Silica antireflection coatings with a tailored refractive index for applications of optics and photovoltaics

Abstract

The antireflection (AR) coatings are of technological importance for applications in optical, electrochemical and sensing devices, however, good performance is restricted to the precise control of the nanoscale structure to suppress the reflection. The applied structures that have been investigated so far are featured by porosity, surface pattern and gradient distribution in the refractive index, but they remain a challenge in large scale fabrication. In this work, an investigation of highly uniform and porous silica AR coating has been systematically presented, and the recent advances of AR technology have also been summarized, including the basic principles of antireflection and its possible applications. A systematic study was carried out to investigate the effect of catalyst and aging time on silica particle size and distribution. It was found that the silica particle size can be tailored well between 9.8 to 91.0nm by the control of the ammonia catalyst ratio. Moreover, to determine the optimal size based on the sol-gel process, a comparative study of the particle effect on the AR properties was undertaken. Where capillary stress was determined to be the dominating factor of the particle assembly, and the refractive index of silica coating was adjusted from 1.16 to 1.47. Based on the current mechanisms of the antireflection and theoretical calculations, two sizes of silica sphere (56.2 and 91.0 nm in diameter) were selected as the main component of the AR coating layer. A peak transmittance achieved 99.57% at the wavelength of 550nm with an average of 98.18% over the visible range (380-800nm). Furthermore, the combination of nano porous AR coatings and the surface pattern on the solar cell covered glass showed vast improvement in the glass transmission efficiency over a wide-angle of incidence ranging from 0 to 68°. The enhancements in the photovoltaic system output efficiency are ≈ 2.5%, as estimated by a comprehensive calculation considering the net effect from transmission spectra, the AM1.5 solar radiation spectrum and silicon solar cell external quantum efficiency spectrum. A freeze test and accelerated weathering stability test showed the strong resistance of the AR coatings which suggests great potential for commercial applications.