A Comparative Study of CdS Thin Films Grown by Chemical-Bath Deposition and Close-Spaced Sublimation
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A Comparative Study of CdS Thin Films Grown by Chemical-Bath Deposition and Close-Spaced Sublimation H.R. Moutinho, D. Albin, Y. Yan, R.G. Dhere, C. Perkins, X. Li, and M.M. Al-Jassim National Renewable Energy Laboratory 1617 Cole Blvd., Golden CO 80401, USA ABSTRACT In this work we study the properties of cadmium sulfide thin films grown by chemicalbath deposition and close-spaced sublimation, on SnO2/borosilicate glass and SnO2/silicon substrates, before and after treatment in vapor CdCl2 at 400°C for 5 minutes. The as-deposited CBD CdS films had cubic structure, poor crystallinity, and high density of planar defects. After the CdCl2 treatment, these films recrystallized to the hexagonal structure, with improved crystallinity and a decrease in the density of planar defects. The as-deposited CSS films had hexagonal structure, better crystallinity, and lower density of planar defects. The main effect of the CdCl2 treatment was a decrease in the intragrain strain in the films. The CBD films had smaller grains and provided a good conformal coverage over the SnO2 films. In contrast to these films, CSS CdS did not have any oxygen or chlorine in its bulk. INTRODUCTION Cadmium sulfide thin films have been intensely used as a window layer in CdTe and Cu(In1-xGax)Se2 (CIGS) solar cells. Most of the CdS films used in solar cells today are grown by chemical-bath deposition (CBD) [1]. However, this method presents some problems for largescale application, such as low yield. An attractive alternative process is close-spaced sublimation (CSS), but the CdTe/CdS solar cells fabricated in our laboratory using CBD CdS still have superior efficiencies. Additionally, the world-record CdTe and CIGS solar cells also use CBD CdS. The objective of this work is to compare the properties of CBD and CSS CdS, to investigate the possible reasons for the best performance of devices using CBD CdS. The use of CdCl2 heat treatment is essential for obtaining high-efficiency CdTe/CdS devices. This treatment promotes recrystallization in highly stressed, small-grain CdTe films, and improves the electro-optical properties [2]. It has also been proven that CdCl2 heat treatment is beneficial for CBD [3] and CSS [4] CdS films. In this work, we treat the CdS films with CdCl2 using a vapor process, which is easier to control and is more effective than the dipping process. EXPERIMENTAL PROCEDURE The substrates used in this work were SnO2/borosilicate glass and SnO2/Si. The former structure is usually used in CdTe/CdS solar cells whereas Si substrates are easier than glass for preparing samples for transmission electron microscopy analysis; Si substrates also allowed us to study the influence of a different material in the formation of SnO2 and, consequently, CdS. To grow CBD CdS films, we immersed the substrate structures for 37 minutes in an aqueous solution with Cd acetate, thiourea, ammonium acetate, and ammonia, with pH of 9.5, maintained at 90ºC. The thiourea was the source of S, and the Cd acetate was the source of Cd. These films had average thicknes
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