Effects of high-temperature annealing on ultra-thin CdTe solar cells Wei Xia, Hao Lin, Hsiang N. Wu, Ching W. Tang ⁎ Department of Chemical Engineering, University of Rochester, Rochester, NY 14627, United States abstract article info Article history: Received 23 February 2011 Received in revised form 23 June 2011 Accepted 28 June 2011 Available online 7 July 2011 Keywords: Cadmium telluride Thin films Annealing Cadmium chloride Surface treatment Chemical treatment Solar cells High-temperature annealing (HTA), a process step prior to vapor cadmium chloride (VCC) treatment, has been found to be useful for improving the crystallinity of CdTe films and the efficiency of ultra-thin CdTe solar cells. Scanning electron microscopy, optical absorption, photoluminescence measurements and analyses on photoluminescence results using spectral deconvolution reveal that the additional HTA step produces substantial grain growth and reduces grain boundary defects. It also reduces excessive sulfur diffusion across the junction that can occur during the VCC treatment. The HTA step helps to produce pinhole-free CdTe films and reduce electrical shorts in ultra-thin CdTe solar cells. An efficiency of about 11.6% has been demonstrated for ultra-thin CdS/CdTe solar cells processed with HTA step. Published by Elsevier B.V. 1. Introduction Thin-film CdS/CdTe solar cells are based on crystalline CdTe films produced by vapor deposition. A common deposition method is close- space sublimation (CSS), where the source and substrate are held at close proximity and their temperature differential is kept relatively small (b 50 °C) in order to promote crystalline CdTe film growth near equilibrium conditions at high temperature (~550 °C). The CSS- deposited film thickness is typically in the range of 4 to 8 μm [1–3], which is much more than necessary for complete light absorption in solar cells. Although thinner CdTe films are desirable, they are not commonly used because it is difficult to produce high-quality crystalline thin films that are also free of pinholes [4,5]. Comparing to the CSS method, other thin-film deposition methods [6,7] such as sputtering are better suited for the deposition of thin films. However, these alternative methods are not widely used for CdTe films because of their low deposition rates and other limitations [8]. From the industry standpoint, reducing CdTe usage in the solar cells would not only lower material cost and extend the tellurium supply, which is a limited reserve [6,9], but also ease the environmental problem of disposing a potentially hazardous material [6]. Apart from absorption issues, CdS/CdTe solar cells based on ultra- thin CdTe films (≤1.5 μm) are generally lower in efficiency as a result of increased grain boundaries and bulk defects in these films, which are necessarily of reduced grain size. It has been shown that excessive sulfur diffusion from CdS to CdTe can occur via the grain boundaries, causing degradation in the p–n junction [10,11]. Metal migration from the back contact to the front junction, also via the CdTe grain boundaries, is known to cause cell degradation [12]. Cadmium chloride treatment (VCC) is a standard process for fabricating thin film CdS/CdTe solar cells, known for improving CdTe crystallinity and reducing the lattice mismatch between CdS and CdTe. Generally it enhances the p-type characteristics of CdTe [13]. In this study, we have investigated the effects of VCC treatment and an additional high- temperature annealing (HTA) on ultra-thin, CSS-deposited CdTe films. We will show that HTA treatment prior to VCC treatment can significantly increase the crystalline quality of ultra-thin CdTe films without introducing pinhole defects and thereby improve the solar cell efficiency. 2. Experimental details CdS films of this work were deposited on fluorine-doped tin oxide glass substrates (Pilkington, TEC-15) using a chemical bath deposition method [14]. They were then subjected to VCC treatment for 3–5 min, which was conducted in a CSS system with ultra-dry CdCl 2 granules as the source. The substrate and source temperatures were maintained at 390–410 °C in an O 2 :N 2 (1:4) ambient of 6.67 kPa. CdTe films were deposited on the CdS-coated substrates by CSS method [15] in oxygen ambient. The CdTe source was CdTe granules (Alfa Aesar, 99.99%) contained in a graphite holder. The gap between the CdTe source and the glass substrate was 4.5 mm. The deposition chamber ambience was typically 200 Pa oxygen. A typical temperature-time profile for CdTe deposition is shown in Fig. 1. The thickness of the CdTe film produced using this profile is approximately 1.5 μm. Thin Solid Films 520 (2011) 563–568 ⁎ Corresponding author. Tel.: + 1 585 2753552; fax: + 1 585 2733237. E-mail address: chtang@che.rochester.edu (C.W. Tang). 0040-6090/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.tsf.2011.06.097 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf