Copper Zinc Sulfur Compound Solar Cells Fabricated by Spray Pyrolysis Deposition for Solar Cells ()
1. Introduction
Compound solar cells as Cu(InGa)Se2 (CIGS), CuInS(or Se)2 (CIS) solar cells have shown high durable and high efficiency (Record efficiency 20.3%) [1]. These solar cells have been normally fabricated by vacuum method. The vacuum process has the high cost of equipment and low process speed for production. In addition, the CIS and CIGS solar cells use expensive materials as gallium and indium. Therefore, Cu-Zn-Sn-S(Se) compound (CZTSSe) which does not use indium has been developed recently (the record efficiency 10.1%) [2]. The advantage of CZTSSe solar cells is using non-vacuum method and without expensive materials. However, CZTS solar cell using four or five elements including Cu, Zn, Sn, S, and Se, is so complex. For this reason, we have tried to develop Cu-Zn-S compound (CZS) with three elements for absorber layer in solar cells; this compound can be simpler in comparison with CZTS. Some groups reported about this material for applications in Inorganic Electro-Luminescence [3,4] or just basic research as semiconductor [5-7]. In our work, the CZS films were prepared by spray pyrolysis method, which is non-vacuum and low cost. This is the first report of CZS for applications in solar cells.
In this paper, we fabricated CZS films with different 
ratios. Optical, electrical and photovoltaic properties were analyzed in detail. Conversion efficiency of 1.72% was obtained at CZS solar cells with 
structure.
2. Experimental
The CZS layers were prepared on glass substrates by spray pyrolysis. The spray solution included 150 mM of 
and 60 ml of Cu-Zn source 
. The 
ratios varied in the range of 0% - 67%. The solution (30 ml) was sprayed onto the substrates at temperature of 277˚C in air ambience. The formation of the CZS crystal was confirmed by X-ray diffraction (XRD; Miniflex 2, Rigaku using CuKα radiation) measurement. The shapes and elemental ratio of CZS crystals were measured using a TEM-EXD system (JEM2100F, JEOL Co. Ltd.). The composition of CZS film was also measure by inductively coupled plasma-mass spectroscopy (ICP-MS). The resistivity of CZS films was calculated from sheet resistance four-probe measurement. The carrier concentration and mobility were obtained by Hall-effect measurement (ResTest8300, Toyo Technica Co. Ltd.). The absorption spectra of CZS films were measured by ultraviolet-visible spectroscopy (Lambda 750 UV/VIS, Perkin-Elmer). The distribution of elements including Cu, Zn, In, S and Ti in the cell cross-section was confirmed by an electron probe micro-analyzer (EPMA). Photovoltaic measurements employed an AM 1.5 solar simulator equipped with a xenon lamp (YSS-100A, Yamashita Denso, Japan). The power of the simulated light was calibrated to 100 mW·cm−2 by using Si solar cell reference. CZS solar cells with structure of 
were fabricated. 300 nm-In2S3 buffer layer and 500 nm-TiO2 compact layers were also fabricated by spray pyrolysis method. Carbon paste was printed on cells as back electrode. Cells were scratched except just under back electrode. Figure 1 shows the samples before and after scratching.
3. Results and Discussions
The composition of the sprayed CZS films measured by Inductively Coupled Plasma (ICP) and TEM-EDX is Cu3.9Zn3.8S5.9 and Cu28.65Zn27.37S43.95, respectively; these elemental results are concluded to
, which corresponds to the ion number of Cu+ and Zn2+. In spite of the deposition under ambient atmosphere, Cu was become Cu+, but not oxidized to Cu2+. This is due to the effect of thiourea.
Figure 2(a) shows a TEM image of spray-pyrolysis CZS. It can be observed that some parts of CZS were crystalized to ca. 10 nm particles. The electron diffraction pattern (Figure 2(b)) shows the Debye-Scherrer ring, which suggests that the spray-pyrolysis CZS is the aggregate of nanoparticles. For the comparison, a TEM image of CZS by ball-milling synthesis is shown (Figure 2(c)). The ball-milling CZS particles were ca. 100 nm, and the particle was close to a single crystal, judged from the diffraction pattern (Figure 2(d)).