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 (<i>V</i>_OC) and CTS has been widely discussed. It is proposed that the CTS energy (<i>E</i>_CT) directly determines the <i>V</i>_OC value; however, the <i>E</i>_CTs measured from different techniques often show considerable discrepancy. Here four methods are applied to probe the <i>E</i>_CT values in five different bulk-heterojunction polymer:fullerene OPVs. It is found that linear relationships exist between different <i>E</i>_CT’s and <i>V</i>_OC values. The detailed energetic meanings of the <i>E</i>_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 <i>V</i>_OC loss by considering the energetic broadening of CTS, bimolecular recombination, and dielectric effect. The results provide a guideline to forecast the <i>V</i>_OC and investigate its loss in OPV