Formation and characterisation of MoSe 2 for Cu(In,Ga)Se 2 based solar cells D. Abou-Ras a,b, * , G. Kostorz a , D. Bremaud b , M. K7lin b , F.V. Kurdesau b , A.N. Tiwari b,1 , M. Dfbeli c a ETH Zu ¨rich, Institute of Applied Physics, 8093 Zu ¨rich, Switzerland b ETH Zu ¨rich, Thin Film Physics Group, Laboratory for Solid State Physics, Technoparkstrasse 1, 8005 Zu ¨rich, Switzerland c Paul Scherrer Institute, c/o ETH Zu ¨rich, Institute for Particle Physics, 8093 Zu ¨rich, Switzerland Available online 8 December 2004 Abstract The reaction kinetics of the MoSe 2 formation have been investigated by selenizing Mo layers in Se vapor at different temperatures and for different durations. The samples were characterized by means of Rutherford backscattering spectrometry, X-ray diffraction (XRD), electron diffraction, and by bright-field and high-resolution transmission electron microscopy. It was found that in all samples, a homogeneous MoSe 2 layer is formed on top of the Mo layer. The temperature dependence shows that the MoSe 2 layer thickness increases strongly for temperatures higher than ca. 550 8C. At a substrate temperature of 450 8C, no difference in the MoSe 2 thickness was detected for samples selenized for different durations. The diffusion constant of Se in MoSe 2 is estimated from the selenization duration dependence of MoSe 2 layer thicknesses at 580 8C. Finally, Cu(In,Ga)Se 2 (CIGS) solar cells in substrate configuration were developed on indium tin oxide (ITO) transparent back contacts. An intentionally grown MoSe 2 intermediate layer on ITO, prior to CIGS deposition, causes a significant efficiency improvement, suggesting that MoSe 2 can facilitate a quasi-ohmic contact. Solar cell efficiencies of up to 11.8% are obtained using an ITO/MoSe 2 back contact. D 2004 Elsevier B.V. All rights reserved. Keywords: MoSe 2 ; CIGS; Solar cells 1. Introduction Molybdenum is widely used as back contact material in Cu(In,Ga)Se 2 (CIGS) based thin-film solar cells in bsubstrate configurationQ. First results of Matson et al. [1] and Russell et al. [2] suggested that Mo back contacts form a Schottky- type barrier to the CuInSe 2 (CIS) absorber, while later Shafarman et al. [3] showed the contact between Mo and CIS to be ohmic, owing to the formation of an intermediate MoSe 2 layer during CI(G)S deposition [4,5]. It is now believed that the formation of a MoSe 2 layer is essential to facilitate a quasi-ohmic electrical contact across the CIGS– Mo interface. Electron microscopy and X-ray diffraction measurements have revealed that the formation and proper- ties of MoSe 2 layer may depend on the CIGS deposition method and the growth recipes [4,6]. An understanding of the reaction kinetics of the MoSe 2 formation is vital for the development of low resistance electrical back contacts for high efficiency CIGS solar cells, since thickness and crystallographic orientation of MoSe 2 layer determine the adhesion and electrical properties of the CIGS–Mo interface. Recent interest in CIGS photovoltaics is aimed at developing multi-junction tandem solar cells which require suitable semi-transparent bbi-facialQ cells grown on trans- parent conducting oxide (TCO) coated glass substrates [7–9]. For an efficient semi-transparent CIGS solar cell in substrate configuration, the TCO layer should form a low resistance ohmic back contact with the CIGS layer. Haug et al. [7] assessed various TCOs and suggested that indium tin oxide (ITO) and SnO x :F can be used to form a quasi-ohmic 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.098 * Corresponding author. Tel.: +41 1 633 21 26; fax: +41 1 633 11 05. E-mail address: daniel.abou-ras@phys.ethz.ch (D. Abou-Ras). 1 Also at: Centre for Renewable Energy Systems and Technology, Department of Electronic and Electrical Engineering, Loughborough University, Leicestershire, LE11 3TU, UK. Thin Solid Films 480–481 (2005) 433 – 438 www.elsevier.com/locate/tsf