Energetics and chemistry at the electron selective interfaces for p-i-n perovskite solar cells: an in situ investigation

Abstract

Perovskite solar cells in an inverted p-i-n architecture are of high interest for developing efficient and affordable single junction photovoltaics and tandem devices. However, commercialization efforts are hampered by low operational stability, largely driven by chemical and electronic changes at critical interfaces, where reactions, ion migration, and energy misalignment can accelerate degradation. Understanding the interfacial chemistry, energetic alignment, and degradation pathways is therefore of primary importance to locate targets for material development and design. In this study, we investigate a partial device structure with clean interfaces, based on single crystal substrates and layers prepared under high-vacuum conditions. Specifically, we present a fully in situ assembled model system for an inverted p-i-n solar cell, extending from the absorber to the back contact. The architecture employs a MAPbI3 (MA = methylammonium) single crystal as the absorber, with sequentially evaporated layers of C60, bathocuproine (BCP), and silver. After each deposition, the material stack is immediately characterized in situ using photoelectron spectroscopy thereby allowing us to directly study the chemical and energetic changes occurring upon interface formation. We find stable interfaces upon deposition of the organic molecules and favorable downward energetic realignment of C60 by 0.3 eV toward the interface with BCP. However, the stability of the final half-cell is limited by reactions of the perovskite occurring upon silver evaporation. We observe the permeation of the perovskite lead cation and iodide into the charge transport layers, as well as the formation of metallic lead. Only the latter can be inhibited by sufficiently thick BCP layers. Furthermore, a complex of cationic silver with BCP is formed after the deposition of the terminal silver layer.