Light-trapping in Thin Film Silicon Solar Cells with a Combination of Periodic and Randomly Textured Back-reflectors
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Light-trapping in Thin Film Silicon Solar Cells with a Combination of Periodic and Randomly Textured Back-reflectors Sambit Pattnaik1, Nayan Chakravarty1, Rana Biswas1, D. Slafer2, Vikram Dalal1 1 Iowa State University, Ames, Iowa; 2 Lightwave Power, Inc., Cambridge, Massachusetts.
ABSTRACT Light trapping is essential to harvest long wavelength red and near-infrared photons in thin film silicon solar cells. Traditionally light trapping has been achieved with a randomly roughened Ag/ZnO back reflector, which scatters incoming light uniformly through all angles, and enhances currents and cell efficiencies over a flat back reflector. A new approach using periodically textured photonic-plasmonic arrays has been recently shown to be very promising for harvesting long wavelength photons, through diffraction of light and plasmonic light concentration. Here we investigate the combination of these two approaches of random scattering and plasmonic effects to increase cell performance even further. An array of periodic conical back reflectors was fabricated by nanoimprint lithography and coated with Ag. These back reflectors were systematically annealed to generate different amounts of random texture, at smaller spatial scales, superimposed on a larger scale periodic texture. nc-Si solar cells were grown on flat, periodic photonic-plasmonic substrates, and randomly roughened photonic-plasmonic substrates. There were large improvements (>20%) in the current and light absorption of the photonicplasmonic substrates relative to flat. The additional random features introduced on the photonicplasmonic substrates did not improve the current and light absorption further, over a large range of randomization features. INTRODUCTION A persistent problem of all silicon-based solar cells is the low absorption of long wavelength red and near infrared photons. For example, at a wavelength of 700 nm the absorption length of photons exceeds 2μm in nc-Si, larger than the absorber layer thickness. It is essential to utilize light trapping approaches to increase the photon path length and harvest red and near-IR photons in thin films Si solar cells[1]. There have been various light trapping strategies used in thin film solar cells. The traditional method is to use a randomly textured back-reflector, utilizing annealed silver [2,3] or etched zinc Oxide (ZnO)[4-7]. Incident light is reflected into all angles uniformly in a Lambertian back reflector. However, roughened Ag/ZnO structures suffer from intrinsic losses of several percent at each reflection [8] that leads to large losses for weakly absorbed photons where multiple reflections from the back-reflector occur. More recently an alternate approach of using a periodically textured back-reflector or photonic-plasmonic crystal has been developed [9-15]. The periodic photonic crystal backreflector strongly diffracts light within the absorber layer, leading to wave guided modes propagating in the plane of the device, that give rise to absorption maxima. In addition, the periodic back-reflector leads
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