Omnidirectional Broadband Antireflection Coating for Silicon Solar Cell Using ITO Nanostructures

Abstract

Anti-reflection coatings (ARCs) in typical solar cells (SCs) adopt single- or multi-layered thin films with intermediate refractive indices between air and SC materials with a thickness corresponding to quarter wavelength. But they are effective only for a limited range of wavelength and incident angle. In this work, we fabricated a variety of ITO nanostructures such as thin films, nanorods, nanotrees etc. through an oblique angle deposition technique using electron-beam evaporator and applied these nanostructures into ARCs for planar p-Si SCs. From SEM and TEM analyses, the ITO nanotree was found to be grown via a self-catalytic mechanism above ~ 200 °C and the branches have an epitaxial relationship with the trunks. We prepared four Si solar cells with different ARCs; a thin film of TiO2 with quarter-wavelength thickness (thin film ARC), dense ITO nanorods (dense ARC), dense ITO nanorods/porous ITO nanotrees (hybrid ARC) along with a reference solar cell without any ARC (no ARC). The reflectance is significantly decreased from 35% (no ARC) to 12% (thin film ARC), 9% (dense ARC) and finally 7% (hybrid ARC), which is responsible for the enhancement of solar cell efficiency to 25% (thin film ARC), 38% (dense ARC), and 47% (hybrid ARC) compared with the reference cell. Also in the angular reflectance measurements from 20° to 70°, the hybrid ARC has a very low reflectance of 7% and excellent omni-directionally reflectance, which is significantly low compared with 11% (dense ARC), 18% (thin film ARC), and 45% (no ARC). By considering the angular dependence of solar cells, the annual power density was calculated based on the meridian transitaltitude of Seoul, Republic of Korea. The power enhancement by omnidirectional light absorption is as high as 33.5% in the hybrid ARC solar cell, compared with 16.3% (dense ARC) and 1.4% (thin film ARC). Our results suggest that onmi-directional ARC can be a breakthough technology for the solar power generation.