Non-fullerene organic solar cells have attracted attention in order to improve the performance of organic thin-film solar cells. It has been reported that an increase in short-circuit current density contributes to the improvement of photoelectric conversion efficiency, and efficient exciton generation and dissociation is the key to increasing short-circuit current density (J_SC). There are two processes by which the generated exciton dissociates [1]. One is the “hot process”, in which the generated exciton reaches the charge separated state quickly, and the other is the “cool process”, in which the generated exciton enters the charge transfer state through relaxation and becomes charge separated when the thermal binding is removed. The key to the selection of these two processes in the strength of the vibronic interaction in the exciton at donor/acceptor interface.

To design optimal D/A interfaces, it is essential to understand exciton dynamics at the D/A stacking structure and the photoelectric conversion mechanism. We hypothesize that charge-transfer (CT) excitons form weakly bound electron-hole polaron pairs, dissociating into free carriers without relaxing into a lower-energy charge-transfer state, driven by nonadiabatic oscillatory interactions.

In this study, we use time-dependent density functional theory theory to investigate the electronic structure, electron-hole distance [2], electron coupling in charge transfer state generated by photoexcitaion and the Huang-Rhys factors of the donor/acceptor complex (PTB7/BTAx, PCTBT/BTAx (x = 1, 3)) [3] in the excited state and the exciton-phonon coupling and nonadiabatic process. Based on the above, we aim to elucidate the role of vibronic interactions in charge transfer excitonic states and the control of J_SC and electron-hole recombination.

We investigate the CT exciton dissociation at the interface of organic thin-film solar cells, modeling CT excitons as pairs of electron and hole polarons and evaluating the Huang-Rhys factor for the bipolaron state. Focusing on PTB7/BTA1 and PTB7/BTA3, we compared the theoretical results for excitation energy transfer and CT distance with experimental data. The absorption spectra of both complexes show that the A → A transition is present around 450 nm, while the oscillator intensities at other wavelengths are dominated by the D → D and D → A transitions. Structural optimization revealed that PTB7/BTA3, with a higher J_SC, has a larger CT distance and electron coupling, while PTB7/BTA1, with a lower J_SC, has greater electron coupling and a larger CT distance. In comparison, PTB7/BTA3 has larger CT distance and charge transfer and more D/A transitions. PTB7/BTA1 has a larger Huang-Rhys factor due to low-wavenumber vibrational modes in the CT state, suggesting possible nonadiabatic relaxation. Understanding exciton dynamics using first-principles calculations will aid in designing high-efficiency organic solar cells.