Over the past decade, perovskite solar cells (PSCs) have moved to the forefront of next-generation renewables, combining high power conversion efficiency with low-cost, solution-based manufacturing. Their lightweight structure also opens up applications beyond traditional panels, including integration into windows, vehicles, and portable devices. 

A major step forward has been the introduction of hole-collecting monolayers (HCMs), ultra-thin interfacial layers that extract positive charges from the perovskite. These materials have helped push single-junction PSC efficiency to 26.9% while enhancing stability.

Even so, the underlying physics remains poorly resolved – in particular, how energy levels align at the electrode–HCM–perovskite interface is still debated. Multiple competing models are used inconsistently, making it difficult to predict performance or design new materials without trial and error.

Unlocking key physics behind perovskite solar cell performance

To tackle this issue, a Chiba University-led team has now developed the first universal model for energy level alignment at electrode/HCM/perovskite interfaces, addressing a key gap in perovskite solar cell research. Led by Professor Hiroyuki Yoshida, the study provides a consistent framework explaining how hole-collecting monolayers work across different material systems and offers design guidelines for improving device performance. 

Researchers combined ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy to measure key energy properties in representative materials. This allowed precise determination of parameters such as work function and ionization energy, improving understanding of charge behavior at critical interfaces.

The new model divides the electrode/HCM/perovskite interface into two separate regions to better explain charge behavior. At the electrode–HCM boundary, energy alignment is dominated by an interface dipole, an electric field formed by the oriented molecular dipoles of the hole-collecting monolayer. 

In contrast, the HCM–perovskite boundary is described using semiconductor heterojunction theory, a standard framework in electronics for understanding how two materials with different energy levels interact when joined together.

Model predicts performance of perovskite solar cells across materials 

According to the researchers, two key factors control hole collection efficiency in perovskite solar cells. The first is band bending, a gradual change in energy levels caused by built-in electric fields at material interfaces. The second is the interfacial energy barrier height, which describes the energy mismatch that can either support or obstruct charge transfer between layers. 

Yoshida notes that these effects depend only on a few fundamental parameters, including the electrode work function and the work functions and ionization energies of the HCM and perovskite. Using this limited dataset, the model consistently explains why some HCM materials deliver better performance than others. 

The team further confirmed its validity by comparing predictions with experimental results across a wide range of material combinations. Taken together, the study offers practical guidance for designing higher-performance materials in next-generation solar technologies. 

Yoshida pointed out that the proposed model provides clear selection rules and molecular design principles for hole-collecting monolayers, helping to optimise interfacial energy alignment while reducing both development time and cost. In turn, this could enable higher power conversion efficiencies and more reproducible device performance across different material systems.