Nano-crystalline silicon solar cell architecture with absorption at the classical 4n2 limit Rana Biswas* and Chun Xu Ames Laboratory, Dept. of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA Microelectronics Research Center, Dept. of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011, USA *
[email protected]
Abstract: We develop a periodically patterned conformal photonicplasmonic crystal based solar architecture for a nano-crystalline silicon solar cell, through rigorous scattering matrix simulations. The solar cell architecture has a periodic array of tapered silver nano-pillars as the backreflector coupled with a conformal periodic structure at the top of the cell. The absorption and maximal current, averaged over the entire range of wavelengths, for this solar cell architecture is at the semi-classical 4n2 limit over a range of common thicknesses (500-1500 nm) and slightly above the 4n2 limit for a 500 nm nc-Si cell. The absorption exceeds the 4n2 limit, corrected for reflection loss at the top surface. The photonic crystal cell current is enhanced over the flat Ag back-reflector by 60%, for a thick 1000 nm nc-Si layer, where predicted currents exceed 31 mA/cm 2. The conformal structure at the top surface focuses light within the absorber layer. There is plasmonic concentration of light, with intensity enhancements exceeding 7, near the back reflector that substantially enhances absorption. ©2011 Optical Society of America OCIS codes: (050.1950) Diffraction gratings; (240.6680) Surface plasmons; (310.6845) Thin film devices and applications ; (350.6050) Solar energy.
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1. Introduction Micro-morph tandem solar cells are a very attractive low-cost thin film tandem solar cell architecture [1] consisting of a nano-crystalline silicon (nc-Si) bottom cell of lower band gap [1,2] coupled with a top high band gap cell of hydrogenated amorphous silicon (a-Si:H). Short wavelength light (blue-green) is absorbed by the top a-Si:H cell whereas the longer wavelengths (upto the band edge of 1100 nm) are absorbed in the bottom nc-Si cell. Advanced manufacturing technology can produce large area cells on both rigid and flexible substrates with a record stabilized cell efficiency of 12% reported [2–4] for micro-morph cells. A serious drawback of thin film silicon solar cells is that red and near-infrared (IR) photons (with λ >650 nm) are very poorly absorbed in thin nc-Si absorber layers. Using experimental wavelength-dependent dielectric functions [1] for nc-Si, the absorption length of near-infrared (IR) photons [5] (la) exceeds 2 μm for λ>700 nm, exceeding the thickness of the nc-Si layer (typically less than 1.5 μm), in micro-morph solar cells. Similar light-harvesting problems exist for thicker c-Si solar cells [6]. Nc-Si is a mixed phase material composed of Si crystallites, that nucleate within an amorphous matrix [7], resulting in nc-Si having higher absorption than c-Si [1,8]. Since long-wavelength photons are absorbed by nc-Si in micromorph cells, it is critical to achieve light trapping in nc-Si rather than a-Si:H. To enhance the absorption of solar photons upto wavelengths of the band edge (λ = 1100 nm i.e.1.1 eV) in nc-Si, a common solution [2] is to use a randomly roughened back-reflector of silver and zinc oxide (Ag/ZnO), formed by etching ZnO, or roughening Ag on a substrate. The randomly roughened Ag/ZnO back reflector, with feature sizes much less than the wavelength, scatters incoming light in nearly Lambertian manner in random directions. This increases the path length of near-IR and red photons, increasing absorption and photo-current. Yablonovitch [9] demonstrated that such Lambertian scattering increases the path length by 4n(λ)2 at each wavelength λ, for completely loss-less conditions, where n(λ) is the refractive index. This enhancement factor can approach ~50 in silicon [10]. However there are
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Received 14 Mar 2011; revised 23 Apr 2011; accepted 29 Apr 2011; published 17 May 2011
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significant losses [11,12] from excitation of surface plasmon modes in the randomly textured back reflector, and it is suggested [13] that experimental enhancements are considerably less than the 4n2 factor. It is a long-standing goal to develop solar cell architectures that can exceed the semiclassical 4n2 limit averaged over all wavelengths in the solar spectrum [14–18]. There has been much activity with periodic photonic crystal based back-reflectors [19–24] where a periodically structured back reflector diffracts light, enhancing the photon path length and dwell time of long-λ photons within the absorber layer. The wavelength of light inside the absorber layer is λ’ = λ/n(λ). When λ’
