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High-Efficiency Non-fullerene Polymer Solar Cells with Bandgap and Absorption Tunable Donor-Acceptor Random Copolymers Da Hun Kim, Thi Thu Trang Bui, Shafket Rasool, Chang Eun Song, Hang Ken Lee, Sang Kyu Lee, Jong Cheol Lee, Won-Wook So, and Won Suk Shin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16202 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 21, 2018

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ACS Applied Materials & Interfaces

High-Efficiency Non-fullerene Polymer Solar Cells with Bandgap and Absorption Tunable DonorAcceptor Random Copolymers Da Hun Kim,a,b Thi Thu Trang Bui,c Shafket Rasool,a,b Chang Eun Song,*,b,d Hang Ken Lee,d Sang Kyu Lee,a,b Jong-Cheol Lee,a,b Won-Wook So,d and Won Suk Shin*,a,b,e

aAdvanced

Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141

Gajeongro, Yuseong, Daejeon 34114, Republic of Korea. E-mail: [email protected] bAdvanced

Materials and Chemical Engineering, University of Science and Technology (UST),

217 Gajeongro, Yuseong, Daejeon 34113, Republic of Korea. cFaculty

of Chemistry, Hanoi University of Industry, 298 Cau Dien, Minh Khai, Bac Tu Liem,

Hanoi, Vietnam. dEnergy

Materials Research Center, Korea Research Institute of Chemical Technology (KRICT),

141 Gajeongro, Yuseong, Daejeon 34114, Republic of Korea. E-mail: [email protected] eKU-KRICT

Collaborative Research Center & Division of Display and Semiconductor Physics &

Department of Advanced Materials Chemistry, Korea University, 2511 Sejong-ro, Sejong 30019, Republic of Korea.

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords: non-fullerene organic solar cell, complementary light absorption, morphology, energy level modulation, D-A random copolymer


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Abstract

Energy levels alignment between donor and acceptor has a critical role to determine open-circuit voltage (VOC) in polymer solar cells (PSCs). Also broad absorption of photoactive layer is required to generate high photocurrent. Herein, non-fullerene PSCs with D-A random copolymers and ITIC has been demonstrated. The D-A random copolymers are composed of 2-ethylhexylthienyl substituted benzodithiophene (BDT) donor unit (D) and ﬂuorinated thienothiophene (TT-F) acceptor unit (A). By controlling D:A unit ratio in polymer backbone, it is attainable to modulate both energy levels and absorption spectra of random copolymers. As the ratio of donor unit in polymer back bone increase, the highest occupied molecular orbital (HOMO) energy level is located in deeper, leading to higher VOC. Also absorption spectra of random copolymers become blue shifted with an increase of donor unit ratio, it compensates the weak absorption region of ITIC. This complementary absorption enhances the photocurrent, leading to higher power conversion efficiency (PCE). Due to optimization of D:A ratio of random copolymers, the notable PCE of 10.27% can be achieved in PSCs with D5A and ITIC.



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1. Introduction For several decades, the advancement of polymer solar cells (PSCs) has been carried out employing fullerene derivatives as electron acceptors, such as phenyl-C61-butyric acid methyl ester (PC61BM) and phenyl-C71-butyric acid methyl ester (PC71BM).1,2 Nevertheless, fullerene derivatives have certain limitations in terms of their inefficacy in tuning of their energy levels, poor stability due to aggregates formations under light and heat, and weak absorption properties in visible region.3,4 In order to overcome these drawbacks, small molecule non-fullerene acceptors (SM-NFAs) have been in the limelight as the replacement for fullerene derivatives because of their striking features which include: (i) favorable molecular energy levels with respect to electron donors, (ii) outstanding absorption in the visible region, (iii) good thermal stability, (iv) potential for low cost synthesis, and all of these factors lead to intensive developments toward non-fullerene PSCs.5-16 Among

the

recently

developed

family

of

SM-NFAs,

3,9-bis(2-methylene-(3-(1,1-

dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-sindaceno[1,2-b:5,6-b’]dithiophene (ITIC) is one of the promising candidate to be employed as an electron acceptor, while among the electron donor polymers, PTB7-Th (P(BDTT-alt-TT-F)) is probably the best candidate. PTB7-Th is most widely studied polymer in PSCs so far due to its reasonably low band-gap and high power conversion efficiency (PCE) when blended with PC71BM.17-23 On the other hand, upon blending PTB7-Th with ITIC, it showed a PCE of only around 6%.10,14 The reason of low PCE lies in the fact that the absorption spectrum of both photoactive materials overlaps, i-e the light absorption spectrum in the range of 500-800 nm is quite similar (Figure 1b). Since, an important prerequisite of photovoltaic applications is broad and strong optical absorption to yield high photocurrent, therefore, there is plenty of scope for


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improvement of efficiency by modulating absorption region of polymer to harness the wide range of light wavelength.24-28 Modulation of the energy levels between the donor polymer and SM-NFA can lead to increased HOMOD-LUMOA offset. This well alignment of energy level affords to produce higher open-circuit voltage (VOC) in non-fullerene (NF) based PSCs (NF-PSCs), leading to their outstanding performances.11 Recently, there have been reports on employing ITIC as electron acceptors with various electron donor polymers and the PCEs with over 11% have been demonstrated.5,29-34 However, in order to explore the compatibility of structurally modified electron donor polymer with ITIC, in this study, we have introduced a high-performance non-fullerene PSCs with random copolymers which are composed of 2-ethylhexylthienyl substituted benzo[1,2-b:4,5-b’]dithiophene (BDT) donor unit (D) and ﬂuorinated thieno[3,4-b]thiophene (TT-F) acceptor unit (A) (P(BDTT-alt-TT-F)). These random copolymers used in this study were already reported in binary systems with PC71BM and polymer acceptor,35,36 but never been utilized with NF-SMAs. The polymers used in this study have been synthesized according to the already reported procedure.35 Since there was almost a complete overlap of absorption region by P(BDTT-alt-TT-F) and ITIC, alteration to the D/A ratio within the donor polymer can effectively modulate the energy levels and thus their absorption regions. With the increase of the D-unit in the polymer backbone from D1A to D5A actually widened the optical bandgap and downshift the HOMO energy level, which contributes to achieve higher VOC upon blending with ITIC SMA in NF-PSCs. In addition, the number of donor units in polymer backbone shifts the absorption maxima and onset wavelength to shorter wavelength. This blue shifted absorption of polymer makes up the weak absorption region of ITIC, leading to complementary absorption and ultimately a higher photocurrent.



An optimized device containing D5A (5:1 donor-acceptor ratio) copolymer exhibited PCE of 10.27% among all D-A copolymers tested with ITIC, which was superior than PCE of D1A:ITIC (8.57%). This remarkable result came from adequate energy levels alignment and complementary absorption with SM-NFA. Furthermore, grazing incidence wide-angle X-ray scattering (GIWAXS) and grazing incidence small-angle X-ray scattering (GISAXS) measurements have shown that the optimum morphology with domain size of nearly 22 nm in D5A:ITIC blend film was observed. This optimum morphology consisting of smaller domains to have higher donor acceptor interfacial area enhances the charge transport and suppresses charge recombination, thus improving electron/hole mobility and the short circuit current (JSC).37,38

2. Experimental Section 2.1 Device fabrication and characterization Following inverted device architecture is used in current study: indium tin oxide (ITO)/zinc oxide nanoparticles (ZnO NPs)/polyethyleneimine ethoxylated (PEIE)/photoactive layer/MoO3/Ag. The device area of the solar cells is 9mm2 and is defined by using the shadow mask. For the measurement of PCEs, Polaronix K201 solar simulator (McScience, Inc.) is used and K3100 spectral IPCE measurement system (McScience Inc.) is used for the external quantum efficiency (EQE) measurement. Before use of the solar simulator, it was calibrated with NREL-certified Si diode and the PCEs are measured under 1 sun in air and at room temperature. 2.2 Synchroton X-ray diffraction analysis


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Beamline PLS-II 3C at Pohang Accelerator Laboratory (PAL), South Korea is used for the GIWAXS and GISAXS analysis. Photoactive films were coated under optimized conditions on the Si substrate and used for the GIWAXS and GISAXS measurements. 3. Results and Discussion The molecular structures of D-A copolymers and ITIC are shown in Figure 1a. To investigate the effect of light absorption by D:A ratio, the pristine and blend films of various D-A copolymers and ITIC (1:1.5 w/w ratio) were prepared with 100 nm thicknesses and normalized absorbances were measured with the wavelength of light, respectively (Figure 1b and c). In Figure 1b, maximum absorption peaks of copolymers gradually blueshifted with the ratio of BDT donor unit (D). And these maximum absorption peaks of copolymers appeared in blend films of Figure 1c. These blue shifted peaks make the colour of blend films from the light to the deep blue with the raise of donor unit due to complementary absorption between copolymers and ITIC (Figure 1d). To probe the photovoltaic performance of random copolymers with ITIC, firstly devices were fabricated with inverted structure (ITO/ZnO NPs/PEIE/polymer:ITIC/MoO3/Ag). The highest efficiencies obtained from the devices with D1A, D3A, D5A, D7A were 8.57, 9.30, 10.27, 9.03%, respectively, which were measured under simulated AM 1.5G at 100 mW cm-2 (Table 1). Current density-voltage (J-V) characteristics of each devices are shown in Figure 2a. JSC and VOC variations with D:A ratio, are represented in Figure 2b. JSC shows that from D1A the higher the donor ratio, the higher the value until D5A, then reduced at D7A. VOC also increased with increasing donor ratio and gained the highest value in D5A, and then dropped slightly in D7A. This result indicates that it’s possible to optimize the efficiency by controlling D:A ratio of copolymer. D5A:ITIC device shows best performance out of random copolymers PSCs, on the other hand, it’s also



worthwhile to study D1A:ITIC blend as a reference because alternative D1A (PTB7-Th) is one of the prominent polymer which is widely used donor material in PSCs field.2 Therefore, this study intensively focuses a discussion on D5A and D1A. In order to optimize the conditions of D5A:ITIC device fabrication, we carried out with varying the blend ratio, annealing temperature, and active layer thickness (Figure S1-3 and Table S1-S3). For commercially available PSCs, the tolerance of the blend ratio, annealing temperature, and active layer thickness is crucial to develop PSCs with excellent photovoltaic performance. Supplementary information shows photovoltaic performance of D5A:ITIC devices exhibits nearly constant with over 9% efficiency depending to the blend ratio, annealing temperature, and active layer thickness. These excellent photovoltaic performance of D5A:ITIC devices makes it a good candidate for commercial application of the PSCs. In Figure 1b, wavelength of maximum absorption (λmax) decreased from 700 to 549 nm and onset wavelength of film states also decreased from 786 to 711 nm as a result of the blue shift from D1A to D5A. The optical bandgap (Egopt) was calculated from onset wavelength in UV-visible spectra of film states and it increased from 1.58 eV for D1A to 1.74 eV for D5A. Cyclic voltammetry (CV) was utilized to investigate the electrochemical properties by assuming the energy level of ferrocene (Fc) is -4.8 eV relative to the vacuum level (Figure S4). The lowest unoccupied molecular orbital (LUMO) energy level was calculated utilizing the relationship of energy levels and Egopt (ELUMO = EHOMO + Egopt).6 The estimated HOMO levels are -5.26, -5.37 eV and LUMO are -3.68, -3.69 eV for D1A and D5A, respectively. As ratio of donor unit increase from D1A to D5A, HOMO level lies in deeper and band gap become wider. These results indicate that it is possible to control the absorption spectrum and energy levels of random copolymer by altering the ratio between donor and acceptor moiety in polymer backbone.24 Energy level diagram of D1A, D5A and ITIC is prepared in Figure 3a. The enhanced open circuit voltage (VOC) value of 0.89 in D5A blend was


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observed compared to 0.81 for D1A blend. This higher VOC value is mainly attributed to deeper HOMO level of D5A polymer than that of D1A polymer as increasing ratio of BDT donor moiety in polymer backbone. Short circuit current densities (JSC) generated in D1A and D5A based devices were 16.09 mA cm-2 and 17.24 mA cm-2, respectively. To confirm the actual JSC generated in each cell at a given wavelength, EQE of the cells are measured. The mismatch of the JSC between the measured solar simulator and the EQE measurement has to be as low as possible.39,40 The calculated JSC from the integration of EQE response are 15.36 mA/cm2 and 16.51 mA/cm2 for NF-PSCs with D1A:ITIC and D5A:ITIC, respectively (Table 1). The measured JSC obtained from AM 1.5 solar simulation are within 5% error to the calculated values. EQE curves are shown in Figure 3b and 3c. To confirm the complementary absorption effect on current density, partial EQE of donor and acceptor were derived from fractional contributions of donor (CD) and acceptor (CA) absorbance.41 The CD and CA were calculated from total absorbance of the blend film (Figure S5 and Table S4), assuming identical internal quantum efficiencies for all component. In EQE spectra of D1A:ITIC, overlapped region of partial EQE between donor and acceptor is dominant. In D5A:ITIC, on the other hand, partial EQE of D5A contributes to EQE around 400~600 nm and ITIC contributes to 600~800 nm, mainly. It represents that D5A polymer makes up the weak absorption region of ITIC, resulting in complementary absorption and higher JSC. This complementary absorption in D5A:ITIC film efficiently quenched photon excitation on 500 and 700 nm wavelength, comparing pristine polymers, ITIC and D1A:ITIC blend film (Figure S6). In addition, hole and electron mobilities of the D1A and D5A blends with ITIC were measured by space charge limited current (SCLC) method (Figure S7). The hole mobilities for D1A and D5A blends were 1.99 × 10-4 and 2.98 × 10-5 cm2 V-1 s-1. The electron mobilities for D1A and D5A



blends were 2.64 × 10-5 and 3.46 × 10-5 cm2 V-1 s-1. Both hole and electron mobilities of D5A blend were higher than that of D1A blend. This higher mobility of D5A blend may contributed to improved current density and FF, due to more favorable charge transfer. In order to have deeper insight of the enhanced PCEs, correlation between charge recombination kinetics and performance of PSCs was studied. The dependence of JSC and VOC with various light intensities (I) provides meaningful information on the recombination process.42-44 It is turned out that JSC shows a power-law dependence on light intensity in PSCs : JSC ∝ Iα, where I is light intensity and α is the exponential factor. Under ideal conditions bimolecular recombination was minimized α values becomes unity, whereas less values of α from unity represents the possibility of bimolecular recombination.43,44 Figure 4a shows the double logarithmic plots of JSC ∝ Iα relationship in D1A and D5A blend devices. The calculated α values of 0.927 was identical in D1A and D5A blend devices. This result indicated that both D1A and D5A blend devices exhibit similar tendency of bimolecular recombination. The J-V characterizations for D1A and D5A blend devices under 3.2~100 mW cm-2 are prepared in Figure S8. The type of recombination behavior can be determined from the slope of VOC versus log(I). The slope of kT/q implies that bimolecular recombination is dominant ; whereas a slope of 2kT/q is seen for monomolecular dominant system.42,45,46 The D1A and D5A blend devices showed slopes of 1.32 and 1.15 kT/q in open circuit condition, respectively. This result represented D5A blend devices had less monomolecular recombination and reduced interfacial trap densities within active layer in device, thus led to relatively higher JSC and FF in D5A blend device than D1A blend device. To further understand the evolution of the active layer morphology in film states, structure orderings of D1A, D5A and ITIC and its blends were characterized by grazing incidence wide-


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angle X-ray scattering (GIWAXS).47,48 The detailed packing parameter calculated form GIWAXS profiles are listed in Table S5. As Figure S9, both of pristine D1A and D5A copolymer films had (010) peak in out-of-plane direction (OOP) and (100) peak in in-plane (IP) direction. Comparing two of pristine random copolymer, D1A had prominent face-on orientations and its π- π stacking distance was shorter than that of D5A. This result can implies that it is beneficial for D1A to yield high carrier mobility via vertical charge transport. It was certain by fabricating hole-only devices with pristine D1A and D5A copolymer (Figure S10). All of the blend films with ITIC, however, exhibited similar molecular orientation : (010) peak in OOP direction became fade and broaden, compared to pristine polymer films and (100) peak in IP direction was maintained and lamellar stacking distance was decreased to 19.76 Å for D1A and 19.37 Å for D5A blend, respectively (Figure 5). Hence significant difference of crystallinity between D1A and D5A blend was not observed as polymers were blended with ITIC in the film states. Additionally, the domain sizes of blend films were estimated via grazing incidence small-angle Xray scattering (GISAXS). Figure 6 shows 2D GISAXS patterns of copolymer:ITIC blend films and corresponding in-plane profiles. At low q region, scattering intensity follows Guniner approximation.49

𝐼(𝑞) = 𝐼(0)exp ( ―

𝑞2𝑅2𝑔 3

)

(1)

The q in equation (1) is IP direction scattering parameter and I(0) is the zero-angle scattering intensity. The domain sizes in blend films, denoted as Rg, were derived from the slopes of inset lines in Figure 6c. The Rg values were 28 nm and 22 nm for D1A:ITIC and D5A:ITIC film. Smaller domain size can afford to generate better inter-mixing and larger interfacial area between donor and acceptor, resulting in higher electron/hole mobility in D5A:ITIC active layer.



The detailed morphology interpretation of blend films was examined by AFM and TEM in nanoscale (Figure 7). Similar morphology and phase separation were observed from AFM phase and TEM images, comparing D1A and D5A blend films. This similarity of morphology between D1A and D5A blend films is consistent with the interpretation of GIWAXS, discussed above. This resemblant morphology of two kind of blend films can imply that the interactions of D1A:ITIC and D5A:ITIC are almost similar. This resemblance of morphological features may come from structure similarity between D1A and D5A, including identical monomer units. From AFM height image, however, D5A and ITIC represented excellent miscibility with each other, D5A blend film exhibited relatively smooth surface with a root-mean-square (RMS) roughness of 2.76 nm than D1A blend film with a RMS roughness of 4.77 nm. The phase separation and surface morphology can be investigated by power spectral density (PSD), obtained from AFM phase images. The average PSD of three different samples are displayed in Figure 8. The PSD curves of active layer films can be expressed by k-correlation (or ABC model). 𝐴

PSD(𝑓) = (1 + 𝐵2𝑓2)(𝐶 + 1)/2

(2)

where f is spatial frequency, A, B, and C are function parameters, summarized in Table S6.50,51 The B parameter determines the knee position in curve, which is related to the size of grain in lateral direction. The D5A:ITIC had smaller value scale of 85 nm than D1A:ITIC of 97 nm. Slightly small grain size seemed to induce smooth surface of film, lower RMS roughness, reflecting well miscibility of donor and acceptor. This well-distributed components may improve the charge transport and separation, and thus lead to increased JSC and FF.10,52,53


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4. Conclusions In summary, D-A random copolymer were developed for high efficiency non-fullerene PSCs. Herein, the simple method to control energy levels and absorption region was introduced by adjusting D:A composition in polymer backbone. By raising the donor ratio in polymer, absorption spectrum became blue shifted and HOMO level deepened, leading to improved VOC and JSC through well-aligned energy levels and complementary light absorption between polymer and SMNFA. The optimized random copolymer:ITIC device exhibited remarkable performance of 10.27% PCE. Thus, this study suggests utility of D-A random copolymers as a one of the effective way to enhance the performance of PSCs without sacrificing structural, electrical, and morphological properties.



Figure 1. a) Chemical structures and b) normalized absorbance of b) neat and c) blend D-A copolymers and ITIC films. d) The image of 100 nm thick D-A copolymers:ITIC blend films (1:1.5 w/w ratio).

Figure 2. a) Current-voltage (J-V) curves and b) VOC and JSC variation of D-A copolymer:ITIC PCSs.


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Table 1. The optimized photovoltaic parameters of devices based on D1A~D7A:ITIC under the illumination of AM 1.5G, 100 mW cm-2. Polymera

VOC [V]

JSC [mA/cm2]

JSC (calc.)c [mA/cm2]

FF [%]

PCE [%]

D1A

0.81 (0.81±0.01)b

16.09 (15.88±0.20)b

15.36

66 (65±1)b

8.57 (8.44±0.13)b

D3A

0.87 (0.87±0.01)b

16.58 (16.39±0.19)b

-

65 (66±1)b

9.30 (9.12±0.22)b

D5A

0.89 (0.88±0.01)b

17.24 (17.13±0.12)b

16.51

67 (67±1)b

10.27 (10.12±0.14)b

D7A

0.88 (0.88±0.01)b

16.71 (16.60±0.10)b

-

62 (62±1)b

9.03 (8.88±0.16)b

aInverted

device architecture is ITO/ZnO NPs/PEIE/Polymer:ITIC(d = ~90nm)/MoOX/Ag.

bThe

average PCE in the brackets is obtained from over 20 independent devices.

cThe

JSC calculated from the integration of EQE measurement.



Figure 3. a) The energy level diagram of D1A, D5A polymer and ITIC. b) and c) Measured EQEs (black dots) and calculated partial EQEs of copolymers (red dots) and ITIC (blue dots) for D1A:ITIC and D5A:ITIC devices.


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Figure 4. a) and b) Dependence of JSC and VOC on light intensity for D1A:ITIC and D5A:ITIC device.



Figure 5. 2D GIWAXS patterns of a) D1A:ITIC and b) D5A:ITIC blend films. c) In-plane and d) out-of-plane line-cut profiles of GIWAXS images.


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Figure 6. 2D GISAXS patterns of a) D1A:ITIC and b) D5A:ITIC blend films. c) In-plane linecut profiles of GISAXS images and fitting lines via Guniner approximation.



Figure 7. Tapping mode AFM topographic and phase images of a), b) D1A:ITIC and d) ,e) D5A:ITIC blend films. TEM images of c) D1A:ITIC and f) D5A:ITIC blend films.

-2

Power Spectral Density (nm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1 0.01 1E-3

D1A:ITIC D5A:ITIC

1E-4 1

10

100 -1

Spatial frequency (μm )

Figure 8. PSD plots for D1A:ITIC and D5A:ITIC blend films in optimized condition of device fabrication. The fitting curves via ABC model are displayed as lines.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed Experimental section, additional figures and tables as mentioned in the manuscript The photovoltaic parameters of D5A/ITIC with different blend ratio, annealing temperature and photoactive thickness; Cyclic voltammetry results of D1A and D5A; UV-vis absorption of D1A:ITIC and D5A:ITIC; Photoluminescence spectra of the pristine polymer, ITIC and blend film; SCLC measurement of neat and blend of the pristine polymer, ITIC and blend film; Dependence of photovoltaic properties on light intensity; 2D GIWAXS results; PSD obtained from AFM phase images. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C.E.S.) *E-mail: [email protected] (W.S.S) Notes The authors declare no competing of financial interest Acknowledgements The authors would like to acknowledge the financial support granted by National Research Foundation (NRF) (NRF-2015M1A2A2056214 & 2015M1A2A2055631) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20173010012960 & 20183010013820) of the Republic of Korea.



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