Direct observation of charge generating regions and transport pathways in bulk heterojunction solar cells with asymmetric electrodes using near field photocurrent microscopy

Abstract

We present a versatile method to examine the donor- acceptor based bulk heterojunction structures using a combination of structural, optical and optoelectronic contrast. The technique relies on current- contrast- optical scanning microscopy on asymmetric photovoltaic device structures with the electrodes extending in orthogonal directions to form a cross- type structure which provides a near- field access for the incident light beam. The method was used to follow changes with annealing and different ratios of the Si- PCPDTBT:PC71BM system, where the correlation between the changes in the morphology and charge carrier generation leading to photocurrent was clearly established. The general viewpoint of increasing heterogeneity between two components and continuous pathways in the entire photovoltaic layer upon thermal annealing are clearly evident from the high resolution optical and current contrast images. Fourier analysis of the images was used to extract the relevant length scales which prevail in these binary mixtures and quantify the changes upon thermal annealing.