Optimization of Textured Photonic Crystal Backside Reflector for Si Thin Film Solar Cells
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0974-CC02-06
Optimization of Textured Photonic Crystal Backside Reflector for Si Thin Film Solar Cells Lirong Zeng, Peter Bermel, Yasha Yi, Ning-ning Feng, Bernard A. Alamariu, Ching-yin Hong, Xiaoman Duan, John Joannopoulos, and Lionel C. Kimerling Massachusetts Institute of Technology, Cambridge, MA, 02139
ABSTRACT In this work, a textured photonic crystal is used as a novel backside reflector for monoand poly-crystalline Si thin film solar cells. The backside reflector has two components, a grating and a distributed Bragg reflector (DBR), both of which enhance light-trapping for the nearinfrared region of crystalline silicon. Simulations based on the scattering matrix method were used to systematically optimize all the key parameters to achieve the highest efficiency for a given solar cell thickness. We found that the optimal length scales in the problem, namely the period of the grating, the etch depth of the grating, the Bragg wavelength of the DBR, and the anti-reflection coating thickness, all decrease linearly as the absorption layer becomes thinner. The optimal value for the dimensionless duty cycle of the grating is found to be around 0.5 for all cell thicknesses. For a 2 µm thick cell, the efficiency enhancement relative to a cell with unpatterned backside can be as high as 53% using the optimized design. INTRODUCTION Thin film solar cells (TFSC) are widely considered the most promising candidates for next generation photovoltaic applications because of their potentially much lower cost [1]. Currently, the efficiencies of TFSC, however, are very low due to their weak absorption of long wavelength photons. For indirect bandgap materials such as Si, this issue is especially severe. To tackle this problem, we invented a new light trapping scheme using a textured photonic crystal as a backside reflector which can enormously elongate the optical path length, resulting in nearly complete light absorption. It is composed of a reflection grating and a distributed Bragg reflector (DBR) [2]. When the DBR is constructed out of Si and SiO2, the stopband can be designed to cover the wavelength range in which crystalline Si exhibits weak absorption, approximately 800-1200 nm. A simple design for a grating targeted at a wavelength λg that will gain the most path length enhancement consists of alternating high and low index blocks of equal width (duty cycle=0.5), with an etch depth of λg /4nSi, and a period of λg /nSi, where λg and nSi are the bandgap wavelength and refractive index of Si, respectively. However, absorption over a wide wavelength range must be taken into account due to the broad span of solar flux [3]. This concept is quantified in the expression for the short circuit current density, J sc = ∫ λλ12 qA(λ ) s (λ )d λ (1), where λ1 and λ2 specify the wavelength range of absorption, q is the electron charge, A(λ) is the absorption at a certain λ, and s(λ) is the number of incident solar photons per unit area per second. Therefore, it is important to strategically place the strong absorption points by numeri
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