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| Fig. 1: Random surface texturing plus a back reflector result in significant absorption enhancement. |
Solar cell technology can provide unlimited, sustainable, clean energy by converting solar radiation into electricity. Despite these advantages, materials cost has been the main obstacle preventing widespread usage of solar technology. To compete with fossil fuel technologies, the cost of solar energy should be reduced by a factor of 2-5. [1] One approach to reduce the cost is to reduce the amount of material used in a cell by thinning the absorber layer, where charge carriers are generated. In addition to less material cost, a thin layer is advantageous in efficiently collecting charge carriers by minimizing recombination and therefore improving efficiency.
Commercial cells are generally much thicker than intrinsic absorption length of the material which allows efficient absorption of the sun light. To get similar performance from thin cells, light trapping schemes have been used. Light trapping provides opportunities for the light to stay in the absorber layer for a long time to increase the chance of it being absorbed and generate charge carriers.
In 1982, a theory of light trapping was developed which established an upper limit for absorption enhancement in solar cells that are multiple wavelengths thick. Treating light as a straight ray, the absorption enhancement limit calculated to be 4n2 where n is the refractive index of the absorbing material. [2,3] As a result, the efficiency of a conventional cell cannot be boosted beyond a certain amount. For example, for silicon with a refractive index of 3.5, the maximum absorption enhancement is about 49. Researchers have investigated various design strategies to approach this limit. Fig. 1 shows an example where scattering by random texturing and a back reflector are added to the absorber layer to scatter the incident light into many different directions. If the light is trapped at an angle beyond the critical angle, then it can never escape the cell and will be ultimately absorbed which significantly improves efficiency. Surface texturing is not possible for thin film cells because it can deteriorate electronic performance of the cell due to increased surface recombination.
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| Fig. 2: (a) A light trapping structure with 60n2 absorption enhancement. The thicknesses of the scattering, cladding, and active layers are 80, 60, and 5 nm, respectively. (b) Electric field intensity of the fundamental mode shows huge enhancement in the absorber layer. [4] |
In 2010, a new theory of light trapping was developed which shows absorption enhancement could substantially exceed the 4n2 limit for ultra thin film solar cells for which some of the assumptions of the previous theory are not valid. [4,5] Instead of treating light as straight rays, this new light trapping theory shows the wave nature of light must be considered for nanometer-thick absorbers and predicts a significantly higher limit for the absorption enhancement. Fig. 2 shows a suggested design with 60n2 absorption enhancement (15 times more than the previous limit) in a 5 nm thick absorber layer sandwiched between a mirror and cladding and scattering layers. This work theoretically showed that properly designed structures can reach absorption enhancements far greater than what was predicted before, opening substantial opportunities for ultra thin high efficiency solar cells, which employ nanowire, plasmonic or other nanophotonic light trapping schemes.
© Farzaneh Afshinmanesh. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] H. A. Atwater and A. Polman, "Plasmonics for Improved Photovoltaic Devices," Nat. Mater, 9, 205 (2010).
[2] E. Yablonovitch and G. D. Cody, "Intensity Enhancement in Textured Optical Sheets for Solar Cells," IEEE Trans. Electron Devices 29, 300 (1982).
[3] E. Yablonovitch, "Statistical Ray Optics," J. Opt. Soc. Am. 72, 899 (1982).
[4] Z. Yu, A. Raman and S. Fan, "Fundamental Limits of Nanophotonics Light Trapping in Solar Cells," Proc. Natl. Acad. Sci. 107, 17491 (2010).
[5] Z. Yu, A. Raman and S. Fan, "Nanophotonic Light-Trapping Theory for Solar Cells," Appl. Phys. A 105, 329 (2011).