Copper Zinc Sulfur Compound Solar Cells Fabricated by Spray Pyrolysis Deposition for Solar Cells

Cu-Zn-S (CZS) films were deposited by the spray pyrolysis method. As-deposited CZS film showed a low crystallinity. The resistivity of CZS film with   Cu Cu Zn  ratio of 50% is around 2 10 cm    n 2 : 2 : 3 . The CZS film elements using spray solution with thiourea was confirmed as  Cu-Zn 1:1   Cu : Zn : S  by ICP-MS and EDX. The band gap of CZS films was varied in the range of 1.8 3.5 ev when the   Cu Cu Zn  ratio was increased from 0% to 67%; CZS film with   Cu Cu Zn  ratio of 50% showed an wide band gap of 2 eV. The photovoltaic characteristics were confirmed with cell structure of 2 2 3 Glass FTO TiO In S CZS carbon . The best cell was observed at the CZS films with   Cu Cu Zn  ratio of 50%; these cells depicted a conversion efficiency of 1.7%.


Introduction
Compound solar cells as Cu(InGa)Se 2 (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][6][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

 
Cu Cu Zn  ratios.Optical, electrical and photovoltaic properties were analyzed in detail.Conversion efficiency of 1.72% was obtained at CZS solar cells with 2 3 2 carbon CZS In S TiO FTO glass structure.

Experimental
The CZS layers were prepared on glass substrates by spray pyrolysis.The spray solution included 150 mM of SC NH and 60 ml of Cu-Zn source Cu Cu Zn  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-visi-ble 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 carbon CZS In S TiO FTO glass were fabricated.300 nm-In 2 S 3 buffer layer and 500 nm-TiO 2 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.

Results and Discussions
The composition of the sprayed CZS films measured by Inductively Coupled Plasma (ICP) and TEM-EDX is Cu 3.9 Zn 3.8 S 5.9 and Cu 28.65 Zn 27.37 S 43.95 , respectively; these elemental results are concluded to

Cu
, which corresponds to the ion number of Cu + and Zn 2+ .In spite of the deposition under ambient atmosphere, Cu was become Cu + , but not oxidized to Cu 2+ .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)).
Figure 3 shows the variation of the resistivity of CZS films with different

 
Cu Cu Zn  ratios.The resistivity of CZS films is rather low.The resistivity was increased with decreasing Cu concentration from 67% to 18.7%.Below 18.7%, we cannot measure the resistivity because of the high resistance of CZS films.
By the Hall-effect measurement using a CZS film with

 
Cu Cu Zn 50%   , the CZS was a p-type semiconductor, and the carrier concentration and mobility was 3.06 × 10 21 cm −3 and , respectively     The carrier concentration was quite high as a p-type semiconductor, which may due to the surface defect on the CZS nano crystal.were not observed from the results as shown in Figure 6.Since the CuInS 2 layer is too thin, the XRD system could not detect the CuInS 2 peaks.
The photocurrent-voltage of CZS cells with and without scratching was shown in Figure 7(a).The CZS layer of this cell was prepared with

 
Cu Cu Zn  ratio of 50%.After scratching, the photocurrent became lower than the sample without scratching.the difference of CZS photocurrent was due to the high conductivity of CZS films, which can collect the generated holes effectively from the surrounding (Figure 7(c)).On the other hand, the conductivity of CuInS 2 layer was lower (ca.0.1 Ω −1 •cm −1 ), and the hole were able to be transferred from the CuInS 2 layer just under the back electrode (Figure 7(d)).
Figure 8 shows the variation of conversion efficiency of CZS solar cells with

 
Cu Cu Zn  ratios.The conversion efficiencies, which were lower than 0.5% in the range below 33% and over 67% of

 
Cu Cu Zn  ratio, performed over 1% from 42% to 58% of

 
Cu Cu Zn  ratio.The cells at 50% ratio of

 
Cu Cu Zn  showed the peak of photoenergy conversion efficiencies.
Figure 9 shows the current density-voltage curves of the best cell with

Figure 1 .
Figure 1.Solar cell structures before and after scratching the CZS layer.

Figure 2 .
Figure 2. A TEM image (a) and an electron diffraction pattern (b) of CSZ by spray pyrolysis deposition, and a TEM image (c) and an electron diffraction pattern (d) of CSZ by ball-milling synthesis.

Figure 3 .
Figure 3. Resistivity of CZS films with different    Cu Cu Zn ratios.

Figure 4 (
a) shows a plot of absorption coefficient as a function of photon energy hc   for the CZS film with different   Cu Cu Zn  ratios.The band gap of CZS films was shifted towards low energy with increasing Cu concentration.The variation of CZS band gap value with   Cu Cu Zn  ratio was shown in Figure 4(b).The band gap value varied in the range from 1.8 eV to 3.6 eV with the variation of   Cu Cu Zn  ratios from 67% to 0%.The results indicate that CZS is wide band gap materials.

Figure 5
shows EPMA results on cross-section of the   Cu Cu Zn 50%   films.The result showed that copper diffused into In 2 S 3 layer, but zinc did not diffuse into In 2 S 3 layer.It was speculated that a thin layer of CuInS 2 may be formed at the interface between CZS and In 2 S 3 ; CuInS 2 and In 2 S 3 can make p-n junction and the cells can generate electricity [8].However, typical CuInS 2 peaks   2 27.9,46.3,55.1   Figure 7(b) shows photocurrent-voltage of CuInS 2 (CIS) spray cells with and without scratching for comparison.After scratching, the photocurrent did not change in the CIS solar cell with the sample without scratching the CIS layer.Therefore,

Figure 4 .
Figure 4. CZS absorption spectra (a) and the band gap variation (b) of CZS with different    Cu Cu Zn ratios.

Figure 5 . 2 CZS
Figure 5. Electron probe micro analysis (EPMA) spectra (a) and the elemental mapping images (b) of 2 3 2 CZS In S TiO FTO glass on cross-section at   %   Cu Cu Zn 50 .