Charge-Transfer State Energy and Its Relationship with Open-Circuit Voltage in an Organic Photovoltaic Device

Charge-transfer state (CTS) plays a very important role in organic photovoltaic (OPV) devices. Especially the relationship between open-circuit voltage (V_OC) and CTS has been widely discussed. It is proposed that the CTS energy (E_CT) directly determines the V_OC value; however, the E_CTs measured from different techniques often show considerable discrepancy. Here four methods are applied to probe the E_CT values in five different bulk-heterojunction polymer:fullerene OPVs. It is found that linear relationships exist between different E_CT's and V_OC values. The detailed energetic meanings of the E_CT values measured from different techniques are discussed, and the origin of their discrepancy is analyzed. Lastly, based on a proposed energy model, a relationship is summarized to estimate V_OC loss by considering the energetic broadening of CTS, bimolecular recombination, and dielectric effect. The results provide a guideline to forecast the V_OC and investigate its loss in OPV.