Extraordinary high broadband specular transmittance of sodalime glass substrate by vapor phase etching

In this paper, we present a simple method to fabricate the antireflective porous surface on sodalime glass using a single step HF-vapor phase etching method. Under optimal conditions, both-sides etched glass substrate exhibited a broadband enhancement in the transmittance with maximum transmittance as high as 99.2% at ∼500 nm with extremely low diffusive scattering. The measured transmittance exceeds by ∼7.6% as compared to plain glass (91.6%). X-ray photoelectron spectroscopy results confirmed the formation of a fluoride layer comprising of NaF and CaF2 on sodalime glass substrate after etching. Field emission scanning electron microscopy results showed the formation of porous structure with randomly distributed pores of size <150 nm. The refractive index of the porous fluoride layer was found to be 1.28 and lowest reflectance of 0.6% has been achieved. Moreover, reflection (measured at 500 nm) remains below 1.5% over a range of incident angles (8-48°), which is ascribed to the fact that refractive index follows a gradual change in the nanoporous surface. The theoretical transmittance of the optimized etched glass determined by finite difference time domain simulation shows a good agreement with the experimental results. The silicon solar cell covered with both-sides optimized etched glass showed a relative increase of ∼4% in power conversion efficiency as compared to a solar cell covered with a plain glass.