Temperature- and Energy-Dependent Separation of Charge-Transfer States in PTB7-Based Organic Solar Cells

A decisive factor for the performance of organic bulk heterojunction solar cells is the competition between charge separation and geminate electron hole recombination through a charge-transfer (CT) state after exciton separation across the heterointerface. By time-resolving the near-infrared emission of the high-performance low-bandgap PTB7:PC71BM material system, we selectively study the properties of CT states formed after optical excitation with a femtosecond laser pulse. We observe that the CT emission yield is higher after 705 urn excitation of the polymer rather than after 400 nm excitation of the fullerene, which can be attributed to better charge separation for excitons generated in fullerene aggregates. Additionally, the CT states are weakly bound with their emission not far red-shifted from that of the exciton. Examining the time-resolved CT emission from room temperature to 10 K, we observe changes in CT state lifetime with energy and temperature, indicative of CT state separation effectively competing with recombination, especially for the higher-energy CT states. Our findings suggest that this weak binding of CT states in the polymer fullerene mixed phase is a key factor for the highly efficient charge separation in this material system.