Comprehensive light trapping study of next generation thin film solar cells
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Comprehensive light trapping study of next generation thin film solar cells
Zhou Zhou1, Jian Zhou1, Xiaowei Sun1, Tim Cheng2, Shengqi Wang2 and Yasha Yi2, 3, 4* 1
Shanghai Institute of Microsystem and Information Technology, Shanghai, China 200050 3G Institute of Renewable Energy, Cambridge, MA 02141 3 CUNY Graduate Center, New York, NY 10016 4 New York University, New York, NY 10012 *Corresponding author: [email protected] 2
ABSTRACT Light trapping is one of the key challenges for the next generation of thin film solar cells. In this work, we have identified the distinct light trapping effects for short and long wavelength solar spectrum ranges, by investigating lighting trapping structures on both sides of Si thin film solar cells. The sub-wavelength moth-eye-like photonic front surface and multi-layer grating photonic crystal reflector on the bottom surface are studied in detail via the Finite Difference Time Domain method for its solar energy absorption characteristics. Our study reveals the drastic difference in the light trapping effects within the solar spectrum wavelength. This work may provide guidance for efficiency enhancement of next generation thin film photovoltaic cells.
INTRODUCTION Next generation thin film solar cells (inorganic or organic) have generated wide interest recently as potential routes to reduce materials usage and significantly lower the cost of electricity generated by solar cells 1-3. One of the main types of thin film solar cells, with Si as the absorbing layer, presents two main challenges for further development; (1) how to achieve broadband antireflection at the front surface; (2) how to achieve broadband light trapping at the bottom surface (especially at longer wavelengths) within the AM1.5 solar spectrum. With the rapid progress of nanotechnology, many nano scale photonic devices as small as 30nm have been realized, which is very promising for achieving manipulation of photons at chip scale and having broad applications in the renewable energy (photovoltaic cells, solid state lighting), telecommunications and the bio-medical fields4. Accordingly, there are two routes to enhance the light absorption for thin films. Various methods of enhancing optical absorption have been proposed, including the use of dielectric photonic structures5-12 or plasmonic metallic nanoparticles13-15; most of which are mainly focused on either front side antireflection film, or light trapping structures on the back side. Few works have been proposed that take into account simultaneously the two challenges stated above; broadband antireflection photonic structures on the front side and light trapping photonic structures at the bottom side, as well as their correlation for enhancing the light absorption for thin film solar cells.
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Antireflection Absorption layer Grating (1D or 2D) Omnidirectional multilayer back reflector
Figure 1: Schematics of thin film solar cell with moth-eye-like pyramid array photonic top surface and multi-layer photonic crystal reflector with grating at bottom surface.
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