Selenium Solar Cells for Semi-Transparent Photovoltaics: Capped Annealing Protocols for Ultra-Thin Devices

This work investigates the impact of annealing protocols on the crystallization, morphology, and device performance of selenium (Se) solar cells, with a focus on the feasibility of transparent devices. Different annealing sequences are explored, varying the thermal processing steps relative to the layer deposition. A complete structural characterization (Raman, X-ray diffraction (XRD), and scanning electron microscopy (SEM)) reveals that uncapped annealing improves interfacial order and grain sizes, while capped conditions favor smoother morphologies. Despite similar bulk crystallinity, these differences significantly influenced the photovoltaic performance, with the best results, 5.6% power conversion efficiency (PCE), achieved when annealing the whole device. Thinner absorber layers (100 and 50 nm) are then processed into devices, enabling semi-transparent devices with promising average photopic visible transmittance (up to 40%) and light utilization efficiencies exceeding 1%, rendering the Se technology interesting for semi-transparency. Interestingly, optical modeling suggests that the optimized annealing conditions yield a nanostructured Se layer with intermediate optical constants, between those of amorphous and crystalline Se, particularly well-suited for semi-transparency, underscoring the key role of synthesis in tuning optical behavior.