This ab initio study investigates laser-induced charge-transfer in a quaterthiophene/fullerene complex at varying intensities of the external pulse. Pulses above 3 GW cm^{− 2} are needed to trigger charge transfer, which is enabled by vibrations in the donor. Under stronger field intensities, transient effects in the system prevail.

Understanding the fundamental mechanisms of ultrafast charge-transfer dynamics in organic solar cells is crucial for developing efficient and sustainable energy technologies. Despite intensive efforts in the last two decades, a complete understanding of the interplays among radiation, electrons, and vibrations has not been achieved yet. In this work, the laser-induced charge-transfer dynamics of quaterthiophene/fullerene, a prototypical donor/acceptor complex, are investigated using real-time time-dependent density functional theory coupled with Ehrenfest nuclear dynamics. Femtosecond radiation in resonance with the lowest-energy transitions in the donor reveals three distinct excitation regimes: weak pulses with intensities below 3 GW cm-2$${}^{-2}$$ fail to initiate charge transfer, medium pulses with intensities between 3 and 36GW cm-2$${}^{-2}$$ effectively trigger charge transfer in agreement with experiments, while stronger fields significantly perturb the system, inducing relevant transient effects. Detailed analysis of the population dynamics highlights the critical role of vibrations in quaterthiophene in facilitating electron transfer to the acceptor. This study demonstrates the high sensitivity of charge-transfer dynamics to pulse intensity and the interplay between electronic and vibronic couplings, providing crucial insights into the initial light-matter interaction processes responsible for charge transfer in organic donor/acceptor interfaces.