Realization of Significant Absorption Enhancement in Thin Film Silicon Solar Cells with Textured Photonic Crystal Backsi
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Realization of Significant Absorption Enhancement in Thin Film Silicon Solar Cells with Textured Photonic Crystal Backside Reflector Lirong Zeng, Peter Bermel, Yasha Yi, Bernard A. Alamariu, Kurt A. Broderick, Jifeng Liu, Ching-yin Hong, Xiaoman Duan, John Joannopoulos, and Lionel C. Kimerling Massachusetts Institute of Technology, Cambridge, MA, 02139 ABSTRACT The major factor limiting the efficiencies of thin film Si solar cells is their weak absorption of red and near-infrared photons due to short optical path length and indirect bandgap. Powerful light trapping is essential to confine light inside the cell for sufficient absorption. Here we report the first experimental application of a new light trapping scheme, the textured photonic crystal (TPC) backside reflector, to monocrystalline thin film Si solar cells. TPC combines a onedimensional photonic crystal, i.e., a distributed Bragg reflector (DBR), with a reflection grating. The near unity reflectivity of DBR in a wide omnidirectional bandgap and the large angle diffraction by the grating ensures a strong enhancement in the absorption of red and near-infrared photons, leading to significant improvements in cell efficiencies. Measured short circuit current density Jsc was increased by 19% for 5 µm thick cells, and 11% for 20 µm thick cells, compared to theoretical predictions of 28% and 14%, respectively. INTRODUCTION Thin film silicon solar cells are already emerging as a leading next generation photovoltaic technology, due to their potential of low cost. However, currently they suffer from low efficiencies due to weak absorption of long wavelength photons. Effective light trapping schemes are crucial to enhance absorption of long wavelength photons and increase cell efficiency. Traditional light trapping schemes are based on geometrical optics elongating optical path length by scattering at the textured front surface [1] and reflecting at the back surface with an aluminum reflector, which has a maximum reflectivity of ~80% when light is incident from Si. Even the combination of an ideally roughened front surface and a lossless rear reflector is theoretically forbidden from enhancing path lengths by more than 50 times the cell thickness [2], while the best experimental result is closer to a factor of ten [3]. Recently we demonstrated the efficiency enhancement in thick Si solar cells using a new light trapping structure, the textured photonic crystal backside reflector [4], which combines a one-dimensional photonic crystal as a distributed Bragg reflector (DBR) and a reflection grating. The design was optimized with coupled wave theory [5] and scattering matrix method [6], and broader theoretical studies on the light trapping properties of photonic crystals were undertaken [7]. We now report the first experimental integration of this structure with optimized parameters onto monocrystalline thin film Si solar cells. This light trapping technique, developed from the foundations of wave optics theory, allows us to effectively target the longer wavel
