Controlling morphology and crystallite size of Cu(In 0.7 Ga 0.3 )Se 2 nano-crystals synthesized using a heating-up method Wei-Hsiang Hsu a , Hsing-I Hsiang a,n , Chih-Ta Chia b , Fu-Su Yen a a Department of Resources Engineering, Particulate Materials Research Center, National Cheng Kung University, Tainan, 70101 Taiwan, People's Republic of China b Department of Physics, National Taiwan Normal University, Taipei, 116 Taiwan, People's Republic of China article info Article history: Received 20 June 2013 Received in revised form 16 September 2013 Accepted 21 September 2013 Available online 27 September 2013 Keywords: Nano-crystals Size controlled Copper indium gallium diselenide Heating-up process abstract CuIn 0.7 Ga 0.3 Se 2 (CIGS) nano-crystals were successfully synthesized via a heating-up process. The non- coordinating solvent (1-octadecene) and selenium/cations ratio effects on the crystalline phase and crystallite size of CIGS nano-crystallites were investigated. It was observed that the CIGS nano-crystallite morphology changed from sheet into spherical shape as the amount of 1-octadecene addition was increased. CIGS nano-crystals were obtained in 9–20 nm sizes as the selenium/cations ratio increased. These results suggest that the monomer reactivity in the solution can be adjusted by changing the solvent type and selenium/cations ratio, hence affecting the crystallite size and distribution. & 2013 Elsevier Inc. All rights reserved. 1. Introduction The I–III–VI chalcopyrite thin film solar cell application was developed more than ten years ago to improve conversion efficiency and reduce cost. CuIn x Ga 1Àx Se 2 (CIGS) is one of the most important semiconductor materials used in thin film photovoltaic cells. It has many remarkable advantages such as high absorption coefficients, good photo-stability, direct band gap semiconductor and demon- strated high conversion efficiency [1–3]. The conversion efficiency for CIGS-based solar cell laboratory devices has reached 19.9%. The module efficiency for large scale industrial products has reached above 13% [4–7]. Developing colloidal routes, such as spin-casting or printing, to fabricate CIGS thin film solar cells that have well controlled stoichiometry, high materials utilization and low proces- sing equipment cost have attracted much attention [8]. The composi- tion, size distribution, morphology and crystalline structure of CIGS particles prepared using the wet-chemical process influence the conversion efficiency of CIGS-based solar cells. The hot-injection process has been widely used in preparing CIGS nano-crystallites with narrow particle size distribution via large over-saturation to induce nuclei burst. However, this synthesis method is difficult to scale up because the monomer injection rate and mass transfer are limited in large quantities [9,10]. A non-injection synthesis (i.e., heating-up process) has recently been the most successful, widely used method in mono-dispersed nano-crystal synthesis [11–14]. Many researchers successfully synthesized and controlled the shape and size of the sulfide/selenide nano-crystallites in non-coordination solvents by increasing the monomer concentration to prevent coar- sening by inhibiting the Ostwald ripening [15–17]. Aso et al. reported that the needlelike SnS particles were synthesized using trioctylpho- sphine (TOP, coordinating solvent) and 1-octadecene (ODE, non- coordinating solvent) as solvents, while plate like SnS particles were obtained using oleylamine (OLA) as the coordinating solvent [18]. Park et al. proposed that metal phosphide nano-rods could be obtained using the cooperative effects of different binding capabilities from two surfactants (trioctylphosphine and oleylamine) [19]. Duan and Nose et al. reported the crystalline phase and crystal size of CuInS 2 nano- crystals synthesized using the heating-up process were controlled by the ligand species of the metallic monomers [20, 21]. If too much capping agent is used nucleation can be completely hindered, ultimately leading to indiscriminate growth of a small population of nuclei [22]. Tang's group synthesized CuInSe 2 nano-particles with a narrow size distribution in oleylamine using asetylacetonate copper and indium and selenium as the precursors. They observed that the reaction mechanism was the oxidation of oleylamine and the reduc- tion of Cu 2 þ -Cu þ and Se-Se 2À [23]. Zhong et al. found that both ligand and non-coordinating solvent were necessary for the prepara- tion of CuInSe 2 nano-crystals [24]. Zhen et al. prepared various aspect ratio CdSe particles via a thermal decomposition process by changing the Cd to Se precursor ratio (Cd/Se ¼ 7/1–1/7) and found that the formation and aspect ratio growth of CdSe nano-wires were reduced with increasing or decreasing selenium content [25]. In summary, the reactivity of the metallic monomer will be affected by the ligand concentration in the bulk solution for the thermal decomposition method. The number of nuclei increases clearly with decreasing ligand concentration during the initial Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jssc Journal of Solid State Chemistry 0022-4596/$ - see front matter & 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jssc.2013.09.033 n Corresponding author. Tel.: þ886 6 275 7575; fax: þ886 6 238 042. E-mail address: hsingi@mail.ncku.edu.tw (H.-I. Hsiang). Journal of Solid State Chemistry 208 (2013) 1–8