Laser-Annealing and Solid-Phase Epitaxy of Selenium Thin-Film Solar Cells

Selenium has resurged as a promising photovoltaic material in solar cell research due to its wide direct bandgap of 1.95 eV, making it a suitable candidate for a top cell in tandem photovoltaic devices. However, the optoelectronic quality of selenium thin-films has been identified as a key bottleneck for realizing high-efficiency selenium solar cells. In this study, we present a novel approach for crystallizing selenium thin-films using laser-annealing as an alternative to the conventionally used thermal annealing strategy. By laser-annealing through a semitransparent substrate, a buried layer of high-quality selenium crystallites is formed and used as a growth template for solid-phase epitaxy. The resulting selenium thin-films feature larger and more preferentially oriented grains with a negligible surface roughness in comparison to thermally annealed selenium thin-films. We fabricate photovoltaic devices using this strategy, and demonstrate a record ideality factor of n=1.37, a record fill factor of FF=63.7%, and a power conversion efficiency of PCE=5.0%. The presented laser-annealing strategy is universally applicable and is a promising approach for crystallizing a wide range of photovoltaic materials where high temperatures are needed while maintaining a low substrate temperature

Laser-Annealing and Solid-Phase Epitaxy of Selenium Thin-Film Solar Cells

Selenium has resurged as a promising photovoltaic material in solar cell research due to its wide direct bandgap of 1.95 eV, making it a suitable candidate for a top cell in tandem photovoltaic devices. However, the optoelectronic quality of selenium thin-films has been identified as a key bottleneck for realizing high-efficiency selenium solar cells. In this study, we present an approach for crystallizing selenium thin-films using laser-annealing as an alternative to the conventionally used thermal annealing strategy. By laser-annealing through a semitransparent substrate, a buried layer of high-quality selenium crystallites is formed and used as a growth template for solid-phase epitaxy. The resulting selenium thin-films feature larger and more preferentially oriented grains with a negligible surface roughness in comparison to thermally annealed selenium thin-films. We fabricate photovoltaic devices using this strategy and demonstrate a record ideality factor of n = 1.37, a record fill factor of FF = 63.7%, and a power conversion efficiency of PCE = 5.0%. The presented laser-annealing strategy is universally applicable and is a promising approach for crystallizing a wide range of photovoltaic materials where high temperatures are needed while maintaining a low substrate temperature

Increasing the Collection Efficiency in Selenium Thin-Film Solar Cells Using a Closed-Space Annealing Strategy

Elemental selenium is an interesting candidate for the top cell in tandem solar cells due to its wide bandgap of EG ≈ 1.95 eV as well as its monatomic simplicity. To realize high-efficiency selenium solar cells, it is crucial to optimize the crystallization process of the selenium thin-film photoabsorber. However, the high vapor pressure of selenium restricts the processing conditions to a compromise between the growth of large crystal grains and the formation of pinholes. In this study, we introduce a closed-space annealing (CSA) strategy designed to suppress the sublimation of selenium, enabling thermal annealing processes at higher temperatures and for longer periods of time. As a result, we consistently improve the carrier collection and the overall photovoltaic device performance in our selenium solar cells. By characterizing the carrier dynamics in our devices, we conclude that the observed improvements result from a reduction in the charge-transfer resistance rather than an increase in the carrier diffusion length. The CSA strategy is a promising method for controlling the surface morphology and roughness without reducing crystal grain sizes, which paves the way for further advancements in the efficiency and reproducibility of selenium thin-film solar cells.</p