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Mehmet Ari Erciyes University, Department of Physics, Kayseri, 38039 Turkey

Selma Erat Laboratory for High Performance Ceramics EMPA—Swiss Federal Laboratories for Materials Testing & Research, CH-8600 Du¨bendorf, Switzerland; and Department of Materials, Nonmetallic Inorganic Materials ETH Zurich—Swiss Federal Institute of Technology, CH-8093 Zurich, Switzerland

Semra Durmus¸ and Mehmet Bozoklu Erciyes University, Department of Physics, Kayseri, 38039 Turkey

Artur Braunb) Laboratory for High Performance Ceramics EMPA—Swiss Federal Laboratories for Materials Testing & Research, CH-8600 Du¨bendorf, Switzerland (Received 1 July 2009; accepted 8 September 2009)

Cadmium sulfide (CdS) photocatalyst films were grown on glass by chemical bath deposition (pH 9.4, 70
C) and then annealed in nitrogen from 423 K to 823 K in steps of 100 K. The XRD crystallite size increases in a sigmoidal manner from 60 nm to 100 nm while the optical band gap energy decreases from 2.42 eV to 2.28 eV. This trend is paralleled by the decreasing Urbach energy, but only up to 623 K, where it increases again. This is the temperature where the Cd effectively surpasses the phase transformation from cubic to hexagonal, and the activation energy for electronic transport drops by a factor of nearly two.

I. INTRODUCTION

Cadmium sulfide (CdS) is a II-VI semiconductor with an energy band gap (Eg) of 2.42 eV in bulk materials at room temperature.1 CdS has been known for many decades as a photocatalytic semiconductor material for solar hydrogen generation in the visible wavelength range.2 CdS is also one of the most promising materials for application in electronic and optoelectronic devices such as solar cells, photo sensors, laser materials, optical waveguides and nonlinear integrated optical devices.3–5 Polycrystalline CdS films are generally used in CuInSe2 (CIS) and CdTe solar cells as a window material for transmitting the light absorbed by CIS or CdTe, and also as the n-type material for p-n junction of solar cells.6 For increased efficiency of solar cells, a semiconductor with optimum band gap, low resistivity, and high absorption Address all correspondence to these authors: a) e-mail: [email protected] b) e-mail: [email protected] This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http:// www.mrs.org/jmr_policy DOI: 10.1557/JMR.2010.0025 J. Mater. Res., Vol. 25, No. 1, Jan 2010

coefficient is required.7 Naturally, many different synthesis processes have been tried such as spray pyrolysis,8,9 vacuum evaporation,10 metal organic chemical vapor deposition,11 and chemical bath deposition.7,12–22 Among these techniques, chemical bath deposition (CBD) is a low-cost and suitable technique to prepare high quality, well adhered film reproducibly. The deposition process is based on the slow release of sulfide ions via the controlled hydrolysis of thiourea [SC(NH2)2] in an alkaline medium in the presence of a Cd salt