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