Effects of Cu 2 S sintering aid on the formation of CuInS 2 coatings from single crystal Cu 2 In 2 O 5 nanoparticles Chia-Ying Su a , Dillip K. Mishra a , Chiu-Yen Chiu b , Jyh-Ming Ting a, c, d, ⁎ a Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan b Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan c Institute of Nanotechnology and Microsystems Engineering, National Cheng Kung University, Tainan, Taiwan d Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan abstract article info Available online 16 March 2012 Keywords: Cu 2 In 2 O 5 Sintering Cu 2 S CuInS 2 Grain growth This article reports the sulfurization of single crystalline Cu 2 In 2 O 5 nanoparticles (NPs) for the formation of CuInS 2 (CIS) coatings. Cu 2 In 2 O 5 NPs were synthesized using a chemical route. Thus formed NPs were applied to Mo-coated glass substrate by a doctor-blading technique. After the doctor-blading, coatings were sulfu- rized under 500 torr of H 2 S at various temperatures. Selected doctor-bladed samples were heat treated at 430 °C in air for 30 min for the removal of carbon residual prior to the sulfurization. During the sulfurization, Cu 2 S (melting point 435 °C) was used as the sintering aid. The weight ratio of Cu 2 S to Cu 2 In 2 O 5 was varied. The effect of Cu 2 S on the CIS crystalline structure was explored using Raman analysis. It was found that Cu 2 S not only improves the sintering but also promotes the formation of desired CIS-chalcopyrite structure and reduces the impurity phase in the resulting CIS coatings. © 2012 Elsevier B.V. All rights reserved. 1. Introduction CIGS(CuIn 1-x GaxSe 2 ) and CIS (CuInS 2 or CuInSe 2 ) are used as ab- sorber layers in thin film solar cells, which have been shown to exhib- it a solar conversion efficiency as high as 20.3% [1]. CIGS/CIS absorber layers can fabricated by vacuum processes, such as co-evaporation [2] and co-sputtering [3]. The resulting cells have high conversion effi- ciencies since these solar absorber layers normally exhibit smooth surfaces, larger grains, and good crystallinity. On the other hand, non-vacuum processes provide advantages of faster deposition, lower equipment costs, and scale-up capability [4–7]. Typical non-vacuum processes include electrodeposition [8], chemical spray pyrolysis [9,10], and various particle-based techniques [11–14], fol- lowed by sulfurization or selenization at elevated temperatures. In the particles-based techniques, the precursor particles include Cu-In alloys (In, Cu 9 In 4 and CuIn 2 ) prepared using melt atomization tech- niques [12,13] and nano-particles of mixed oxides of Cu, In and Ga prepared using an acid dissolution process [14]. Most of the coatings are applied onto soda-lime glass substrates, which post a limit in the sulfurization/selenization or sintering temperature. As it is desirable to obtain large grains in CIS [15–17], a sintering aid is thus often used. A mixture of Cu 2 Se (melting point = 523 °C) and CuSe (melting point = 1130 °C) was used for the sintering of CuInSe 2 coatings prepared by screen printing [18]. The sintering temperature ranges from 450 to 650 °C. CuInSe 2 having grain sizes up to 20 μm was obtained. Cu 2 Se deposited on a Cu-In-Ga metal precursor layer was found to enhance the grain growth during selenization at 550 °C in a Se vapor atmosphere [19]. CuS was evapo- rated on CuInS 2 coatings for subsequent heat treatment at 500 °C in the presence of a sulfur atmosphere to allow grain growth [20]. It appears that various sintering aids have been used during the sintering of CIS nanoparticles or the heat treatment of CIS coatings. In this study, a sintering aid of Cu 2 S was used during the sulfurization sintering of single crystalline Cu 2 In 2 O 5 nanoparticles in the presence of H 2 S. The Cu 2 In 2 O 5 nanoparticles were obtained using a smeltering process which costs less than the formation of CuIn alloy particles. Also, the use of H 2 S eliminates the reduction step. Cu 2 S has a lower melting point (435 °C) than the CuS (507 °C) and is therefore a more effective sintering aid at 500 °C. We also show that Cu 2 S not only serves as a sintering aid but also reduces the impurity and the undesirable phases. The effects of Cu 2 S addition on the characteristics of the CIS are addressed and discussed. 2. Experimental For the synthesis of single-phase Cu 2 In 2 O 5 nanopowders (NPs), an aqueous solution of copper and indium chlorides was first prepared. The ratio of Cu to In in the solution was equal to a predetermined value of 1.1:1. A 2 M NaOH solution was then added into the solution for the formation of precipitates. The precipitate-containing solution was then dried in an oven at 100 °C for 8 h. The dried product was then milled and subsequently calcined at 850 °C for 8 h. The obtained Surface & Coatings Technology 231 (2013) 517–520 ⁎ Corresponding author at: Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan. E-mail address: jting@mail.ncku.edu.tw (J.-M. Ting). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.03.031 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat