Hole-collecting monolayers (HCMs) for inverted perovskite solar cells have attracted significant attention because they offer superior stability, conductivity, and transparency compared with conventional polymer-based hole-collecting materials such as PTAA and PEDOT:PSS. By incorporating HCMs, a power conversion efficiency (PCE) of 26.9% has been achieved for single-junction solar cells[1]. To further improve HCM performance, elucidating the energy-level alignment at the electrode/HCM/perovskite interfaces is crucial. Efficient hole collection cannot be realized without proper energy-level alignment, even when other requirements—such as film uniformity and defect-passivation capability—are satisfied. Several models, including vacuum-level alignment, Fermi-level alignment, and Schottky-type models, have been proposed to explain and predict energy-level alignment; however, they are often applied selectively to rationalize one’s own photovoltaic performance, and no consensus model has established within the community. Therefore, developing an appropriate model remains essential.

In this study, we propose an energy-level alignment model for electrode/HCM/perovskite interfaces. We find that the model is consistent with HCM-dependent photovoltaic performance in terms of hole-collection efficiency and electron-blocking capability. In our framework, the electrode/HCM/perovskite interface is separated into two interfaces: electrode/HCM and HCM/perovskite. The electrode/HCM interface is treated in terms of interface dipole formation, whereas the HCM/perovskite interface is described by a semiconductor heterojunction model[2].

To demonstrate the validity and generality of the proposed model, we first tested it using three representative carbazole-derived HCMs—2PACz, MeO-2PACz, and 3PATAT-C3[3]. We precisely determined the energy parameters in the solid states of these HCMs and a mixed-cation perovskite by ultraviolet photoelectron spectroscopy (UPS) and low-energy inverse photoelectron spectroscopy (LEIPS). Reliable energy parameters obtained under controlled conditions enable rigorous validation of the model. We then assessed the model by comparing photovoltaic performance with hole-collection efficiency and electron-blocking capability predicted from the derived energy-level alignment. This comparison shows that our model for electrode/HCM/perovskite interfaces explains the experimentally measured photovoltaic performance, whereas the three earlier models could not. To further test the model’s universal applicability, we evaluated a broad range of HCMs, including Me-PhpPACz, Br-2EPT, MeO-BTBT, 4PADCB, ID-Cz, Py3, MPA-BT-XA, and 4-XPBA, using photovoltaic and energy parameters reported in the literature. These results confirm the model’s universality for electrode/HCM/perovskite interfaces.

This study enables prediction of HCM photovoltaic performance in terms of energy-level alignment and provides guidelines for the design and selection of HCMs for high-efficiency perovskite solar cells.