Exploring Selenium Viability for Semi-Transparent Photovoltaics

Selenium (Se) holds historical significance as the first material used in a solar cell [1], 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% [2] with the following Au/Se/TiO₂/FTO architecture. In 2017, Todorov improved the device architecture with more suitable selective contacts (Au/MoO_x/Se/ZnMgO/FTO) and achieved the current worldwide efficiency record of 6.5% [3]. 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, due to low temperature processing (melting point around 200 °C) and a direct bandgap of approximatively 1.95 eV. However, Se appears also well suited for semi-transparent and/or indoor applications. Although its energy bandgap may initially not seem ideal for semi-transparency, 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 0.7% [4]. Notably, the current worldwide efficiency record involved a 100 nm layer, but there is variability in the literature regarding the optimal absorber thickness, ranging from 100 nm to several micrometers, which could compromise its suitability for semi-transparent applications.<br/><br/>To check the viability of Se for semi-transparent PV, several amorphous layers of different thicknesses were prepared using an evaporation system. These layers were optically characterized before and after hot plate crystallization at temperatures around 200 °C, a process widely reported in literature. This crystallization step, both on glass and FTO, resulted in the formation of separated Se crystallized islands. In an effort to enhance the Se adhesion during crystallization, we initially avoided the use of Te, known to improve it, but potentially detrimental for device performance. Additionally, we explored the option of performing the crystallization step on complete device architectures, incorporating various selective contact layers before Se deposition, both in substrate and superstrate configurations, yielding valuable insights. All the results of this work will be presented at the MRS Meeting and the viability of selenium for semi-transparent photovoltaics discussed.<br/><br/>[1] C. E. Fritts, On a new form of selenium cell, and some electrical discoveries made by its use, Am. J. Sci. 26, 465 (1883).<br/>[2] A. Kunioka et al., Polycrystalline thin-film TiO2/Se solar cells, Jpn. J. Appl. Phys. 24, L536–L538 (1985).<br/>[3] T. K. Todorov et al., Ultrathin high band gap solar cells with improved efficiencies from the world’s oldest photovoltaic material, Nature Communications 8, 682 (2017).<br/>[4] A. Lopez-Garcia et al., Ultrathin a-Si:H/Oxide Transparent Solar Cells Exhibiting UV-Blue Selective-Like Absorption, Solar RRL 7, 2200928 (2023).