New research sheds light on voltage deficit in selenium solar cells

Scientists from Havard University, the Swiss Federal Laboratories for Materials Science and Technology (Empa) and Denmark's DTU have investigated the origins of voltage deficit in selenium (Se) solar cells and have found that the well-known intrinsic point defects that characterize these PV devices are not a crucial factor impacting their performance.

“In particular, we found that vacancies form several deep defect levels in the bandgap, but incredibly, we also ascertained that selenium is tolerant to all of these point defects,” one of the research's leading authors, Rasmus Nielsen, told pv magazine. “This is an incredibly promising trait, as we’ve eliminated point defects as the killers of the photovoltaic performance in selenium thin-film solar cells.”

Using time-of-flight secondary ion mass spectroscopy (TOF-SIMS), the researchers initially identified the impurities present in one of the best-performing selenium thin-film photoabsorber they built, which demonstrated a world-record open-circuit voltage of 0.992 V.

“As expected, we found selenium, tellurium and oxygen – no surprise given how we fabricate our photovoltaic devices – but we also detected halogens, namely fluorine and chlorine,” Nielsen explained. “These impurities were unexpected, but their presence alone doesn’t prove they’re killer defects.” 

The research group then teamed up with computational materials scientists from Imperial College London, University of Birmingham, and Harvard University – Aron Walsh, David O. Scanlon, and Seán R. Kavanagh – to conduct defect calculations on trigonal selenium (t-Se). “Using highly sophisticated and new approaches to first-principles density functional theory (DFT) defect calculations, Seán took a deep dive into both the intrinsic defects in selenium that are unavoidable such as vacancies and interstitials, and the extrinsic elements we found experimentally.”

Through their analysis, the scientists found that the t-Se thin film crystal dimensionality plays a key role in the behavior of defects, as it “favors” self-interstitials and chalcogen substitutions and “disfavors” vacancies and heterovalent defects, due to its structural flexibility.

Analyzing the effect of a range of relevant extrinsic impurities in t-Se samples and computing the electronic and energetic properties of these species and other likely impurities, the team found that most extrinsic species are inactive for doping, either due to electrical inactivity or self-compensation.

“Naturally, this conclusion raises the question about what then limits performance,” Nielsen said. “Our results point to interfaces and extended defects – which, in principle, is much easier to engineer.”

Looking forward, he said there are “promising avenues” to understand the origins of defects and improve the related passivation strategies. “So, selenium is still very much in the game for tandem and indoor PV,” he concluded.

The research's findings can be found in “Intrinsic point defect tolerance in selenium for indoor and tandem photovoltaics,” published in Energy & Environmental Science. The research team also included scientists from Aarhus University.

The research group from DTU presented in December 2022 a 0.30 cm2 selenium solar cell with a world record open-circuit voltage of 0.99 V. A few months later, the same team showcased a selenium solar cell built with laser-annealing achieving a record fill factor of 63.7%.

In April, the same group from DTU teamed up with researchers from IBM and presented the results of research aimed at understanding the optoelectronic properties and carrier dynamics of selenium solar cells. In this work, they found that carrier mobilities in selenium are significantly more promising than commonly thought.

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