Photonic crystal back reflector in thin-film silicon solar cells
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1153-A03-05
Photonic crystal back reflector in thin-film silicon solar cells O. Isabella1, B. Lipovšek2, J. Krč2, and M. Zeman1 1 2
Delft University of Technology, EEC Unit / DIMES, 2600 GB Delft, The Netherlands University of Ljubljana, Faculty of Electrical Engineering, SI-1000 Ljubljana, Slovenia
ABSTRACT One-dimensional photonic crystals having desired broad region of high reflectance (R) were fabricated by alternating the deposition of amorphous silicon and amorphous silicon nitride layers. The effect of the deposition temperature and angle of incidence on the optical properties of photonic crystals deposited on glass substrate was determined and an excellent matching was found with the simulated results. The broad region of high R of photonic crystals deposited on flat and textured ZnO:Al substrates decreases when compared to the R of photonic crystals deposited on glass. The performance of amorphous silicon solar cells with 1-D photonic crystals integrated as the back reflector was evaluated. The external quantum efficiency measurement demonstrated that the solar cells with the photonic crystals back reflector had an enhanced response in the long wavelength region (above 550 nm) compared to the cells with the Ag reflector.
INTRODUCTION For obtaining high conversion efficiency of thin-film solar cells the proper light management inside the solar cell structures is of great importance. In today’s thin-film solar cells light management is accomplished by implementing light-trapping techniques that are based on the introduction of surface-textured substrates and the use of special layers called back reflectors. Reflection at the textured back contact is a critical issue, since conventional metal reflectors (Ag, Al) suffer from undesired plasmon absorption, limiting the long wavelength response of the solar cell [1]. Recently, novel concepts for wavelength-selective manipulation of the reflection and transmission at a particular interface inside a solar cell have been investigated, such as the use of Photonic Crystals (PCs) [2]. PCs in the role of a Distributed Bragg Reflector (DBR) have attracted attention as back reflectors in thin-film silicon solar cell [3, 4] or as intermediate layer in double-junction silicon-based solar cell [5]. PCs with the desired wavelength-selective behavior can be designed by using optical simulations that help to tune the thickness of layers, the number of alternating pairs, and the combination of refractive indexes [2]. In this contribution we investigate the optical properties of one dimensional (1-D) PCs based on alternating layers of hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (a-SiNx:H). The total reflectance of the PCs deposited at different substrate temperatures and on different substrate carriers was measured under various angles of incidence. The experimental results of wavelength-selective high reflectance were compared to the simulation results and an excellent matching has been obtained. The 1-D PC in the role of DBR was applied
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