Time-Resolved Charge-Transfer State Emission in Organic Solar Cells: Temperature and Blend Composition Dependences of Interfacial Traps

Charge-transfer (CT) states across organic heterojunctions play an important role in determining the efficiency of organic solar cells. These states can be the precursors of free charges or lead to geminate recombination losses. Here, we use time-resolved photoluminescence measurements to study charge-transfer states in a sequence of polythiophene:fullerene derivative blends with different mixing ratios, over the temperature range from 10 to 290 K, and after excitation with various photon energies. Our results show that (1) excess fullerene leads to a higher probability of CT state formation per absorbed photon and (2) the CT emission intensity is temperature-dependent whereas its emission lifetime is temperature-independent. Observation 1 cannot be explained solely by the increased fraction of excitations formed on fullerene-derivatives in the fullerene-rich blends, suggesting that disruption of the polymer packing at high fullerene loadings can negatively influence charge separation. Observation 2 suggests that relaxed, emissive CT states are formed in this system through a short-lived intermediate state whose separation into free-charges or relaxation into a bound CT state is temperature-dependent. Analysis of the temperature dependence suggests that there is no single activation energy for the CT*-to-CS transition, but rather a wide variety of different sites ranging in activation energy.