Influence of vibronic interaction of charge transfer excitons in PTB7/BTA-based nonfullerene organic solar cells

We studied photoinduced charge transfer (CT) states and their dissociation processes at the donor/acceptor (D/A) interface of PTB7/BTAx (x = 1 and 3) nonfullerene organic thin-film solar cells using density functional theory (DFT) and time-dependent DFT calculations. We focused on the CT distances and electron coupling in the CT state generated by photoexcitation and the Huang-Rhys (HR) factors that describe the nonadiabatic processes associated with vibronic interactions. The PTB7/BTA3 system with a large short-circuit current density (JSC) exhibited a large charge CT distance and electronic coupling. Contrastingly, the PTB7/BTA1 system with a low JSC has a large HR factor because of the low-wavenumber vibrational modes in the CT state of the D/A complex and is prone to nonadiabatic relaxation to the ground state. Systematic theoretical analysis of the excitonic states in the D/A complex has provided insight into the control of CT exciton dynamics, namely JSC and electron-hole recombination.