Organic-inorganic lead halide perovskites are used in research solar cell devices which promise to deliver affordable and sustainable energy. To bridge the gap to scale-up, p-i-n architecture solar cells are investigated which utilize facile fabrication methods with low temperature processing. However, challenges remain at the device interfaces which present sites for charge recombination and chemical reactions. These processes limit device performance as well as long-term stability. Understanding interfacial processes and material interactions is therefore crucial to achieve reliable perovskite solar cells (PSCs) for green electricity generation. Insights can be gained with photoelectron spectroscopy (PES). The quantitative and qualitative information contained in the PE spectra can be related to the electronic structure and composition of solid samples.[1]

In this talk, I will present high-quality synchrotron-based PES data measured on a well-defined model system. As a substrate, a MAPbI₃ single crystal was used to study fundamental perovskite properties.[2] During the experiment, electron extracting layers of C₆₀, bathocuproine (BCP), and silver were evaporated in situ while PE measurements were performed after each deposition step. Via this approach, each interface could be characterized individually and directly. This way the energetic alignment and chemical reactions in the dark could be studied: at the perovskite/C₆₀ interface, downward energetic alignment was found for C₆₀. This suggests a barrier for electron transport away from the perovskite. However, the effect is reduced after the deposition of BCP which induces a flattening of the C₆₀ energetic states. After the deposition of silver, significant ion migration of iodide and even the lead cations toward the metal contact is observed next to the formation of metallic lead. These findings show the decomposition of the underlying perovskite which limits the stability of the entire device.

Moreover, the interfaces were investigated under illumination to examine if additional photo-induced effects are present. Here, the migration of iodide and lead cations is already observed for MAPbI₃/C₆₀/BCP. Additionally, the data suggests the migration of the deposited silver into the adjacent layers.

The talk demonstrates how PES can be used to reveal interfacial energy alignment and degradation pathways in p-i-n PSCs. The permeability of the electron transporting layers to perovskite ions is identified as a key target for future material design.