Light Trapping in Silicon Nanowire Solar Cells

TLDR

Thin‑film solar cells reduce cost but require high absorption or light trapping; semiconducting nanowire arrays have low reflectance, yet their light‑trapping capability has not been quantified. The study demonstrates that ordered silicon nanowire arrays can increase incident light path length by up to 73×, as shown by optical transmission and photocurrent measurements on thin silicon films. The nanowire arrays achieve a 73× light‑trapping enhancement exceeding the Lambertian limit, surpassing other methods, and yield efficiencies above 5% with higher short‑circuit currents than planar controls, though absorption gains can be offset by surface recombination.

Abstract

Thin-film structures can reduce the cost of solar power by using inexpensive substrates and a lower quantity and quality of semiconductor material. However, the resulting short optical path length and minority carrier diffusion length necessitates either a high absorption coefficient or excellent light trapping. Semiconducting nanowire arrays have already been shown to have low reflective losses compared to planar semiconductors, but their light-trapping properties have not been measured. Using optical transmission and photocurrent measurements on thin silicon films, we demonstrate that ordered arrays of silicon nanowires increase the path length of incident solar radiation by up to a factor of 73. This extraordinary light-trapping path length enhancement factor is above the randomized scattering (Lambertian) limit (2n(2) approximately 25 without a back reflector) and is superior to other light-trapping methods. By changing the silicon film thickness and nanowire length, we show that there is a competition between improved absorption and increased surface recombination; for nanowire arrays fabricated from 8 mum thick silicon films, the enhanced absorption can dominate over surface recombination, even without any surface passivation. These nanowire devices give efficiencies above 5%, with short-circuit photocurrents higher than planar control samples.