Optimization of molybdenum thin ﬁlms for electrodeposited CIGS solar cells M. Jubault n , L. Ribeaucourt, E. Chassaing, G. Renou, D. Lincot, F. Donsanti Institut de Recherche et Developpement sur l’Energie Photovoltaı ¨que (IRDEP), UMR 7174, EDF-CNRS-Chimie Paristech, 6 quai Watier, 78401 Chatou, France article info Available online 31 December 2010 Keywords: Molybdenum Back contact RF sputtering DC sputtering Electrodeposition CIGS solar cells abstract Molybdenum thin ﬁlms are widely used as back contact for CIGS-based solar cells. In this paper, the properties of Mo layers deposited by DC and RF sputtering are investigated in view of a speciﬁc optimization of electrodeposited CIGS solar cells. In the ﬁrst part of the paper RF and DC ﬁlms are grown on soda lime glass, in a pressure range from 2 to 20 mTorr, and for various RF power and DC current. They are then characterized by optical, electrical and structural methods. It appears that the ﬁlms deposited by RF mode sputtering are more reﬂective, conductive and adherent than those obtained by DC mode. Structurally, they present different behaviors with respect to nucleation and growth of CIGS precursor layers by electrodeposition. A large difference is observed in the photovoltaic properties of completed cells, with much better performances obtained with DC Mo layers. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Cu(In 1 x Ga x )Se 2 (CIGS) thin ﬁlm solar cells usually use molyb- denum back electrodes. Substrates of Mo-coated soda lime glass can be purchased from commercial sources. However, it has been shown that the properties of the molybdenum thin ﬁlms can strongly affect the performances of the solar cell. Indeed, many studies point out the role of molybdenum as the main transport gate for Na diffusion during the annealing of CIGS, on the enhancement of the solar cell performances [1–3]. The reaction of Mo with Se to form a thin MoSe 2 layer also plays a positive role for promoting the ohmicity of the back contact. Former studies of our laboratory have shown the close dependence between molybdenum properties and electrodeposited CIGS growth. The purpose of our study is to investigate the effect of the Mo coating conditions over the overall properties of the CIGS- based solar cell. The objective is to adapt the Mo back contact for our CIGS electrodeposition method [4]. In comparison to the molybdenum substrate commonly used in PVD deposition processes of CIGS, the one needed for electro- chemical processes requires a stronger chemical and physical resistance. To obtain the more suitable molybdenum for our process, we optimized the sputtering deposition by varying the different parameters (pressure, excitation mode, power, etc.). In order to compare more accurately the inﬂuence of the Mo morphology on the growth of CIGS, a study of the nucleation at different steps of the electrochemical deposition process was carried out. 2. Experimental 2.1. Molybdenum deposition Molybdenum thin ﬁlms were deposited by a RF (13.56 MHz) or DC magnetron sputtering device using rectangular Mo target (purity, 99.95%). The soda lime glass substrates were positioned onto a substrate-holder moving parallel to the target surface at a distance of 55 mm, with ﬁxed scan rate. Prior to molybdenum deposition, an argon plasma treatment was performed in order to remove contaminations and activate the surface. The pressure was varied between 2 and 20 mTorr by adjusting the argon ﬂow (purity, 99.9997%), keeping RF power to 600 W, or DC current at 1.0 A. Then pressure was ﬁxed to 2 mTorr whereas RF power was varied between 450 and 900 W or DC current from 0.5 to 1.5 A. 2.2. Films characterization The thickness was obtained by X-ray ﬂuorescence measure- ments. A four-point probe was used to determine the resistivity of the ﬁlm. SEM-FEG and X-ray diffraction analysis were performed to characterize morphological modiﬁcations of the Mo thin ﬁlms. The optical reﬂectance was measured with unpolarized light in the wavelength range of 250–2000 nm. The adhesion of the Mo was then tested by ultrasonic bath at 50 1C in presence of RBS 25 (alkaline detergent) diluted at 2% v/v in deionized water. 2.3. Solar cells elaboration and characterization Copper indium gallium diselenide precursor ﬁlms are deposited on the molybdenum substrates by electrodeposition from acidic aqueous solutions containing dissolved salts. Nucleation processes Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2010.12.011 n Corresponding author. E-mail address: marie.jubault@edf.fr (M. Jubault). Solar Energy Materials & Solar Cells 95 (2011) S26–S31