Studies And Synthesis Of Cu₂ZnGeS₄ Thin Films For Solar Cells

As a novel absorber layer material in thin film solar cells, quaternary semiconductor Cu2ZnGeS4 of the CU2-Ⅱ-Ⅳ-Ⅵ4 family has attracted great interest due to its high absorption coefficients, an optimal direct band gap and the abundance of elements in the earth’s crust. In this work, CZGS thin films were fabricated by sulfurization of radio-frequency (RF) magnetron sputtered precursors. And the structural, morphological and optical properties of CZGS thin films have been investigated. The results are displayed as follow:1. CZGS thin film was first fabricated by sulfurization of radio-frequency magnetron sputtered precursor, which indicates that it is a suitable process for the growth of CZGS thin films.The CZGS thin film has been prepared on glass substrate by sulfurization of stacked precursor which is deposited sequentially by the sputtering method. The EDX mapping results reveal that all the constituent elements are uniformly distributed. The surface micrograph shows a compact and densely packed morphology with the grain size of positively 500 nm. Both the results of XRD pattern and Raman spectrum confirm the pure phase of the CZGS thin film. The existence of the microstrain and crystal disorder in Cu2ZnGeS4 is confirmed by Williamson-Hall analysis method. The presence of a disordered phase with I42m symmetry is reflected in the appearance of a dominant broadened Al-symmetry peak at lower frequency than the peak of the main Al-symmetry mode at 358 cm-1 in Raman spectrum. Furthermore, the absorption spectrum demonstrates that the value of the band gap for Cu2ZnGeS4 thin film is 1.85 eV. These results indicate that sulfurization of magnetron sputtered precursor is a suitable process for the growth of Cu2ZnGeS4 thin films.2. The influence of Cu content on the structural, morphological and optical properties of CZGS thin films has been investigated. The results indicate that Cu:ZnGeS4 thin films are well crystallized and present a visible increase of grain size with increasing Cu content, and the corresponding square resistance of these films is found to decreased. The band gap can be fine tuning by varying Cu content.CZGS thin films with different Cu contents (Cu/(Zn + Ge)= 0.93,1.00,1.05,1.13) were fabricated by varying sputtering time of Cu target. Microstructural characterizations using scanning electron microscopy and X-ray diffraction reveal that all of the as-prepared Cu2ZnGeS4 thin films are well crystallized and present a visible increase of grain size with increasing Cu content. In this case, the corresponding square resistance of these films is found to decrease from 14.2 to 1.3 KΩ/□. Furthermore, ZnS phase can be observed in the Cu-poor sample determined by Raman spectroscopy. Band gaps of the films decrease from 1.97 eV to 1.80 eV with increasing of Cu/(Zn+Ge) ratio from 0.93 to 1.13. These results are helpful to further study on Cu2ZnGeS4 thin films that are applicable for manufacturing solar cell.3. Cu2ZnGeS4 thin film growth process has been investigated by varying the holding time of sulfuration. The results indicates that the CZGS thin films are synthesized by Cu2GeS3 and ZnS.The CZGS thin films have been prepared with different holding time of sulfuration. The results of EDX and Raman reveal CZGS thin film growth process as follow: Cu2-xS+GeS2→Cu2GeS3 Cu2GeS3+ZnS→Cu2ZnGeS4These result are helpful to further study the growth process of CZGS thin films that are applicable for manufacturing solar cells.4. The CZGS thin film solar cells have been prepared with the efficiency of 0.45%. Through optimize the CZGS absorber layer, the efficiency has been improved 4 times. The low efficiency may can be attributed to the poor absorber layer, big leakage current, small short-circuit current and so on.The CZGS thin film solar cells have been successfully prepared. The optical properties of CZGS thin films solar cells have been investigated. The efficiency is increased from 0.1% to 0.45% via optimizing the absorber layer. Besides, the cause for the low efficiency has been investigated in this work, which may be related to the poor thin film, big leakage current, small short-circuit current and so on.