Heat is an essential part of manufacturing modern thin-film solar cells—but more heat does not necessarily mean better performance. A research team at DESY has now revealed in unprecedented detail how the element selenium spreads through the material at different processing temperatures. Their findings show that while higher temperatures can improve individual regions of a solar cell, they may also reduce its overall efficiency. The results could help manufacturers further optimize future production processes.

More than 140 years after the first selenium-based solar cells were developed, the element is making a comeback in photovoltaics. Today, selenium plays a key role in modern thin-film solar cells, which require significantly less material than conventional technologies. However, little has been known about how selenium redistributes during manufacturing—or how the annealing temperature affects the local performance of the finished device.

To answer these questions, researchers at DESY investigated three nearly identical solar cells that differed only in their annealing temperature. The samples were fabricated by the National Laboratory of the Rockies (NLR) in close collaboration with the DESY team and analysed using high-resolution scanning X-ray microscopy at DESY's PETRA III storage ring. By simultaneously mapping the distribution of selenium and the optical properties of the material, the researchers were able to examine, on the microscale, how chemical composition influences device performance.

"We wanted to understand how the local chemical composition directly affects the material properties," says Michael Stuckelberger, team leader for Semiconductor Characterization at DESY. "Our measurement techniques allow us to investigate both aspects simultaneously, point by point."

The study builds on several years of method development. In earlier work, the team developed and continuously refined the experimental approach. Today, the technique is also available to external users, enabling researchers to address a wide range of questions in materials science.

When More Heat Delivers Less

At higher annealing temperatures, selenium diffuses further into the solar cell and locally improves the material quality. Surprisingly, however, the corresponding device showed the lowest conversion efficiency of all samples studied.

The reason lies in the uneven distribution of selenium. While some regions benefit from the higher temperature, others remain comparatively selenium-poor. As a result, the solar cell becomes imbalanced: its best-performing regions cannot compensate for weaker areas elsewhere in the device.

"A solar cell is ultimately only as good as its weakest regions," says Niklas Pyrlik, a doctoral researcher at DESY and co-author of the study. "For the first time, we were able to show how selenium redistributes depending on the annealing temperature and how this locally affects the material properties."

The findings provide valuable guidance for further improving the technology while also raising new research questions. The National Laboratory of the Rockies and DESY are already continuing their collaboration to investigate these effects in greater detail.

"To continuously improve solar cells, we need to understand how individual manufacturing steps influence the material," says Eric Colegrove of the National Laboratory of the Rockies. "The results of this study provide valuable insights that help us further develop the next generation of our technology."

Looking Ahead

The study also highlights the current limits of today's measurement capabilities. At present, the researchers can investigate only small regions of a solar cell at high spatial resolution and then extrapolate their findings to the entire device. In the future, the team aims to extend the method into three dimensions. The planned PETRA IV X-ray source would enable even more robust analyses while making the currently demanding technique accessible to a much broader research community.

"Today, we focus on small sections of a solar cell and hope that they are representative of the entire device," says Stuckelberger. "With PETRA IV, we could investigate areas hundreds to thousands of times larger—or study many more solar cells within the same amount of time to better understand the complex interplay of different manufacturing steps. That would open entirely new opportunities for understanding the materials needed for the next generation of semiconductor devices."

Original publication

J. L. Barp Jr. et al., Impact of the Annealing Temperature on the Local Se Distribution and Optical Properties in Solar Cells by Multimodal X-ray Microscopy, PRX Energy 5, 033011 (2026), DOI: 10.1103/56gn-4mgn.