Harvesting Photons in Thin Film Solar Cells with Photonic Crystals
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Harvesting Photons in Thin Film Solar Cells with Photonic Crystals Dayu Zhou1, and Rana Biswas1,2 1 Microelectronics Research Center and Department of Electrical and Computer Engineering, Iowa State University, Ames, IA, 50011 2 Department of Physics and Astronomy, Ames Laboratory, Iowa State University, Ames, IA, 50011 ABSTRACT Enhanced light absorption and improved photon harvesting is a major avenue to improving solar cell performance. We simulate and design photonic crystal based loss-less back reflectors. The photonic crystal is a 2-dimensional photonic crystal combined with a distributed Bragg reflector. We have designed and simulated a thin film a-Si:H solar cell with the photonic crystal reflector and an antireflection coating. The photonic crystal has square lattice symmetry and generates strong diffraction of near band edge photons in the absorber layer. The absorption of red and near-IR photons is increased by more than an order of magnitude by the photonic crystal. The photonic crystals are composed of ITO and can easily serve as a conducting back contact. This scheme can be easily extended to other solar absorber layers. We have optimized the geometry of the photonic crystal to maximize absorption using rigorous scattering matrix simulations. The optical path length with the photonic crystal can improve over the limit for a random roughened scattering surface. INTRODUCTION A critical need for all solar cells is to maximize the absorption of the solar spectrum. Optical enhancements and light trapping is a cross-cutting challenge applicable to all types of solar cells. Traditionally optical enhancements have involved use of anti reflecting coatings coupled with a metallic back reflector. Solar cell efficiencies are improved by textured metallic back reflectors which scatter incident light through oblique angles, thereby increasing the path length of photons within the absorber layer [1]. A completely random loss-less scatterer is predicted [2] to achieve an enhancement of 4n2 (n is the refractive index of the absorber layer), which has the value near 50 in a-Si:H. However the idealized limit of loss-less scattering is not possible to achieve in solar cells, and it is estimated that optical path length enhancements of ~10 are achieved in practice [3]. Textured back reflectors of Ag coated with ZnO have intrinsic losses resulting from surface plasmon modes of the granular interface. Optical measurements by Springer et al [4] have estimated losses of 3-8% with every reflection. Such losses accumulate rapidly. With a loss of 4% with each pass, 34% of the light is lost in 10 passes. It is thus necessary to examine alternative schemes for light trapping. Although the analysis in this paper can be applied to any semiconductor absorber, we focus on a-Si:H, where the optical constants have been well-determined [5]. For a-Si:H with an energy gap of 1.6 eV typical of mid-gap cells, photons with wavelengths below the band edge of 775 nm are absorbed. Short wavelength solar photons in the blue and green
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