Performance of Hydrogenated a-Si:H Solar Cells with Downshifting Coating
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PERFORMANCE of HYDROGENATED a-Si:H SOLAR CELLS with DOWNSHIFTING COATING Bill Nemeth1, Yueqin Xu1, Haorong Wang2, Ted Sun2, Benjamin G. Lee1, Anna Duda1, and Qi Wang1 1 National Renewable Energy Laboratory, Golden, CO, 80401 2 Sun Innovations, Inc, Fremont, CA 94539 ABSTRACT We apply a thin luminescent downshifting (LDS) coating to a hydrogenated amorphous Si (a-Si:H) solar cell and study the mechanism of possible current enhancement. The conversion material used in this study converts wavelengths below 400 nm to a narrow line around 615 nm. This material is coated on the front of the glass of the a-Si:H solar cell with a glass/TCO/p/i/n/Ag superstrate configuration. The initial efficiency of the solar cell without the LDS coating is above 9.0 % with open circuit voltage of 0.84 V. Typically, the spectral response below 400 nm of an a-Si:H solar cell is weaker than that at 615 nm. By converting ultraviolet (UV) light to red light, the solar cell will receive more red photons; therefore, solar cell performance is expected to improve. We observe evidence of downshifting in reflectance spectra. The cell Jsc decreases by 0.13 mA/cm2, and loss mechanisms are identified. INTRODUCTION Researchers continue to develop novel approaches to drive the economics of solar cells to be more competitive with traditional fossil fuel power generation. One of the primary limitations of performance for all solar cells is the response to the spectral output of the sun, and many tactics have been utilized or theorized in an effort to circumvent this. Multi-junction cells are tailored to absorb specific wavelength ranges of sunlight, whereas wavelength conversion layers can be used to tailor the sunlight to the particular device. By doing so, lattice thermalization and material transmission losses can be minimized [1]. Light conversion techniques can utilize quantum dots, rare earth ions, as well as various organic dye materials [2], and can be broadly placed into downconversion (high energy to lower energy) and upconversion (vice versa) processes. A subcategory of downconversion is downshifting (or photoluminescence [3]), which occurs at sub-unity quantum efficiencies. The narrow emission lines of rare earth ion light converters [4] lend their utility to single junction solar cells; however, narrow absorption bands limit the likelihood that broadband conversion is likely to occur for a single rare earth ion type. Many solutions have been implemented to films and phosphors with a few applications to solar cells giving mixed results in device performance [5]. Amorphous silicon (a-Si:H) solar cells show peak quantum efficiencies in wavelengths between 500 and 600 nm with sharp declines approaching 350 and 750 nm. The highest laboratory scale stable amorphous silicon (a-Si:H) single junction cell efficiency is in excess of 10% [6] with a theoretical efficiency limit between 15% [7] to 22% [8]. In this paper, we address a luminescent downshifting (LDS) layer applied to a single junction amorphous silicon solar cell to determine performance chan
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