J1: Exploring the Limits of Absorption Enhancement
Monday AM, November 28, 2011
Republic B (Sheraton)
9:30 AM - **J1.1
A Thermodynamic Approach to Artificial Photonic Materials for Solar Energy Conversion.
Harry Atwater 1 Show Abstract
1 Applied Physics and Materials Science, California Institute of Technology, Pasadena, California, United States
Artificial photonic materials can enhance light-trapping and absorption, as well as increase the open circuit voltage and enhance quantum efficiency in solar photovoltaic structures. We describe a thermodynamic approach to understanding the opportunities for artificial photonic materials to increase solar energy conversion efficiency. This approach focuses on the control of the increases in photon entropy in light-matter interactions, as a means of minimizing free energy losses in photovoltaics.From thermodynamic arguments, Yablonovitch and Cody in 1982 determined the maximum absorption enhancement in the ray optics limit for a bulk material to be 4n2, where n is the index of refraction of the absorbing layer . Stuart and Hall in 1997 expanded this approach to study a simple waveguide structure; however, for the waveguide structures they considered, the maximum absorption enhancement was <4n2 . Using a combination of analytical and numerical methods, we describe why these structures do not surpass the conventional ergodic limit, and show how to design structures that can. The conventional light trapping limit can be exceeded in waveguide-like structures when the active region has an elevated local density of optical states (LDOS) compared to that of the bulk, homogeneous material. Additionally, to practically achieve light trapping exceeding the ergodic limit, the modes of the structure must be appreciably populated via an appropriate incoupling mechanism. We find using full wave simulations that ultrathin solar cells incorporating a plasmonic back reflector can achieve spatially averaged LDOS enhancements of 1 to 3, and a metal-insulator-metal (MIM) structure can achieve enhancements over 50 at a wavelength of 1100 nm, near the the band edge of Si. Interestingly, incorporating the active solar cell material within a localized metallodielectric plasmonic or metamaterial resonator can lead to nearly spatially uniform LDOS enhancements above 1000 within the active material. Another opportunity for increased photovoltaic efficiency lies in the control of the angular distribution of absorbed and emitted light interacting with a solar cell. In particular, we illustrate how artificial structures placed on top of a thin solar cell that control the light emission angle can increase the open circuit voltage and cell efficiency.Overall, we find many opportunities for increasing photovoltaic efficiency by adopting a thermodynamic perspective on light-matter interactions. These results can guide future solar cell designs that incorporate dispersive dielectric, plasmonic and metamaterial artificial photonic structures. Yablonovitch and Cody. IEEE Trans. Elect. Dev. 29 300 (1982) Stuart and Hall J. Opt. Soc. Am A 14 3001 (1997)
10:00 AM - J1.2
Ergodicity of Light-Trapping in Nanocrystalline Silicon Solar Cells.
Hui Zhao 1 , Eric Schiff 1 , Baojie Yan 2 , Jeff Yang 2 , Subhendu Guha 2 Show Abstract
1 , Syracuse University, Syracuse, New York, United States, 2 , United Solar Ovonic. LLC, Troy, Michigan, United States
Nanocrystalline silicon solar cells (nc-Si:H) are thick enough that a simple "classical" estimate of the maximum photocurrent enhancement due to light-trapping is a useful guide. This maximum enhancement is 4n2, where n is the refractive index of nc-Si:H; this limit is based on an ergodic argument that all the electromagnetic modes in the cell at a given wavelength are equally excited by sunlight.We have prepared nc-Si:H solar cells using several different texturing and back reflector schemes, and analyzed their quantum efficiencies in terms of a simple enhancement metric Y that can be compared to this 4n2 result. We have also analyzed published nc-Si:H cell properties from other laboratories. While optical measurements have shown the full Y=4n2 effect, photocurrents in thin-film nc-Si:H cells do not. We find that the largest values for Y are 15 for 1.0 micron thick cells, and about 25 for 2.5 micron cells.Since enhancements of these magnitudes can be achieved using a variety of implementations, we suggest that this convergence indicates that the best light- trapping implementations for thin-film nc-Si:H cells are close to ergodicity, even though the Y-values are well below 4n2. We account for this difference by parasitic absorption in the cell (by doped layers, reflectance losses, etc.) and by imperfect anti-reflection coatings. We discuss three approaches to further increasing light-trapping: reducing parasitic absorption, improving anti-reflection coatings, and implementing true "supraclassical" designs involving evanescent electromagnetic excitations beyond the modes considered for the 4n2 limit [1,2]. Martin A. Green, Prog. Photovolt: Res. Appl (2010). Zongfu. Yu, Aaswath Raman, Shanhui Fan - Proc. Nat. Acad. Sci. (Oct 2010 Vol 107 #41).
10:15 AM - J1.3
Development of Photonic and Plasmonic Designs to Surpass the 4n2 Light Trapping Limit.
Jeremy Munday 1 , Dennis Callahan 1 , Harry Atwater 1 Show Abstract
1 Applied Physics, CALTECH, Pasadena, California, United States
Recently there has been great interest in the nanotexturing solar cells in an attempt to surpass the traditional light trapping limit as described by Yablonovitch. Because this limit is only valid for bulk absorbers, it does not apply to the new generation of subwavelength solar absorbers including wire-based, photonic crystal-based, or plamonic-based cells. Her