This paper presents a systematic numerical analysis of the Band Gap Narrowing (BGN) effect in indium gallium nitride (InGaN) solar cells for two indium molar fractions (x = 0.12 and x = 0.28) at T = 300 K. By sweeping the donor doping concentration N from 1016 to 3 × 1018 cm?3, we quantify the reduction of the effective band gap energy (Egeff) and the resulting red-shift of the optical cutoff wavelength (λcut). Results show that BGN intensifies non-linearly with doping and is systematically more pronounced for higher indium content. At the highest doping level (3 × 1018 cm?3), λcut shifts by +154 nm for x = 0.12 and by +378 nm for x = 0.28, extending absorption deep into the visible spectrum. These findings underline the necessity of a careful trade-off between spectral coverage and material quality degradation in the design of high-efficiency InGaN photovoltaic devices.
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