In the past decade, metal halide perovskites have attracted tremendous attention in the field of photovoltaics and light emission due to their superb optoelectronic properties and potentially low manufacturing costs. Perovskite solar cells have shown remarkable progress in recent years with rapid increases in power conversion efficiency, from reports of about 3% in 2009 to over 25% in 2020, making it the fastest improving solar cell technology in history. The understanding of surface and interface properties plays a very important role in the design of high-efficiency solar cells. To advance the development of such devices, the ability to understand, control and manipulate surface and interface properties is essential.
The work presented here addresses the challenges and rewards of characterizing metal halide perovskite surfaces with high sensitivity measurements, such as Kelvin probe contact potential difference and photoemission spectroscopy measurements. The work highlights issues such as film degradation, nonequilibrium situations, and compensation for parasitic contributions to photoemission data, which can potentially occur during the experiments. We develop strategies that help to measure the electronic structure of perovskite surfaces accurately and avoid erroneous interpretation of experimental results. With these high sensitivity techniques, for the first time, our work establishes the link between potentially detrimental defect states and an additive commonly used in perovskite film processing. Last but not least, we demonstrate molecular doping as a facile and effective method to tune the surface and interface properties of perovskites.