Start Date: September 2012
James S. Harris and Shanhui Fan, Department of Electrical Engineering; Yi Cui and Mark Brongersma, Department of Materials Science and Engineering, Stanford University
This program will address the challenge of nanophotonic light trapping in high-performance, multi-junction solar cells with the goal of achieving greater than 32 percent efficiency. Researchers will (1) develop novel fabrication techniques that enhance light trapping without degrading the electronic properties of the cells; and (2) combine low-cost crystalline silicon (c-Si) technology with gallium arsenide (GaAs) in a thin film a multi-junction cell. These advancements will have significant impact in the critical area of high-efficiency thin film solar cells.
Advances in device physics and nanophotonic light management represent crucial steps toward reducing the cost of solar energy conversion. Significant improvement in solar-conversion efficiency using thin-film, multi-junction cells and nanophotonic light management will greatly reduce the size, material cost and installed system cost of solar cells. Figure 1 illustrates a simple light-trapping scheme in a silicon film about 1 wavelength thick. Absorption is greatly enhanced in the presence of the grating (b), producing an absorption spectrum (c) that consists of a collection of sharp spectral peaks, each corresponding to a resonance of the structure. Since the solar spectrum is typically much wider than the width of the individual peak, it is the aggregate contribution of the resonance, rather than the behavior of the resonance at a single wavelength, that determines the response of the overall light-trapping effects.
In recent years, nanophotonic structures have been extensively explored for light-management purposes in solar cells1-4. However, significant challenges remain. First, there is a critical need to develop a strategy for enhancing light absorption over the entire solar spectrum for use in ultra-thin active layers. Second, the electronic properties of the nanophotonic solar cells need to be optimized. This program directly targets these fundamental challenges.
To realize a practical, high-efficiency thin film solar cell, the project team will adopt a coordinated research effort spanning a broad range of areas – from optics and device theory, to low-cost material growth and processing development, to experimental characterization. This effort will build upon significant achievements of the team members. Fan will design metallo-dielectric super scatterers that will enable control of the angular distribution of the scattered light. To minimize the initial experimental cost of optimizing these scattering structures, Brongersma will synthesize super-scatterers and employ an inexpensive Si-based photodetector platform that quantifies their light trapping ability. Cui and Harris will develop state-of-the-art GaAs and Si/GaAs tandem cells, as well as low-cost nanostructuring and epitaxial lift-off/transfer techniques that will enable facile incorporation of the optimized scatterers. The goal is to achieve a significant increase in efficiency (1.5 times that of crystalline silicon and double that of thin film cells) and develop low-cost fabrication processes that are applicable at scale. These processes include peel-off fabrication for GaAs thin films, which reuse the GaAs substrate, and a low-cost lithography approach based on self-assembly of silica nanospheres that enable large-area nanostructure patterning (Figure 2).
Success in demonstrating a high-efficiency, low-cost photovoltaic cell will drastically reduce the cost of solar energy conversion systems as measured in dollars/Watt and ultimately enable much greater penetration of photovoltaics into the global energy market.
 Z. Yu, A. Raman and S. Fan, “Fundamental limit of nanophotonic light trapping in solar cells”, Proc. National Academy of Sciences 107, 17491 (2010).
 Z. Ruan and S. Fan, “Super-scattering of light from sub-wavelength nano-structures”, Phys. Rev. Lett. 105, 013901 (2010).
 J. J. Schermer, G.J. Bauhuis, P. Mulder, E.J. Haverkamp, J. van Deelen, A.T.J. van Niftrik, P.K. Larsen, “Photon confinement in high-efficiency, thin-film III-V solar cells obtained by epitaxial liftoff”, Thin Solid Films 511, 645 (2006).
 J. Zhu, C. M. Hsu, Z. Yu, S. Fan, and Y. Cui, “Nanodome solar cells with efficient light management and self-cleaning”, Nano Letters 10, 1979 (2010).