New Light Trapping in Thin Film Solar Cells Using Textured Photonic Crystals
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New Light Trapping in Thin Film Solar Cells Using Textured Photonic Crystals Lirong Zeng, Yasha Yi, Ching-yin Hong, Xiaoman Duan and Lionel C. Kimerling Department of Materials Science and Engineering, Massachusetts Institute of Technology Cambridge, MA, 02139, U.S.A. ABSTRACT A novel light trapping scheme is developed to enhance the optical path length in solar cells by using a photonic structure as the backside reflector. This structure combines a reflection grating on the substrate with an over-deposited distributed Bragg reflector (DBR). With this structure, the optical path length can be enhanced by more than 104 times with very little reflection loss. In turn, solar cell efficiency is predicted to be enhanced enormously. INTRODUCTION Thin film solar cells are leading candidates for next generation photovoltaic applications due to lower materials cost, simpler device processing and manufacturing technology for largearea modules and arrays [1]. Currently, however, thin film cells suffer from low efficiencies. For example, the best thin film Si cells have an efficiency of less than 13% [2], compared to a theoretical maximum of 30%. One major bottleneck for Si thin film solar cell efficiency is insufficient absorption of long wavelength photons, because of the short optical path length imposed by the limited thickness of the active material, which is usually on the order of 1µm. As the incident wavelength λ increases, the absorption coefficient drops rapidly, leading to a very long absorption length. For instance, in Si, at λ=800 nm, the absorption length L is about 10 µm; whereas for λ near Si bandgap, L jumps to near 10 cm. The small film thickness makes it almost impossible for these photons to be absorbed. The key solution to realizing complete light absorption is to enhance the optical path length by highly trapping light within the cell. However, current light trapping schemes are far from effective. They can increase the path length at most by 50 times [3]. Our goal in this research is to realize path length enhancement by up to 105 times for complete light absorption. DESIGN OF NEW LIGHT TRAPPING SCHEME We designed a new backside reflector combining a reflection grating and a distributed Bragg reflector. Figure 1 is a schematic of this design. The grating can diffract normally incident light almost parallel to the surface of the cell, increasing the path length by more than 104 times. Our DBR has an extremely high reflectivity of more than 99.96%, almost completely eliminating reflection loss. Therefore, nearly complete light absorption can be realized.
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Incident light
Diffracted light
air Si
Diffracted light
air
Figure 1. Schematic of a grating DBR
Grating design
For a reflection grating, the key parameters are the period p and etch depth t. We set the grating period based on the grating equation rλ = p (sin α + sin θ ) , (1) where r is the diffraction order in integers, λ is incident wavelength, p is the period of grating, α and θ are incidence and diffraction angles, respective
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