Hybrid bulk heterojunction solar cells (HBHJs) that combine metal oxide acceptors with polymeric donors provide a low-cost solid state device architecture and controllable donor/accepter interface morphology. These two benefits propel the potential of these devices over that of other so-called next generation solar cells including dye-sensitized solar cells (DSSCs) and traditional bulk heterojunctions (BHJs). Although HBHJs have been studied since the late 1990s, these potential advantages have not been fully realized and efficiencies of only 2% have been reported. Indeed, the photocurrents in these hybrid devices have been surprisingly low (5.6 mA/cm2). To elucidate the underlying properties that result in this low current, a detailed understanding of interfacial properties is required. Presented herein is a detailed spectroelectrochemical and x-ray photoelectron spectroscopic study of the interfacial energetics that results upon heterjunction formation. The results quantify a vacuum level offset of 0.8 eV between the polymer and the TiO2 created by spontaneous charge transfer from the polymer to deep trap states in the TiO2. This offset creates an approximate 0.7 eV barrier to charge separation at the TiO2-polymer interface and explains the poor performance of cells to date. New materials for improved photocurrent are proposed by tuning the energetics of the heterojunction.

Controlling interfacial energetics for efficient hybrid bulk heterojunction solar cells

249th National Meeting (2015)

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