New Solar Cells with Novel Light Trapping via Textured Photonic Crystal Back Reflector
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0891-EE06-06.1
New Solar Cells with Novel Light Trapping via Textured Photonic Crystal Back Reflector Lirong Zeng, Yasha Yi, Ching-yin Hong, Bernard A. Alamariu*, Jifeng Liu, Xiaoman Duan, Lionel C. Kimerling Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, U.S.A. * Microsystems Technology Laboratory, Massachusetts Institute of Technology ABSTRACT We have successfully developed a new light-trapping scheme for solar cells that can enhance the optical path length by more than 104 times using a textured photonic crystal structure as a backside reflector. Top-contacted crystalline Si solar cells integrated with the new back reflector were designed, fabricated and characterized. Both external quantum efficiency and power conversion efficiency of the cells have shown significant improvement due to the path length enhancement furnished by the new back reflector despite of the 675 mm thick wafers and relatively short minority carrier diffusion length. INTRODUCTION Thin film solar cells are the best candidates for next generation photovoltaic applications because of lower cost. However, they also suffer from very low efficiency because of limited absorption of long wavelength photons imposed by the small film thickness, which is usually around 1 mm. In thin film Si cells, this is especially severe due to the indirect bandgap, which causes the absorption coefficient to drop rapidly as l increases. For instance, for l between 800 nm and 1170 nm, the absorption length increases from 10 mm to 10 cm. Light trapping is imperative in order to utilize these photons. Unfortunately, traditional light trapping techniques can at best increase the path length by 50 times [1]. We developed a new light trapping scheme that can enhance the optical path length by more than 104 times via textured photonic crystal back reflector. NEW BACK REFLECTOR DESIGN AND PERFORMANCE Back reflector design
Our new back reflector is a combination of reflection grating and distributed Bragg reflector (DBR). The grating can diffract normally incident light to almost parallel to the surface of a cell. When the light hits the front surface of the cell, it will be totally internally reflected back. On the other hand, the DBR has near unity reflectivity in the whole desired wavelength range, so that no light can leak out from the back side. By the unique combination of grating and DBR, light can be highly trapped in the solar cell, effectively changing the path length from the thickness of the cell to its lateral dimension. Assuming a solar cell with a dimension of 1 mm thickness and 2 cm x 2 cm width, the path length enhancement can be as huge as 104 times the cell thickness. Therefore, almost complete absorption can be realized.
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Period and etch depth are the two key parameters for a grating. The grating period is set as the bandgap wavelength of Si divided by its refractive index, i.e., lg/nSi. The etch depth is l/4nSi such that normally incident light can be strongly bent by 47°(at
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