Chlorine Doping of Cadmium Sulfide on the Example of CBD CdS
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Chlorine Doping of Cadmium Sulfide on the Example of CBD CdS Jaan Hiie1, Federico Quinci2, Vanni Lughi2, Valter Sergo2, Vello Valdna1, Valdek Mikli1, Erki Kärber1 and Taavi Raadik1 1 Tallinn University of Technology, Department of Materials Science, Ehitajate tee 5, 19086 Tallinn, Estonia 2 University of Trieste, Department of Materials and Natural Resources, via Valerio, 2-6 34127 Trieste, Italy ABSTRACT The role of thermal annealing and of CdCl2 as a main source of electrically active but vaporizable chlorine doping in chemical bath deposited CdS thin films is studied. The films were deposited on glass substrates from aqueous solution of either CdCl2, NH4Cl, NH4OH, and thiourea, or CdSO4, (NH4)2SO4, NH4OH, and thiourea. Films deposited in the presence of CdCl2 and annealed in H2 atmosphere at 310 and 420 °C show a resistivity lower than 10 Ω·cm, one order of magnitude less than for identically annealed films deposited in absence of CdCl2. Annealing at 420 °C in closed ampoules, where a counter pressure of CdCl2 builds up, leads to a lower resistivity on the order of 10-1 Ω·cm, confirming the key role of chlorine on the electronic properties. However, further characterization via photoluminescence raises new questions about chlorine-related defects and their role in the mechanisms that govern film resistivity. INTRODUCTION Both leading silicon-free thin film photovoltaic technologies, CdTe-based and CIGS-based solar cells, rely upon CdS films to play the role of the n-side of the junction [1]. Chemical bath deposition (CBD) has proven, over the years, to be a simple, reliable technique to provide good quality films – high crystallinity and texture, uniformity, good adhesion and coverage – even for large-scale production [1, 2]. However, processing CdTe thin-film solar cells in a superstrate configuration requires exposure of CBD CdS film to high temperature in the presence of CdCl2 vapor and O2 [1]. One important open issue concerns the role of CdCl2 on the electrical properties of the CdS films, one of the keys to high performance solar cells. In this paper, we investigate the effect of the presence of chlorine in the deposition bath, as well as the effect of post-deposition thermal processes on the film resistivity, and interpret the results in terms of chlorine-related defects. Monitoring the photoluminescence intensity of the band at around 800 nm, associated to chlorine defects [3, 4], provides further insight on the role of chlorine on film resistivity. EXPERIMENTAL DETAILS Sample preparation Film deposition was carried out in a 500 ml, flat bottom glass jar with hermetically screwclosed coverlid (for stopping vaporization of ammonia) immersed in a stainless steel water bath on a hot plate with magnetic stirrer. The substrates were 25 x 37 mm white soda lime float glass microscope slides, 1 mm thick. Two plates with their back sides facing each other were introduced in a quartz tubular sleeve, diameter 26 mm, length 30 mm. Prior to CdS deposition two of such sleeves, containing a total of 4 subst
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