Enlarged bilayer interfaces from liquid-liquid dewetting for photovoltaic applications

Abstract

The details of the arrangement of mixtures of semiconducting materials in thin-films have a major influence on the performance of organic heterojunction solar cells. Here, we exploit the phenomenon of spinodal dewetting during spin coating of blends of PCBM and a cyanine dye for the design of phase separated morphologies with increased interfacial area. AFM snapshots of as-prepared films and after selective dissolution suggest that the solution separates into transient bilayers, which destabilize due to long-range intermolecular interactions. We propose that film destabilization is effectively driven by electrostatic forces that build up due to mobile ions that cross the junction and dissolve partially in PCBM. The resulting morphology type is mainly dependent on the ratio between the layer thicknesses, whereas the dominant wavelengths formed are determined by the absolute film thickness. Solar cells were fabricated from films with known structure and a power conversion efficiency of η = 0.29 % was measured for a vertically segregated film consisting of a cyanine layer covering the anode and an upper phase composed of dewetted PCBM domains. We explain the merits of this structure in contrast to a lateral separated blend morphology where the efficiency was 3 times smaller.