Selenium (Se) holds historical significance as the first material used in a solar cell, in 1883, marking the initial exploration of materials capable of harnessing solar energy. However, it took over a century before being seriously considered for photovoltaics, achieving at this time an efficiency of 5% with the following Au/Se/TiO2/FTO architecture. In 2017, IBM improved the device architecture with more suitable selective contacts (Au/MoOx/Se/ZnMgO/FTO) and an impressive efficiency of 6.5% was achieved. This breakthrough reignited interest in selenium for photovoltaics, resulting in many recent research publications, with a particular focus on its potential use as a top cell in tandem configurations, and much more recently as indoor devices. During the last year, two new world-record efficiencies have been reported: 7.2% and 8.1% under AM1.5 illumination, and more than 20% under indoor conditions. However, all these results were achieved for thick absorber layer (higher than 1 um) except for the IBM device, where the impressive 6.5% efficiency was achieved with a thickness of just 100 nm.

Selenium benefits from a low temperature processing (melting point around 200 ºC) and a direct bandgap of approximatively 1.95 eV. Although this bandgap may initially not seem ideal for semi-transparent applications, recent findings suggest that very thin (less than 50 nm) amorphous silicon devices can achieve high visible transparency and impressive efficiencies, with promising light utilization efficiencies (LUE) of 1%. An initial optical simulation of a complete device architecture using a 100 nm thick selenium indicated an average visible transmittance (AVT) of 51% across the visible spectrum. This could potentially lead to LUE values higher than 2.5%, which is an outstanding result, suggesting that c-Se is inherently compatible for semi-transparent applications.

To check the viability of Se for semi-transparent PV, different strategies for the synthesis of Se layers have been established. A baseline processing technology allowing the fabrication of state-of-the-art Se devices has been developed and optimized. Subsequently, strategies to get transparency have been implemented and developed. All the results of this work will be presented at the Matsus Fall 25 conference where the viability of selenium for semi-transparent photovoltaics will be discussed.