Model unifies competing theories of perovskite solar cell interfaces

Apr 30, 2026

A new universal model predicts which hole-collecting monolayers boost perovskite solar cell efficiency, replacing trial-and-error design.

(Nanowerk News) Hole-collecting monolayers, the ultra-thin films that pull positive charges out of perovskite solar cells, have helped single-junction devices reach 26.9 percent power conversion efficiency. But no one could explain why some monolayers work and others don’t. A team led by Professor Hiroyuki Yoshida at Chiba University has now built the first universal model of how energy levels align inside these devices, unifying three competing theories that researchers had been applying interchangeably without clear justification.

Key Findings

Perovskite solar cells have become one of the most actively studied renewable energy technologies of the past decade. They convert sunlight efficiently, weigh little, and can be produced through low-cost solution processing, which opens uses well beyond rooftop panels. Researchers are targeting applications such as building windows, vehicle surfaces, and portable electronics. The newly developed model revealed that both the band bending phenomenon and the energy barrier height at the interface between the perovskite and the hole-collecting monolayer are critical factors in hole collection efficiency, which in turn determines the efficiency of the solar cell. (Image: Hiroyuki Yoshida, Chiba University) The hole-collecting monolayers (HCMs) driving recent efficiency records have also improved device stability. But their molecular and electronic behavior has remained poorly understood. The way energy levels align where the electrode, the monolayer, and the perovskite meet largely determines how easily charge moves through the device. Several competing theories, including vacuum level alignment, Fermi level alignment, and the electrode-modified Schottky model, have been applied interchangeably without clear justification, making it difficult to predict which monolayer materials will perform well or to design new ones systematically. The new study, published in the Journal of Materials Chemistry A (“A universal model for energy level alignment at interfaces of hole-collecting monolayers in p-i-n perovskite solar cells”), addresses that gap. The work was co-authored by Aruto Akatsuka of Chiba University, Minh Anh Truong and Atsushi Wakamiya of Kyoto University, and Gaurav Kapil and Shuzi Hayase of The University of Electro-Communications. The team set out to build a framework that holds across diverse material combinations rather than one tailored to a single chemistry. To anchor the model in measured data, the researchers used ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy to characterize representative monolayer and perovskite materials. These techniques allowed them to determine the work function, the energy difference between the Fermi level and the vacuum level of a solid, and the ionization energy, the energy required to remove an electron from the surface of a material to the vacuum. The framework treats the electrode, monolayer, and perovskite stack as two distinct regions. At the electrode and monolayer boundary, behavior is governed by an interface dipole, an electric field generated mainly by the aligned dipole moments of the monolayer molecules. At the monolayer and perovskite boundary, the analysis draws on semiconductor heterojunction theory, the established description of how two materials with different energy properties interact when joined. Two physical quantities emerged as decisive for hole collection efficiency. The first is band bending, a gradual shift in the energy landscape near the junction caused by built-in electric fields. The second is the interfacial energy barrier height, the energetic mismatch between materials that either eases or blocks charge transfer. “These quantities are determined solely by a limited set of fundamental parameters, namely the work function of the electrode and the work functions and ionization energies of the HCM and perovskite,” explains Yoshida. The team tested the framework against experimental data from a broad range of monolayer and perovskite combinations, and performance differences between materials traced back to those parameters. “Using only these parameters, our model successfully and self-consistently explains why certain HCMs lead to superior solar cell performance whereas others do not,” Yoshida says. That reduction to a small set of measurable parameters is what makes the framework useful for device development. “The proposed model offers clear selection criteria and molecular design guidelines for HCMs, enabling optimized interfacial energy levels and reducing development time and cost. This will ultimately lead to higher power conversion efficiency and improved reproducibility,” Yoshida adds. The same energy alignment principles apply to other devices in which thin organic layers meet semiconductors. The authors point to light-emitting devices and transistors as candidates for similar treatment. “Beyond photovoltaics, this framework can be extended to other semiconductor electronic devices, establishing a new foundation in materials science that contributes to sustainable energy technologies,” Yoshida concludes.