Probing Exciton Dynamics of Naphthalene-Imide-Based Nonfullerene Acceptors for Organic Photovoltaics

Highly efficient organic photovoltaic (OPV) devices were recently reported for bis(naphthalene-imide)arylenelidenes (BNIAs), a class of nonfullerene acceptors (NFAs) with fused (indacenodithienothiophene or indecenodithiophene-benzothiadiazole) and nonfused (thienylene vinylene) ring donor moieties. The BNIA with a nonfused ring donor moiety (NITV) has an OPV device efficiency approximately two times lower compared to the BNIAs with fused ring donor moieties (NIDT and NIBT). Herein, we report the use of ultrafast time-resolved laser spectroscopy and theoretical calculations to investigate the exciton dynamics of the BNIAs. Time-resolved fluorescence upconversion results show that the BNIAs have relatively fast fluorescence lifetimes. Femtosecond transient absorption results show NIBT has 5×–6× slower exciton dynamics─such as exciton decay and recombination─when compared to NITV and NIDT. Theoretical calculations and fs-TA results show reorganization energy and energy difference between the exciton and intramolecular charge transfer excited states of NIBT is approximately 31% smaller compared to the other BNIAs. Despite different donor moieties, NITV and NIDT have similar reorganization energies, energy differences, and exciton dynamics. Our results show benzothiadiazole π-bridges on the donor moiety of NIBT lead to strong exciton dynamics; thus, creating an efficient charge generation and OPV device. The results discussed in this paper will increase the understanding between the structure of small molecule NFAs and the efficiency of OPVs.