  Elemental selenium is a promising wide-bandgap ($E_\mathrm{G}\approx$ 1.95
eV) photovoltaic material for the next generation of thin-film solar cells. 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 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 charge transfer resistance rather than an increase in carrier
diffusion length. The CSA strategy is a promising method for controlling
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.
