Chemical synthesis of Cu(In) metal inks to prepare CuInS2 thin films and solar cells
Research highlights
▶ CuInS2 (CIS) thin films with optical band gap of 1.5 eV are favored for the production of high-efficiency single-cell systems with large open circuit voltage and of top cells in tandem systems. To reduce the cost of the thin-film solar cells non-vacuum techniques are intensively studied. Among a variety of non-vacuum techniques, nanoparticle-based ink printing is attractive, given the convenience for large-scale production and sufficient utilization of materials. Thereinto, nanoparticles play an important role in the preparation of inks. Although some methods have been proposed to prepare the CIS related nanoparticles, rare examples have been succeed in the fabrication of Cu, In and Cu–In nanoparticles. ▶ In this paper, we present a novel low-cost method to prepare Cu, In or Cu–In nanoparticles, which then are used to fabricate CIS films. Modified polyol method is successfully employed to synthesis metal nanoparticles. The metal nanoparticles are then dispersed in solvents to form inks. Later on, metal films can be subsequently obtained by drop-casting Cu inks and In inks in sequence or Cu–In inks, analogous to the subsequent or simultaneous evaporation in industry, respectively. Finally, the sulfurization of the metal films leads to the formation of CIS films, which are compact with large grains and pure phases. CIS solar cell with an efficiency of 0.7% is also prepared and investigated.
Introduction
CuInS2 (CIS) thin films with optical band gap of 1.5 eV are favored for the production of high-efficiency single-cell systems with large open circuit voltage and of top cells in tandem systems [1]. However, expensive vacuum equipments are employed in industrial production of CIS films, usually by subsequent evaporation of elemental layers (i.e. Cu, In, etc.), or simultaneous co-evaporation of these elements [2], [3]. To reduce the cost of the thin-film solar cells non-vacuum techniques are intensively studied [4], [5], [6]. Among a variety of non-vacuum techniques, nanoparticle-based ink printing is attractive, given the convenience for large-scale production and sufficient utilization of materials [7], [8], [9]. Thereinto, nanoparticles play an important role in the preparation of inks.
Up to now, three methods have mainly been reported for the preparation of nanoparticles related to CIS films. The first is that CIS particles are directly synthesized by sol–gel or mechanical alloying (MA). However, the sol–gel process leads to porous CIS aggregates, which are readily oxidized [10]. Moreover, large distribution of particle sizes is also a serious problem in the MA [11]. The second is that metal precursors such as metal oxides or metal salts are initially prepared. After reduction and selenization/sulfurization, separate particles are sintered together to form integrated films. However, since metal-oxide impurities may remain in the films to initiate cracks and shrinkages, an extra reduction process must be carried out [12]. The third is that fine Cu–In alloy particles are produced by a physical top-down method (MA or gas atomization) [13]. In this case, again, the MA cannot lead to a narrow size distribution of Cu–In particles. It is usually difficult to handle gas atomization, especially given the fact that the boiling point of In is low [14]. Therefore, simple and cost-effective methods to prepare particles related to CIS films still need to be developed.
In this paper, we present a novel low-cost method to prepare Cu, In or Cu–In nanoparticles, which then are used to fabricate CIS films. Modified polyol method is successfully employed to synthesis metal nanoparticles. The metal nanoparticles are then dispersed in solvents to form inks. Later on, metal films can be subsequently obtained by drop-casting Cu inks and In inks in sequence or Cu–In inks, analogous to the subsequent or simultaneous evaporation in industry, respectively. Finally, the sulfurization of the metal films leads to the formation of CIS films, which are compact with large grains and pure phases. CIS solar cell is also prepared and investigated.
Section snippets
Experimental
Analytical grade reagents were used in this work. They were indium trichloride, InCl3·4H2O; copper dichloride, CuCl2·2H2O; poly(vinyl pyrrolidone) (PVP, K30); sodium borohydride (NaBH4); diethylene glycol (DEG); tetraethylene glycol (TEG); ethylene glycol (EG), 2-methoxyethanol (MeEtOH), and methyl alcohol (MeOH); sulfur (S). The nanoparticles were synthesized by an approach similar with the commonly used polyol method [15]. 1 mmol metal salt (InCl3·4H2O or CuCl2·2H2O) and 0.2 g PVP were firstly
Results and discussion
In the experiments, Cu, In or Cu–In nanoparticles are firstly synthesized, respectively. The XRD pattern of the Cu nanoparticles shown in Fig. 1(a) indicates that the Cu is face-centered-cubic (fcc) crystal structure, the same as that in Ref. [16]. Their SEM image shown in Fig. 2(a) displays that the Cu nanoparticles are uniform with particle size of about 50 nm. Moreover, In nanoparticles with tetragonal phase are synthesized, as indicated from the XRD pattern in Fig. 1(b). In the pattern, no
Conclusions
A simple chemical method to prepare Cu/In or Cu–In nanoparticles inks is developed. These metallic inks are applied to prepare CIS films. Both stacking and mixing methods are employed to prepare the metal films, corresponding to the two main states of art methods in industry. It is proven that all the methods based on our metal inks are feasible to prepare CIS films. Moreover, the Cu11In9 phase in the mixing method is beneficial for the sulfurization, exhibiting little impurity phases. CIS
Acknowledgements
The authors would like to appreciate the financial supports from 973 Project (No. 2007CB613403). We thank Jieru Wang and Guoliang Xu for their kind help for characterization and Jipeng Cheng for his deep discussion in experiments.
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