Photonic and plasmonic crystal based enhancement of solar cells- overcoming the Lambertian classical 4n 2 limit
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Photonic and plasmonic crystal based enhancement of solar cells- overcoming the Lambertian classical 4n2 limit Rana Biswas1,2, Chun Xu1, Sambit Pattnaik2, Joydeep Bhattacharya2, Nayan Chakravarty1, Vikram Dalal1 1
Dept. of Physics and Astronomy and Ames Laboratory, Iowa State University, Ames, Iowa 50011 2 Microelectronics Research Center and Department of Electrical and Computer Engineering, Iowa State University, Ames, IA 50011 ABSTRACT Long wavelength photons in the red and near infrared region of the spectrum are poorly absorbed in thin film silicon cells, due to their long absorption lengths. Advanced light trapping methods are necessary to harvest these photons. The basic physical mechanisms underlying the enhanced light trapping in thin film solar cells using periodic back reflectors include strong diffraction coupled with light concentration. These will be contrasted with the scattering mechanisms involved in randomly textured back reflectors, which are commonly used for light trapping. A special class of conformal solar cells with plasmonic nano-pillar back reflectors will be described, that generates absorption beyond the classical 4n2 limit (the Lambertian limit) averaged over the entire wavelength range for nc-Si:H. The absorption beyond the classical limit exists for common 1 micron thick nc-Si:H cells, and is further enhanced for non-normal light. Predicted currents exceed 31 mA/cm2 for nc-Si:H. The nano-pillars are tapered into conical protrusions that enhance plasmonic effects. Such conformal nc-Si:H solar cells with the same device architecture were grown on periodic nano-hole, periodic nano-pillar substrates and compared with randomly textured substrates, formed by annealing Ag/ZnO or etched Ag/ZnO. The periodic back reflector solar cells with nano-pillars demonstrated higher quantum efficiency and higher photo-currents that were 1 mA/cm2 higher than those for the randomly textured back reflectors. Losses within the experimental solar architectures are discussed. INTRODUCTION Thin film silicon solar cells utilize a micro-morph solar architecture composed of a top cell of high band gap amorphous silicon (a-Si:H) followed by a bottom cell of nano-crystalline silicon (nc-Si:H) [1,2,3]. The longer wavelength red and near-infrared photons are absorbed in the bottom cell, whereas the top cell absorbs the shorter wavelengths. A ubiquitous problem in all silicon based solar cells is the poor absorption of long-wavelength red and near-infrared photons [1,2,3,4]. The absorption length of these photons exceeds the thickness of the nc-Si:H layer (typically 1 μm), and the quantum efficiency of these solar cells decreases rapidly at wavelengths near the band edge (1.1 μm in nc-Si:H) [1,2,3]. Similar problems are well known in a-Si:H solar cells where the active layer thickness is limited by the hole diffusion length to be ~300nm. The absorption length of red and near-IR photons with wavelengths beyond 650 nm exceeds the thickness of the cell, and light trapping
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approaches are essential. Crystalline silic
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