Exciton blocking layers in organic photovoltaic devices

Exciton blocking layers are essential for efficient organic photovoltaic device operation, where they serve to block excitons at the anode or cathode contact in order to reduce losses in the cells. At the anode side, exciton blocking layers are typically implemented in contact with a hole transporting layer, such as molybdenum oxide (MoOx), where they prevent recombination at the metal oxide interface. Exciton blocking layers that also transport electrons have at the same time shown to efficiently reduce losses at the cathode contact.

In this work, the use of different organic interlayers integrated as exciton blocking layers in organic photovoltaic cells is presented. The exciton blocking layers are integrated either between MoO3 hole contact layer and the active organic layer, or as cathode contact layers between ITO and the active organic layer. Besides organic solar cell device development and characterization, the integration of the layers is studied through hole-only and electron-only devices, optical spectroscopy analysis and morphology studies, in combination with complete optical and electrical device modeling. From the characterization and modeling, ideal thicknesses of the organic exciton blocking layers are found, based on a delicate interplay between the exciton blocking properties, the series resistance and the exciton generation profiles in the devices. Significantly improved device performances are observed in small molecule DBP/C70 based organic photovoltaic cells when including specific organic interlayers (one of them being 4P-NPD) in combination with MoO3, due to the added exciton blocking properties at the hole contacts. In addition, we evaluate here the effect of different exciton blocking layers on the performance of solar cells with up-scaled device areas. This is specifically area-dependent performance behavior observed in BCP based cathode contact layers, where BCP based exciton blocking and electron transporting stacks are introduced to improve the performance of up-scaled devices. The work thus addresses the need and challenge of incorporating exciton blocking layers in organic solar cells, and addresses the issue of device scale-up, which is needed for further implementing these layers in industrially relevant organic solar cells modules.