Benzothiadiazole – an excellent acceptor for indacenodithiophene based polymer solar cells

Abstract

Two tetradodeoxyphenyl-substituted indacenodithiophene (IDT) based polymers, PIDT3T and PIDTDTBT, were achieved by copolymerizing IDT with terthiophene (3T) or di-2-thienyl-2′,1′,3′-benzothiadiazole (DTBT). Although these two polymers show significantly different UV-vis absorption spectra and band gaps (2.08 eV and 1.75 eV), the HOMO levels (−5.35 eV and −5.30 eV) of these polymers are almost the same. Polymer solar cells (PSCs) based on polymers with the benzothiadiazole (BT) unit show relatively high short-circuit current density (J_sc) due to the relatively wide and high photo-electronic response and high hole mobility. Thanks to the four long aryl side chains on IDT, the polymer thin film shows an amorphous nature, and the AFM root-mean-square roughness (RMS) value of the polymer/PCBM blend film is only around 0.3 nm which can contribute to the homogenous bulk heterojunction structures without significant phase separation. Finally, decent power conversion efficiency (PCE) of 4.52% is achieved by the benzothiadiazole based polymer and PC₇₁BM composite. By comparison study, we demonstrate why BT is an excellent acceptor unit for indacenodithiophene-based PSCs.