Experimental Demonstration of 4<i>n</i>² Classical Absorption Limit in Nanotextured Ultrathin Solar Cells with Dielectric Omnidirectional Back Reflector

Abstract

The experimental demonstration of the 4n2 classical absorption limit in solar cells has been elusive for the last 30 years. Especially the assumptions on front and internal rear reflectance in a slab of absorbing material are not easily fulfilled unless an appropriate light-trapping scheme is applied. We propose an advanced metal-free light-trapping scheme for crystalline silicon wafers. For different bulk thicknesses, at the front side of the wafers we applied a nanotexture known as black-silicon. At the rear side, we implemented a random pyramidal texture coated with a distributed Bragg reflector. Such a dielectric back reflector was designed to exhibit a maximized omnidirectional internal rear reflectance in the region of weak absorption of crystalline silicon. Integrating the measured absorptance spectra of our wafers with the reference solar photon flux between 400 and 1200 nm, we could calculate the so-called implied photogenerated current densities. For wafers thinner than 35 μm, we achieved more than 99% and up to 99.8% of the implied photogenerated current density based on the theoretical 4n2 classical absorption limit. Successful implementation of our maskless and metal-free light-trapping scheme in crystalline silicon solar cells requires the adequate surface passivation of the front nanotexture. For this purpose we used thermal silicon oxide, but we discuss also the usage of aluminum oxide. Our findings, applied in a solar device structure where front side losses are minimized, open the way for the realization of next-generation high-efficiency, cost-effective, and ultrathin crystalline silicon solar cells.