Controlled bandgap CuIn 1 - x Ga x (S 0.1 Se 0.9 ) 2 (0.10 ≤ x ≤ 0.72) solar cells from electrodeposited precursors João C. Malaquias a, ⁎, Dominik M. Berg b , Jan Sendler a , Marc Steichen a , Nathalie Valle c , Phillip J. Dale a a University of Luxembourg, Physics and Materials Science Research Unit, 41, rue du Brill, L-4422 Belvaux, Luxembourg b University of Delaware, Institute of Energy Conversion, 451 Wyoming Rd., Newark, DE 19716, USA c Centre de Recherche Public Gabriel Lippmann, 41, rue du Brill, L-4422 Belvaux, Luxembourg abstract article info Available online 25 October 2014 Keywords: Electrodeposition CIGS Gallium segregation Annealing Photovoltaics Ionic liquids Deep eutectic solvent Electrodeposition and post-annealing is a potentially low-cost industrial growth route for Cu(In,Ga)(S,Se) 2 solar cells. Nevertheless, this process is limited by the difficulty to introduce gallium in the precursor and by the segregation of gallium during the annealing step. Previously, we countered the former problem by co- electrodepositing In and Ga from a Deep Eutectic Solvent, accurately controlling the Ga/(Ga + In) (referred to as Ga/III) ratio of the precursor. In order to avoid segregation we employed a three-step annealing procedure, in- troducing a limited quantity of sulphur on the surface of the absorber. In this work, absorbers and solar cells orig- inating from electrodeposited precursors with 0.10 ≤ Ga/III ≤ 0.72 are characterised. X-ray diffraction results show that the Cu(In,Ga)Se 2 112 peak shifts to higher angles with increasing Ga content, in agreement with the expected composition values. Additionally, these results show identical incorporation of sulphur in all samples. Photoluminescence, external quantum efficiency, and current–voltage measurements corroborate the X-ray dif- fraction results. Controlled incorporation of Ga, over a large Ga/III range, is achieved for electrodeposited and post-annealed Cu(In,Ga)(S,Se) 2 absorbers. A maximum solar cell efficiency of 9.8% was obtained. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Solar cells based on Cu(In,Ga)(S,Se) 2 (CIGSSe) are one of the best performing thin film technologies, achieving efficiencies of 21% in the laboratory environment [1]. However, these devices are produced by vacuum methods, which prevent cost reduction at industrial level. Electrodeposition and post-annealing (EDA) is a low-cost growth meth- od which can replace these evaporation methods. This process is hin- dered by two factors: i) The electrodeposition of Ga from aqueous electrolytes is hindered by the occurrence of the competing Hydrogen Evolution Reaction during deposition, reducing the process efficiency [2] and leading to the incorporation of oxides and hydroxides; and ii) Ga commonly segregates to the back of the absorber layer during annealing, forming a CuInSe 2 /CuGaSe 2 system. In our previous work, we reported on the efficient electrodeposition of Cu(In,Ga) metal pre- cursors with controlled composition (0 ≤ Ga/(Ga + In) ≤ 1), by using a non-toxic and inexpensive deep eutectic solvent as electrolyte [3–5]. In the following discussion, the Ga/(Ga + In) ratio will be referred to as Ga/III, for simplicity. Gallium segregation typically occurs in two-step processes such as EDA [2] and sputtering and annealing [6], severely hindering solar cell performance due to the open circuit voltage (V oc ) limitation. Recently Kim et al. reported a three-step annealing process, for co- sputtered metal precursors, avoiding Ga segregation and delivering absorber layers with nearly flat Ga profile in the bulk and a slight Ga depletion near the surface [7,8]. In this work, we apply the annealing method developed by Kim and co-workers to electrodeposited metal precursors. We show that this process is adequate to treat our precur- sors and demonstrate that CIGSSe, with varying Ga content, can be grown from electrodeposited precursors. To this end, semiconductor properties and solar cell electrical parameters were studied as a function of the Ga/III ratio in CIGSSe absorbers, since these consistently change with different Ga content. In this manuscript the acronym for the chalcopyrite compounds will end in “Se” for pure selenide materials (e.g. CISe, CGSe) and in “S” for pure sulphide materials (e.g. CIS, CGS). 2. Experimental The Cu(In,Ga) metal precursors used in this study were deposited by electrodeposition from a deep eutectic solvent. Extensive details can be found in Ref. [4]. Briefly, a copper layer was electrodeposited on a soda lime glass (SLG) substrate with a sputtered Mo coating, followed by in- dium and gallium coelectrodeposition. These metal precursors were then annealed at the Institute of Energy Conversion of the University of Delaware (IEC) with the process described in Ref. [8]. In short, this process comprises three different steps. The first step consists of anneal- ing the precursor in the presence of H 2 Se, followed by a second thermal Thin Solid Films 582 (2015) 2–6 ⁎ Corresponding author. Tel.: +352 466644-5860; fax: +352 466644 36156. E-mail address: joao.malaquias@uni.lu (J.C. Malaquias). http://dx.doi.org/10.1016/j.tsf.2014.10.068 0040-6090/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf