Investigating charge extraction dynamics in hybrid perovskite solar cells using time-resolved spectroscopy

Abstract

The widespread production of clean and efficient solar panels is becoming essential for overcoming the present threat of climate change. With this comes a growing need for the development of our fundamental understanding of charge extraction processes that occur within solar panel devices. These processes consist of charge diffusion then transfer and are crucial steps in the operation of solar cells. This thesis details work primarily performed with time-resolved photoluminescence measurements to study electron diffusion in hybrid perovskite films and subsequent transfer to the adjacent electron extraction layer. These processes can be hard to distinguish, but by exciting with an incident laser from each side of the sample we can separate them and identify which process limits charge extraction. One of the key challenges in designing new devices is realising which modifications to device architecture truly enhance the charge carrier extraction properties of the device as opposed to purely improving the device performance. A method such as this for accurately determining diffusion and transfer properties would make a powerful tool for the optimisation of such photovoltaic devices.

Samples of methylammonium lead iodide (MAPbI₃) on a SnO₂ electron extraction layer were first explored with this model and this revealed that electron extraction was limited by electron transfer from MAPbI₃ to the SnO₂ layer. Then a fullerene monolayer was introduced between the MAPbI₃ and SnO₂ layers which greatly increased the electron transfer velocity between them to the extent that electron diffusion limited the rate of electron extraction instead.

This analysis method was then applied to a wide scope of different modifications to both the perovskite film and electron extraction layer. Our results suggest that although electron diffusion and transfer can be complimentary processes it is important to understand which is limiting electron extraction when designing high performance perovskite solar cells.