A universal model for energy level alignment at interfaces of hole-collecting monolayers in p-i-n perovskite solar cells

Abstract

Hole-collecting monolayers (HCMs) have attracted considerable attention in p-i-n (inverted structure) perovskite solar cells. Although HCMs exhibit superior performance compared with widely used polymers, there is confusion regarding energy level alignment at electrode/HCM/perovskite interfaces, which is crucial for achieving 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 carbazole-based HCMs: 2PACz, MeO-2PACz, and 3PATAT-C3. Based on precisely determined energy parameters, we propose a universal model for energy level alignment at electrode/HCM/perovskite interfaces. The key feature of this model is the application of the semiconductor heterojunction theory, in which both the HCM and perovskite layers are treated as semiconductors, whereas the electrode/HCM interface is treated in terms of interface dipole formation. The model is further tested on HCMs beyond the carbazole-based types, confirming its universal applicability. The origins of the 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 provide a unified understanding of energy level alignment at electrode/HCM/perovskite interfaces and offer guidelines for the rational selection and design of HCM materials.