Comparison of optical and electrical gap of electrodeposited CuIn(S,Se) 2 determined by spectral photo response and I–V–T measurements Z. Djebbour a,d, ⁎ , A. Migan Dubois a , A. Darga a , D. Mencaraglia a , C. Bazin a , J.P. Connolly b , J.-F. Guillemoles b , D. Lincot b , B. Canava c , A. Etcheberry c a Laboratoire de Génie Electrique de Paris (LGEP, UMR 8507 CNRS), LGEP-Supélec, Universités Paris VI & Paris XI, Plateau de Moulon, 11 rue Joliot Curie, 91192 Gif-sur-Yvette, France b Institut de Recherche et Développement sur l'Energie Photovoltaïque (IRDEP) UMR 7174, EDF-CNRS-ENSCP, 6 quai Watier, BP 49, 78401 Chatou Cedex, France c Institut Lavoisier (UMR 8180 CNRS UVSQ), Université de Versailles Saint Quentin, 45 av. des Etats Unis, 78035 Versailles, France d Département de Physique, Université de Versailles Saint Quentin, 45 av. des Etats Unis, 78035 Versailles, France Available online 16 December 2006 Abstract The knowledge of the absorber band gap of heterojunction solar cells is of crucial importance, because it helps to investigate the band offsets at the buffer/absorber heterointerface. Usually this gap is deduced from optical techniques such as Spectral Photo Response (SPR). However, for inhomogeneous materials, this method of estimating the band gap is dominated by the lowest value somewhere inside the absorber layer, and SPR alone cannot give any indication about the spatial location of the region concerned with this measured value. Nevertheless, to determine the nature of band offsets at the heterointerface, we need a more accurate and local value of the gap next to the interface. In this study, dark I–V–T measurements were performed to characterize the gap within the CuIn(S,Se) 2 (CISS) Space Charge Region (SCR) near the CdS buffer layer. Under forward bias conditions, several recombination mechanisms occur in the CdS/CISS solar cells, each of them being characterised by a specific activation energy. One of these activation energies is related to the gap of the CISS SCR near the buffer/absorber heterointerface. We present a detailed study of the gap of electrodeposited CISS, determined from I–V–T measurements and its evolution with various CdS deposition conditions. The obtained gap values are found to be 200 to 300 meV higher than those derived from SPR. We explain this difference by the occurrence of graded CISS gap, with a gap value next to the heterointerface higher than the bulk gap value. © 2007 Elsevier B.V. All rights reserved. Keywords: CISS; Cliff-type alignment; Spike-type alignment; I–V–T measurements; Tunnelling enhanced recombination mechanisms 1. Introduction One of the most promising strategies for lowering PV costs is the use of thin-film technologies in which thin films of photoactive materials (typically b 5 μm in thickness) are deposited on inexpensive large-area substrates as window glass for instance [1,2]. Chalcopyrite compounds Cu(In,Ga) (Se,S) 2 (CIGS) have led to the highest laboratory efficiencies for thin film solar cells (N 19%) [3] and CIGS modules have been successfully produced on an industrial scale [4–6]. Although CIGS compounds are comparable in performance and stability to existing crystalline silicon devices, their market share is still very small (b 1%) [7]. Electrodeposited CISS thin film solar cells are a promising technology to low cost solar energy as vacuum deposition techniques are avoided [8]. Efficiencies above 11% have already been obtained [9] for small area electrodeposited CISS based solar cells. To further increase this efficiency and also to extend it to large area solar cells, one of the key parameters is the heterojunction formation with the Chemical Bath Deposited (CBD) CdS buffer layer which is still not fully understood. The conduction band alignment type between the CISS absorber and Thin Solid Films 515 (2007) 6233 – 6237 www.elsevier.com/locate/tsf ⁎ Corresponding author. Laboratoire de Génie Electrique de Paris (LGEP, UMR 8507 CNRS), LGEP-Supélec, Universités Paris VI & Paris XI, Plateau de Moulon, 11 rue Joliot Curie, 91192 Gif-sur-Yvette, France. Tel.: +33 1 69 85 16 42; fax: +33 1 69 41 83 18. E-mail address: djebbour@lgep.supelec.fr (Z. Djebbour). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.12.154