Fullerene Acceptor Blend with a Tunable Energy State

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 25570−25579

Nonfullerene/Fullerene Acceptor Blend with a Tunable Energy State for High-Performance Ternary Organic Solar Cells Min Kim,† Jaewon Lee,† Dong Hun Sin,† Hansol Lee,† Han Young Woo,‡ and Kilwon Cho*,† †

Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 37673, Korea Department of Chemistry, Korea University, Seoul 02841, Republic of Korea

‡

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S Supporting Information *

ABSTRACT: Ternary blending is an effective strategy for broadening the absorption range of the active layer in bulk heterojunction polymer solar cells and for constructing an efficient cascade energy landscape at the donor/acceptor interface to achieve high efficiencies. In this study, we report efficient ternary blend solar cells containing an acceptor alloy consisting of the indacenodithiophene-based nonfullerene material, IDT2BR, and the fullerene material, phenyl-C71-butyric acid methyl ester (PC71BM). The IDT2BR materials mix fully with PC71BM materials, and the energy state of this phase can be tuned by varying the blending ratio. We performed photoluminescence and external quantum efficiency studies and found that the ternary charge cascade structure efficiently transfers the photogenerated charges from the polymer to IDT2BR and finally to PC71BM materials. Ternary blend devices containing the IDT2BR:PC71BM acceptor blend and various types of donor polymers were found to exhibit power conversion efficiencies (PCEs) improved by more than 10% over the PCEs of the binary blend devices. KEYWORDS: ternary organic solar cells, acceptor blends, nonfullerene, indacenodithiophene, fullerene

1. INTRODUCTION In the past decades, the organic solar cell (OSC) has attracted considerable attention as a promising next-generation photovoltaic (PV) technology because of the possibility of mass production and because it can be light in weight, low cost, and flexible.1−3 The light-absorbing layer of an OSC consists of a donor material and an acceptor material structured as a bulk heterojunction with a finely phase-separated morphology and a bicontinuous charge transport network.4−9 One effective strategy for enhancing the power conversion efficiencies (PCEs) of OSCs is the preparation of the ternary-blend OSCs, which has been widely tested because of the potential to extend the scope of active layer light absorption and to enhance the short-circuit current densities (Jsc) and PCEs in single-junction devices.10−13 This strategy integrates both enhanced photon harvesting, through the incorporation of multiple organic materials in tandem solar cells, and the simplicity of the fabrication process for single-junction solar cells. The third component of ternary blend OSCs can be either a second electron donor material or a second acceptor material. These third components have versatile functions, including complementary light harvesting, the facilitation of exciton dissociation, the enhancement of charge and energy transfer, and the optimization of the film morphology.10,11 Ternary blends containing a secondary donor material have been studied extensively, but it has been found that the secondary donor material can dilute the light absorption of the host donor material.14−17 Moreover, depending on the compati© 2018 American Chemical Society

bility of the two donor materials, the overall highest occupied molecular orbital (HOMO) energy level of the donor material blend can be pinned to the lower energy level of one component, which results in energy losses from the chargetransfer complexes.13,18−20 Secondary acceptor materials have also been investigated as ternary components.19,21−24 A representative example is that of ICBA:PCBM, which forms a well-mixed blend phase and can be used to finely adjust the lowest unoccupied molecular orbital (LUMO) energy level of the acceptor alloy phase.25,26 However, fullerene acceptors exhibit low absorption and only limited energy level tuning. In recent years, various types of nonfullerene acceptor materials have been reported that exhibit impressive advantages, for example, strong light absorption in the visible region, high miscibility with polymers to ensure nanomorphology formation, low energy losses, and high opencircuit voltages, and thus their associated devices exhibit excellent PCEs.6,27−33 Such nonfullerene acceptor materials can also be used as a ternary component because they do not dilute the donor material, which means not only that the strong light-absorbing properties of the donor material are retained but also that an efficient cascade energy structure for charge transfer is formed.34,35 However, nonfullerene molecules have a planar molecular structure and anisotropic charge transport properties, so for applications in PV cells, they must Received: April 20, 2018 Accepted: July 7, 2018 Published: July 7, 2018 25570

DOI: 10.1021/acsami.8b06445 ACS Appl. Mater. Interfaces 2018, 10, 25570−25579

Research Article

ACS Applied Materials & Interfaces

Figure 1. Chemical structures and relative energy level of (a) first acceptor, PC71BM, second acceptor, IDT2BR, and donor polymer PPDT2FBT and (b) UV−vis absorption of the neat PPDT2FBT, neat IDT2BR, and neat PC71BM films and blend films (c) with different IDT2BR blending ratios.

2. EXPERIMENTAL SECTION

be carefully optimized to prevent strong aggregation, whereas fullerene derivatives have long been used for electron acceptors in OSCs because of their complete miscibility with conjugated polymers.36−40 Thus, ternary blends using both fullerene and nonfullerene acceptors have the potential to exhibit improved organic PV s. In this work, we employed IDT2BR containing the indacenodithiophene (IDT) unit as the core with 5-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)-3-ethyl-2-thioxothiazolidin-4-one (BR) units as end-capping electron-withdrawing groups in combination with the fullerene acceptor phenyl-C71butyric acid methyl ester (PC71BM), which forms an electroncascade structure with an electron donor polymer, PPDT2FBT. The IDT2BR and PC71BM materials form an intermixed alloy phase with energy levels that are tunable by varying IDT2BR:PC71BM blending ratio, which enables the effective control of the open-circuit voltage of the associated ternary OSCs. This alloy acceptor could also enable efficient cascade charge transfer from PPDT2FBT to PC71BM, which would increase the internal quantum efficiency (IQE). We investigated the carrier transport and recombination dynamics of the resulting PPDT2FBT:IDT2BR:PC71BM ternary OSCs. Finally, the alloy acceptor was combined with various polymers [i.e., poly-3-hexyl-thiophene (P3HT), PTB7-th, and PffBT4T2OD] and found to increase the PCEs of the associated devices. Thus, the IDT2BR:PC71BM acceptor alloy can be used to fabricate high-performance ternary OSCs.

2.1. Material Preparation. The PPDT2FBT polymer and IDTbased IDT2BR were synthesized by following previously described procedures.41,42 P3HT was purchased from Rieke Metals and used as received. The polymers PTB7-th and PffBT4T-2OD were purchased from 1-Material. PC71BM (99.5% pure; Nano-C) was used as received. 1,8-Diiodooctane, diphenyl ether, 1-chloronaphthalene, and chlorobenzene were purchased from Sigma-Aldrich. 2.2. Sample Preparation. The ternary blend polymer:IDT2BR:PC71BM was dissolved with various weight ratios in chlorobenzene (1:1.5 donor to acceptor weight ratio, 25 mg·mL−1 in total) and stirred at 60 °C overnight, and then a volume of 20 μL of a solvent additive, either diphenyl ether or 1,8-diiodooctane, was added to 1 mL of each solution, which was stirred for 30 min. PTB7th:PC71BM (1:1.5 weight ratio, 25 mg·mL−1 in total) and PffBT4T2OD:PC71BM (1:1.2 weight ratio, 22 mg·mL−1 in total) were dissolved in chlorobenzene. 2.3. Device Fabrication. Glass substrates coated with indium tin oxide (ITO) were cleaned sequentially with detergent, distilled water, acetone, and isopropyl alcohol. To prepare devices with the standard structure (ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al), poly(3,4ethylenedioxythiophene)−poly(styrene sulfonate) (PEDOT:PSS, Baytron P VP AI4083, Clevios) was spin-coated after UV−ozone treatment onto the substrates and then baked at 120 °C for 30 min in a convection oven. The thicknesses of the PEDOT:PSS layer were measured to be ∼40 nm by using a surface profiler (Alpha-Step 500, Tencor). The polymer and PC71BM blend solution were spin-coated onto the PEDOT−PSS-coated substrates and then soft-baked at 70 °C for 10 min. To deposit the electrodes on the active layer, the samples were transferred into a vacuum chamber (pressure < 1 × 10−6 Torr), and then LiF (0.6 nm)/Al (100 nm) was deposited sequentially on top of the thin films by using thermal evaporation. 25571


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Figure 2. (a) J−V characteristics, (b) EQE and IQE spectra, (c) Jsc−Voc values, and (d) photocurrent density Jph vs effective applied voltage of the ternary photovoltaic devices with different IDT2BR blending ratios. For the inverted structure (ITO/ZnO/polymer:PC71BM/MoO3/ Ag), ZnO nanoparticle solution was synthesized following a procedure described previously,40 spin-coated onto the substrates after filtration through a 0.2 μm polytetrafluoroethylene filter, and then thermally annealed at 200 °C for 1 h. The polymer and PC71BM blend solution was spin-coated onto the ZnO-coated substrates. After transferring the films coated with active layers into a vacuum chamber, MoO3 (3 nm)/Ag (100 nm) was deposited sequentially on top of the thin films by performing thermal evaporation. The electrical characteristics were measured with a source/measure unit (Keithley 4200) in the dark and under 100 mW·cm−2 AM1.5 solar illuminations in a N2-filled glovebox. The light was generated with an Oriel 1 kW solar simulator referenced by using reference cell PVM 132 calibrated at the US National Renewable Energy Laboratory. 2.4. Film Characterization. Grazing incidence X-ray diffraction (XRD) was performed at the 5A and 9C beamlines at the Pohang Accelerator Laboratory (PAL). The two-dimensional grazingincidence X-ray scattering images of the films were analyzed according to the relationship q = 2π/d between the scattering vector q and the d spacing. The grazing incidence wide-angle X-ray scattering images shown are normalized with respect to the exposure time. The X-ray adsorption spectroscopy, ultraviolet photoelectron spectroscopy, and near-edge X-ray absorption fine structure (NEXAFS) spectra were measured at the 4D and 8A2 beamlines at PAL. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) images were obtained by using a MultiMode 8 scanning probe microscope (Veeco Instruments Inc.) and a JEOL JEM-2200FS (with Image Cs-corrector), respectively. UV−vis spectra were recorded on a Varian CARY-5000 UV−vis spectrophotometer. Photoluminescence (PL) was measured with an FP-650 (JASCO Corporation).

Figure 1a. The LUMO and HOMO energy levels of IDT2BR are located between those of PPDT2FBT and PC71BM, which is expected to facilitate charge transfer. The LUMO offset between PPDT2FBT and IDT2BR is 0.18 eV, and that between IDT2BR and PC71BM is 0.23 eV. These values are sufficient to drive electron transfer from the donor to the acceptor.43,44 The absorption spectra of pristine PPDT2FBT, IDT2BR, and PC71BM films were recorded (Figure 1b). The absorption profiles of the three films complement each other, with complete coverage from 300 to 750 nm. The spectrum of the neat PC71BM film contains a strong absorption peak at 300 nm and a continuously decreasing profile for longer wavelengths up to 700 nm. The neat PPDT2FBT film exhibits the typical absorption behavior of conjugated donor/acceptor type copolymers, with two broad absorption peaks at 350−450 nm for band I originating from the π−π* transition of the conjugated backbone and 500−700 nm for band II originating from intramolecular charge transfer. The optical band gap of the PPDT2FBT polymer was determined to be 1.76 eV based on its λonset of 705 nm. The spectrum of the pristine IDT2BR film also contains two distinct absorption peaks, 350−450 nm for band I and 500−750 nm for band II, with a band edge at 750 nm that is slightly red-shifted with respect to that of PPDT2FBT. The optical band gap estimated from the absorption edge of the IDT2BR film is 1.65 eV. As the relative proportion in the ternary blend film of IDT2BR with respect to PC71BM increases, the absorption edges shift to a longer wavelength from 700 to 750 nm, and the intensities of the absorption due to the PC71BM material at 300 nm decrease (Figure 1c). To investigate the PV properties of the ternary blend films, we fabricated PV devices with the standard structure ITO/ PEDOT:PSS/PPDT2FBT:IDT2BR:PC71BM/LiF/Al. The J− V curves and the PV parameters of ternary solar cells with various weight ratios of IDT2BR with respect to PC71BM (0, 10, 20, 40, 60, 80, and 100% IDT2BR) are shown and summarized in Figure 2a and Table 1, respectively. The Jsc

3. RESULTS AND DISCUSSION In this study, we selected three conjugated semiconductors as constituents of the ternary blend system: the polymer PPDT2FBT as the electron donor, IDT2BR as the nonfullerene electron acceptor, and PC71BM as the fullerene electron acceptor (Figure 1).41,42 IDT2BR and PC71BM acceptor blends with various compositions were mixed with the PPDT2FBT polymer. The HOMO and LUMO energy levels of PPDT2FBT, IDT2BR, and PC71BM are compared in 25572


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1.97 3.92 3.26 9.32 3.10 1.63 3.36

× × × × × × ×

10−7 10−8 10−8 10−9 10−9 10−10 10−11

1.77 1.69 1.71 1.64 1.63 1.54 1.65

2.59 2.27 2.49 7.53 13.4 22.0 19.3

124 176 227 434 1014 3042 1867

0.821 0.856 0.876 0.888 0.917 0.972 1.115

The efficiencies were averaged from 12 devices. The values in brackets are average and standard deviation. bJ0, n, Rs, and Rp were calculated from the measured J−V curves under dark using Shockley ÅÄÅ ÑÉÑ V − J R q(V − Jd R s) − 1ÑÑÑÑ + R d s , where Jd is the current density through the diode, J0 is the saturation current density, k is the Boltzmann constant, T is the temperature in kelvin, V is equation, Jd = J0 ÅÅÅÅexp nkT ÅÅÇ ÑÑÖ p the external bias applied to the diode, n is the ideality factor of the diode, Rs is the serial resistance, and Rp is the parallel resistance. cCalcd Voc: calculated Voc was obtained based on the equation qVoc = nkT ln(Jsc/J0).

values of the ternary OSCs are enhanced remarkably by the incorporation of IDT2BR and are highest at 10% IDT2BR. This increase is presumably due not only to the extension of the light absorption range to ∼750 nm but also to the formation of a charge cascade energy landscape. In particular, the addition of IDT2BR to the acceptor phase is expected to enhance the charge collection of the blend, as is confirmed by the high external quantum efficiency (EQE) and IQE of the 10% IDT2BR ternary blend (the dashed lines in Figure 2b). The IQE values of the ternary blend in the absorption range from 600 to 750 nm are more than 10% higher than those of the binary blend. The Voc values of the ternary OSCs continuously increase from 0.78 to 1.11 V as the proportion of IDT2BR increases (Figure 2c). The diode parameters were obtained by fitting the dark J−V characteristics of the devices with the Shockley equation (Figure S1 and Table 1).45,46 If we assume that Rs is small and Rp is large, then Voc is strongly dependent on J0 as given by the equation47,48

0.70 (0.69 ± 0.014) 0.71 (0.70 ± 0.009) 0.69 (0.68 ± 0.008) 0.60 (0.58 ± 0.019) 0.42 (0.44 ± 0.019) 0.39 (0.41 ± 0.007) 0.55 (0.55 ± 0.002)

ji J zy qVoc ≈ nkT lnjjjj sc zzzz j J0 z k {

0.79 (0.79 ± 0.004) 0.82 (0.82 ± 0.004) 0.84 (0.83 ± 0.005) 0.88 (0.88 ± 0.006) 0.91 (0.90 ± 0.005) 0.98 (0.98 ± 0.005) 1.11 (1.10 ± 0.008)

)

a

(

14.75 (14.37 ± 0.427) 15.50 (15.35 ± 0.413) 14.88 (13.93 ± 0.417) 13.68 (12.91 ± 0.746) 10.23 (9.13 ± 0.298) 7.71 (6.99 ± 0.376) 8.40 (7.97 ± 0.511) 8.16 (7.78 ± 0.300) 9.02 (8.84 ± 0.335) 8.62 (7.95 ± 0.278) 7.22 (6.60 ± 0.589) 3.91 (3.60 ± 0.282) 2.95 (2.79 ± 0.122) 5.16 (4.86 ± 0.355) 0:10 1:9 2:8 4:6 6:4 8:2 10:0

(1)

where J0 is the saturation current density, Jsc is the short-circuit current density under illumination, k is the Boltzmann constant, T is the temperature in kelvin, and n is the ideality factor. The Voc values calculated from J0 are in good agreement with the measured Voc values with a small relative error. As the proportion of IDT2BR increases, J0 decreases from 1.97 × 10−7 to 3.36 × 10−11 mA·cm−2 because J0 is primarily dependent on the energy level difference, ΔE, between the HOMO of the donor and the LUMO of the acceptor with the equation J0 = J00·exp(−ΔE/nkT), where J00 is the pre-exponential factor. These results indicate that variations in the short-circuit current and the resistances have only minimal effects on the measured Voc and that the characteristics of the diode in the dark are useful for the analysis of the Voc values of these ternary blend OSCs. To investigate the photon-harvesting and exciton dissociation process in the active layers, the photocurrent density (Jph) versus effective voltage (Veff) curves were determined for the ternary OSCs (Figure 2d). Here, Jph is defined by the equation Jph = Jl − Jd, where Jl and Jd are the current densities under 100 mW·cm−2 illumination and in the dark, respectively. Veff is defined by the equation Veff = V0 − Va, where V0 is the voltage when Jph = 0 and Va is the applied bias. It is apparent that the Jph values of the PPDT2FBT:PC71BM-based binary solar cells and of the ternary solar cells with an IDT2BR blending ratio of 10 wt % rapidly reach the saturation state at Veff = ∼0.2 V, which indicates that efficient exciton dissociation and charge carrier collection occur in the corresponding cells. IDT2BR blending ratios higher than 20 wt % produce an unsaturated state even at Veff = 1 V, which is correlated with less efficient exciton dissociation and charge collection. The Gmax values were determined from the equation Jsat = qLGmax, where q is the elementary charge, L is the thickness of the active layers, and Jsat is the saturation current density at Veff = 2 V, and compared. The Gmax values of the PPDT2FBT:PC71BM-based and the optimized PPDT2FBT:IDT2BR:PC71BM-based solar cells were found to be 0.809 × 1028 and 0.932 × 1028 m−3·s−1, respectively, which suggests that the ternary active layers with an IDT2BR blending ratio of 10 wt % exhibit increased overall

14.31 15.15 14.52 12.96 9.57 6.98 8.94

J0b (mA·cm−2) FF Voc (V) calcd Jsc from EQE (mA·cm−2) Jsc (mA·cm−2) efficiencya (%) acceptor ratio (IDT2BR:PC71BM)

Table 1. Photovoltaic Parameters of PPDT2FBT:IDT2BR:PC71BM Solar Cells with Various Ratios of IDT2BR:PC71BM

n

Rs (Ω·cm2)

Rp (kΩ·cm2)

calcd Vocc (V)


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Figure 3. (a) EQE spectra and (b) specific EQE values at the selected wavelengths of the ternary photovoltaic devices with different IDT2BR blending ratios. (c,d) PL spectra of the neat polymer, neat IDT2BR, neat PC71BM, and binary blend films (1:1 ratio) under the specific excitation wavelengths of 600 and 750 nm.

blend film exhibits complete PL quenching with an efficiency of 96%, which indicates that the charge transfer from IDT2BR to PC71BM is efficient. To further investigate the charge-transfer properties of the ternary blend films, the PL emission under a selective excitation wavelength for the IDT2BR material (λex = 750 nm) was measured (Figure 3d). The PL spectra of neat IDT2BR and the IDT2BR:PDPT2FBT (1:1) blend film have similar intensities, whereas the IDT2BR:PC71BM (1:1) blend film exhibits effective quenching. These results confirm that the excited charge carriers in IDT2BR are effectively transferred toward PC71BM but less effectively toward PPDT2FBT. This conclusion is in agreement with the observation that the IDT2BR-loaded ternary blend exhibits an enhanced EQE in the 700−750 nm region only when it is blended with less than 40% IDT2BR. We suggest that an IDT2BR:PC71BM acceptor alloy with a low IDT2BR proportion will form an efficient charge cascade energy landscape exhibiting charge flow from PPDT2FBT to IDT2BR and PC71BM. We confirmed that the binary blends PPDT2FBT:IDT2BR and IDT2BR:PC71BM exhibit PV behaviors with high Voc of 1.1 and 0.86 V, respectively, although the IDT2BR:PC71BM device exhibits low Jsc and fill factor (FF) because of its unoptimized and overly intermixed nanomorphology (Figure S2). When IDT2BR is introduced into the ternary blend, the resulting Voc increases with the weight ratio in the blend of IDT2BR with respect to PC71BM. This variation in Voc can be correlated with the LUMO energy level of the acceptor alloy phase. The HOMO and LUMO energy levels of the acceptor alloys with various IDT2BR:PC71BM weight ratios were measured in an ultrahigh vacuum (base pressure 2 × 10−10 mbar) by using ultraviolet photoelectron spectroscopy and NEXAFS spectroscopy (Figures 4a and S3),49−51 which showed that there is a linear increase in the LUMO and HOMO energy levels of the acceptor alloy with increases in the IDT2BR:PC71BM blend ratio. The details of the calculation of energy levels are explained in the Supporting

exciton generation, which is presumably due to the increased light absorption of the ternary blend layer. To clarify the role of IDT2BR in the internal charge transfer, EQEs and PLs of the blend films were measured (Figure 3). The EQE spectra of the ternary PV devices with various IDT2BR blending ratios are compared in Figure 3a. The EQE values of the ternary PV devices continuously decrease as the blending ratio of IDT2BR rises above 20%. To distinguish charge transfer involving IDT2BR from those of other materials, we focused on the EQE values in the wavelength range from 700 to 750 nm, which is the dominant absorption range only for IDT2BR among the blend components. In this absorption region, the EQE values increase as the blending ratio of IDT2BR increases up to 40% and then decrease for higher IDT2BR proportion. The EQE values at the selected wavelengths of 720 and 750 nm are plotted in Figure 3b to illustrate this trend. These results indicate that a PC71BMabundant acceptor alloy (up to 40% IDT2BR) can efficiently transfer the photogenerated charge carriers through the IDT2BR materials, whereas an IDT2BR-abundant acceptor alloy (higher than 40% IDT2BR) transfers the photogenerated charge carriers inefficiently to the charge-collecting electrodes. To gain deeper insights into these charge-transfer characteristics, the PL emissions of the IDT2BR:PC71BM blend films with various compositions but identical thicknesses were recorded (Figure 3c). The neat IDT2BR film exhibits strong PL emission with a sharp emission peak at approximately 730 nm. The neat PPDT2FBT film produces a rather weak PL emission composed of two broad emission peaks at ∼725 and ∼760 nm. The PL spectra of those two films partially overlap with slightly different peak maxima. For the PPDT2FBT:IDT2BR (1:1) blend film, the intensity of the PL emission is quenched more than 60% compared to that of the neat IDT2BR film, but the PL spectrum of the blend film contains the PL peak shape of the neat IDT2BR film, which indicates that hole transfer from IDT2BR to PPDT2FBT occurs incompletely. In contrast, the IDT2BR:PC71BM (1:1) 25574


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2 is IDT2BR. Ne is the total electron density of states of unit mass. Ne of PC71BM and IDT2BR can be calculated by using the equation Ne = nl, where n is the molecular number of unit mass and l is the number of quasi-degenerate LUMOs (1 V and High Efficiency >10% in Fullerene-Free Polymer Solar Cells via Energy Driver. Adv. Mater. 2017, 29, 1605216. (25) Mollinger, S. A.; Vandewal, K.; Salleo, A. Microstructural and Electronic Origins of Open-Circuit Voltage Tuning in Organic Solar Cells Based on Ternary Blends. Adv. Energy Mater. 2015, 5, 1501335. (26) Ko, S.-J.; Lee, W.; Choi, H.; Walker, B.; Yum, S.; Kim, S.; Shin, T. J.; Woo, H. Y.; Kim, J. Y. Improved Performance in Polymer Solar Cells using Mixed PC61BM/PC71BM Acceptors. Adv. Energy Mater. 2015, 5, 1401687. (27) Lin, Y.; Zhao, F.; He, Q.; Huo, L.; Wu, Y.; Parker, T. C.; Ma, W.; Sun, Y.; Wang, C.; Zhu, D.; Heeger, A. J.; Marder, S. R.; Zhan, X. High-Performance Electron Acceptor with Thienyl Side Chains for Organic Photovoltaics. J. Am. Chem. Soc. 2016, 138, 4955−4961. (28) Singh, R.; Lee, J.; Kim, M.; Keivanidis, P. E.; Cho, K. Control of the molecular geometry and nanoscale morphology in perylene diimide based bulk heterojunctions enables an efficient non-fullerene organic solar cell. J. Mater. Chem. A 2017, 5, 210−220. (29) Fan, Q.; Wang, Y.; Zhang, M.; Wu, B.; Guo, X.; Jiang, Y.; Li, W.; Guo, B.; Ye, C.; Su, W.; Fang, J.; Ou, X.; Liu, F.; Wei, Z.; Sum, T.

enhances both their Jsc and Voc values. This study has demonstrated the potential of the ternary blend solar cell with nonfullerene molecules, which have tunable energy states and form well-mixed morphologies with various types of donor polymers and PC71BM.

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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.8b06445. Dark J−V curves, UPS and NEXAFS spectra, AFM images, XRD patterns, SCLC curves, water contact angle, and TEM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Han Young Woo: 0000-0001-5650-7482 Kilwon Cho: 0000-0003-0321-3629 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.K. and J.L. contributed equally to this work. The authors thank the Pohang Accelerator Laboratory (PAL) for providing the synchrotron radiation sources at the 3C and 9A beamlines for XRD measurements and the 9D beamline for UPS and NEXAFS measurements. This work was supported by a grant (code no. 2011-0031628) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science and ICT, Korea.

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REFERENCES

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Fullerene Acceptor Blend with a Tunable Energy State

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