Study On Compositional Dependence Of Properties For Cu₂ZnSnS（Se）₄ Thin Film Solar Cell

Cu2ZnSn(S,Se)4 (CZTSSe) is a promising material for the absorber layer of thin film solar cells. The CZTSSe compound is the crystal-structural descendant from Cu(In,Ga)Se2 (CIGS) by substitution of Zn, Sn for In, Ga. All constituents in this CZTSSe film are abundant in the crust of the earth, and are nontoxic. Furthermore, CZTSe film has high absorption coefficients (>104 cm-1), an optimal direct band gap. Hence, CZTSe has attracted great interest in recent years. Up to now the best power conversion efficiency (PCE) for CZTSSe thin film solar cells has been reached 12.6%, which was fabricated by chemical solution-based method. This work is focous on the issue on the composition control for CZTSSe thin film. The innovative results are displayed as follows:1. The composition dependence of structure and optical properties of CZTSSe solid solutions has been systematically investigated and our research results give the direct experimental evidence to the related theoretical calculations.The evolution of structural and optical properties of Cu2ZnSn(SxSe1-x)4 (CZTSSe) solid solutions in a wide composition range (0≤x≤1) has not been fully elucidated. We have performed comprehensive characterization on the CZTSSe powders with different S/Se ratios, which were synthesized by the solid state reaction method. X-ray diffraction patterns demonstrate that the lattice parameters a and c of CZTSSe decrease lineally when S replace Se gradually, which obeys the Vegard’s rule. The A1 Raman modes of CZTSSe show a typical two-mode behavior. The absorption spectra reveal that the band gap of CZTSSe can be tuned monotonously between 0.96 and 1.5 eV with almost linearity, and a small band gapbowing constant (b≈0.08 eV) is deduced. These experimental results agree well with the theoretical prediction by the first-principles calculation.2. CZTSSe thin films have been successfully grown directly by pulsed laser deposition method using one-step process. The composition dependence of structure and optical properties of CZTSSe thin films has been systematically investigated.CZTSSe thin films have been successfully grown on the heated quartz substrates directly by pulsed laser deposition method using one-step process. The results of energy dispersive X-ray spectroscopy indicate that there are some discrepancies of the elemental compositions between targets and thin films. This observation can be explained by different atomic mass of the species, the re-evaporation of some elements and substrate-target distances. However, the structural and optical properties of CZTSSe thin films are insensitive to the composition deviation from stoichiometry. The lattice parameters of CZTSSe thin films are smaller than those of CZTSSe bulks, and the main Raman peaks of CZTSSe films shift slightly towards the low frequency in contrast to those of bulks. These phenomena indicate that the internal compressive stress exists in CZTSSe thin films. Analysis of transmission spectra for CZTSSe thin films demonstrates that the optical band gaps of thin films are slightly higher than those of bulks. It is due to the existence of internal compressive stress and the deficiency of Cu content in the films. Our research results reveal that it is a feasible method to fabricate CZTSSe films using one-step process by PLD method.3. CZTSSe thin films have been successfully grown directly by RF magnetron sputtering process. The issue on change in composition duing the process has been systematically studied. The solar cell fabricated with the CZTS absorber grown at an optimized process shows a conversion efficiency of 2.85%.In sputtering deposition, the stoichiometry of the thin films is found to be difficult to control when growing quaternary compound films using one-step RF magnetron sputtering process. In order to obtain the stoichiometric ratio of CZTS thin films, we changed the atomic ratio of sputtering target, which was expected to be capable of growing a stoichiometric CZTS films. Based on using a non-stoichiometric CZTS target, CZTS thin films were deposited on heated substrates by RF magnetron sputtering process without post-annealed process. As for sputtering process, one-step RF magnetron sputtering process without post-selenization process is novel method with the advantage of the simplicity, environmentally friendly and good uniformity. However, the as-deposited CZTS thin films have a poor crystalline quality. Further, the crystalline quality of as-deposited CZTS thin films was improved using a conventional slow thermal process (STP). However, the composition ratio of CZTS thin film transformed from Cu-poor state to Cu-rich state using STP process due to volatilization of Zn and Sn elements during the STP process. To limit the losses of Zn and Sn elements in CZTS thin film, a short annealing process has been implemented. The effects of sulfurization temperature on compositional, microstructural, morphological and optical properties of CZTS thin films have been studied. The compositional analysis shows that all the CZTS thin films are in Cu-poor and Zn-rich state and RTP technique can remarkably reduce the volatilization of Zn and Sn elements compared to the conventional STP process. The solar cell fabricated with the CZTS absorber grown at an optimized sulfurization temperature of 550 ℃ shows a conversion efficiency of 2.85%. These results show that this method is an alternative, environmental friendly, low elemental loss and simple process to fabricate CZTS thin films for solar cell applications.4. CZTS thin films were grown using the sequential deposition of metallic precursor layers done by RF magnetron sputtering followed a final sulfurization process. Utilization sulfurization of metallic precursor prepared by sputtering of metal targets can easily control amounts of deposited metals. The best solar cell fabricated at an optimized process shows a conversion efficiency of 3.78%.The effects of sulfurization temperature and pressure on composition, morphology and microstructure of quaternary kesterite Cu2ZnSnS4 thin films obtained by sulfurization of the metallic layers have been investigated. The optimized sulfurization temperature is at 570 ℃. The results of EDX and SEM indicate that the S content in the CZTS thin films sulfurized under low sulfurization pressure have S-poor state with a bilayer structure and the thin films sulfurized under high sulfurization pressure have sufficient amounts of sulfur with grain growth throughout the entire absorber film. The analysis of XRD patterns indicates lower sulfurization pressure during the CZTS grain growth process may lead to formation of more structural defects in the CZTS lattice and the CZTS film sulfurized under high sulfurization pressure have more random orientation. The cell based on the CZTS thin films derived from lower sulfurization pressure shows relatively poor performance. The solar cell fabricated with the CZTS film under 10 Torr sulfurization pressure shows the best conversion efficiency of 3.52%. Since utilization sulfurization of metallic prepared by sputtering of metal targets can easily control amounts of deposited metals by changing sputtering durations of each metallic layer, further investigations on the relationship between conversion efficiency and composition along this process have been done. High conversion efficiency devices are found in a relatively narrow region in Cu/(Zn+Sn)=0.8～0.9 and Zn/Sn=1.1～1.3.