Fabrication of Photonic Crystal based Back-reflectors for Light Management and Enhanced Absorption in Amorphous Silicon
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Fabrication of Photonic Crystal based Back-reflectors for Light Management and Enhanced Absorption in Amorphous Silicon Solar Cells Benjamin Curtin1, Rana Biswas1,2 and Vikram Dalal1 1 Microelectronics Research Center; Dept. of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011, U.S.A. 2 Dept. of Physics & Astronomy; Ames Lab, Iowa State University, Ames, Iowa 50011, U.S.A. ABSTRACT Photonic crystal based back-reflectors are an attractive solution for light management and enhancing optical absorption in thin film solar cells, without undesirable losses. We have fabricated prototype photonic crystal back-reflectors using photolithographic methods and reactive-ion etching. The photonic crystal back-reflector has a triangular lattice symmetry, a thickness of 250 nm, and a pitch of 765 nm. Scanning electron microscopy images demonstrate high quality long range periodicity. An a-Si:H solar cell device was grown on this back-reflector using standard PECVD techniques. Measurements demonstrate strong diffraction of light and high diffuse reflectance by the photonic crystal back-reflector. The photonic crystal backreflector increases the average photon collection by ~9% in terms of normalized external quantum efficiency, relative to a reference device on a stainless steel substrate with an Ag coated back 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]. 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 [4]. For a-Si:H with an energy gap of 1.75 eV typical of mid-gap cells, most photons with wavelengths below the band edge of 700 nm are absorbed. Short wavelength solar photons in the blue and green regions of the spectrum have absorption lengths less than 250 nm and are effectively absorbed within the thin absorber layer. However, the absorption length of photons grows rapidly for red light (λ > 600 nm) and even exceeds 6-7 µm for photons near the band edge. These red and near-IR photons are very difficult to absorb in thin a-Si:H layers and light-trapping schemes are critical to harvest these long-wavelength photons. Similar physical
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