Thin Solid Films 457 (2004) 391–396 0040-6090/04/$ - see front matter  2004 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090Ž03.01033-2 Electrosprayed and selenized Cu y In metal particle films M. Kaelin *, H. Zogg , A.N. Tiwari , O. Wilhelm , S.E. Pratsinis , T. Meyer , A. Meyer a, a a b b c c Thin Film Physics Group, Laboratory for Solid State Physics, ETH Technoparkstr 1, PFA H43, CH-8005 Zurich, Switzerland a ¨ Department of Mechanical and Process Engineering, Particle Technology Laboratory, ETH Zentrum, CH-8092 Zurich, Switzerland b ¨ Solaronix SA, Rue de l’ouriette 129, 1170 Aubonne, Switzerland c Received 20 December 2002; received in revised form 24 April 2003; accepted 16 July 2003 Abstract Nano-particulate copper and indium metal layers of 1–2 mm have been deposited by non-vacuum techniques such as doctor blade, screen printing and electrospray using alcoholic suspension pastes. Electrospray showed a high efficiency of material usage and yielded the most uniform morphology. The metal precursor layers were subjected to a thermal treatment (500–600 8C) in selenium vapor to convert the porous metal layers into CuInSe compound layers. The chemical conversion, investigated by X- 2 ray diffraction, showed the presence of the In O impurity phase in the precursor as well as in the selenized layers. 23  2004 Elsevier B.V. All rights reserved. Keywords: Electrospray; Copper indium selenide; Selenization; Nanoparticles 1. Introduction Thin metal films of ;2 mm thickness are generally deposited with chemical or physical vapor deposition methods that require sophisticated and expensive vacu- um systems. These methods can be used to grow layers with precise thickness and composition, but require high investment for equipment. Low cost methods, such as screen printing, doctor blade, also known as tape casting w1x, and spraying are mostly used to deposit thick layers ()10 mm) on large area substrates. Following the advances in nano-materials technology, these non-vacu- um deposition methods become quite attractive for industrial low cost and high speed deposition of thin films on large area. Polycrystalline CuInSe (CIS) and its alloys with Ga 2 and S (CIGS, CIGSS, here CIS is used for CuInSe 2 only) are excellent thin film photovoltaic materials because of their favorable optoelectronic properties such as a tunable direct band gap in the 1–1.7 eV range and a high absorption coefficient w2x. Earlier attempts to use screenprintingordoctorblademethodsfailedtoproduce thin dense CIS films mainly for the lack of suitable *Corresponding author. Tel.: q41-1-445-14-81; fax: q41-1-445- 14-99. E-mail address: kaelin@phys.ethz.ch (M. Kaelin). precursor materials w3x. Recent developments have shown promising results: Eberspacher et al. w4x and Kapur et al. w5x sprayed metal oxide nanoparticles to grow a precursor film, which is transformed to CIGS by reduction and seleni- zation of the oxides. This method yielded solar cells with efficiencies up to 13% w5x, indicating the potential of non-vacuum deposition methods. However, strong and toxic reducing agents such as H Se are used in the 2 process, restricting the application for industrial produc- tion in many countries. CuInS films, grown on a heated 2 substrate (300 8C) by spray pyrolysis of CuCl , InCl 2 3 and thiourea in aqueous solution were produced by Krunks et al. w6x. These films contained organic impu- rities and required an annealing in H at 400–500 8Cto 2 turn the as-deposited n-type films to p-type. A modified spray pyrolysis method has been used to grow various oxide films as indium tin oxide, titanium oxide and non- oxide films as ZnS, CdS and CdSe w7–9x. This process involves atomizing a solution of a precursor salt mixture in ethanol and spraying precursor droplets from an electrostatically charged nozzle across an electric field towards the grounded substrate. The advantage of this method is the control of the gas phase reaction near the heated substrate, good film adhesion, and a very high efficiency of material usage w10x.