Relation of Photoabsorption and Excitation Energy Transfer in Fluorinated Non-Fullerene Type Organic Thin Film Solar Cells

The π-conjugated system is important as a semiconductor material in organic electronics such as organic solar cells (OSCs). Recently, it has been reported that the bulk heterojunction OSCs, which contain fluorinated naphtho [1,2-c:5,6-c'] bis [1,2,5] thiadiazole-based non-fullerene acceptors has a better photovoltaic performance [1]. In this study, we theoretically investigate the photovoltaic performance of fluorinated acceptor. We performed density functional theory (DFT) calculations with ωB97XD/6-31G(d) using Gaussian16. At first, we compared NTz-T_eh-FA and FNTz-T_eh-FA fluorinated to NTz which are non-fullerene acceptors. Consequently, fluorination of the central π-conjugated system part in non-fullerene acceptor improved photoabsorption because of electron-attracting. In addition, it is expected that it provides high charge-transfer in the complex with the fluorinated acceptor. Next, we calculated the donor-acceptor excitation energy transfer (EET) with DFT. we used PCPD-TBT for donor molecule. Comparing fluorination with non-fluorination, it increased EET coupling. Therefore, it is expected that J_SC [mA/cm²] will be increased. It provides the key to improve power conversion efficiency in OSCs.
[1] S. Chatterjee, Y. Ie, et al., NPG Asia Mater. 10, 1016-1028 (2018).