A Universal Model for Energy Level Alignment at Interfaces of Hole-Collecting Monolayers in p-i-n Perovskite Solar Cells

Hole-collecting monolayers (HCMs) have attracted considerable attention in p-i-n (inverted structure) perovskite solar cells. Although HCMs show superior performance to widely used polymers, there is confusion in the understanding of energy level alignment at electrode/HCM/perovskite interfaces, which is crucial to hole collection efficiency through HCM. In this work, ultraviolet photoelectron spectroscopy, low-energy inverse photoelectron spectroscopy, and metastable atom electron spectroscopy are employed to investigate prototypical HCMs: 2PACz, MeO-2PACz, and 3PATAT-C3. In addition, a model to describe energy level alignment at the HCM/perovskite interface is proposed, applying the semiconductor heterojunction theory to treat both layers as semiconductors. The origins of key energy parameters in the model are analyzed to establish design rules for HCM development. Particular attention is given to the oft-overlooked work function of the transparent conductive electrode. The results impart a unified understanding of energy level alignment at electrode/HCM/perovskite interfaces and provide guidelines for the rational selection and design of HCM materials.