Enhancing Light-trapping and Efficiency of Solar Cells with Photonic Crystals
- PDF / 238,358 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 40 Downloads / 213 Views
0989-A03-02
Enhancing Light-trapping and Efficiency of Solar Cells with Photonic Crystals Rana Biswas1, and Dayu Zhou2 1 Dept of Physics, Microelectronics Res Ctr, Ames Laboratory, Iowa State University, Dept of Electrical & Computer Engineering, Ames, IA, 50011 2 Dept. Electrical and Computer Engineering, Microelectronics Res. Center., Iowa State University, Ames, IA, 50011 ABSTRACT A major route to improving solar cell efficiencies is by improving light trapping in solar cell absorber layers. Traditional light trapping schemes involve a textured metallic back reflector that also introduces losses at optical wavelengths. Here we develop alternative light trapping schemes for a-Si:H thin film solar cells, that do not use metallic components, thereby avoiding losses. We utilize low loss one dimensional photonic crystals as distributed Bragg reflectors (DBR) at the backside of the solar cells. The DBR is constructed with alternating layers of crystalline silicon and SiO2. Between the DBR and the absorber layer, there is a layer of two dimensional photonic crystal composed of amorphous silicon and SiO2. The 2D photonic crystal layer will diffract light at oblique angles, so that total internal reflections are formed inside the absorber layer. We have achieved very high optical absorption throughout optical wavelengths (400ñ700 nm) and enhanced light-trapping at near-infrared (IR) wavelengths (700ñ800 nm) for amorphous silicon solar cell. The optical modeling is performed with a rigorous three dimensional scattering matrix approach where Maxwellís equations are solved in Fourier space. INTRODUCTION Enhancing light-trapping is a major route to improving solar cell efficiency. Typically, enhancing light-trapping in thin film solar cells is achieved by a back reflector that confines light within the absorber layer. The back reflector is usually textured to scatter light at the interface through large reflected angles. This increases the optical path length within the cell ilayer, and it is necessary to scatter as much light as possible in oblique directions. A typical metallic back reflector consisting of Ag coated with ZnO [1], suffers from intrinsic losses. The granularity of the interface [1] produces small metallic nanoparticles that can exhibit surface plasmon modes. Surface plasmons of free Ag nano-particles are at ultraviolet wavelengths. Ag coated with a dielectric (with refractive index n), has surface plasmon wavelengths lowered by ~1/n, and can reside at optical wavelengths. Such surface plasmon modes induce intrinsic loss with every light passage in the cell, which was measured by Springer et al [2] to be 3% to 8% at 650 nm for different surface roughness of the silver back reflector. The losses accumulate and become severe at infrared wavelengths where the absorption length of photons in a-Si:H is long and multiple optical passes are required. Even a small loss of 4% with each reflection in a metallic back-plane incurs a severe loss of 1-(0.96)50 or 87% with 50 passes. These considerations have motivated us t
Data Loading...
