All-Polymer Solar Cells Employing Non-Halogenated Solvent and

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All-Polymer Solar Cells Employing Non-Halogenated Solvent and Additive Yan Zhou,† Kevin L. Gu,† Xiaodan Gu,†,‡ Tadanori Kurosawa,† Hongping Yan,‡ Yikun Guo,§ Ghada I. Koleilat,† Dahui Zhao,§ Michael F. Toney,‡ and Zhenan Bao*,† †

Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States § College of Chemistry, Peking University, Beijing 100871, China ‡

S Supporting Information *

ABSTRACT: Herein, we report an all-polymer solar cell with a PCE of over 5% fabricated with non-halogenated solvent. Our method of polymer side-chain engineering using polystyrene enhanced the solubility of polymers in toluene. The phase separation size of the polymer−polymer blend was controlled by tuning the additive concentration. Three different additives were employed and studied. To the best of our knowledge, this is the highest performing all-polymer solar cell fabricated with both nonhalogenated solvent and non-halogenated additive, which highlights its potential toward environmentally friendly manufacturing of allpolymer organic solar cells.

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polymers due to changes in thin film morphology.12,13 For better solubility in non-halogenated solvents, bulky substitutions and larger torsion angles between two aromatic rings are usually required.13,12 These modifications tend to result in low carrier mobilities within the polymer films due to weak π−π interactions and low crystalline nature. On the other hand, complex cosolvent mixtures may introduce more irreproducibility during the fabrication process, and complicate drying dynamics that are essential for phase separation formation.10,14 In our previous work, we reported a polymer side-chain engineering approach to modify an isoindigo-based polymer.12 The polymer solubility was significantly enhanced by incorporation of 5−10 mol % of polystyrene side-chains (Mn = 1300) while at the same time the high charge carrier mobility was maintained.12 One additional advantage of the polymer side-chain was that the phase separation size in the all-polymer donor−acceptor blend for photovoltaic devices was reduced using PiI-2T (chemical structure shown in Figure 1A) with 5 mol % of polystyrene (PS) covalently attached to the polymer backbone. In this work, a higher molar ratio of the PS sidechain is employed to further increase the solubility of the isoindigo polymer in non-halogenated solvents. Without PS side-chains, the solubility of PiI-2T in toluene is poor (Figure 1B), swelling but not dissolving at 90 °C. With 5% of PS side-

INTRODUCTION Organic solar cells (OSCs) containing non-fullerene polymeric acceptors have attracted significant interest in the research community over the past few years. The power conversion efficiencies (PCEs) of such cells have evolved from less than 2% to more than 5%.1−6 Polymeric acceptors not only provide possibilities for low-cost processing but also further enable a vast number of combinations of donor/acceptor pairs.7 Polymer blends potentially have better phase separation stability than polymer/small molecule blends due to the lower diffusivity of polymers.7 However, reported high performance all-polymer solar cells typically utilize toxic halogenated solvents for the fabrication process, which is unfavorable for large-scale production.1−5 Consequently, an allpolymer solar cell system with high PCE that can be processed with non-halogenated solvents and subsequently adapted to printing techniques is highly desirable. The solubility of conjugated polymers in non-halogenated solvents is usually limited due to the highly polarizable conjugated backbones.8 Two strategies exist to increase the solubility of a conjugated polymer in non-halogenated solvents: (1) chemical modification of either the backbone or side-chains to increase the fraction of non-polarizable groups and (2) use of a cosolvent to tune the solubility based on the Hansen solubility parameters.9−11 The chemical modification method is straightforward for improved solubility in non-halogenated solvents.11 However, the main drawback lies in the difficulty to preserve good electronic properties of the modified conjugated © 2016 American Chemical Society

Received: May 2, 2016 Revised: June 23, 2016 Published: June 26, 2016 5037

DOI: 10.1021/acs.chemmater.6b01776 Chem. Mater. 2016, 28, 5037−5042

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Figure 1. (A) Molecular structures of the donor and acceptor polymers. (B) Solution of PiI-2T-PS10 in toluene, (C) gel of PiI-2T-PS5, and (D) swelled solid of PiI-2T-PS0. showed similar results. Absorption spectra were recorded on a Cary 6000i Spectrometer. Solar Cell Fabrication and Testing. Glass substrates coated with patterned indium-doped tin oxide (ITO) with a sheet resistance of 13 Ω/□ were purchased from Xin Yan Technology Lt. Before device fabrication, the ITO/glass substrates were subjected to a series of wetcleaning processes in an ultrasonic bath sequentially in acetone, detergent, deionized water, and isopropanol. After drying in the vacuum oven at 80 °C for 10 min, followed by a 20 min UV-ozone treatment, ZnO in ammonium solution was spin-coated onto the ITO surface at a speed of 5000 rpm for 30 s.17 The ZnO was baked at 90 °C for 10 min in air, forming a 10 nm thick film. The polymers were dissolved into toluene with stirring for 3 h at 70 °C. Next, 10 mg of each polymer is dissolved into 1 mL of toluene separately and the solutions are filtered via a 0.45 μm PTFE syringe filter. Two polymer solutions, freshly mixed, are heated at 40 °C for 30 min before the spin-coating process. Different amounts of additives were added to the freshly mixed solution, and the total concentration of the polymers was kept the same by adding different amounts of toluene. All the active layers were spin-coated at 700 rpm for 45 s and 3000 rpm for 40 s in the glovebox. The dried films were subsequently annealed in the glovebox at 180 °C for 5 min before they were transferred to a vacuum evaporator for electrode deposition. The thickness of the active layer was 90 nm for all the different conditions measured by a Dektak surface profiler. A MoO3 layer (15 nm), followed by an Ag layer (150 nm), were thermally deposited at a pressure of 8 × 10−6 Torr. The device active areas are 4.0, 8.0, and 16.0 mm2. All devices were tested inside a nitrogen glovebox after encapsulation. The PCE was tested under AM 1.5G illumination with an intensity of 100 mW cm−2 (Newport Solar Simulator 94021A) calibrated by a Newport certified silicon photodiode covered with a KG5 filter with active area of 0.0663 cm2, comparable to our device area of 0.04 cm2. The J−V curves were recorded with a Keithley 2400 semiconductor analyzer. The IPCE and double-pass absorption were measured under monochromic illumination, and the calibration of the incident light intensity was performed with a calibrated silicon photodiode. The integrated JSC were calculated from the IPCE and standard AM1.5G spectrum. The double-pass absorptions measurements were carried out in an integrating sphere. SCLC Measurements. The mobilities were measured by SCLC diodes, and calculated from the typical expression

chain attached (PiI-2T-PS5), a homogeneous solution is formed at 90 °C. However, below 90 °C, gelling still occurs, making reproducible film fabrication difficult. With 10 mol % PS side-chain, the polymer becomes easily dissolved in toluene, and the solution remains stable at room temperature for hours, making it a suitable candidate for solution processing under ambient conditions. In this work, PiI-2T-PS10 with 10 mol % monomers modified with polystyrene side-chains is used as the donor material to take advantage of its good solubility in toluene. P(TP) polymer with a swallow tail alkyl chain substitution is used as the acceptor (Figure 1A). We show that PCE up to 5% is obtained with this polymer blend using both nonhalogenated solvent and non-halogenated additive. The phase separation domain size in the blend was found to depend on the amount of additives.

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EXPERIMENTAL SECTION

General Method. All the polymers were synthesized according to previously reported procedures.5 The donor polymer (PiI-2T-PS10) was purified via preparative size exclusive chromatography (SEC) at room temperature with chloroform as the solvent at a concentration of 7 mg/mL. The molecular weight and polydispersity index (PDI) of all polymers are measured by high temperature GPC with 1,2,4trichlorobenzene as the solvent and polystyrenes as the calibration standards at 160 °C. 2D-GIXD images were collected in reflection mode with a planar area detector in a He atmosphere at beamline 11− 3 of the Stanford Synchrotron Radiation Lightsource. The sample− detector distance was nominally set to 400 mm, and the incidence angle was 0.12°. The X-ray wavelength was 0.9758 Å. Slits were set to 150 and 50 μm in the horizontal and vertical directions, respectively. RSoXS measurements were performed at beamline 11.0.1.2 at the Advanced Light Source. The sample films used for RSoXS were spincast on sodium polystyrene sulfonate covered silicon as substrate. To carry out RSoXS measurements in transmission, the film is floated off in deionized water and picked up with a 1.5 mm by 1.5 mm silicon nitride window. The film is then dried in air before being transferred into the vacuum chamber for RSoXS at 285.2 eV. The energy range was swept from 270 to 290 eV at 0.2 eV steps, and we found that the highest contrast was obtained at 285.2 eV. The contrast function 5038

DOI: 10.1021/acs.chemmater.6b01776 Chem. Mater. 2016, 28, 5037−5042

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Table 1. Device Characteristics of All-Polymer Solar Cells with Various Volume Ratios of Different Additives vs Toluene under AM 1.5G Solar Spectruma additive vol % none 0.2% CN 0.5% CN 1.0% CN 2.0% CN 1% THN 5% THN 10% THN 0.5% MN 1.0% MN 2.0% MN

PCE (PCEmax) 2.19 3.48 3.99 4.61 4.95 2.77 3.00 3.66 4.05 4.50 4.88

± ± ± ± ± ± ± ± ± ± ±

0.14 0.06 0.07 0.11 0.03 0.07 0.14 0.21 0.14 0.11 0.10

(2.20) (3.52) (4.10) (4.79) (5.10) (2.80) (3.20) (3.74) (4.20) (4.55) (5.00)

JSC/mA cm−2 (EQE) −5.44 −7.82 −8.72 −9.43 −9.93 −6.21 −6.70 −7.98 −8.53 −9.44 −9.77

± ± ± ± ± ± ± ± ± ± ±

0.33 0.32 0.15 0.34 0.09 0.29 0.25 0.40 0.33 0.23 0.25

(5.56) (7.50) (8.94) (9.23) (10.1) (6.33) (6.78) (7.73) (8.40) (9.54) (9.97)

VOC/V 0.97 0.98 0.98 0.99 0.98 0.98 0.98 0.99 0.97 0.97 0.98

± ± ± ± ± ± ± ± ± ± ±

0.01 0.01 0.01 0.00 0.00 0.01 0.02 0.00 0.01 0.00 0.00

Rsh/Ω cm−2

FF 0.42 0.46 0.47 0.50 0.51 0.45 0.46 0.46 0.49 0.49 0.51

± ± ± ± ± ± ± ± ± ± ±

0.01 0.02 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

37 22 18 12 9 23 22 16 18 8 7

a

The blend ratio of the polymers is 1:1 by weight, and the total concentration is 10 mg/mL in toluene. Integrated JSC calculated from External Quantum Efficiency (EQE) spectra and AM 1.5G spectrum.

J=

9εr ε0μV 2 8l 3

The hole mobilities were measured by a hole-only diode with a structure of ITO/PEDOT:PSS/Blend film/MoO3/Ag, and the electron mobility was measured by an electron-only diode with a structure of ITO/ZnO/Blend film/LiF/Al. Results and Discussion. Device Performance with Various Additives and Concentrations. The weight ratio of the donor and acceptor polymers is fixed for this work at 1:1. Three different additives, namely, 1-chloronaphthalene (CN), 1,2,3,4-tetrahydronaphthalene (THN), and 1-methyl-naphthalene (MN), were investigated for morphology tuning. The final thickness of the active films with different additives and concentrations are fixed to approximately 90 nm by tuning the spin-coating speed. The device structure is the typical inverted structure: ITO/ZnO/Active layer/MoO3/Ag. The ZnO and the active layers were all spin-coated in air.15 The active films were annealed at 180 °C for 30 s before deposition of the back electrode. Different volume ratios 0.2−2.0% of CN were added to the mixed solutions prior to the spin-coating process. A dramatic increase in the PCE was observed with different CN concentrations. As shown in Table 1, without any CN addition, the average PCE of the control devices was only 2.19%, with a moderate JSC of 5.44 mA cm−2. With only 0.2% (v/v) amount of CN added to the solution, the PCE increased to 3.48%. The average JSC and the FF improved by 43% and 10%, respectively. The PCE and JSC achieved a maximum of 5% and 10.0 mA cm−2, respectively, with a CN concentration of 2.0% (v/v). CN is thus a very effective additive leading to more than doubling of the efficiency of the control cells. However, CN still contains chlorine halogen atoms, though it is only a small amount. Therefore, we proceeded to investigate the influence of two additional additives that are halogen-free with our all-polymer blend system (Figure 2). The JSC of the devices with THN only slightly improved compared to the control devices. Even with 10% v/v THN in toluene solution, where the THN now acts as a cosolvent instead of an additive, the JSC only reached around 8 mA cm−2. The FF of the devices improved but is still 10% lower than that of the best devices with CN. In contrast, addition of only 2% of MN resulted in an average PCE up to 4.88%, with champion devices of 5.0% PCE. Both the JSC and FF of the devices were found to be sensitive to the additive concentrations, while VOC remained constant. Thus, we conclude that absorption of the active layer, phase separation, and charge transport are likely the most three important factors affected by the additives in our system.16 Thin Film Characterization. The UV−vis absorption spectra are nearly identical in devices with different concentrations of CN as additive (Figure S1). The maximum deviation of the thickness of the active layer from different conditions is less than 10%, and thus much smaller than the deviation of the short-circuit currents.

Figure 2. J−V curve (A) and EQE spectra (B) of devices with different additives, control device (blue), 10% THN v/v in toluene (red), 2% MN v/v in toluene (green), and 2% CN v/v in toluene (black). There was no observable shift for all the three major absorption features from donor and acceptor polymers at 578, 636, and 701 nm, indicating that the ground states of the polymer donor and acceptor remained unchanged with different concentrations and types of the additives. We can thus conclude that the differences in the JSC observed are not caused by the different thicknesses, or aggregation forms. Another mechanism that influences current generation is exciton collection efficiency at the donor/acceptor interface. The more excitons are able to diffuse to the donor/acceptor interface without quenching, the more charge carriers can be generated. Since exciton collection depends on domain size, we move to determine the phase separation sizes in the films, which ideally should be comparable to exciton diffusion length for efficient exciton collection. The Resonant Soft XRay Scattering (RSoXS) data show a broad peak which is indicative of a broad distribution of domain spacing between the PII2T-PS-10 domains and P(TP) domains. The average domain spacing is obtained from the peak position of RSoXS intensity vs scattering vector plot (Figure 3). 5039

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nm, with the device generating JSC of 8 mA cm−2. In comparison to CN, we concluded that THN is not very efficient in reducing the phase separation size. Actually, a minimum of 10% THN in the blend is required to obtain a similar small domain size that forms with only 0.5% of CN in the solution. On the other hand, in similar trend to CN, the phase separation domain size reduces significantly at low concentrations of MN. The differences between these three additives in the all-polymer blend systems appear to be affected by the “solubility” of donors in acceptor polymer. Compared with THN, with CN and MN added, the donor mixes better with the acceptor. Small amounts of CN and MN in toluene give smaller phase separated domains and, therefore, indicate better mixing between the two materials under these conditions. Charge Carrier Mobilities. After investigating the effects of domain size in our systems, we proceed to characterize the transport properties in our various films. Balanced and efficient charge carrier transport in BHJ solar cells is required for high FF and JSC. We employ space-charge-limited current (SCLC) to evaluate the carrier mobilities in our solar cells (Figure 4). The device structures of hole and electron-only diodes are ITO/PEDOT:PSS/Active layer/MoO3/Ag and ITO/ZnO/Active layer/LiF/Al, respectively. With all the three different additives, the hole mobilities in the active layer decreased

Figure 3. RSoXS spectra of active films with different additive types and concentrations. (A) Films prepared with CN, (B) with THN, and (C) with MN. Thus, from the RSoXS measurements, we observed that the domain size of the phase separation in the blend films decreased with increasing CN concentration in the solution. The domain size in the blend film without any additive is estimated as ∼150 nm, which is much larger than the optimal domain size in a solar cell due to the typical short exciton diffusion length. With 0.5% of CN dispersed in toluene, the peak of the scattering signal decreases to that corresponding to a domain size ∼ 100 nm, and the JSC increases from 5.4 to 8.7 mA cm−2. The significant decrease in phase separation size occurs when the concentration of the CN in the blend reaches 1%. The smallest phase separation size in our system is ∼50 nm with addition of 2.0% of CN in toluene. In the latter case, we obtained an average 10 mA cm−2 JSC in the solar cell devices. Our films made with the additives THN and MN are also examined with RSoXS. Reduction of the phase separation domain size in the films containing THN is not as significant as that with CN. In fact, the smallest domain size with THN as the additive is observed to be 80

Figure 4. SCLC charge carrier mobilities of the devices processed with different additives and concentrations. Empty squares for hole mobility, filled squares for electron mobility, filled circles for FF, and filled triangle for JSC. Films with CN (A), THN (B), and MN (C). 5040

DOI: 10.1021/acs.chemmater.6b01776 Chem. Mater. 2016, 28, 5037−5042

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Chemistry of Materials compared to those of the control film containing no additives. On the other hand, the electron mobilities scaled well with increasing concentration of additives. However, the trend with different additives is different. With CN and MN as the additives, the electron mobility increased about 1 order of magnitude to around 1 × 10−4 cm2 V−1 s−1. The ratio of hole mobility to electron mobility decreased to 1.5 and 1.1 with 2% CN and MN as the additive, respectively. Now, with THN in the blend, the electron mobility only doubled with 10% addition, which is still 5 times lower than the hole mobility. In the latter films, the ratio of hole mobility to electron mobility remained at 5 even with a high concentration of THN added to the solution. Balanced charge transport, which we nearly observed in the cases of CN and MN, is necessary to reduce recombination near the electrodes, helping to improve FF. Other than phase separation, JSC is also limited by the lower-mobility carrier type, which, in our case, is the electron mobility in the all-polymer solar cells. Thus, the 1 order of magnitude increase in electron mobility with either CN or MN addition is directly related to the JSC increase from 5.4 mA cm−2 to around 10 mA cm−2 in those devices.

were carried out at Beamline 11.0.1.2 at the Advanced Light Source, which is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy, under Contract No. DE-AC02-05CH11231, and at Beamline 11-3 of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-76SF00515.

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(1) 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, 2466−2471. (2) 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, 2359−2365. (3) 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− 3317. (4) Diao, Y.; Zhou, Y.; Kurosawa, T.; Shaw, L.; Wang, C.; Park, S.; Guo, Y.; Reinspach, J. A.; Gu, K.; Gu, X.; Tee, B. C. K.; Pang, C.; Yan, H.; Zhao, D.; Toney, M. F.; Mannsfeld, S. C. B.; Bao, Z. Flowenhanced solution printing of all-polymer solar cells. Nat. Commun. 2015, 6, 7955. (5) Zhou, Y.; Kurosawa, T.; Ma, W.; Guo, Y.; Fang, L.; Vandewal, K.; Diao, Y.; Wang, C.; Yan, Q.; Reinspach, J.; Mei, J.; Appleton, A. L.; Koleilat, G. I.; Gao, Y.; Mannsfeld, S. C.; Salleo, A.; Ade, H.; Zhao, D.; Bao, Z. High Performance All-Polymer Solar Cell via Polymer SideChain Engineering. Adv. Mater. 2014, 26, 3767−3772. (6) 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, 1501991. (7) Facchetti, A. Polymer donor−polymer acceptor (all-polymer) solar cells. Mater. Today 2013, 16, 123−132. (8) Kumar, A.; Takashima, W.; Kaneto, K.; Prakash, R. Nanodimensional self assembly of regioregular poly (3-hexylthiophene) in toluene: Structural, optical, and morphological properties. J. Appl. Polym. Sci. 2014, 131, 40931. (9) Burgues-Ceballos, I.; Machui, F.; Min, J.; Ameri, T.; Voigt, M. M.; Luponosov, Y. N.; Ponomarenko, S. A.; Lacharmoise, P. D.; CampoyQuiles, M.; Brabec, C. J. Solubility Based Identification of Green Solvents for Small Molecule Organic Solar Cells. Adv. Funct. Mater. 2014, 24, 1449−1457. (10) Chueh, C.-C.; Yao, K.; Yip, H.-L.; Chang, C.-Y.; Xu, Y.-X.; Chen, K.-S.; Li, C.-Z.; Liu, P.; Huang, F.; Chen, Y.; Chen, W.-C.; Jen, A. K. Y. Non-halogenated solvents for environmentally friendly processing of high-performance bulk-heterojunction polymer solar cells. Energy Environ. Sci. 2013, 6, 3241−3248. (11) Chen, K.-S.; Yip, H.-L.; Schlenker, C. W.; Ginger, D. S.; Jen, A. K. Y. Halogen-free solvent processing for sustainable development of high efficiency organic solar cells. Org. Electron. 2012, 13, 2870−2878. (12) Fang, L.; Zhou, Y.; Yao, Y.-X.; Diao, Y.; Lee, W.-Y.; Appleton, A. L.; Allen, R.; Reinspach, J.; Mannsfeld, S. C. B.; Bao, Z. Side-Chain Engineering of Isoindigo-Containing Conjugated Polymers Using Polystyrene for High-Performance Bulk Heterojunction Solar Cells. Chem. Mater. 2013, 25, 4874−4880. (13) Mei, J.; Bao, Z. Side Chain Engineering in Solution-Processable Conjugated Polymers. Chem. Mater. 2014, 26, 604−615.

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CONCLUSIONS Incorporation of polystyrene as side-chains in isoindigo conjugated polymer results in a processable material in nonhalogenated solvent. Different concentrations of the additives are employed to further tune the phase separation domain size in the all-polymer hulk heterojunction blend films. With a volume ratio of 2% of additive in the blend solution, devices with spin-coated films exhibited a PCE of up to 5.1%, and a JSC as high as 10 mA cm−2. The phase separation size reduces significantly with the addition of three different additives, ensuring efficient exciton separation. We also achieved a more balanced electron−hole (e−h) charge carrier transport in our devices as the SCLC electron mobility increased over 1 order of magnitude. Both reduced phase separation domain size and a more balanced e−h charge carrier transport were directly influenced by the additives in the blends, resulting in a high performance all-polymer solar cell employing non-halogenated solvent and additives.

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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.6b01776. UV−vis absorption spectra, 2D scattering profiles for RSoXS, and SCLC curves (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.Z. and X.G. were supported by DOE EERE (grant no. DEEE0005960). K.L.G. was supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. T.K. was supported by the Office of Navy Research (grant no. N0001414-1-0142). H.Y. was supported by the National Science Foundation Materials Genome Program (grant no. 1434799). Y.G. and D.Z. were supported by the National Natural Science Foundation of China (No. 51473003). Portions of this work 5041

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Chemistry of Materials (14) Synooka, O.; Eberhardt, K. R.; Hoppe, H. Chlorine-free processed high performance organic solar cells. RSC Adv. 2014, 4, 16681−16685. (15) Ka, Y.; Lee, E.; Park, S. Y.; Seo, J.; Kwon, D.-G.; Lee, H. H.; Park, Y.; Kim, Y. S.; Kim, C. Effects of annealing temperature of aqueous solution-processed ZnO electron-selective layers on inverted polymer solar cells. Org. Electron. 2013, 14, 100−104. (16) Vandewal, K.; Tvingstedt, K.; Inganas, O. In Quantum Efficiency in Complex Systems, Part II: From Molecular Aggregates to Organic Solar Cells; Wurfel, U., Thorwart, M., Weber, E. R., Eds.; Elsevier/Academic Press: Boston, MA, 2011; Chapter 7, pp 227−271. (17) Bai, S.; Wu, Z.; Xu, X.; Jin, Y.; Sun, B.; Guo, X.; He, S.; Wang, X.; Ye, Z.; Wei, H.; Han, X.; Ma, W. Inverted organic solar cells based on aqueous processed ZnO interlayers at low temperature. Appl. Phys. Lett. 2012, 100, 203906.

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DOI: 10.1021/acs.chemmater.6b01776 Chem. Mater. 2016, 28, 5037−5042