Synthesis and microstructural characterization of $Cu_2ZnSnS_4$ photovoltaic absorber layer

Abstract

Thin film solar cells such as CdTe and $Cu(In,Ga)Se_2$ (CIGS) have the great potential to contribute sus-tainable energy resources rather than fossil fuels. However, to accommodate the demands for worldwide needs it is important to consider fabrication cost of solar cell, suggesting that alternative absorber materials should be developed using non-toxic and earth-abundant elements with competitive efficiency. One of them, $Cu_2ZnSnS_4$ (CZTS) absorber is the most suitable material because it has high absorption coefficient and direct band gap of 1.4 -1.6 eV closed to the best value for maximum power conversion efficiency (PCE). So far, Shin et al. reported CZTS based solar cell with 8.4 % PCE by thermal evaporation, and Winkler et al. reported that band gap engineered approaches, solution processed $Cu_2ZnSnS(S,Se)4$ based solar cell led to performance improved to 12.04 % PCE. Furthermore, there were various attempts to enhance PCE of CZTS based solar cell: control of the composition ratio, grading of band gap, addition of another elements (Se, Ge and In), and removal of the secondary phases. Compared with CdTe (19.6 %) and CIGS (20.4 %) based solar cell, there are still enough room for the improvement of PCE in CZTS based solar cell. In this research, two big categories of analysis were organized to provide valuable design criteria of in CZTS based solar cell. One is the investigation of the microstructural characterization in CZTS absorber layer by taking account of sulfurization conditions, including the crystallization behavior during the sulfurization process and defective structures. The other is that the correlation between Cu to metal ratio, crystallization kinetic, and electronic band structure was investigated. The detailed accomplishments are listed as follows. First, crystallization behavior and microstructural characteristic of CZTS absorber layer were demon-strated by transmission electron microscopy (TEM). After the sulfurization process of co-sputtered precursor layer below $450^\circ C$, it was clearly seen that the precursor layers were separated into the crystallized surface and partially crystallized bottom region (almost amorphous state). Moreover, crystallization process needed the sufficient time for growth of CZTS grain even though the sulfurization process was conducted over $450^\circ C$. Based on the microstructural characterization and chemical analysis, the crystallization mechanism of CZTS absorber layer was suggested to understand the two-step synthesis process. The understanding of crystallization behavior may provide information that is expected to be helpful for the preparation of CZTS absorber layer with the improved microstructure. Second, defective structures in CZTS absorber layer have been investigated using high-resolution TEM and selected area electron diffraction patterns. Under the low temperature process, it was confirmed that the precursor layers were not completed crystallization process, and contained a large amount of amorphous region and bright contrast (BC) region via bright-field TEM. In addition, these BC region had less atomic con-tents compared to the surrounding region and no crystallinity. At the final stage of the sulfurization, BC re-gions might contribute to the formation of voids in CZTS absorber layer. Furthermore, void could have facet planes like CZTS grain, and a lot of stacking faults were observed near the void. Also, the secondary phase such as wurtzite type ZnS was observed in CZTS absorber layer sulfurized under overheating condition. Ob-served ZnS existed inside CZTS grain surface with an epitaxial relationship: (0002) ZnS // (112) CZTS and [112(-)0] ZnS // [021(-)] CZTS. With these defects, CZTS based solar cells can be more vulnerable to the defects compared with Cu(In,Ga)Se2 based solar cell due to the low dielectric constants. Understanding these defec-tive microstructures in CZTS absorber layers is considered to be essential to synthesized CZTS based solar cell with excellent performance. Third, influence of Cu/metals ratio in CZTS materials on the crystallization kinetics was investigated using non-isothermal and isothermal DSC. It was clearly seen that the transformation temperatures showed a dependence on the heating rate rather than Cu/metals ratio. Actually, factor of Cu/metals ratio could not contribute to the crystallization kinetics. The average activation energy was determined by the Kissinger plot of peak temperature obtained from continuous heating to be 109.97 and 253.30 kJ/mol for CTS and CZTS, respectively. Under isothermal condition, it is clearly seen that all composition for all temperatures experience an incubation time before the detectable phase transformation starts. Herein, transformation kinetics could be described by the Johnson-Mehl-Avrami model. The average value of the Avrami exponent throughout the various composition ratios was over 2.5, indicating that crystallization behavior showed three-dimensional growth with increasing nucleation rate according to diffusion controlled theory. Detailed results of nucleation and growth behaviors are discussed in terms of the local Avrami exponent versus crystallized volume fraction. Lastly, We have demonstrated the effects of Cu/metals ratio on the energy band alignment at CdS/CZTS interface using ultraviolet photoemission spectroscopy (UPS) depth profiles and ultraviolet-visible (UV-Vis) spectroscopy. . Using extrapolating the edges of the UPS depth profiles, the valence band maximum (VBM) levels could be directly traced relative to Fermi level. Valence band offset (VBO) of Cu-poor, -correct and -rich with CdS buffer layer was estimated to 1.74, 1.70 and 1.63 eV, and valence band bending of CZTS absorber layer at the interface region showed 0.17, 0.13 and 0.094 eV with the positive curvature which arose from the secondary phase like ordered vacancy compound. According to these results, CdS/CZTS interface could make the electronic band structure with the cliff type junction only. To acquire the spike type junction at the interface, VBM and VBO of hydrothermally grown ZnO and its doped ZnO (S and Se) were evaluated. Our results can propose the improved design of the p-n junction by using Cd-free buffer layer (ZnSe) and band gap modified absorber layer (CZTSSe).