Speaker
Description
In the past few years, organic-inorganic metal halide perovskites employed as light absorber have achieved an impressive breakthrough in the field of photovoltaics, particularly due to a rapid improvement of the power conversion efficiencies of up to 22 % obtained from perovskite-based solar cells. However, perovskite solar cells are confronted with air stability and reproducibility issues, which represent a serious obstacle for establishing reliable property-function relationships and long-term applications. Thus, the understanding of the processes underlying these issues is indispensable and it is equally important to gain information that best possibly reflects the perovskite material behavior in an actual device-operation environment.
Here, by means of photoelectron spectroscopy (PES), we investigated (i), the influence of pure oxygen and water exposure on the electronic structure of CH$_3$NH$_3$PbI$_{3-x}$Cl$_x$ mixed halide perovskite film surfaces, and (ii), that of ambient air exposure, where both oxygen and water act in combination.
While the perovskite surfaces were originally n-type, pure oxygen exposure leads to a shift of the Fermi-level towards a mid-gap position, i.e., the surface became significantly less n-type. Upon water vapor exposure, the n-type character of the surface was accentuated. This change is mostly reversible after mild heating in ultra-high vacuum. We further discuss the impact of water on the perovskite thin films by monitoring structural changes in-situ at high relative humidity levels above 80 % by means of grazing-incidence X-ray diffraction. We find indications for the formation of a monohydrate phase before severe material degradation sets in.
Importantly, after air exposure we observe the prevailing effect of oxygen over water on the electronic properties of the perovskite films. Such variations of the electronic structure of perovskite film surfaces will certainly affect the energy-level alignment at the interface between the perovskite and typical charge-transport materials.
