Optical absorbance enhancement by electrochemical surface roughening of CuInS 2 thin films
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endahan and Marcel Pasquinelli Universite´ d’Aix-Marseille I, II, III – CNRS, UMR 6242, Institut Mate´riaux Microe´lectronique Nanosciences de Provence, Centre de Saint Je´r^ ome, Marseille 13397, France
Philippe Knauth Universite´ d’Aix-Marseille I, II, III – CNRS, UMR 6264, Laboratoire Chimie Provence, Centre de Saint Je´r^ ome, Marseille 13397, France (Received 25 March 2009; accepted 12 May 2009)
This work presents a simple electrochemical etching method to increase the surface roughness of copper indium disulphide (CIS) thin films using low concentration of hydrochloric acid. Film morphologies were investigated using scanning electron microscopy and atomic force microscopy. The structure of films was investigated using x-ray diffraction. A comparative study of optical properties of as-deposited and roughened CIS thin films by transmittance and reflectance experiments show a strong enhancement of absorbance for wavelengths between 600 and 900 nm.
I. INTRODUCTION
High-efficiency silicon solar cells are usually obtained using two final steps. The first step is the chemical surface etching in acidic or alkaline solution to decrease the silicon defect density, resulting from wafer cutting.1 The second step is the deposition of an antireflective coating to increase optical absorption and to improve photoconversion.2 Etching processes using different mixtures can combine both steps: removing top layers with high-defect density and decreasing reflectance by improving the surface morphology.3 A pyramidal morphology is observed for simple etching on monocrystalline silicon,3 which can be improved using sonic waves.4 Similar results are obtained for reactive ion etching on polycrystalline silicon.5,6 A honeycomb texture is obtained by a photolithographic process on monocrystalline silicon.7 Many other morphologies can be achieved by laser etching.8 In addition to surface etching, antireflective coatings are deposited on the front contact to increase light absorption.9 Thin films are made of different materials, including TiO2, CsI, PbF2, MgF2, and Y2O3; most of these layers are obtained by sputtering or CVD.9 It is interesting to note that some antireflective coatings are synthesized with simple and low-cost chemical techa)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0387
3044
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 10, Oct 2009 Downloaded: 16 Mar 2015
niques such as sol-gel.10,11 To increase the optical absorbance, the possibility to prepare highly textured surfaces for other p-type semiconductors than silicon has already been reported for ZnTe.12 CIS solar cells are composed of chalcopyrite compounds such as CuInS2 and CuInSe2. These materials are generally used as films of approximately 1- to 2-mm thickness. This low thickness, compared with silicon, is due to the fact that CuInS2 is a direct band gap semiconductor with an optical absorbance 100 times higher than that of silicon.13–16 For CIS solar cells, surface texturing was ne
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