Charge transfer in the Ag-polymer-fullerene system of organic solar cells (OSCs) observed by surface-enhanced Raman spectroscopy: donor/acceptor concentration-dependent

Currently, modulating the charge transfer (CT) process poses a major challenge for many organic solar cells (OSCs) and monitoring the CT dynamics demands further research. The surface-enhanced Raman spectroscopy (SERS) technique was successfully used herein to monitor the CT process, which was strongly dependent on the carrier density in the Ag-polymer-fullerene system. However, the carrier density can be controlled by the acceptor and donor concentrations. As the acceptor and donor concentrations were changed, significant frequency shifts in the SERS peaks were observed. The donor and acceptor concentration changes exhibited the opposite effects on the frequency shift due to donor-to-acceptor and acceptor-to-donor CT. In addition, the peak intensity at 1450 cm(-1) increased significantly owing to the CT contribution. Thus, the CT process induced the Raman frequency shift and monitoring the peak intensity increase can elucidate the interaction mechanism between the acceptor and donor in OSCs. This study provides a novel insight into the theoretical foundation of CT dynamics in metal-polymer-fullerene systems.