Improved Efficiency of Polymer Solar Cells by Modifying the Side Chain of Wide-Band Gap Conjugated Polymers Containing Pyrrolo[3,4- f]benzotriazole-5,7(6 H)-dione Moiety

Two novel wide-band gap donor-acceptor-type conjugated copolymers, PTzBI-S and PTzBI-Ph, are designed and synthesized, based on alkylthio-thienyl- or alkylphenyl-substituted benzodithiophene (BDT) derivatives as the electron-donating unit and pyrrolo[3,4- f]benzotriazole-5,7(6 H)-dione as the electron-withdrawing unit. The as-generated copolymers show the comparable optical and electrochemical properties. The alkylthio-thienyl-substituted BDT unit facilities a benign decrease of the highest occupied molecular orbital (HOMO) levels. This consequently enhances open-circuit voltages ( V_OC) over 0.9 V in relevant solar cells with the fullerene acceptor ([6, 6]-phenyl-C71-butyric acid methyl ester, PC₇₁BM) or the nonfullerene acceptor (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2',3'- d']- s-indaceno[1,2- b:5,6- b']dithiophene, ITIC). The combination studies of Fourier transform photocurrent spectroscopy and electroluminescence further rationalize the V_OC difference between solar cells with fullerene and nonfullerene acceptors. An impressively high power conversion efficiency of 10.19% is obtained for the device based on PTzBI-Ph:ITIC, outperforming the 8.84% achieved by the PC₇₁BM-based device. Our results demonstrate that the modification of substituents of BDT units can effectively decrease the HOMO level and consequently improve V_OC, ultimately allowing the attainment of high-efficiency polymer solar cells.