Autogenic single p/n-junction solar cells from black-Si nano-grass structures of p-to-n type self-converted electronic configuration

Abstract

Photovoltaic performance of solar cells automatically improves when the absorber layer itself simultaneously acts as the anti-reflection nanostructure with an enhanced active absorber area on the front surface. Combined physical and chemical etching of p-c-Si wafers by (Ar + H₂) plasma in inductively coupled low-pressure plasma CVD produces various nanostructures with subsequent minimization of reflectance. At a reduced temperature, the rate constant of thermal diffusion of atomic-H in the Si-network becomes smaller, leading to enhanced chemical etching reactions that further increase at an elevated RF power. Regrowth of the SiH_n precursors produced by etching and subsequent hydrogenation in the plasma develops a high density of elongated nano-grass structures, which further align with sharp tips via Ar⁺ ion bombardment and elimination of loosely bound amorphous over-layers, on application of negative dc substrate bias during real-time etching and regrowth. A significantly reduced reflectance (∼0.5%) via coherent light trapping within the uniformly distributed vertically aligned nano-grass surfaces evolves truly black-silicon (b-Si) nanostructures, which further self-convert from the p-type to n-type electronic configuration via etching-mediated modification of B–H bonds from BH₁ to BH₂ and/or BH₃ states, producing autogenic p/n junctions. Using (Ar + H₂) plasma etched b-Si nano-grass structures at low temperature (∼200 °C), one-step fabrication of autogenic single p/n-junction proof-of-concept solar cells is accomplished. There is plenty of room for further progress in device performance.