Universidad. National Autonoma de Mexico . Temixco Mor. 62580. Mexico ... by NATIONAL AUTONOMOUS UNIVERSITY OF MEXICO (UNAM) on 04/17/15.
Mod. Phys. Lett. B 2001.15:609-612. Downloaded from www.worldscientific.com by NATIONAL AUTONOMOUS UNIVERSITY OF MEXICO (UNAM) on 04/17/15. For personal use only.
Modern Physics Letters B, Vol. 15, Nos. 17, 18 & 19 (2001) 609-612 © World Scientific Publishing Company
F O R M A T I O N OF C O N D U C T I V E CdO T H I N FILMS O N P H O T O C O N D U C T I V E CdS T H I N FILMS F O R W I N D O W LAYER A P P L I C A T I O N S I N S O L A R CELLS O. GOMEZ DAZA, *A. ARIAS-CARBAJAL READIGOS, J. CAMPOS, M. T. S. NAIR, P. K. NAIR Department of Solar Energy Materials, Centro de Investigation enEnergiat Universidad National Autonoma de Mexico . Temixco Mor. 62580. Mexico
We report the formation of a thin layer of CdO on chemically deposited CdS thinfilmsduring air anneal at 370°C to 500°C for 5 min to 120 min. During a 5 min anneal, the sheet resistance of the CdS thinfilmsdrops from about 1013 D/9 to 3.5 kD/D (370°C) and 470 D/D (500°C> X-ray diffraction studies showed that this is associated with the formation of a thin layer of CdO layer, which occurs at temperatures above 370°C. The CdS, which remains under the conductive CdO top layer, is photosensitive - with photo-to-dark current ratio of 103 - 104. Essentially the air annealing converts the highly resistive and highly photosensitive intrinsic (i) CdS thin film into a (i)CdS-(n+)CdO layer. The technique offers prospects to convert the top part of a chemically deposited CdS thinfilmwindow layer of high photosensitivity, deposited on an absorber layer, to a conductive layer. This is of interest in thinfilmsolar cell technology.
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Introduction
In the past, chemically deposited CdS thin films have been successfully incorporated as a buffer-window layer in polycrystalline thin film solar cells by various research groups [1, 2]. The requirement of a small thickness for the buffer layer to achieve high conversion efficiency in solar cells has been established in the 1980's [3]. The advantage of using chemical bath deposition technique to obtain such a coating on an underlying absorber layer has been suggested and demonstrated subsequently [1, 4]. It has also been suggested that CdS thin film in a solar cell structure does not just act as buffer layer, but contributes to carrier generation due to its high photosensitivity [5,6]. In this paper we report that a brief air anneal, for about 5 min at a temperature 370°C or above, of a chemically deposited CdS thin film can result in the formation of a thin layer of CdO. 2
Experimental
CdS thin films were deposited from a chemical bath containing citratocadmium complex and thiourea reported in a previous paper [6]. The bath was prepared by the sequential addition of 30 ml of 0.1 M cadmium nitrate, 12 ml of 1 M sodium 609
Mod. Phys. Lett. B 2001.15:609-612. Downloaded from www.worldscientific.com by NATIONAL AUTONOMOUS UNIVERSITY OF MEXICO (UNAM) on 04/17/15. For personal use only.
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citrate, 10 ml of 1.5 M ammonia (aq.), 8 ml of 1 M thiourea and the rest deionized water to make 100 ml deposition bath. Clean microscope glass slides (75 mm x 25 mm x 1 mm) were placed vertically in a 100 ml beaker containing the bath, guided though an acrylic lid with slots made for this purpose. The deposition was made at 75°C inside an air oven. A very uniform film producing greenish blue reflection in daylight was deposited in 4 h. The film thickness was determined using Alpha Step 100 (Tencor, CA); it was approximately 400 nm CdS thin film prepared as above was annealed at different temperatures, 100°C to 500°C in a Thermolyne furnace for different duration, 5 min to 4 hours. For comparison, annealing of the film at 400°C was done for 4 h in a vacuum oven (TM Vacuum Products) under 100 millitorr nitrogen pressure The X-ray diffraction pattern (XRD) of the films were recorded on a Siemens D500 diffractometer using Cu-Kv radiation. To record the photocurrent response and evaluate sheet resistance, pairs of silver print electrodes, 5 mm long at a separation of 5 mm, were applied on the film surface. A Keithley 619 multimeter and a Keithley digital voltage source, interfaced with a PC, were used for this measurement. The bias applied was 1 V across the electrodes. A tungsten halogen lamp, which produced an intensity of illumination of 2 kW/m2 at the plane of the sample, was used for illumination C d S thin films a n n e a l e d in air at 4 0 0 0 C
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