  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.
