Exploring Selenium for Semi-Transparent and Indoor Photovoltaics

Small autonomous systems or Internet of Things (IoT) devices incorporating photovoltaics (PV) mostly rely on amorphous silicon (a-Si) cells. Crystalline silicon, with higher efficiency and stability, faces limitations like weight, design aesthetics and mass production capability for small devices, especially under low light conditions where their performance decreases. For indoor, a-Si is the dominant technology thanks to its wider bandgap energy, matching the spectra of artificial lights, but it has some performance limitations and degradation issues over time. On the other way, a-Si cells are also the preferred choice for solar windows with selected transparency grades, achieved by partially removing the thick active layer using laser techniques. Recently, very thin a-Si devices (< 50 nm) have shown high visible transparency without material removal, offering encouraging efficiencies and promising light utilization efficiencies (LUE) of 0.7%.

Indoor and semitransparent applications require absorber materials with bandgaps between 1.6-2.2 eV. Materials like perovskite, organic and dye-sensitized layers have such energies but face challenges regarding stability and upscaling still need to be addressed. Thin film chalcogenides (CdTe, CIGS, and CZTS) also offer tunable bandgap energies within this range (adjusting their composition or forming alloys) but few devices have demonstrated high efficiencies with such bandgap energies, and with limited evidence of semitransparency and few study under indoor light. Additionally, these technologies often involve high-temperature processing, posing compatibility issues with transparent contacts and low-temperature substrates.

Elemental selenium (Se) semiconductor possesses numerous advantages for these applications thanks to its bandgap (1.95 eV) and low temperature processing (200°C). To check the viability of Se for semi-transparent and indoor PV, a baseline fabrication process of Se devices has been first developed, including the optimization of the selenium recrystallization process, which enabled the demonstration of Se devices with efficiencies up to 4.4% (for Se layer thickness of 300 nm). Experiments were then conducted to increase the transparency by reducing the thickness, achieving an efficiency of 4% for a 100nm-thick Se device yielding a LUE of 1.2%. Preliminary characterization of (unoptimized) Se devices under various indoor lighting conditions revealed that they maintain high fill factor and open circuit voltage values under most of the conditions. All the results of this work will be presented during the MRS meeting, where the viability of selenium for these niche PV applications discussed.