Understanding the Role of Charge Mobility and Recombination in Organic Photovoltaics

Current organic photovoltaics based on bulk heterojunctions have an active absorbing layer that is typically only 70 to 100 nm thick. While this results in efficient extraction of photogenerated charge carriers, the active layer is too thin for efficient absorption of light. If the active layer were 300 nm thick, close to 100% of the light would be absorbed. However, when devices are made with thicker active layers, recombination of charge carriers reduces the overall efficiency of the device.

In this project, I conduct an in-depth analysis of the effect of charge mobility and recombination in organic photovoltaics. Using a combination of experiments and a 1D simulation software, Seftos, I explain why bulk heterojunction organic photovoltaics suffer from poor performance when made with thick active layers. I experimentally demonstrate charge mobility’s effect on reducing solar cells, then utilize it to extract values of the recombination rate constant for two organic photovoltaic systems. These results provide an overview on the role of charge mobility and recombination in device performance and a road map for designing the next iteration of bulk heterojunction systems.