Correlation between Phase-Separated Domain Sizes of Active Layer

Jul 7, 2016 - Correlation between Phase-Separated Domain Sizes of Active Layer and Photovoltaic Performances in All-Polymer Solar Cells. Changyeon Lee...
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Correlation between Phase-Separated Domain Sizes of Active Layer and Photovoltaic Performances in All-Polymer Solar Cells Changyeon Lee,† Yuxiang Li,§ Wonho Lee,† Youngmin Lee,∥ Joonhyeong Choi,† Taesu Kim,† Cheng Wang,⊥ Enrique D. Gomez,*,∥ Han Young Woo,*,§ and Bumjoon J. Kim*,† †

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 306-701, Republic of Korea § Department of Chemistry, Korea University, Seoul 136-701, Republic of Korea ∥ Department of Chemical Engineering and Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802, United States ⊥ Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: The control of the bulk-heterojunction (BHJ) morphology in polymer/polymer blends remains a critical hurdle for optimizing all-polymer solar cells (all-PSCs). The relationship between donor/acceptor phase separation, domain size, and the resulting photovoltaic characteristics of PDFQx3T and P(NDI2OD-T2)-based all-PSCs was investigated. We varied the film-processing solvents (chloroform, chlorobenzene, o-dichlorobenzene, and p-xylene), thereby manipulating the phase separation of all-polymer blends with the domain size in the range of 30−300 nm. The different volatility and solubility of the solvents strongly influenced the aggregation of the polymers and the BHJ morphology of polymer blends. Domain sizes of all-polymer blends were closely correlated with the short-circuit current density (JSC) of the devices, while the open-circuit voltage (0.80 V) and fill factor (0.60) were unaffected. All-PSCs with the smallest domain size of ∼30 nm in the active layer (using chloroform), which is commensurate with the domain size of highly efficient polymer/fullerene solar cells, had the highest JSC and power conversion efficiency of 5.11% due to large interfacial areas and efficient exciton separation. Our results suggest that the BHJ morphology was not fully optimized for most of the previous high-performance all-PSC systems, and their photovoltaic performance can be further improved by fine-engineering the film morphology, i.e., domain size, domain purity, and polymer packing structure.

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producing high device efficiency.12,22−31 The large phase separation in all-PSCs (compared to polymer/fullerene based solar cells) has been an obstacle to overcome for optimizing the device properties. Various approaches, including developing film-processing protocols3,26−28,32 and designing new polymer donors and acceptors with high compatibility and solubility,8−11,24,33 have been attempted to control the BHJ morphology of all-PSCs. It has been suggested that the ideal BHJ morphology should involve interconnected network of phase-separated domains with large donor/acceptor (D/A) interfacial areas and the dimension comparable to the exciton diffusion length (∼10 nm)34,35 for efficient charge generation and transport.29,30 However, in comparison to the case of the polymer/fullerene based solar cells, achieving such an ideal phase-separated morphology in all-PSCs is very challenging due to the significantly reduced entropic contribution by two macromolecular species (polymer donor−polymer acceptor)

INTRODUCTION All-polymer solar cells (all-PSCs), based on the binary blend of an electron-donating polymer and an electron-accepting polymer, are of emerging interest due to various virtues over conventional polymer/fullerene devices including complementary absorption, flexible tunability of chemical structures/energy levels, and excellent mechanical endurance.1−20 Recently, allPSCs have demonstrated high power conversion efficiency (PCE) of up to ∼8% with exceptionally high short-circuit current density (JSC = 18.8 mA cm−2) and good fill factor (FF = 0.70).3,4 More importantly, all-PSCs can have significantly better mechanical durability and high stretchability compared to polymer/fullerene solar cells, highlighting their potentials in flexible and portable photovoltaic devices.5,21 An opportunity exists for the development of new photoactive materials and processing optimization to achieve both higher efficiency and stability in all-PSCs, and eventually all-PSCs are expected to have comparable or better device performances than polymer/ fullerene devices. Controlling the bulk-heterojunction (BHJ) morphology in the active layer of all-PSCs is a critical requirement to © XXXX American Chemical Society

Received: May 20, 2016 Revised: June 27, 2016

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Figure 1. (a) Chemical structures of PDFQx3T and P(NDI2OD-T2) and processing solvents with their boiling points. (b) UV-absorption spectra of PDFQx3T and P(NDI2OD-T2) in thin film.

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that energetically disfavors their mixing.36 Indeed, most of the reported high performance all-PSCs showed relatively large domains near 50−200 nm, which is much larger than the length scales (10−30 nm) in the active layer of optimized polymer/ fullerene devices.6,7,11,12 Also, because the molecular geometry of 2-dimensional polymer chains is different from that of spherical-shaped fullerenes, electron transport within polymer acceptors is strongly dependent on packing orientations in thin films, and charge dissociation at the D/A interface is also greatly affected by the molecular orientation at the interface.12,23,37 Thus, it remains a grand challenge to achieve an ideal BHJ morphology for all-PSCs, and efforts are urgently needed to develop a quantitative understanding of the influence of morphological parameters on device performance, including the role of domain length scales, domain purity, and chain orientation in thin films.8,10,12 Here we aim to elucidate the fundamental correlations between blend morphology (in particular, domain spacing) and device performance in all-PSCs. We varied the film-processing solvents to systematically manipulate the BHJ morphology of PSCs.38−43 All-PSCs, which are based on the blend of poly{2,5di(2-thienyl)thiophene-alt-6,7-difluoro-2,3-bis(3,4-bis(octyloxy)phenyl)quinoxaline)} polymer donor (PDFQx3T) and poly{N,N′-bis(2-octyldodecylnaphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl)-alt-5,5′-(2,2′-bithiophene)} polymer acceptor P(NDI2OD-T2), were fabricated using four different processing solvents of chloroform (CF), chlorobenzene (CB), o-dichlorobenzene (DCB), and p-xylene (XY) (Figure 1a). The different volatility of the processing solvents and polymer solubility affect the kinetics and thermodynamics of BHJ film evolution. Therefore, the final phase-separated domain sizes of four all-polymer blends were tuned from 30 to over 300 nm length scales. The domain sizes were quantitatively determined by combined measurements of atomic force microscopy (AFM), transmission electron microscopy (TEM), and resonant soft X-ray scattering (RSoXS). Our blend systems were ideal for extracting the correlation between morphology and charge generation because the JSC of devices varies as a function of the domain size in the active layer while the opencircuit voltage (VOC = 0.80 V) and fill factor (FF = 0.60) are unaffected. The finest phase separation with an ∼30 nm length scale was achieved by casting the blend film from CF, resulting in a PCE of 5.11%, which is the highest photovoltaic performance for the quinoxaline polymer-based all-PSCs.

RESULTS AND DISCUSSION We have chosen the quinoxaline-based semicrystalline PDFQx3T as the donor and the naphthalenediimide-based P(NDI2OD-T2)11,16,17,44,45 as the acceptor. PDFQx3T was synthesized by Stille polycondensation of 5,8-bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(3,4-bis(octyloxy)phenyl)quinoxaline and 2,5-bis(trimethylstannyl)thiophene using Pd2(dba)3/P(o-tolyl)3 as a catalyst in toluene (yield: 73%). Figure 1a displays the chemical structures of polymers and various solvents (CF, CB, DCB, and XY) used in this study. Table S1 summarizes basic properties of the polymers. The number-average molecular weights (Mn) of PDFQx3T and P(NDI2OD-T2) are 113 and 61 kg mol−1, respectively, as determined by size exclusion chromatography (SEC) using DCB as the eluent at 80 °C. The PDFQx3T polymer has the lowest unoccupied molecular orbital/highest occupied molecular orbital (LUMO/HOMO) energy levels of −3.66 eV/−5.39 eV, while P(NDI2OD-T2) has a LUMO/HOMO of −4.02 eV/ −5.52 eV, producing sufficient energy offsets between the LUMOs and HOMOs for efficient exciton dissociation. Figure 1b shows optical absorption of neat PDFQx3T and P(NDI2OD-T2) polymer films. The blend of two polymers shows a broad absorption over a wide wavelength range, from 400 to 900 nm, enabling high JSC in all-PSCs. We first monitored photovoltaic current density−voltage (J− V) characteristics of the PDFQx3T:P(NDI2OD-T2) solar cells where the active layers were cast from CF, CB, DCB, and XY. All-PSCs were prepared with an inverted architecture of ITO/ ZnO/active layer/MoO3/Ag, and the device characteristics were measured under 100 mW cm−2 AM 1.5G solar illumination in ambient air. We optimized all-PSCs with each different processing solvent by controlling the polymer concentration, spinning condition, the blend ratio of donor/ acceptor polymers, and the volume fraction of solvent additive. The optimal weight ratio of PDFQx3T:P(NDI2OD-T2) blend was determined to be 1:1. The thicknesses of active layers were around 100−110 nm for all of the solvent systems, which minimizes a variation in the contribution of the light absorption of the active layers (due to different thickness) to the device efficiency (Figure S1). We used diphenyl ether (DPE) as a processing additive to promote crystalline orderings of polymers in thin films and thus improve charge transport.11,46 The details for the device fabrication are described in the Supporting Information. The photovoltaic characteristics of allPSCs are summarized in Table 1, and the representative J−V B

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vertical direction, parallel to the surface normal.48,49 Also, four all-polymer blends have (001) peaks in the in-plane direction (Figure S2d), which is associated with the scattering of P(NDI2OD-T2) polymer chain backbone.47 In the line cuts of four samples, the (010) and (001) scattering intensities were gradually enhanced in the order of films cast from CF, CB, DCB, and XY. This is likely due to the longer drying time of polymer films with increasing boiling point of the casting solvent that influences the formation of crystalline polymer domains. Nevertheless, this modest change in the crystalline structures of the active layer is not likely to be strongly correlated to the significant changes in the JSC values of devices. We speculate that the different film-drying kinetics could affect phase separation within polymer blends; thus, we subsequently investigated the blend morphologies of films cast from CF, CB, DCB, and XY. Imaging using AFM and TEM was employed to characterize the morphological differences of the films cast from different solvents and to investigate the correlation between the photovoltaic properties of devices and the blend morphology in the active layer. First, AFM was used to image the surface structure of four different blend films (Figure 3a−d and Figure S3a−d). It is evident that processing solvents have a significant impact on the phase separation in PDFQx3T:P(NDI2OD-T2) blends. The film cast from CF exhibits a fine nanoscale morphology and a very smooth film with an average root-meansquare (RMS) roughness of 0.7 nm. In contrast, the blend films prepared from other solvents have relatively coarse morphologies in the order of CB < DCB < XY with the RMS surface roughness of 1.4 for CB, 1.4 for DCB, and 3.2 nm for XY. We also conducted TEM measurements to characterize the structure of the blends that is present throughout the entire film (Figure 3e−h and Figure S3e−h). The TEM results corroborate the AFM experiments; the blend film cast from CF has the smallest domain sizes, while films cast from CB, DCB, and XY exhibit larger granular domains. The small domain size in the film cast from CF is important for efficient exciton dissociation, thus leading to the highest JSC values in devices. Overall, the domain sizes observed through AFM and TEM vary significantly (30−300 nm) for the various samples. These dramatic morphological changes originate from the different solvent volatility and polymer solubility. For example, fast evaporation of CF quickly freezes the blend morphology at an early stage of phase separation before significant demixing of the polymer donor and polymer acceptor occurs. In contrast,

Table 1. Photovoltaic Chracterisitcs of Four Different AllPSC Systems Processed with Different Solvents processing solvents a

CF CBb DCBb XYb

VOC (V)

JSC (mA cm−2)

FF

PCE (PCEavg)c (%)

0.80 0.79 0.80 0.79

10.58 9.12 8.44 7.91

0.60 0.61 0.60 0.61

5.11 4.43 4.04 3.83

(5.08 (4.36 (3.88 (3.78

± ± ± ±

0.03) 0.05) 0.23) 0.08)

a

All-PSCs were fabricated with 1 vol % DPE. bAll-PSCs were fabricated with 2 vol % DPE. cThe average PCEs were obtained from at least five different devices for each system.

and external quantum efficiency (EQE) curves are shown in Figure 2. The highest PCE value of 5.11% (VOC = 0.80 V, JSC = 10.58 mA cm−2, and FF = 0.60) was obtained when the active layer was cast from CF, and device efficiencies were significantly lower when the active layer was cast from CB (PCE = 4.43%), DCB (PCE = 4.04%), and XY (PCE = 3.83%). Changes in the VOC and FF in all-polymer devices were negligible (VOC ≈ 0.80 V and FF ≈ 0.60) with the casting solvent of the active layer, such that changes in the JSC were mainly responsible for the difference in the overall device performance (7.91 mA cm−2 for XY, 8.44 mA cm−2 for DCB, 9.12 mA cm−2 for CB, and 10.58 mA cm−2 for CF). The photocurrents calculated from the intergration of EQE responses (Figure 2b) are well matched with the measured JSC within 2% error and follow the same trends with changing casting solvents as those highlighted in Table 1. Grazing incidence X-ray scattering (GIXS) was performed to compare the polymer packing structure and orientation in allPSC films cast from different solvents. All of the PDFQx3T:P(NDI2OD-T2) polymer blends cast from CF, CB, DCB, and XY were prepared under identical conditions as those for optimized devices. Pristine PDFQx3T and P(NDI2OD-T2) films cast from CF were also examined to analyze the scattering peaks in the blend samples. Figure S2 shows the line cuts of 2D GIXS patterns of pristine PDFQx3T, P(NDI2OD-T2), and four different all-polymer blend films along the out-of-plane and in-plane directions. As given in Figure S2c, all of the blend films have pronounced (010) π−π stacking peaks along the outof-plane direction, which were attributed to the strong face-on crystalline orientation of PDFQx3T (q = 1.57 Å−1) and P(NDI2OD-T2) (q = 1.72 Å −1 ) polymers. 47 This is hypothesized to promote efficient charge transport in the

Figure 2. (a) J−V curves and (b) EQE responses of the optimized all-PSCs processed with CF, CB, DCB, and XY. C

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Figure 3. AFM phase (a−d) and TEM (e−h) images of PDFQx3T:P(NDI2OD-T2) blends prepared from CF, CB, DCB, and XY processing solvents.

Figure 4. (a) RSoXS profiles of PDFQx3T:P(NDI2OD-T2) blends prepared from CF, CB, DCB, and XY processing solvents. (b) Correlation between domain size and JSC of different devices with CF, CB, DCB, and XY.

less volatile solvents (i.e., DCB) slowly evaporate and plasticize the film longer during casting and allow for phase separation into large aggregates. In addition, the relatively poor solvent, XY, is expected to accelerate self-aggregation/demixing of the polymers by promoting crystallization in solution. To investigate the aggregation behaviors of polymers in different solvents, we measured the temperature-dependent UV−vis absorption at a fixed concentration of 0.02 mg mL−1.50 As given in Figure S4a, the 0−1 and 0−0 transition peaks of PDFQx3T at 610 and 665 nm were progressively decreased for all of the solvent systems as the temperature was elevated from 20 to 60 °C. It is because the polymer chains became gradually disaggregated with increasing temperature. Interestingly, the change in the optical density of PDFQx3T was decreased in the order of DCB > CF ≈ CB > XY, indicating that the PDFQx3T polymers were strongly aggregated in XY. We found similar trend for the aggregation behavior of P(NDI2OD-T2) in different solvents (Figure S4b). For example, a strong absorption peak of P(NDI2OD-T2) centered at ∼710 nm still remained in XY at the elevated temperature of 60 °C, reflecting significant aggregation of the polymer chains. At the actual condition for device fabrication (20 °C), the absorption peak at 710 nm became more prominent. These significant preaggregations of both PDFQx3T and P(NDI2OD-T2) in XY are likely to cause large-scale phase separation in the XYprocessed film. Photoluminescence (PL) quenching of the blend films processed by different solvents was also compared. The PL

spectra were obtained by exciting at 700 nm, which corresponds to the dominant absorption of the P(NDI2ODT2) polymer acceptor (Figure S5). While all of the PL emissions of the four polymer blends were significantly quenched via photoinduced hole transfer from P(NDI2ODT2) to PDFQx3T, a decrease in the quenching efficiencies is clear in the order of CF > CB > DCB > XY, consistent with increasing domain spacings of the blend film. The quenching efficiencies were measured to be 87.5% for the CF processed blends, 81.7% for CB, 79.2% for DCB, and 77.6% for XY. These results can be interpreted by that smaller domain size facilitates efficient exciton dissociation,51−53 and the resulting PL quenching trends agree well with the morphology data from AFM and TEM. A quantitative measure of the BHJ morphology of allpolymer blends was obtained using RSoXS.31,54 Four blend samples (cast from CF, CB, DCB, and XY) were prepared at the same film fabrication condition for the optimized device. Data were obtained at a photon energy of 287 eV to enhance the scattering contrast between the two polymer domains. Figure 4a displays the RSoXS profiles as a function of scattering vector q. For the blend cast from CF, no discernible peak is apparent in the range of q = 0.01−0.001 Å−1. This is consistent with the AFM and TEM results, suggesting that the PDFQx3T:P(NDI2OD-T2) film has a domain spacing smaller than the measurable q range (q = 0.01 Å−1, domain size = 31.4 nm) (Figure 4a). On the other hand, for the blends cast from CB and DCB, distinct scattering peaks are apparent at q = D

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Macromolecules Table 2. Domain Sizes and Photovoltaic Properties of All-PSC Blends and PDFQx3T:PC71BM Blend RSoXS (nm) AFM (nm) TEM (nm) JSC (mA cm−2) FF VOC (V) PCE (%) a

CF blend

CB blend

DCB blend

XY blend

300 332 ± 64 320 ± 70 7.91 0.61 0.79 3.83

PDFQx3T:PC71BM 22.4

12.59 (12.23 ± 0.70)a 0.69 (0.70 ± 0.031)a 0.72 (0.72 ± 0.010)a 6.30 (6.15 ± 0.13)a

The average photovoltaic properties of PDFQx3T:PC71BM devices were obtained from at least five different devices.

0.0031 Å−1 for the CB cast film and 0.0019 Å−1 for the DCB cast film, which correspond to 101 and 165 nm in domain sizes, respectively. Assuming that there are approximately equal volume fractions of the two different polymer domains in the blend, the size of each polymer domain is half of the domain spacing (d = 2πq−1). The peaks for the blend cast from DCB become more pronounced compared to those cast from CF and CB, indicating a higher purity in the phase-separated domains. Interestingly, two different peaks are visible for the XY cast film; one broad scattering peak centered at q = 0.0026 Å−1 and a noticeable strong scattering peak at q value lower than 0.001 Å−1. Even though we could not assign the exact peak position of the low-q signal due to our the RSoXS instrumentation resolution, it suggests that most of the domains of the XY blend film are larger than 310 nm. Nevertheless, the length scales extracted from RSoXS are consistent with those from AFM and TEM measurements. Table 2 summarizes the average domain sizes obtained from RSoXS, AFM, and TEM with the device characteristics where the active layer was cast from four different solvents. The procedures for determining the average domain size are described in Figure S6, based on the analysis of multiple images using standard software (ImageJ). The histograms of the estimated domain sizes from the AFM phase and TEM images can be found in Figure S7. Both AFM and TEM results suggest that the domain size varies with casting solvent, where the domain sizes from TEM images are 27 ± 10 (CF), 126 ± 52 (CB), 170 ± 74 (DCB), and 320 ± 70 nm (XY). Also, the domain sizes are in good agreement for AFM and TEM measurements and match well with those from RSoXS; i.e., for the blend cast from CB, the average domain size is estimated to be 129, 126, and 101.3 nm from AFM, TEM, and RSoXS. For the blends cast from DCB, the average domain size is 178, 170, and 165.3 nm for AFM, TEM, and RSoXS measurements. Our quantitative estimate of the domain sizes allows for exploration of the correlation between morphology and the JSC in the PDFQx3T:P(NDI2OD-T2) all-PSCs. Overall, tuning the processing solvent to reduce the length scale for phase separation leads to an enhancement in the JSC values by 34% (7.91 → 10.58 mA cm−2) (Figure 4b). We expect that the nanoscale phase-separated morphology (∼30 nm) of the active layer cast from CF must be beneficial for efficient charge separation due to the larger D/A interface area.33,55,56 Many of the previously reported all-PSC systems have relatively large degrees of phase separation,6,11,12 suggesting their photovoltaic performance can be further improved by tuning the film morphology. For example, Ye et al. have reported highperformance all-PSCs based on PBDTBDD-T:PNDI blends with 5.8% PCE, but RSoXS measurements revealed that the allpolymer blend had a large domain size of ∼200 nm.12 In addition, the active layer in high efficiency PPDT2FBT:P-

(NDI2OD-T2) all-PSCs reported by our group had a large domain size of ∼115 nm. 11 Clearly, engineering the morphology can lead to improvements in the photovoltaic properties. Here our work demonstrates that tuning the processing solvents could be a simple but powerful method to modulate the morphology of all-polymer blends. Next, we compare the domain sizes in the active layers of PDFQx3T:P(NDI2OD-T2) and the fullerene-based PDFQx3T:PC71BM device based on the same polymer donor. The optimized PDFQx3T:PC71BM device showed a finely phase-separated interpenetrating structure in the active layer with the domain size of 22.4 nm (q = 0.014 Å−1), which was measured from the RSoXS experiment (Figure S8). This characterestic length is comparable to that of PDFQx3T:P(NDI2OD-T2) film cast from CF. Nevertheless, a PDFQx3T:PC71BM device has a PCE value of 6.30%, with higher JSC and FF values compared to the PDFQx3T:P(NDI2OD-T2) device (Table 2). The differences are ∼2 mA cm−2 for the JSC and 0.10 for FF values. We speculate that high electron transport property of PC71BM is important for the higher JSC value. It was observed that the PDFQx3T:PC71BM blend had much higher μe of 1.4 × 10−3 cm2 V−1 s−1 than the values from all-polymer blends (μe = (1.4−2.2) × 10−4 cm2 V−1 s−1) (Figure S9 and Table S2). In addition, lower charge dissociation efficiencies in all-PSCs could result in lower FF and JSC values by increasing geminate recombination. In this case, the orientation of P(NDI2OD-T2) at the D/A interface should be optimized to produce strong π-orbital overlap with face-toface stacking between D and A polymers.12,23,57 Furthermore, other factors, such as domain purity and vertical segregation within PDFQx3T:P(NDI2OD-T2) thin film, should be considered to further enhance the PCE in all-PSCs.58,59

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CONCLUSIONS We delineated a straightforward relationship between blend morphology and photovoltaic device performance in all-PSCs based on a model system of the PDFQx3T:P(NDI2OD-T2) blend. We systematically controlled the length scales of the phase separation in the all-PSC blends from 30 to over 300 nm by using different kinds of casting solvents (domain sizes: ca. 27 nm with CF, 126 nm with CB, 170 nm with DCB, and 320 nm with XY). The highly volatile CF solvent enabled to form kinetically frozen morphologies with smaller domain sizes, whereas slow evaporation of CB and DCB allowed sufficient time for demixing of the polymers, leading to larger domain sizes. JSC values significantly increased from 7.91 to 10.58 mA cm−2 with decreasing the domain size. As a result, the CFprocessed PDFQx3T:P(NDI2OD-T2) film with the smallest average domain size of 30 nm had the highest PCE of 5.11%. This characteristic domain size in the active layer of all-PSC is comparable to that of the fullerene-based counterpart (i.e., E

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(7) Deshmukh, K. D.; Qin, T. S.; Gallaher, J. K.; Liu, A. C. Y.; Gann, E.; O’Donnell, K.; Thomsen, L.; Hodgkiss, J. M.; Watkins, S. E.; McNeill, C. R. Performance, Morphology and Photophysics of High Open-Circuit Voltage, Low Band Gap All-Polymer Solar Cells. Energy Environ. Sci. 2015, 8, 332. (8) Hwang, Y. J.; Earmme, T.; Courtright, B. A. E.; Eberle, F. N.; Jenekhe, S. A. N-Type Semiconducting Naphthalene Diimide-Perylene Diimide Copolymers: Controlling Crystallinity, Blend Morphology, and Compatibility toward High-Performance All-Polymer Solar Cells. J. Am. Chem. Soc. 2015, 137, 4424. (9) Jung, J. W.; Jo, J. W.; Chueh, C. C.; Liu, F.; Jo, W. H.; Russell, T. P.; Jen, A. K. Y. Fluoro-Substituted N-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%. Adv. Mater. 2015, 27, 3310. (10) Lee, C.; Kang, H.; Lee, W.; Kim, T.; Kim, K. H.; Woo, H. Y.; Wang, C.; Kim, B. J. High-Performance All-Polymer Solar Cells Via Side-Chain Engineering of the Polymer Acceptor: The Importance of the Polymer Packing Structure and the Nanoscale Blend Morphology. Adv. Mater. 2015, 27, 2466. (11) Kang, H.; Uddin, M. A.; Lee, C.; Kim, K. H.; Nguyen, T. L.; Lee, W.; Li, Y.; Wang, C.; Woo, H. Y.; Kim, B. J. Determining the Role of Polymer Molecular Weight for High-Performance All-Polymer Solar Cells: Its Effect on Polymer Aggregation and Phase Separation. J. Am. Chem. Soc. 2015, 137, 2359. (12) Ye, L.; Jiao, X.; Zhou, M.; Zhang, S.; Yao, H.; Zhao, W.; Xia, A.; Ade, H.; Hou, J. Manipulating Aggregation and Molecular Orientation in All-Polymer Photovoltaic Cells. Adv. Mater. 2015, 27, 6046. (13) Benten, H.; Nishida, T.; Mori, D.; Xu, H.; Ohkita, H.; Ito, S. High-Performance Ternary Blend All-Polymer Solar Cells with Complementary Absorption Bands from Visible to Near-Infrared Wavelengths. Energy Environ. Sci. 2016, 9, 135. (14) Li, S.; Zhang, H.; Zhao, W.; Ye, L.; Yao, H.; Yang, B.; Zhang, S.; Hou, J. Green-Solvent-Processed All-Polymer Solar Cells Containing a Perylene Diimide-Based Acceptor with an Efficiency over 6.5%. Adv. Energy Mater. 2016, 6, 201501991. (15) Zhou, N. J.; Dudnik, A. S.; Li, T. I. N. G.; Manley, E. F.; Aldrich, T. J.; Guo, P. J.; Liao, H. C.; Chen, Z. H.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Olvera de la Cruz, M.; Marks, T. J. All-Polymer Solar Cell Performance Optimized via Systematic Molecular Weight Tuning of Both Donor and Acceptor Polymers. J. Am. Chem. Soc. 2016, 138, 1240. (16) Lee, W.; Lee, C.; Yu, H.; Kim, D.-J.; Wang, C.; Woo, H. Y.; Oh, J. H.; Kim, B. J. Side Chain Optimization of Naphthalenediimide− Bithiophene-Based Polymers to Enhance the Electron Mobility and the Performance in All-Polymer Solar Cells. Adv. Funct. Mater. 2016, 26, 1543. (17) Mori, D.; Benten, H.; Okada, I.; Ohkita, H.; Ito, S. Highly Efficient Charge-Carrier Generation and Collection in Polymer/ Polymer Blend Solar Cells with a Power Conversion Efficiency of 5.7%. Energy Environ. Sci. 2014, 7, 2939. (18) Jung, I. H.; Lo, W.-Y.; Jang, J.; Chen, W.; Zhao, D.; Landry, E. S.; Lu, L.; Talapin, D. V.; Yu, L. Synthesis and Search for Design Principles of New Electron Accepting Polymers for All-Polymer Solar Cells. Chem. Mater. 2014, 26, 3450. (19) Earmme, T.; Hwang, Y.-J.; Murari, N. M.; Subramaniyan, S.; Jenekhe, S. A. All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor. J. Am. Chem. Soc. 2013, 135, 14960. (20) Zhao, R.; Dou, C.; Xie, Z.; Liu, J.; Wang, L. Polymer Acceptor Based on B←N Units with Enhanced Electron Mobility for Efficient All-Polymer Solar Cells. Angew. Chem. 2016, 128, 5399. (21) Savagatrup, S.; Printz, A. D.; O’Connor, T. F.; Zaretski, A. V.; Rodriquez, D.; Sawyer, E. J.; Rajan, K. M.; Acosta, R. I.; Root, S. E.; Lipomi, D. J. Mechanical Degradation and Stability of Organic Solar Cells: Molecular and Microstructural Determinants. Energy Environ. Sci. 2015, 8, 55. (22) Schubert, M.; Dolfen, D.; Frisch, J.; Roland, S.; Steyrleuthner, R.; Stiller, B.; Chen, Z.; Scherf, U.; Koch, N.; Facchetti, A.; Neher, D. Influence of Aggregation on the Performance of All-Polymer Solar

PDFQx3T:PC71BM), but the JSC and FF values are still lower due to the lower μe value of P(NDI2OD-T2) in all-PSCs. While we mainly focused on finding the relationship between the blend morphology and the device performance, the domain size does not solely influence the device characteristics; other morphological factors including the domain purity in D and A phases as well as the polymer orientation at the D/A interface should be considered together for optimization of all-PSCs. Our results highlight the potential of all-PSCs for further improvement because many of the high performing all-PSCs (with PCEs higher than 5−7%) still have unoptimized morphologies with length scales much greater than 20−30 nm.

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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.macromol.6b01069. Materials and methods, detailed experimental procedures, and additional data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (E.D.G.). *E-mail [email protected] (H.Y.W.). *E-mail [email protected] (B.J.K.). Author Contributions

C.L. and Y.L. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation Grant (2012M3A6A7055540, 2015M1A2A2057506, and 2015R1A2A1A15055605), funded by the Korean Government. This research was supported by the Research Projects of the KAIST-KUSTAR and the CRH (Climate Change Research Hub) of KAIST. Y.L. and E.D.G. acknowledge funding from the Office of Naval Research, United States, under Contract N000141410532.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b01069 Macromolecules XXXX, XXX, XXX−XXX