was chosen from the Jph at a Veff of 4 V with a value of 10.10 mA cm−2), which is higher than the device with the DT method (Jph,sat = 9.21 mA cm−2, Jph,SC/Jph,sat = 81.5%). The enhanced charge collection efficiency of the device with DI method may result from the high and balanced hole/electron mobilities. The bimolecular recombination behaviors were evaluated by the light intensity (I)-dependent JSC, where JSC and I follow the power law JSC ∝ Iα (α is the power-law exponent).60,61 As shown in Figure 3b, the α values of the fitted line for the allPSC devices with DT and DI methods are 0.93 and 0.95, respectively. The larger α value indicates the free charge carriers are more efficiently swept out and collected prior to recombination for the all-PSC device with the DI method, which agrees well with the high and balanced hole/electron mobilities in the active layer with the DI method. The improved charge generation, transport, and collection are consistent with the enhanced JSC and FF in the all-PSC device D

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Figure 4. 2D-GIWAXS of (a) pure J61 film and (b) pure PBN-10 film and J61:PBN-10 blend films with different solution preparing methods: (c) DT and (d) DI. (e) 1D linecuts of the corresponding 2D-GIWXAS patterns in the in-plane and out-of-plane directions.

Table 2. 2D-GIWAXS Characterization Data of the Pure J61 Film, Pure PBN-10 Film, and J61:PBN-10 Blend Films with Different Solution Preparing Methods 100 films J61 PBN-10 blend with DT blend with DI

directions out-of-plane in-plane out-of-plane in-plane out-of-plane in-plane out-of-plane in-plane

−1

location [Å ]

d-spacing [Å]

010 −1

fwhm [Å ]

CL [Å]

0.24 0.26 0.26

25.82 24.28 23.99

0.086 0.038 0.043

66 148 131

0.26

24.42

0.056

101

0.26

24.22

0.046

122

−1

location [Å ]

d-spacing [Å]

fwhm [Å−1]

CL [Å]

1.74

3.62

0.185

31

1.73 1.73 1.72

3.64 3.63 3.64

0.135 0.115 0.165

42 49 34

1.73

3.64

0.159

36

Figure 5. (a) R-SoXS profiles and AFM height images of J61:PBN-10 blend films with different solution preparing methods: (b) DT and (c) DI. TEM images of J61:PBN-10 blend films with different solution preparing methods: (d) DT and (e) DI.

Å−1 (d= 3.64 Å) in the out-of-plane direction is attributed to the overlapped π−π stacking of J61 and PBN-10. The CL of (100) lamellar and (010) π−π stacking are 101 and 34 Å for the blend film with the DT method. In comparison, the CL of (100) lamellar and (010) π−π stacking increase to 122 and 36 Å for the blend film with the DI method. These results suggest that the two polymers possess higher crystallinity in the active

films with different processing methods show similar reflection peak locations but different coherence lengths (CL) (Figure 4c,d). Both the blend films show weak (100) and (200) reflection peaks of PBN-10 in the in-plane direction but without the (300) reflection peak, indicating that the crystallinity of PBN-10 is somewhat suppressed in the blend films. The (010) reflection peak of the blend film at q of 1.73 E

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dicarboximide-2,6-diyl)-alt-5,5′-(2,2-bithiophene)] (N2200)31,69 (Figure S4) blends were fabricated. The J−V plots of the all-PSC devices based on these two blends with DT and DI methods are shown in Figure 6, and the

layer processed with the DI method in comparison with the DT method. This improved crystallinity is consistent with the improved charge carrier mobilities of the active layer processed with the DI method. Figure 5a displays the R-SoXS profiles of the two active layers. The blend film prepared with the DT method shows a low-q peak at 0.076 nm−1 and a high-q peak at 0.41 nm−1 corresponding to two domain sizes of 41 and 7.7 nm, respectively. The former peak is assigned to the polymer donor and/or polymer acceptor-rich domains, and the latter peak can be ascribed to the fine phase separation inside these domains.63 For the blend film with the DI method, there are three scattering peak at 0.016, 0.067, and 0.44 nm−1, which correspond to domain sizes of 196, 47, and 7.1 nm, respectively. It suggests that large phase domains and complex morphology are formed for the active layer prepared with the DI method. Most importantly, the relative domain purity of the active layer increases from 0.71 with the DT method to 1.00 with the DI method. The larger domain size and higher domain purity of the active layer with the DI method than that of the DT method are consistent with the slightly decreased fluorescence quenching efficiency (Figure S3). The AFM height images of the two active layers are shown in Figure 5b,c. Both the active layers show fibrous networks of the polymers. Homogeneously distributed small fibrils are observed in the film with the DT method, which gives a rootmean-square (rms) roughness of 0.92 nm. For the film with the DI method, the fibrils tend to form distinguishable bundles and the rms roughness slightly increases to 1.18 nm. The TEM images are shown in Figure 5d,e. Homogenous fibrous networks are observed in the blend film with the DT method, while the fibrils aggregate into small grains with the size of 40−80 nm in the blend film with the DI method, which are consistent with that in the AFM images. All the above results indicate that the J61:PBN-10 active layers processed with the DT and DI methods display different phase separation morphology. It is believed that the solution preparation process before spin-coating can affect the active layer morphology of polymer blends.64−66 Huang et al. have reported that aggregation of polymer donors or polymer acceptors are important for the high-performance all-PSCs.43 In this work, though both J61 and PBN-10 exhibit aggregation in solution, the diffusion and intermixing of the aggregated polymer chains in solution should be very slow due to the overlapping and entanglement of the long polymer chains. Thus, when J61 and PBN-10 are dissolved individually and mixed instantly before spin-coating, the aggregated J61 and PBN-10 chains are unable to deaggregate and intermix completely in a short time.67 The aggregation state of the polymer chains in solution is partially preserved in the thin film. Therefore, the blend film prepared by the DI method exhibits higher phase domain purity and larger phase domain size than those prepared by the regular DT method, which leads to the enhanced photovoltaic performance of the resulting all-PSC devices. Generality of the DI Method. To demonstrate the generality of the DI method, all-PSCs based on poly[5,6difluoro-2-(2-hexyldecyl)-4-(5-(6-methyl-4,8-bis(5-(tripropylsilyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophen-2-yl)thiophen-2-yl)-7-(5-methylthiophen-2-yl)-2H-benzo[d]1,2,3triazole] (J71):68PBN-10 and poly[(ethylhexyl-thiophenyl)benzodithiophene-(ethylhexyl)thienothiophene] (PTB7Th):poly[(N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalene-

Figure 6. J−V plots of the all-PSCs based on (a) J71:PBN-10 and (b) PTB7-Th:N2200 blends with different solution preparing methods.

corresponding photovoltaic parameters are summarized in Table S2. For both blends, the all-PSC devices with the DI method show improved JSC and FF. As a result, the J71:PBN10 and PTB7-Th:N2200 based all-PSC devices with DI method exhibit PCEs of 5.78% and 4.75%, which is higher than those with DT method (the PCEs are 5.13% and 4.02%). These results indicate that the DI method is a general approach to optimize the active layer morphology and boost the device performance of all-PSCs.

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CONCLUSIONS In summary, we have developed a simple method to improve the active layer morphology and enhance the photovoltaic performance of all-PSCs. In this DI method, the polymer donor and polymer acceptor are dissolved individually and then blended immediately before spin-coating. Owing to the low diffusion coefficients of polymer chains in solution, the aggregated state of polymer chains in solution are partially preserved in the thin film with the DI method. Compared with the regular DT method, the DI method leads to enhanced polymer ordering and higher domain purity, which improve the charge transport and suppress the charge recombination. The resultant all-PSC device based on J61:PBN-10 blend from F

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the DI method exhibits a PCE of 7.09%, which represents a 32% increase compared with the device from the DT method. The DI method is applicable for different polymer donor/ polymer acceptor systems and explores an new approach toward high-efficiency all-PSCs.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00057.

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Characterization of J61 and PBN-10, all-PSC device fabrication, hole- and electron-only devices fabrication and mobility measurements, 2D-GIWAXS and R-SoXS measurements (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.D.). *E-mail: [email protected] (J.L.). ORCID

Xiaojing Long: 0000-0002-9899-1820 Wei Ma: 0000-0002-7239-2010 Jun Liu: 0000-0003-1487-0069 Lixiang Wang: 0000-0002-4676-1927 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (No. 21625403, 21875244, 51873204, 21761132020, 21504066, and 21534003), the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDB12010200), and Jilin Scientific and Technological Development Program (No. 20170519003JH). Z.D. thanks the Youth Innovation Promotion Association of Chinese Academy of Sciences. W.M. is thankful for the support from the Ministry of Science and Technology (No. 2016YFA0200700). 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 DE-AC0205CH11231. The authors 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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DOI: 10.1021/acs.macromol.9b00057 Macromolecules XXXX, XXX, XXX−XXX