Influence of Annealing in H 2 Atmosphere on the Electrical Properties of Thin Film CdS
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Influence of Annealing in H2 Atmosphere on the Electrical Properties of Thin Film CdS Natalia Maticiuc1, 2, Jaan Hiie1, Tamara Potlog2, Vello Valdna1, Aleksei Gavrilov1 1 Department of Materials Science, Tallinn University of Technology, 5 Ehitajate tee, Tallinn, 19086, Estonia. 2 Department of Applied Physics and Informatics, Moldova State University, 60 A. Mateevici street, Chisinau, MD 2009, Moldova. ABSTRACT Chemical bath-deposited (CBD) thin film CdS has been widely used as a buffer and n-type window layer in CdS/CIGS and CdS/CdTe thin film solar cells. Annealing of CBD CdS assigns to the layers required concentration and mobility of electrons, crystallinity, structural stability and perfect Ohmic front contact in TCO/CdS interface. But always annealing reduces band gap (Eg) of solution-deposited CdS and lowers current density of the CdS/CdTe PV device due to optical absorption within the CdS layer. We have studied systematically dynamics of changes in CBD CdS/glass thin film structural, optical and electrical properties in annealing process in H2 ambient at normal pressure in preheated ceramic tubular furnace. Here we’ll present electrical, characterization results of annealed CBD CdS/glass thin films, 300 nm thick. The films were deposited with thiourea from ammoniacal 1 mM dilute solution of CdSO4 and 0.001 at. % of NH4Cl relative to Cd for Cl doping. We found high concentration of electrons 1-4 E19 cm-3 in the layers annealed at 200 - 450 o C, while for 200 oC the long time of annealing over 60 min is needed, but for high temperature region 350 - 450 oC only for short 10 min annealing this concentration region of electrons was achieved. In the high temperature region rapid decrease of electron concentration and conductivity will go on with increasing annealing temperature and time. Mobility of electrons will decrease from 9 to 5 cm2/V·s in the annealing region 200-350 oC, which is probably connected with disordering of lattice. On the basis of acquired results we propose an hypothesis about substitutional incorporation of OH group on S site in CdS lattice in deposition process and that (OH)s complex defect acts as a donor defect like Cl and we believe that the both defects are responsible for changes of thin film CdS electrical, optical and structural properties in the annealing process. Thermal annealing in hydrogen atmosphere is a convenient and appropriate method for precise control of CdS thin film electrical properties, and also for creation of n/n+ CdS window layers in the substrate configuration of a solar cell. INTRODUCTION CdS thin films are used as n-type window layers in CdTe [1] and CIGS [2] based solar cells due to wide band gap, high transparency and reasonable mobility of electrons. The performance of solar cells that use chemically deposited CdS are superior to those using evaporated CdS films [3]. This is primarily due to a shift of the absorption edge of the film to shorter wavelengths, in comparison with PVD (physical vapor deposited) CdS [4] – an advantage in the role of as deposited CBD
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