Electrical and optical properties of Cu2ZnSnS4 thin films prepared by rf magnetron sputtering process

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Abstract

Cu2ZnSnS4 thin films were deposited on corning 7059 glass substrates without substrates heating by rf magnetron sputtering. The Cu/(Zn+Sn) ratio of the thin film sputtered at 75 W was close to the stoichiometry of Cu2ZnSnS4. However, the S/(Cu+Zn+Sn) ratio was less than the stoichiometry. The as-deposited films were amorphous and annealed in the atmosphere of Ar+S2 (g). The annealed (1 1 2), (2 0 0), (2 2 0), (3 1 2) planes were conformed to all the reflection of a kesterite structure. A preferred (1 1 2) orientation was observed with the increase of the annealing temperature. The optical absorption coefficient of the thin film was about 1.0×104 cm−1. The optical band energy was derived to be 1.51 eV. The optical absorption coefficient of the sputtered Cu2ZnSnS4 thin films was less than that of CuInS2 thin film, however, the band gap energy was more appropriate for photovoltaic materials.

Introduction

CuInS2 (CIS) is the chalcopyrite-type semiconductors which has a direct energy band gap (Eg=1.08 eV) and a large optical absorption coefficient. However, we know In in this system is expensive element for commercializing. In the study, we synthesized Cu2ZnSnS4 (CZTS) compounds on the glasses by rf magnetron sputtering for reducing the cost of CIS solar cell. CZTS compound was derived by replacing the half of indium (In) atoms with zinc (Zn) atoms and the other half with tin (Sn) atoms in the chalcopyrite-type lattice of CIS. It is known that CZTS is one of the potential photovoltaic materials for use as low-cost thin films solar cells due to appropriate band gap of 1.4–1.5 eV, as well as a good absorption coefficient. However, the study on CZTS cells has not been studied for several years due to the lack of the detailed understanding of the parameters governing the performance and low-efficiency. In this study, we investigated the physical, electrical and optical properties of these thin films for a solar cell application.

Section snippets

Experimental

CZTS thin films were deposited on corning 7059 glass substrates without substrates heating by rf magnetron sputtering. The target, composed of finely mixed Cu2S, ZnS and SnS2 was cold-pressed at 250 MPa. After a deposition, thin films were annealed in atmosphere of Ar+S2(g) at 250–400°C. (Φ30 mm quartz tube, 800 sccm), the composition of thin films was determined by energy dispersive spectroscopy (Fisons, dispersive spectroscopy, Fisons, Kevex Super Dry) for the chemical composition and X-ray

Results and discussions

In the case of sputtering target with Cu2S:ZnS:SnS2=1:1:1, the Zn and Sn contents of the thin films increased but the Cu content decreased. Thus we changed the atomic ratio of the sputtering target with Cu2S:ZnS:SnS2=2:1.5:1 to obtain the stoichiometric ratio of CZTS thin films. Fig. 1 shows atomic ratio in the CZTS thin films deposited on corning 7059 glass substrates at room temperature as the function of rf power.

The atomic ratio of the thin films obtained at rf power between 50 and 100 W was

Conclusion

We obtained that as-deposited films without substrate heating were amorphous, while all annealed films showed the distinct diffraction peaks of CZTS, which were identified to the (1 1 2), (2 0 0) and (2 2 0), (3 1 2) planes, respectively. The peak of a preferred (1 1 2) orientation was stronger than that of diffraction peaks of CIS cells with increasing the annealing temperature.

With the increase of annealing temperature, the crystallinity of CZTS thin films was improved, and the sheet resistance

For further reading

The following references may also be of interest to the reader: [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19].

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