Control of Donor-Acceptor Photophysics through Structural Modification of a "Twisting" Push-Pull Molecule

In contemporary organic solar cell (OSC) research, small A-D-A molecules comprising electron donor (D) and acceptor (A) units are increasingly used as a means to control the optoelectronic properties of photovoltaic blends. Slight structural variations to these A-D-A molecules can result in profound changes to the performance of the OSCs. Herein, we study two A-D-A molecules, BTCN-O and BTCN-M, which are identical in structure apart from a subtle difference in the position of alkyl chains, which force the molecules to adopt different equilibrium conformations. These steric effects cause the respective molecules to work better as an electron donor and acceptor when blended with benchmark acceptor and donor materials (PC71BM and PBDB-T). We study the photophysics of these "D:A" blends and devices using a combination of steady-state and time-resolved spectroscopic techniques. Time-resolved photoluminescence reveals the impact of the molecular conformation on the quenching of the A-D-A emission when BTCN-O and BTCN-M are blended with PBDB-T or PC71BM. Ultrafast broadband transient absorption spectroscopy demonstrates that the dynamics of charge separation are essentially identical when comparing BTCN-M and BTCN-O based blends, but the recombination dynamics are quite dissimilar. This suggests that the device performance is ultimately determined by the morphology of the blends imposed by the A-D-A conformation. This notion is supported by X-ray scattering measurements on the "D:A" films, electroluminescence data, and pump-push-photocurrent spectroscopy on the "D:A" devices. Our findings provide insight into the remarkable structure-function relationship in A-D-A molecules and emphasize the need for careful morphological and energetic considerations when designing high-performance OSCs.