Development of transparent conductive oxide materials for improved back reflector performance for amorphous silicon base
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Development of transparent conductive oxide materials for improved back reflector performance for amorphous silicon based solar cells. Scott J. Jones, David Tsu, Tongyu Liu, Jeff Steele, Rey Capangpangan and Masat Izu Energy Conversion Devices, Inc., Troy, MI 48084 ABSTRACT A new back reflector comprised of an Al/(multi-layered stack)/ZnO structure is being developed to replace Al/ZnO used in manufacturing and boost conversion efficiencies with improved back reflector performance. Use of the multi-layered stack should lead to improved reflectivity which will in turn improve solar cell currents and efficiencies. The results from studies of different transparent conductive oxides (TCOs) which comprise the multi-layered stack are reported with emphasis on ZnO alloys. Alloying with Si or MgF2 and using moderately high substrate temperatures, TCOs with low indices of refraction between 1.6 and 1.7 have been fabricated. The Si, Mg and F contents for these alloys were near 14, 12 and 33 at. %. Structural analysis demonstrates that alloys with MgF2 have smother surfaces and finer morphologies than those for ZnO. The expected high values for multi-layered structures with these alloys have yet to be achieved but this is likely due to properties of layers in the structure other than the ZnO alloys which have yet to be fully optimized. INTRODUCTION In the manufacturing of amorphous Si based solar modules for terrestrial applications, Al/ZnO back reflectors rather than Ag/ZnO are used because of poor long term stability and yield issues related to the Ag/ZnO structure. The problems in using Ag as the metal layer of the back reflector have been well documented [1]. The degradation of the module yield has been attributed to diffusion of the Ag through the ZnO layer to the semiconductor layers leading to a shorting of the device. One solution to this problem would be to devise and insert a diffusion barrier between the ZnO and Ag layers to halt the propagation of Ag. However, attempts to use such barriers have thus far led to high series resistances and low fill factor (FF). Besides the ease of diffusion, Ag is also not desirable because of its soft nature and relative high cost. However, because of the poorer reflectance properties of Al as compared with Ag, roughly a 10% drop in current and cell efficiency is observed with the use of the Al/ZnO back reflector. In an attempt to improve the performance of the back reflector without using Ag, studies of a back reflector design comprised of an Al metal layer, a multi-layered structure of films with different composition and a ZnO layer have been made[2]. The multi-layered structure was comprised of semiconductor and oxide layers with high and low indices of refraction (n) with the contrast in n leading to enhanced reflection. In particular using an MgF2/Si/MgF2 multi-layer structure, an increased reflectance in the red portion of the spectrum between 600 and 1000 nm as compared with reflectance values for Al/ZnO was demonstrated. Since all back reflectors for triple-junction
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