Japan -- Perovskite solar cells, PSCs, have emerged as one of the most promising renewable energy technologies of the past decade. Besides their remarkable power conversion rates, perovskites are lightweight in nature and can be manufactured through low-cost solution processing methods. Thus, they offer greater versatility for applications that go beyond rooftop solar cell installations, such as integration into building windows, vehicle surfaces, and portable electronics. A recent key breakthrough in PSCs has been the development of hole-collecting monolayers, or HCMs -- ultra-thin layers that collect positive electrical charges, or "holes", from the perovskite material. HCMs have pushed single-junction PSCs to 26.9% power conversion efficiency while improving device stability.
Despite these advances, scientists do not fully understand the fundamental mechanisms governing their molecular and electronic behavior. The way energy levels align at the interface between the electrode, the HCM, and the perovskite layer plays a central role in determining how efficiently charges move through the device. However, several competing theories, such as vacuum level alignment, Fermi level alignment, and the electrode-modified Schottky model, have been used interchangeably to model energy levels at the interface, often without clear justification. As a result, scientists today struggle to predict which HCM materials would perform well or design new ones without relying heavily on trial and error.
Fortunately, a research team led by Professor Hiroyuki Yoshida from the Graduate School of Engineering, Chiba University, Japan, has addressed this knowledge gap by developing the first universal model for energy level alignment at electrode/HCM/perovskite interfaces. Their findings, published in the Journal of Materials Chemistry A on 14 March 2026, establish a physically consistent framework that explains and provides guidelines for HCM performance across diverse material combinations. The study was co-authored by Mr Aruto Akatsuka from Chiba University, Dr Minh Anh Truong, Professor Atsushi Wakamiya from Kyoto University, and Professor Shuzi Hayase from The University of Electro-Communications, Japan.
To build this model, the researchers used advanced techniques, including ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy, to precisely measure key energy properties of representative HCM materials and perovskites. These measurements allowed them to determine important quantities in the materials, such as the work function, energy difference between the Fermi level and the vacuum level of a solid material, and the ionization energy, the energy needed to remove an electron from the surface of a material to the vacuum.
The proposed model treats the electrode/HCM/perovskite interface as two distinct regions. The boundary between the electrode and the HCM is governed by the formation of an interface dipole, which is an electric field created mainly by the dipole moment of the orientationally aligned HCM molecules. Meanwhile, the boundary between the HCM and the perovskite is analyzed through the lens of semiconductor heterojunction theory, a well-known concept in conventional semiconductor-based electronics where two materials with different energy properties meet.
The model identified two critical factors that determine hole collection efficiency. The first is a phenomenon known as 'band bending', which refers to a gradual shift in the energy landscape caused by built-in electric fields at the junction. The second factor is the interfacial energy barrier height, which is the energetic mismatch between materials that can either facilitate or hinder 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," Yoshida explains. "Using only these parameters, our model successfully and self-consistently explains why certain HCMs lead to superior solar cell performance whereas others do not." Notably, the team validated the model by testing it against experimental data from a diverse range of materials and perovskite combinations.
Overall, this study provides practical guidance for designing materials with improved performance for emerging solar technologies. "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," says Yoshida.
The researchers also note that the impact of their work may extend beyond solar cells. The same principles could be applied to light-emitting devices and transistors. "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.
