Fundamental and practical efficiency limits of selenium solar cells under outdoor and indoor illumination

Selenium (Se), particularly its trigonal phase, offers a wide-bandgap platform for outdoor and indoor photovoltaics. This work quantifies its spectrum-dependent efficiency limits using reciprocal optical absorption, radiative and nonradiative recombination, finite thickness, and electrical resistance, with separate module-temperature and degradation sensitivities. At a bandgap of 1.85 eV and 300 K, the one-sun air mass 1.5 global Shockley–Queisser limit is 26.10%; the nominal 500-lux limits are 52.86% for a white light-emitting diode (LED) and 53.16% for a compact fluorescent lamp. A representative experimental absorption spectrum gives one-micrometer-thick planar and textured radiative limits of 26.71% and 28.00%, respectively. A non-unique calibration reproduces three independent performance metrics of the certified 10.3% device. Parameter-conditioned advanced-target and practical-limit scenarios give 20.5%–26.9% outdoors and 26.5%–42.2% under the LED spectrum. These are isothermal, external-response-conditioned results, not forecasts of device performance. The full Shockley–Read–Hall expression and finite-doping calculations distinguish low-injection behavior from the intrinsic baseline. The radiative and recombination maps assume spatially uniform carrier densities and quasi-Fermi-level splitting through the absorber, together with complete extraction unless an empirical external-response factor is stated; they are zero-dimensional optical-recombination limits rather than drift–diffusion predictions for low-mobility Se films. Temperature-dependent optical-edge shifts are treated as illustrative sensitivities rather than calibrated material parameters. An ideal sub-bandgap reflector can reject 56.3% of incident solar power if an opaque module would otherwise absorb it, compared with only 1.30% under the modeled LED. The analysis identifies distinct outdoor and indoor priorities in absorption-edge quality, recombination, carrier extraction, leakage, and rear-stack optics, and provides reproducible parameter maps for interpreting future Se measurements.