Simultaneously Enhanced Efficiency and Stability of Polymer Solar

Research Article www.acsami.org

Simultaneously Enhanced Efficiency and Stability of Polymer Solar Cells by Employing Solvent Additive and Upside-down Drying Method Qianqian Sun,† Fujun Zhang,*,† Qiaoshi An,† Miao Zhang,† Xiaoling Ma,† and Jian Zhang*,‡ †

Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing 100044, People’s Republic of China ‡ School of Material Science and Technology, Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, 1 Jinji Road, Guilin, Guangxi 541004, People’s Republic of China S Supporting Information *

ABSTRACT: The morphology of active layer plays an important role in determining the power conversion efficiency (PCE) and stability of polymer solar cells (PSCs), which strongly depend on the dynamic drying process of active layer. In this work, an efficient and universal method was developed to let active layer undergo upside-down drying process in a covered glass Petri dish. For the PSCs based on PTB7Th:PC71BM, the champion PCEs were improved from 8.58% to 9.64% by mixing 3 vol % 1,8-di-iodooctane and further to 10.30% by employing upside-down drying method. The enhanced PCEs of PSCs with active layers undergoing upside-down drying process are mainly attributed to the optimized vertical phase separation, the more ordered and tightly packed π−π stacking of polymer molecules. Meanwhile, PC71BM molecules can be frozen in more ordered and tightly packed π−π stacking polymer network, which lead to the enhanced stability of PSCs. The universality of upside-down drying method can be solidly confirmed from PSCs with PTB7:PC71BM, PffBT4T2OD:PC71BM, or PBDT-TS1:PC71BM as active layers, respectively. The molecular packing and phase separation of blend films with different solvent additives and drying methods were investigated by grazing incidence X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. KEYWORDS: polymer solar cells, efficiency, stability, upside-down, drying method

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layers.23,24 The solution processed interfacial layers can markedly improve the performance of PSCs, the underlying mechanisms should be attributed to the enhanced charge collection by interfacial dipole layer and the optimized vertical phase separation assisted by volatilization of solvent or solvent additive.25−27 In addition, some strategies were developed to suppress the morphology degradation of active layers such as improving organic materials purity,28,29 using nonfullerene acceptors,30,31 and mixing third components.32,33 Therefore, an ideal morphology of active layer is prerequisite for achieving highly efficient and stable PSCs. In this work, an efficient and simple drying method of blend film was developed to improve the PCE and stability of PSCs, named as upside-down drying method. After conventional spincoating process of blend film, the pristine blend film was rapidly placed in a covered glass Petri dish with the top surface downward to let the pristine blend film undergo upside-down drying process. During upside-down drying process, donor

INTRODUCTION Polymer solar cells (PSCs) have become one of the potential candidates to generate electricity utilizing solar energy.1−3 Very recently, power conversion efficiencies (PCEs) of PSCs have exceeded 12% at a laboratory-scale through lots of efforts from material synthesis and device physics.4−6 The long stability of PSCs is still a great challenge for academic and industrial purposes.7−9 The morphology of active layer plays an important role in determining the efficiency and stability of PSCs, which can be finely optimized by adjusting the dynamic drying process of active layer. Various efficient strategies were developed to adjust the morphology of active layer such as using solvent additive, employing thermal annealing, and adopting solvent vapor annealing treatment.10−15 Among the various strategies, it has been demonstrated that using high boiling point (BP) solvent additive is a widely used strategy to improve the efficiency of PSCs.16−18 However, the residual solvent additive in active layer may lead to the instability of PSCs because fullerenes derivatives are more willing to aggregate during the slow volatilization of solvent additive.19−22 It has been confirmed that the stability of PSCs can be improved by removing the residual solvent additive from active © 2017 American Chemical Society

Received: January 11, 2017 Accepted: February 23, 2017 Published: February 23, 2017 8863

DOI: 10.1021/acsami.7b00510 ACS Appl. Mater. Interfaces 2017, 9, 8863−8871

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Chemical structures of used organic materials and solvent additives; (b) schematic diagram of active layer drying methods; (c) schematic configuration of PSCs.

Figure 2. J−V curves of PSCs based on PTB7-Th:PC71BM without or with solvent additive, (a) employing conventional drying method; (b) employing upside-down drying method.

undergoing upside-down drying process exhibited good stability compared to the PSCs with active layers undergoing conventional drying process.

molecular packing and acceptor molecular redistribution can be optimized to improve the PCE and stability of PSCs, which were solidly confirmed from four kinds of PSCs with PTB7Th:PC71BM, PTB7:PC71BM, PffBT4T-2OD:PC71BM, or PBDT-TS1:PC71BM as active layers. To clarify the working mechanism of upside-down drying method, three kinds of solvent additives (1,8-diiodooctane (DIO), p-anisaldehyde (AA), and N-methylpyrrolidone (NMP)) and different drying methods were synergistically employed in PTB7-Th:PC71BM system. The AA and NMP are nonhalogenated green solvents, which have lower BP (248 and 204 °C) than that of DIO (333 °C).34−36 The chemical structures of used organic materials, solvent additives, schematic diagram of active layer drying methods, and schematic configuration of PSCs are shown in Figure 1. During upside-down drying process, donor molecules prefer to form the more ordered and tightly packed π−π stacking network in the active layers. Therefore, the hole hopping transport between molecules can be markedly enhanced by the decreased intermolecular distance, which can be further demonstrated from the enhanced hole mobility based on the corresponding hole-only devices. Meanwhile, the joint effect between the gravity and driving force of solvent and solvent additive volatilization on PC71BM can further optimize its redistribution to form an appropriate vertical phase separation. The appropriate vertical phase separation may be frozen to improve the stability of PSCs. The discussions can be well supported by the experimental results of grazing incidence X-ray diffraction (GIXD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The champion PCEs of PSCs arrived to 10.30%, 10.42%, or 10.20% for PTB7-Th:PC71BM, PffBT4T-2OD:PC71BM, or PBDTTS1:PC71BM as active layers undergoing upside-down drying process. After 500 h of storage, all the PSCs with active layers

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RESULTS AND DISCUSSION

The dynamic drying process of active layer plays a crucial role in optimizing morphology, which strongly determines the performance of PSCs. A series of PTB7-Th:PC71BM based PSCs was fabricated; three typical solvent additives (DIO, AA, or NMP) and different drying methods (upside-down and conventional) were employed to adjust the morphology of active layer. The current density−voltage (J−V) curves of PSCs were measured under 1 sun AM 1.5 G illumination with the light intensity of 100 mW cm−2, as shown in Figure 2. For the active layers undergoing conventional drying process, the PCEs of PSCs were markedly improved by mixing appropriate solvent additive, as shown in Figure 2a. The PSCs with 3 vol % DIO exhibited a champion PCE of 9.64%, corresponding to an approximate 12.4% PCE improvement compared with the PSCs without solvent additive. The similar phenomenon was also observed from the PSCs with active layers undergoing upside-down drying process. The PCE improvement of PSCs was commonly reported by mixing solvent additive, and the underlying reasons are mainly attributed to the optimized molecular packing and phase separation during slow volatilization of solvent additive.16−18 It is worthwhile to highlight that the PCEs of PSCs without or with different solvent additives were markedly enhanced by employing upside-down drying method. The champion PCEs of the PSCs without solvent additive were slightly improved from 8.58% to 8.95% by employing upside-down drying method. The champion PCEs of PSCs were improved from 9.64% to 10.30% for the active layers with DIO as solvent additive, from 9.62% to 10.01% for 8864


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Table 1. Photovoltaic Parameters of PSCs Based on PTB7-Th:PC71BM without or with Solvent Additive and Employing Different Drying Methods PCE (%) drying method conventional

upside-down

a

solvent additive without 3 vol % 1 vol % 3 vol % without 3 vol % 1 vol % 3 vol %

DIO AA NMP DIO AA NMP

JSC (mA cm−2)

VOC (V)

FF (%)

best

averagea

17.34 17.91 17.92 18.15 17.69 18.87 18.47 18.40

0.83 0.80 0.80 0.79 0.83 0.80 0.80 0.80

59.59 67.29 67.13 65.43 60.93 68.21 67.73 66.42

8.58 9.64 9.62 9.38 8.95 10.30 10.01 9.77

8.51 9.59 9.56 9.28 8.86 10.17 9.95 9.57

Average values were calculated according to 20 cells.

Figure 3. (a) EQE spectra of PSCs based on PTB7-Th:PC71BM without or with 3 vol % DIO; (b) absorption spectra of corresponding blend films, the inset is the zoomed-in absorption spectra in long wavelength range.

Figure 4. (a) Out-of-plane and (b) in-plane GIXD profiles of PTB7-Th:PC71BM blend films without or with 3 vol % DIO undergoing different drying processes, the inset image is zoomed-in GIXD profiles.

confirmed from the four group’s independent experimental results. The PCE improvement of PSCs mainly arose from the increased JSC and FF, which may be attributed to the optimized donor molecular packing and acceptor molecular redistribution during upside-down drying process of active layers. The external quantum efficiency (EQE) spectra of PSCs without or with solvent additive were measured and are shown in Figure 3a and Figure S3. It is apparent that the EQE of PSCs can be enhanced by mixing solvent additive or employing upside-down drying method, especially in long wavelength range. The absorption spectra of neat PTB7-Th and PC71BM films were measured and are shown in Figure S4; neat PTB7Th film exhibits a strong absorption in long wavelength range. To better investigate the effect of upside-down drying method

the active layers with AA as solvent additive, and from 9.38% to 9.97% for the active layers with NMP as solvent additive by employing upside-down drying method, respectively. On the basis of the 20 cells for each kind of PSCs, the photovoltaic parameters of all PSCs are summarized in Table 1. To further exhibit solid experimental results, the J−V curves of 20 optimized PSCs with 3 vol % DIO and employing upsidedown drying method are shown in Figure S1. The photovoltaic parameters and the statistical histogram are summarized in Table S1 and in Figure S2. The average values of PCE, short circuit current (JSC), open circuit voltage (VOC), and fill factor (FF) based on 20 optimized PSCs were 10.17%, 18.77 mA cm−2, 0.80 V, and 68.04%, respectively. The positive effect of upside-down drying method on PCE can be adequately 8865


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Figure 5. (a) XPS survey scans of PTB7-Th:PC71BM blend films without or with 3 vol % DIO undergoing different drying processes; (b) C/S atomic ratios and weight ratios of PC71BM to PTB7-Th near the top surface of blend films without or with 3 vol % DIO undergoing different drying processes.

drying method. It is known that charge transport along backbone and π−π stacking directions is considerably more efficient than that along lamellar structure direction.43 It is worth noting that the positions of (010) diffraction peaks along out-of-plane and in-plane directions were shifted to higher diffraction vector (q) values by employing upside-down drying method. The higher q values of (010) diffraction peaks indicate smaller d-spacing distances of π−π stacking, as listed in Table S2.44−46 Therefore, the more ordered lamellar structure and tightly packed π−π stacking of PTB7-Th can be enhanced for the active layer with solvent additive undergoing upside-down drying process, which can further improve the JSC and FF of corresponding PSCs. To investigate the effect of solvent additive and upside-down drying method on PC71BM redistribution in blend films, the XPS survey scans of blend films were measured and are shown in Figure 5a. The composition variation near the top surface of blend films could be estimated according to the atomic ratio of carbon/sulfur (C/S) because S atom is the characteristic element of PTB7-Th. On the basis of the XPS survey scans, the C/S atomic ratios near the top surface of blend films can be calculated according to the peak areas and sensitivity factors (0.11 for C 1s, 0.20 for S 2p). The weight ratios of PC71BM to PTB7-Th near the top surface of blend films can be calculated according to the C/S atomic ratios. The detailed calculations are described in the Supporting Information. The calculated C/ S atomic ratios and the weight ratios of PC71BM to PTB7-Th near the top surface of blend films are exhibited in Figure 5b. For the blend films undergoing conventional drying process, the weight ratios of PC71BM to PTB7-Th near the top surface of blend films were slightly decreased from 0.97 to 0.86 by mixing 3 vol % DIO, which is less than the real mixing weight ratio of 1.5 in the blend solution. It indicates that PC71BM molecules may be more willing to migrate toward the down surface during slow drying process of active layer.47,48 Lee and co-workers demonstrated that surface energy of material plays the key role in determining vertical phase separation in organic blend films.49,50 According to the principle of total system energy minimization, materials with low surface energy are more willing to migrate toward top (air) surface, and materials with high surface energy are more willing to migrate toward down surface. To investigate the effect of surface energy on vertical phase separation, the water contact angles (WCAs) of neat PTB7-Th and PC71BM films were measured and are exhibited in Figure S7. The WCAs of neat PTB7-Th and

on the performance of PSCs, the following discussions focus on the PSCs without or with 3 vol % DIO. According to Figure 3a, the EQE of PSCs were enhanced in long wavelength range by mixing 3 vol % DIO or employing upside-down drying method, especially for the active layers employing the combined method. The enhanced EQE should be attributed to improved photon harvesting dependent on the optimized molecular packing. The absorption spectra of PTB7-Th:PC71BM blend films were measured and are shown in Figure 3b. The absorption intensity of PTB7-Th in blend films was enhanced by mixing 3 vol % DIO and employing upside-down drying method or the combined method. To clearly observe the enhanced absorption intensity, the zoomed-in absorption spectra are exhibited in the inset of Figure 3b. The enhanced absorption intensity of PTB7-Th in blend films can be confirmed from the almost overlapped absorption spectra in the short wavelength range, corresponding to the absorption of PC71BM. The enhanced absorption intensity may be attributed to the more ordered PTB7-Th molecular packing during prolonged drying process due to mixing high BP solvent additive and employing upside-down drying method.37−39 The similar phenomenon can be observed from the blend films with AA or NMP as solvent additive, as shown in Figure S5. To investigate the effect of solvent additive and upside-down drying method on PTB7-Th molecular packing, the GIXD in out-of-plane direction and in-plane direction were performed on all blend films. The corresponding GIXD profiles of blend films without or with 3 vol % DIO are shown in Figure 4, and GIXD profiles of blend films with 1 vol % AA or 3 vol % NMP are exhibited in Figure S6. Both the (100) and (010) diffraction peaks along out-ofplane direction and the (010) diffraction peaks along in-plane direction can be clearly observed in all blend films, indicating that PTB7-Th molecules prefer to edge-on molecular packing in blend films.40 The (100) and (010) diffraction peaks represent the lamellar structure and the π−π packing direction of PTB7-Th, respectively.41,42 The (100) diffraction intensity of blend films was enhanced by mixing solvent additive, indicating that the PTB7-Th lamellar structure become more ordered under the driving force of solvent additive volatilization. The more ordered lamellar structure of PTB7-Th is beneficial to hole transport along the normal direction of active layer. For blend films without or with solvent additive, the diffraction intensities of (010) peaks along out-of-plane and in-plane directions were obviously enhanced by employing upside-down 8866


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ACS Applied Materials & Interfaces PC71BM films are 90.7° and 80.2°, respectively. According to the WCAs, the calculated surface energy is 22.94 mJ cm−2 or 35.95 mJ cm−2 for neat PTB7-Th or PC 71BM films, respectively. It means that PC71BM should be more willing to migrate toward the interface between active layer and substrate. Meng et al. developed a smart strategy to optimize the vertical phase separation by employing a cross-linkable donor polymer as the underlying layer.51,52 Actually, the redistribution of PC71BM during the drying process of active layer may depend on the molecular structures of PC71BM and polymer. The globular structure of PC71BM is beneficial to its migration in the network of polymer chains under the effects of gravity and driving forces of solvent and solvent additive volatilization. The weight ratios of PC71BM to PTB7-Th near the top surface of blend films were increased from 0.97 to 1.17 or from 0.86 to 1.02 for blend films without or with 3 vol % DIO by employing upside-down drying method, respectively. It means that some PC71BM molecules were elevated toward the top surface of blend films during upside-down drying process, which should be attributed to the joint effect of gravity and the driving force of solvent and solvent additive volatilization.53,54 It means that the upside-down drying method should be an efficient strategy to optimize vertical phase separation for better charge transport and collection, resulting in the increased FF of PSCs. It can be envisaged that the morphology of blend films should be affected by the variation of PTB7-Th molecular packing and PC71BM distribution during upside-down drying process, which can be seen from TEM images, as shown in Figure 6 and Figure S8.

the morphology became more homogeneous with the smaller bright or dark domains, which suggested that bicontinuous interpenetrating network and more appropriate phase separation were formed for more efficient exciton dissociation and charge transport. According to the XPS experimental results, vertical phase separation can be optimized during upside-down drying process due to more PC71BM migration toward the top surface of active layer. The synergistic effect of solvent additive and upside-down drying method can simultaneously optimize PTB7-Th molecular packing and PC71BM redistribution, which would result in the more ordered lamellar structure, the more ordered and tightly packed π−π stacking of PTB7-Th, and the more ideal vertical phase separation. To investigate the effect of PTB7-Th molecular packing and PC71BM redistribution on charge transport in active layer, the hole-only and electron-only devices were fabricated with the structures of ITO/ PEDOT:PSS (40 nm)/active layer (100 nm)/MoO3 (10 nm)/Ag (100 nm) and ITO/ZnO (40 nm)/active layer (100 nm)/Al (100 nm). The J−V curves of the hole-only and electron-only devices were measured under dark and are shown in Figure S9. The hole mobilities (μh) and electron mobilities (μe) in the active layers were calculated according to the space charge limited current (SCLC) method, as listed in Table S3. For the active layers without solvent additive, the hole mobilities and electron mobilities were increased from 4.26 × 10−4 to 8.58 × 10−4 cm2 V−1 s−1 and from 3.49 × 10−4 to 7.77 × 10−4 cm2 V−1 s−1 by employing upside-down drying method. For the active layers with 3 vol % DIO, the hole mobilities and electron mobilities were increased from 1.42 × 10−3 to 2.11 × 10−3 cm2 V−1 s−1 and from 1.05 × 10−3 to 1.73 × 10−3 cm2 V−1 s−1 by employing upside-down drying method. The hole mobilities in the active layers without or with solvent additive can be enhanced by employing upside-down drying method, which should be attributed to the more ordered and tightly packed π−π stacking of PTB7-Th. This finding well accords with the observed results from GIXD experiment. The distribution of PC71BM in the active layers can be optimized to form the more continuous channels for electron transport, resulting in the enhanced electron mobilities in the active layers undergoing upside-down drying process. According to the ratios of hole mobility to electron mobility (μh/μe), charge transport should become more balanced in the active layer undergoing upside-down drying process. It means that the more bicontinuous interpenetrating network can be formed in the active layers undergoing upside-down drying process. To freeze the optimized morphology of active layers, PFN methanol solution was spin-coated onto active layers to remove residual solvent and solvent additive.22,25 The optimized and fixed morphology may be beneficial to improve the stability of PSCs. To investigate the stability of PSCs, the J−V curves were repeatedly measured for 500 h in high-purity nitrogen-filled glovebox. According to the J−V curves, the VOC values of all PSCs were almost kept constant with approximate ±0.02 eV fluctuation, as shown in Figure S10. The degradation tendencies of JSC, FF, and PCE were fitted according to the power function (y = α(1 + t)β) as the solid lines exhibited in Figure 7 and Figure S11. Here, the independent variable is storage time (t); the dependent variable (y) is photovoltaic parameter; the coefficient (α) is the initial value of corresponding photovoltaic parameter; and the absolute value of index (β) is the degradation coefficient of corresponding photovoltaic parameter, which reflects the degradation rate.

Figure 6. TEM images of PTB7-Th:PC71BM blend films with different preparation conditions: (a) without solvent additive and employing conventional drying method; (b) with 3 vol % DIO and employing conventional drying method; (c) without solvent additive and employing upside-down drying method; (d) with 3 vol % DIO and employing upside-down drying method.

It is generally believed that bright and dark regions in TEM images represent donor-rich and acceptor-rich domains due to their different electron densities, respectively.55 It is apparent that the bright regions in TEM images of blend films with 3 vol % DIO became more apparent compared with those of blend films without solvent additive, which suggested the more ordered PTB7-Th molecular packing. The observations from TEM images well accord with the GIXD experimental results. For the blend films undergoing upside-down drying process, 8867


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Figure 7. Photovoltaic parameters (a) PCE, (b) JSC, and (c) VOC as a function of storage time (t) (scatters, measured values; solid lines, fitted curves according to the power function y = α(1 + t)β).

Obviously, the corresponding fitted degradation curves well accord with the measured data. The fitted degradation tendency of photovoltaic parameters as a function of time well accords with the reported phenomena in the literature.56−58 The β values of all photovoltaic parameters are listed in Table 2. The

PSCs. Zhan’s group reported that mixing appropriate solvent additive may be an effective strategy to increase the stability of PSCs.32 For the active layers undergoing upside-down drying process, the preserved percentages of PCEs after 500 h storage were 76.9%, 79.0%, 82.9%, and 85.6% for corresponding PSCs without solvent additive, with 3 vol % DIO, 1 vol % AA, or 3 vol % NMP, respectively. The PSCs with solvent additive displayed better stability compared with the PSCs without solvent additive. It should be attributed to the optimized morphology of active layer by mixing solvent additive. For the active layers with solvent additive, the PBT7-Th molecules may prefer to form the more ordered lamellar structure compared with the active layers without solvent additive. Meanwhile, the distribution of PC71BM can be finely tuned in the optimized PTB7-Th network during the volatilization of solvent additive to form the more ideal phase separation. After that, the ideal phase separation can be frozen by spin-coating PFN methanol solution. Meanwhile, the stability of PSCs can be markedly enhanced by employing upside-down drying method. The enhanced stability of PSCs may be attributed to the limited PC71BM aggregation in active layers undergoing upside-down drying process due to the more ordered and tightly packed π−π stacking of PTB7-Th network.60,61 It means that the more ordered and tightly packed PTB7-Th network can freeze PC71BM molecules to impede large aggregation formation. To further confirm the universality of upside-down drying method, the PCE and stability of PSCs were investigated with PTB7:PC 7 1 BM, PffBT4T-2OD:PC 7 1 BM, or PBDTTS1:PC71BM as active layers, respectively. The champion performance of PTB7:PC71BM based PSCs was obtained for the active layers with 3 vol % DIO. The champion performances of PffBT4T-2OD:PC 71 BM or PBDTTS1:PC71BM based PSCs were obtained for the active layers without solvent additive. The J−V curves and PCEs as a function of storage time are shown in Figure 8. The photovoltaic parameters of PSCs are summarized in Table 3. The champion PCEs of PSCs based on PTB7:PC71BM, PffBT4T-2OD:PC71BM, or PBDT-TS1:PC71BM were increased to 7.44% from 7.01%, to 10.42 from 9.66%, and to 10.20% from 9.44% by employing upside-down drying method. The EQE spectra of corresponding PSCs with conventional or upside-down drying process are shown in Figure S12. For the PSCs based on PTB7:PC71BM, PffBT4T-2OD:PC71BM, or PBDT-TS1:PC71BM after 500 h storage, the preserved percentages of PCEs were 62.6%, 70.2%, and 69.1% for the PSCs with active layers undergoing conventional drying process, respectively. Meanwhile, the preserved percentages of PCEs after 500 h storage were increased to 68.8%, 85.1%, and

Table 2. Degradation Coefficients (β) of Photovoltaic Parameters as a Function of Storage Time (t), PCEs after 500 h Storage (PCE*), and Preserved Percentages of PCEs after 500 h Storage (PCE*/PCE)a β (×10−2) drying method conventional

upside-down

a

solvent additive without 3 vol % DIO 1 vol % AA 3 vol % NMP without 3 vol % DIO 1 vol % AA 3 vol % NMP

JSC

FF

PCE

PCE* (%)

PCE*/PCE (%)

1.73 1.14

4.29 3.12

6.30 4.50

5.92 7.12

69.0 73.9

1.07

3.24

4.29

7.21

75.0

0.79

2.22

2.80

7.84

83.6

1.18 0.99

2.72 2.64

4.09 3.67

6.87 8.14

76.9 79.0

0.95

2.00

2.98

8.30

82.9

0.72

2.09

2.55

8.36

85.6

The average values were calculated according to 20 cells.

small β value indicates the slow degradation rate of the corresponding photovoltaic parameter. For all PSCs, the β values of FFs were larger than those of JSC, which indicated that the degradation of PSCs should be mainly attributed to morphology evolution induced by PC71BM aggregation.59,60 The β values of photovoltaic parameters were decreased by mixing solvent additive or employing upside-down drying method, which indicated that the stability of PSC can be enhanced by the individual or combined method. To clarify the effect of solvent additive and upside-down drying method on the stability of PSCs, the PCEs after 500 h storage (PCE*) and the preserved percentages of PCEs after 500 h storage (PCE*/PCE) are summarized in Table 2. For the active layers undergoing conventional drying process, the preserved percentages of PCEs after 500 h storage were 69.0%, 73.9%, 75.0%, and 83.6% for corresponding PSCs without solvent additive, with 3 vol % DIO, 1 vol % AA, or 3 vol % NMP, respectively. It indicates that the pristine morphology of active layer plays the key role in determining the stability of 8868


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Figure 8. (a−c) J−V curves of PSCs with different active layers; (d−f) PCEs as a function of storage time for corresponding PSCs (scatters, measured values; solid line, fitted curves according to power function y = α(1 + t)β).

Table 3. Photovoltaic Parameters of PSCs with Different Active Layers PCE (%) active layer PTB7:PC71BM PffBT4T-2OD:PC71BM PBDT-TS1: PC71BM a

−2

drying method

solvent additive

JSC (mA cm )

VOC (V)

FF (%)

best

averagea

PCE* (%)

PCE*/PCE (%)

conventional upside-down conventional upside-down conventional upside-down

3 vol % DIO 3 vol % DIO without without without without

15.37 15.95 18.89 19.23 18.16 19.35

0.74 0.74 0.75 0.76 0.83 0.81

61.72 62.98 68.15 71.32 62.64 65.06

7.01 7.44 9.66 10.42 9.44 10.20

6.88 7.29 9.54 10.27 9.25 10.09

4.39 5.12 6.78 8.87 6.52 7.95

62.6 68.8 70.2 85.1 69.1 77.9

The average values were calculated according to 20 cells.

preserved percentages of PCEs for the PSCs with active layers undergoing upside-down drying process were larger than those for the PSCs with active layers undergoing conventional drying process. The enhanced PCEs of PSCs with the active layers undergoing upside-down drying process should be attributed to the optimized vertical phase separation, the more ordered and tightly packed π−π stacking of polymer molecules. The enhanced stability of PSCs with the active layers undergoing upside-down drying process should be mainly due to the more ordered and tightly packed π−π stacking of polymer and the frozen PC71BM distribution in polymer network. The upsidedown drying method should be an efficient and universal strategy to improve PCE and stability of PSCs.

77.9% for the PSCs with active layers undergoing upside-down drying process, respectively. To intuitively exhibit the effect of upside-down drying method on the stability of PSCs, the initial PCEs and the preserved PCEs after 500 h of PSCs with different polymers as donor and PC71BM as acceptor are shown in Figure S13. The universality of upside-down drying method can be solidly confirmed from four kinds of PSCs with different active layers, the upside-down drying method should be an efficient and universal strategy to improve PCE and stability of PSCs, even for large-area production.

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CONCLUSIONS The morphology of active layer plays crucial role in determining the PCE and stability of PSCs, which can be optimized by mixing solvent additive or employing different drying methods. In this work, active layer upside-down drying method was employed to improve the PCE and stability of PSCs, which was solidly confirmed from four kinds of cells with PTB7-Th:PC71BM, PTB7:PC71BM, PffBT4T-2OD:PC71BM, or PBDT-TS1:PC71BM as active layers. The PCEs of PTB7Th:PC71BM based PSCs were improved from 8.58% to 9.64% by mixing 3 vol % DIO, further to 10.30% by employing upside-down drying method. After 500 h of storage, the

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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/acsami.7b00510. Detailed experimental procedures, J−V curves of PSCs, photovoltaic parameters, EQE spectra, absorption spectra, GIXD profiles, WCAs images, TEM images, J− V curves of hole-only and electron-only devices, hole and 8869


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electron mobilities, degradation characteristics of photovoltaic parameters (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: 0086-13552149521. ORCID

Fujun Zhang: 0000-0003-2829-0735 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China (61675017, 61377029); Fundamental Research Funds for the Central Universities (2016YJS157); and Bagui Scholars Program of Guangxi, Guangxi Natural Science Foundation (2015GXNSFGA139002). The authors gratefully acknowledge the assistance of the scientists at Diffuse X-ray Scattering Station (1W1A) and Photoelectron Spectroscopy Station (4B9B), BSRF during the experiments.

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