Spectral-Thermodynamic Detailed-Balance Modeling of Single-Junction Photovoltaics Under ASTM AM1.5G and E490 AM0 Spectra

This paper develops a reproducible spectral-thermodynamic model for ideal single-junction photovoltaic conversion under the ASTM G173 AM1.5G terrestrial spectrum and the ASTM E490 air-mass-zero (AM0) extraterrestrial spectrum. The formulation converts tabulated spectral irradiance to photon flux, evaluates step-function absorption, and enforces radiative detailed balance through the cell blackbody emission current. A bounded voltage-retention factor, 0 < Vf ≤ 1, is introduced as a phenomenological sensitivity parameter for aggregate voltage losses without permitting energy creation. Spectral Shannon entropy is retained only as a grid-controlled distribution descriptor; it is not subtracted directly from an energy flux. At 300 K and with one-sided radiative emission, the calculated AM1.5G maximum is 33.71% near a 1.34-eV bandgap, while the E490 AM0 model window yields 30.60% near 1.26 eV and a larger absolute power density. For AM1.5G, reducing Vf from 1.00 to 0.85 lowers the optimum efficiency from 33.71% to 28.18%. A silicon temperature study using a Varshni bandgap relation predicts decreasing open-circuit voltage and radiative-limit efficiency from −100 °C to 100 °C. The resulting model unifies terrestrial, space, voltage-loss, spectral-distribution, and temperature analyses while clearly separating theoretical limits from realizable device performance.