Tracking the evolution of materials and interfaces in perovskite solar cells under an electric field

Stable performance is a key requirement for solar cell devices. Here, spectroscopy combined with depth profiling reveals I-2 and PbI2 are distributed evenly in a perovskite solar cell under an electric field, while the electric field itself promotes chemical heterogeneity and device degradation. What causes the instability of perovskite solar cells has been a puzzling problem impeding the development of commercial panels. So far there is limited evidence on the link between device instability and the various materials in each of the stacked layers. Here, we study the chemistry and distribution of various species and the integrity of the functional layers in high-performance inverted perovskite solar cells, with and without an electric field. The distribution of the diffusion species and its impact on the chemical and electronic structures through the transporting layers are measured by photoemission spectroscopy combined with damage-free ion beam sputtering. We find that various species, such as I-2 and PbI2, are distributed throughout the organic transporting layers toward the electrode interface. These species are found to be charge neutral, have no impact on the Fermi level, and react little with copper. An electric field, however, can catalyze the electro-decomposition of the perovskite, causing chemical heterogeneity and degradation in device performance.