Supporting Information Non-Fullerene/Fullerene Acceptor Blend with Tunable Energy State for High- Performance Ternary Organic Solar Cells Min Kim 1, Jaewon Lee 1, Dong Hun Sin 1, Hansol Lee 1, Han Young Woo 2, Kilwon Cho 1* 1 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 37673, Korea 2 Department of Chemistry, Korea University, Seoul 02841, Republic of Korea *E-mail: kwcho@postech.ac.kr S-1
Figure S1. Dark J-V curves of the ternary blend films with respect to acceptor blending ratios from 0:10 to 10:0. Red lines are fitted curves by using Shockley equation. S-2
Current density (ma cm -2 ) 5 0-5 -10-15 PPDT:PCBM PPDT:IDT IDT:PCBM 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Voltage (V) Figure S2. J-V curve of the photovoltaic devices fabricated from the PPDT:PCBM, PPDT:IDT2BR (IDT), and IDT2BR:PCBM blend films. S-3
Figure S3. (a) UPS spectra (valence band) and (b) NEXAFS 1C spectra of the binary blend films with the blending ratios of IDT2BR:PCBM from 0:10 to 10:0. The energy range related to HOMO/LUMO energy level are magnified to clearly see (c) the valence band and (d) the C1s to π* transition. We used synchrotron radiation to perform Ultraviolet Photoelectron Spectroscopy (UPS) and Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS). The valence bands from UPS measurements were normalized to the fermi energy level of gold, obtained at the same round, which gives HOMO energy levels of the blend films. 1 For molecules with π- electron systems, the transition C1s - C2p(π*) from inner-atomic to the lowest unoccupied molecular orbital (LUMO) appears in NEXAFS as an intense, sharp peak a few ev below the carbon K-edge. 2 Through NEXAFS measurements, we could observe the variation of LUMO level as a function of composition in the IDT2BR:PC 71 BM blend. The absolute values of the LUMO energy level for the neat materials were correlated to the calculated LUMO energy S-4
levels from HOMO energy level and optical band gap. The LUMO energy level of the neat IDT2BR film obtained from CV measurement (-4.63 ev) was slightly higher than the calculated LUMO value (-4.87 ev) from UPS and optical band gap. This difference arises from the fundamental differences in the measurements, which has been generally accepted in organic semiconductors. 3,4 Electrochemical cyclic voltammetry (CV) was conducted on a CHI-730B electrochemistry workstation with glassy carbon disk, Pt wire, and Ag/Ag+ electrode as the working electrode, counter electrode, and reference electrode, respectively in a 0.1 M tetrabutylammonium hexafluorophosphate (n-bu 4 NPF 6 )-anhydrous acetonitrile solution at a potential scan rate of 50 mv s 1. The potential of Ag/AgCl reference electrode was internally calibrated by using the ferrocene/ferrocenium redox couple (Fc/Fc + ). The electrochemical energy levels were estimated by using the empirical formula: E HOMO = (4.80 + E onset, ox ) and E LUMO = (4.80 + E onset, red ). 5 As shown in Fig. S4, the onset oxidation and reduction potentials of IDT2BR are 0.74 V and -1.13 V versus FeCp 0/+ 2, respectively. Assuming the absolute energy level of FeCp 0/+ 2 to be 4.8 ev below vacuum, the HOMO and LUMO energy levels are estimated to be -5.54 ev and -3.67 ev from onset oxidation and reduction potentials, respectively, which are comparable with the values from literature. 6 The LUMO energy level of IDT2BR is higher than that of PC 71 BM (-4.1 ev), leading to an improved open circuit voltage (V OC ). Figure S4. Cyclic voltammetry curves of the thin film of the IDT2BR material. S-5
Reference 1. Ishii, H.; Sugiyama, K.; Ito, E. ; Seki, K. Energy Level Alignment and Interfacial Electronic Structures at Organic/Metal and Organic/Organic Interfaces. Adv. Mater. 1999, 11, 605. 2. Fratesi, G.; Lanzilotto, V.; Stranges, S.; Alagia, M.; Brivioe, G. P.;Floreano, L. High resolution NEXAFS of perylene and PTCDI: a surface science approach to molecular orbital analysis. Phys. Chem. Chem. Phys. 2014, 16, 14834 3. Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E. Measurement of the lowest unoccupied molecular orbital energies of molecular organic semiconductors. Org. Electron. 2009, 10, (3), 515 4. Sworakowski, J. How accurate are energies of HOMO and LUMO levels in smallmolecule organic semiconductors determined from cyclic voltammetry or optical spectroscopy? Syn. Met. 2018, 235, 125 5. Zhou, H.; Yang, L.; Stuart, A. C.; Price, S. C.; Liu, S.; You, W. Development of fluorinated benzothiadiazole as a structural unit for a polymer solar cell of 7 % efficiency. Angew. Chem. Int. Ed. 2011, 50, 2995. 6. Wu, Y.; Bai, H.; Wang, Z.; Cheng, P.; Zhu, S.; Wang, Y.; Mac, W.; Zhan, X. A planar electron acceptor for efficient polymer solar cells. Energy Environ. Sci., 2015, 8, 3215. S-6
Figure S5. AFM height images of the neat IDT2BR and IDT2BR:PCBM blend films. S-7
Figure S6. Grazing-incidence XRD images of the neat materials and the blend films. S-8
Figure S7. SCLC device characteristics of hole-dominant (left panel) and electron-dominant (right panel) devices. Hole-dominant device structure: ITO/PEDOT:PSS/active layer/pd, Electron-dominant device structure: ITO/Al/active layer/lif/al. Table S1. Calculated SCLC mobilities for the acceptor blend devices. Acceptor ratio (IDT2BR : PCBM) Electron mobility (cm 2 V -1 s -1 ) Hole mobility (cm 2 V -1 s -1 ) 0 : 100 1.47 10-3 9.11 10-4 10 : 90 1.47 10-3 6.61 10-4 20 : 80 8.64 10-4 3.21 10-4 40 : 60 3.84 10-4 1.87 10-4 60 : 40 7.70 10-5 1.12 10-4 80 : 20 7.03 10-6 8.43 10-5 100 : 0 4.51 10-6 3.85 10-5 S-9
Figure S8. Optical images for contact angle measurement on the pristine PPDT2FBT, IDT2BR, and PCBM films. S-10
Figure S9. AFM height images of the ternary blend films with respect to acceptor blending ratios from 0:10 to 10:0. S-11
Figure S10. Bright field TEM images of the ternary blend films with respect to acceptor blending ratios from 0:10 to 10:0. S-12
Figure S11. Magnified TEM and AFM images of (a, d) the binary blend (PPDT2FBT:PCBM), (b, e) the ternary blend with 10% addition of IDT2BR, and (c, f) the binary blend with (PPDT2FBT:IDT2BR). S-13