| Summary: | 博士 === 國立彰化師範大學 === 機電工程學系所 === 104 === During the preparation of Cu2ZnSn(S,Se)4 thin film, the difference kinds of components, stacking orientation or the conditions of selenization would lead reactions into different paths. It also further effects to the nucleation of secondary phase. By far, we’re not fully understand about the secondary phase, not only its nucleation site is not predictable, but also the influence to the device from that different secondary phase sites. Besides, to gain a solar cell with high photoelectric conversion efficiency, Adjusting band gap (Eg) of absorption layermight be a solution. By increasing the sulfur concentration in the Cu2ZnSn(S,Se)4 thin film, the Eg would be increased, but it is difficult to control the crystal phases and composition. It might complicate the preparation reactions, and even generate more secondary phase in the film.
This study carried out with two-stage preparation of CZTSe(S) thin film. First, CuZnSn layer were prepared by sputtering method as the precursors layer on the molybdenum-coated glass substrate electrode, and then the selenization treatment would follow right after. The research of this thesis would be divided into the following three parts:
First, the study of CZTSe growth mechanism. In this study, the CuZnSn ternary alloy sputtering target was used to prepare a unified CZTSe thin film. With the different selenization temperature, trying to reveal the CZTSe growth mechanism. Results showed that, with the low temp. of 350 oC, The precursor layer were identified as CZTSe phase.
Follow up is the study with different composition of CZTSe film. By adjusting the ratio of [Zn]/[Sn] to investigate the effect of the composition of the secondary phase. Discussion about the influence of the performance impacted by secondary phase and its different nucleation sites. The results showed that under the zinc-rich preparation condition ([Zn]/[Sn]≥1.11), these devices possessed the similar conversion efficiency (≤4.4%). With the tin-rich condition ([Zn]/[Sn]≦0.75), the efficiency was too low to detect. According to theanalysis, with the composition of [Zn]/[Sn]≦0.75, there were great amount of secondary phase, SnSe2and Cu2SnSe3were found at the surface. Base on the C-AFM test result, These secondary phase would cause leakage currentover large area. With the composition of [Zn]/[Sn]≥1.11, large amount of ZnSe were generated at the bottom of the layer and limited the conversion efficiency. Besides, the composition tended to affect effect the thickness of interface layer from CZTSe/Mo, MoSe2. With the ratio of [Zn]/[Sn]≥1.11, the thickness of MoSe2 were less than 1μm, due to the CuSe layer, which is closed to the interface of CZTSe/MoSe2, blocked element selenium from entering counter-electrode and reacting with element molybdenum. Alternatively, with the composition of [Zn]/[Sn]≥1.11, the grain size at the bottom of the layer were smaller, it increased the pathway for element sodium accumulating at the bottom of the layer and even reacting with selenium to fabricate NaSe. It increased the difficulty for element selenium reaching the counter-electrode. Based on the above discussion, the reason why the conversion efficiencyof CZTSe thin film with the composition condition of [Zn]/[Sn]≦0.75 was not detectable was concluded with three reasons, the great amount of secondary phase, such as SnSe2 and Cu2SnSe3, generated at the surface and caused leakage current over large area, too thick of MoSe2 inter layer and poor crystallinity
Finally, the study of influence from the ratio of [S]/[S]+[Se] to CZTSe thin film. By adjusting the ratio of [S]/[S]+[Se] to control the Eg, trying to improve the performance of conversion efficiency. With the increasing content of sulfur, the higher the open-circuit voltage(Voc), it revealed that the sulfur-rich surface lower the short-circuit density (Jsc), due to the defect increasing in the film. The pure CZTSe thin film possesses the highest fill factor (F.F), it is due to almost no leakage current at its surface. On the other hand, adding element sulfur into CZTSe thin film modified the crystal structure and chemical composition. The higher sulfur content, the ratio of Zn/Sn lower, and the secondary phase, such as SnSe2, CuSn(S,Se) and ZnS appeared. At the preparation condition of [S]/[S]+[Se]= 0.55, The device demonstrated the best conversion efficiency of 5.2%. It proved that the influence of Eg comes greater than that from secondary phase.
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