Advanced Light Trapping in Thin-film Silicon Solar Cells
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1245-A03-03
Advanced Light Trapping in Thin-Film Silicon Solar Cells
M. Zeman 1, O. Isabella1, K. Jaeger1, R. Santbergen1, R. Liang1, S. Solntsev1, J. Krc2, 1 Delft University of Technology, DIMES, P.O. Box 5053, 2600 GB Delft, The Netherlands 2 University of Ljubljana, Faculty of Electrical Engineering, SI-1000 Ljubljana, Slovenia ABSTRACT Photon management is one of the key issues for improving the performance of thin-film silicon solar cells. An important part of the photon management is light trapping that helps to confine photons inside the thin absorber layers. At present light trapping is accomplished by the employment of the refractive-index matching layers at the front side and the high-reflective layers at the back contact of the solar cells and scattering of light at randomly surface-textured interfaces. In this article key issues and potential of light management in thin-film silicon solar cells are addressed. Approaches for light trapping are presented such as i) surface textures based on periodic diffraction gratings and modulated surface morphologies for enhanced scattering and anti-reflection, ii) metal nano-particles introducing plasmonic scattering, and iii) onedimensional photonic-crystal-like structures for back reflectors. INTRODUCTION The efficiency of thin-film silicon based solar cells has to achieve a level of 20% on a laboratory scale in order to stay competitive with bulk crystalline silicon solar cells and other thin-film solar cell technologies. Photon management is one of the key issues for improving the performance of thin-film silicon solar cells and decreasing the production costs by shortening deposition times and using less material. The aim of the photon management is the effective use of the energy of the solar radiation and the maximization of absorption in desired parts of a solar cell that are called absorbers. Photon management in thin-film solar cells is accomplished by a number of techniques that are related to the following areas: i) Effective use of the energy of the solar spectrum; ii) Minimization of absorption outside the absorber layers; iii) Trapping of photons inside the absorber layers. Effective use of the energy of the solar spectrum There are two principal optical losses that strongly reduce the energy conversion efficiency of today’s solar cells. Both of these losses are related to the spectral mismatch of the energy distribution of photons in the solar spectrum and the band gap of a semiconductor material that serves as the absorber in a solar cell. The first loss is the non-absorption of photons with energy lower than the band gap energy of the absorber. These photons are in principle not absorbed in the absorber and therefore do not contribute to the energy conversion process. The second process is the thermalization. In this process the electrons and holes generated by photons with energy higher than the band gap of the absorber release the extra energy as heat into the semiconductor atomic network. Multi-junction (also known as tandem) solar cells, photo
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