Nanoscale Insight into Performance Loss Mechanisms in P3HT:ZnO Nanorod Solar Cells

Abstract

Hybrid solar cells based on semiconducting polymers and metal oxides offer the possibility of combining the best attributes of both constituents to achieve efficient and low-cost solar energy harvesting. Active layers consisting of ZnO nanorods and semiconducting polymer poly(3-hexylthiophene), P3HT, have been studied extensively over the past decade. This type of solar cell, however, still exhibits a relatively low performance and poor device-to-device reproducibility. For insight into the performance bottlenecks in P3HT:ZnO nanorod solar cells, we employ a point-by-point current–voltage mapping method using conductive atomic force microscopy to probe the local photovoltaic properties of hybrid P3HT:ZnO nanorod active layers with nanoscale spatial resolution. We observe that the short-circuit current density, open-circuit voltage, fill factor, and power conversion efficiency are highly heterogeneous and sensitive to the local thickness of the P3HT hole transport layer that sits atop the active layer, with local power conversion efficiencies reaching as high as double those seen in macroscopic devices. We further demonstrate that this hybrid system exhibits a high charge photogeneration rate, which approaches that in high-performance organic solar cells; however, photocurrent is limited by a low charge collection probability under short-circuit conditions. These experiments suggest the potential for significant performance gains in hybrid organic–inorganic nanorod solar cells through improvements in active layer uniformity and charge extraction efficiency.