Periodic textures for enhanced current in thin film silicon solar cells
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Periodic textures for enhanced current in thin film silicon solar cells Franz-Josef Haug, Thomas Söderström, Oscar Cubero, Vanessa Terrazzoni-Daudrix, Xavier Niquille, Stephanie Perregeaux, and Christophe Ballif Institute of Microtechnology, University of Neuchatel, Rue A.-L. Breguet 2, Neuchatel, Switzerland
ABSTRACT For thin film silicon solar cells it is vital to increase the optical path of light in the absorber because this allows for thinner cells with better stability and higher production throughput. We discuss the effect of periodically textured interfaces for the case of thin film silicon solar cells in n-i-p configuration using embossed plastic substrate which allows us studying the effect of a wide range of random or periodic textures. Due to the moderate thickness of the individual layers the texture is carried into each interface with a high degree of conformity even for the front contact which is the last layer. Solar cells on such periodically structured back reflectors have already showed promising gains in performance. Furthermore, the periodicity serves as a useful tool for the study of light management because the underlying phenomena like diffraction or grating coupling to plasma excitations of the metallic back reflector are governed by a relatively low number of well defined parameters like the periodicity and the amplitude of the grating. INTRODUCTION Soon after the first fabrication of an amorphous solar cell by Carlson [1] it was suggested that the utilization of incoming light in the device could be enhanced by light scattering at rough interfaces [2]. Further on, light trapping developed into an essential feature because the adverse effects of light induced degradation are less severe in thin cells [3]. Since then a wealth of structuring methods has been proposed and tested. Most often structuring is achieved in the contact layers during their deposition. For example, pin devices usually employ transparent front contact materials that develop a texture during their deposition on the glass substrate [4-6]. Similarly, the natural textures of polycrystalline metal layers are used for nip devices [7]. Recently, light scattering by the excitation of localized plasma oscillation in metallic nanoparticles has gained considerable interest; enhanced intensities of the electric field in their vicinity has been found useful in organic semiconductor solar cells where poor charge carrier mobility necessitates very thin absorber structures [8-10]. The effect was also reported to increase the photocurrent in silicon thin film solar cells [11].
Besides randomly textured interfaces, also periodic structures have been suggested for enhancing the performance [12-14]. Compared to random interfaces, the prediction of some of their properties is possible without undue complications in the mathematical modeling. This makes them a powerful tool towards a better understanding of light trapping in the device, but they could also become an interesting option for production because large area roll t
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