Isomerizing thieno[3,4-b]thiophene-based near-infrared non-fullerene acceptors towards efficient organic solar cells

With the rapid growth in the requirement for emerging photovoltaic technology like semitransparent solar cells applied for integrated smart windows, there is an urgent need to develop near-infrared (NIR) non-fullerene acceptors (NFAs). To address this issue, thieno[3,4-b]thiophene (TT), which has a stable quinoid structure to minimize the energy difference between two resonance structures corresponding to the band gap, is introduced into the push-pull molecular architecture as a bridge unit to narrow the band gap of the derived acceptors. Due to the different linkage positions (4- or 6-position) of asymmetric TT, these acceptors are classified into two types of isomers, namely 4TIC, 4T4F, 6TIC and 6T4F, of which all have strong absorption in the NIR range. By incorporation with polymer donor PTB7-Th, the devices based on 6-position isomers exhibit superior photovoltaic performance, wherein a champion device based on 6T4F is obtained with a power conversion efficiency of 10.74%. With detailed investigations on inherent optoelectronic properties as well as structural and morphological variation, this performance diversity induced by isomerism is determined by the evident difference in the packing order, which will impact the charge mobility and fill factor. This work presents a class of high-performance NIR acceptors in which the regioisomeric backbone will significantly impact the optoelectronic properties.