Fluorination of Benzothiadiazole–Benzobisthiazole Copolymer Leads to Additive-Free Processing with Meliorated Solar Cell Performance

Abstract

Processing solvents and conditions have unique importance in the performance of bulk heterojunction organic solar cells. In the present work, we have investigated the role of a primary solvent and solvent additive in the device performance of two benzobisthiazole (BBTz)-based push–pull type polymers. In an inverted cell structure, the BBTz-<i>co</i>-fluorinated benzothiadiazole polymer (PBBTzFT) with a PC₇₁BM acceptor showed additive-free enhanced performance with a power conversion efficiency (PCE) of 6.4% from a 1,2-dichlorobenzene solvent, while the BBTz-<i>co</i>-pyridylthiadiazole polymer (PBBTzPT) showed maximum performance from a chlorobenzene (CB) solution with a 1,8-diiodooctane (DIO) additive (PCE = 2.3%). The detailed investigation by atomic force microscopy and two-dimensional grazing incidence X-ray diffraction corroborates that the fluorination of benzothiadiazole brought about optimal morphology without a solvent additive, the PCE of which is comparable with the previous nonfluorinated analogue (PCE = 6.5%) processed from CB with DIO