Enhancement of solar cells with photonic and plasmonic crystals - overcoming the Lambertian limit
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Sambit Pattnaik Microelectronics Research Center and Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011
Chun Xu Department of Physics and Astronomy and Ames Laboratory, Iowa State University, Ames, Iowa 50011
Joydeep Bhattacharya, Nayan Chakravarty, and Vikram Dalal Microelectronics Research Center and Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011 (Received 1 September 2012; accepted 8 February 2013)
Red and near-infrared photons of longer wave lengths are poorly absorbed in thin film silicon cells and advanced light trapping methods are necessary. The physical mechanisms underlying the light trapping using periodic back reflectors are strong light diffraction, coupled with plasmonic light concentration. These are contrasted with the scattering mechanisms in randomly textured back reflectors. We describe a class of conformal solar cells with nanocone back reflectors with absorption at the Lambertian 4n2 limit, averaged over the “entire” wave length range for hydrogenated nanocrystalline silicon (nc-Si:H) thin-film solar cells. The absorption is theoretically found for 1-lm nc-Si:H cells, and is further enhanced for off-normal incidence. Predicted currents exceed 31 mA/cm2. Nc-Si:H solar cells with the same device architecture were conformally grown on periodic substrates and compared with randomly textured substrates. The periodic back reflector solar cells with nanopillars demonstrated higher quantum efficiency and photocurrents that were 1 mA/cm2 higher than those for the randomly textured back reflectors. I. INTRODUCTION 1
Plasmonics is currently a very actively studied area as the combined nature of the electric and photonic excitation inherent in a surface plasmon mode allows manipulation of photons in nanophotonics, integrated plasmonic circuits, antennas, optical imaging and quantum information transfer.1 In this article we focus on plasmonic and photonic structures to enhance the performance of thin film siliconbased solar cells. The common solar cell architecture for thin film silicon solar cells usually utilizes a micromorph architecture composed of a top cell of high band gap amorphous silicon (a-Si:H) followed by a bottom cell of hydrogenated nanocrystalline silicon (nc-Si:H).2–4 The red and nearinfrared (IR) photons of longer wave lengths are absorbed in the bottom cell, whereas the top cell absorbs the shorter wave lengths, with current matching between the cells. A
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Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2013.41 J. Mater. Res., Vol. 28, No. 8, Apr 28, 2013
ubiquitous problem in all silicon-based solar cells is the poor absorption of red and near-IR photons of longer wave lengths.2–5 The absorption length of these photons exceeds the thickness of the nc-Si:H layer (typically 1 lm), and the quantum efficiency of these solar cells decreases rapidly at wave lengths near the band edge (1.1 lm in nc-Si:H).2–4 Similar pr
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