Microstrutural characterization of kesterite Cu₂ZnSnS₄(CZTS) thin films using transmission electron microscope = 투과전자현미경을 이용한 kesterite Cu₂ZnSnS₄(CZTS) 박막의 미세구조 분석

다국어 초록 (Multilingual Abstract)

Earth abundant, non-toxic, and cost-effective CZTS absorber material has been attracted much attention for thin film solar cells. Rapid development in achieving the high efficiency in CZTS-based solar cells is hindered by the narrow phase stability of the quaternary phase, Cu2ZnSnS4, and the existence of other complex secondary phases and defects. TEM is a powerful technique to provide information on the microstructure and chemistry of defects, interfaces, and junctions in polycrystalline thin-film solar cells. This information can provide crucial insights for understanding the performance of thin film solar cells. In this thesis, microstructural characterization of the CZTS absorbers by sputtering method for the wide production of kesterites solar cells have been studied using TEM.
First, the temperature dependent phase evolution of CZTS films is systematically studied the stacked ZnS/SnS2/Cu films sulfurized at different temperatures. Based on the experimental evidence the plausible phase evolution mechanism is proposed.
Second, The phase evolution process in sulfurized CZTS thin films prepared using different sulfurization times was described through ex-situ analysis, and a mechanism for Cu-S and Zn-S based secondary phase segregation was proposed. In addition to performing conventional XRD and RAMAN characterization, a sulfurization temperature-dependent phase evolution mechanism was proposed. The formation of secondary phases (Cu2S and ZnS) in CZTS films with longer annealing times was shown by TEM and STEM-mapping analyses. The driving force for Cu-S and Zn-S based secondary phase segregation was the favored Sn (II) oxidation state at high and long sulfurization temperatures.
Third, the phase evolution pathways have proposed for kesterite CZTS and CZTSe thin films by annealing of sputtered metallic precursor under chalcogenide atmospheres through experimental observation during the annealing process. Unlike phase evolution pathway for kesterite CZTS thin film by annealing of S-contained precursor, the Cu2-xS secondary phase was observed in the thin films sulfurized at temperature of 450 °C, even though no observation of this phase was made using XRD patterns and Raman spectra. Conversely, the Cu2SnSe3 phase was not observed during selenization process. The binary phases including Cu2-xSe, ZnSe, and SnSe2 were directly transformed to the kesterite CZTSe phase and they became large with micronsized grains. This study suggests variable and useful information for the preparation of kesterite CZTS and CZTSe thin films that can be used as high quality absorber materials in the high efficiency and low cost TFSC devices.
Finally, the CZTS-based thin film solar cells were fabricated Al/AZO/i-ZnO/CdS/CZTS-Se/Mo/Glass multi-structure and recorded a cell efficiency 2.10 % for CZTS TFSC and 2.44 % for CZTSe TFSC, respectively.

목차 (Table of Contents)

• Contents
• List of Figure ⅳ
• List of Table ⅸ
• Abstract x
• 1. Introduction 1
• Reference 9
• 2. Properties of CZTS 11
• A. General Properties 11
• B. Structural Properties 11
• C. Phase diagram 15
• D. Detrimental Phases during CZTS Synthesis 19
• Reference 21
• 3. Characterizations of CZTS Thin Film 23
• A. Transmission Electron Microscopy 23
• 1) Bright-Field( BFI ) and Dark-Field Image( DFI ) 25
• 2) Selected-Area Electron Diffraction (SAED) 27
• 3) High-Resolution Transmission Electron Microscopy (HRTEM) 27
• 4) Analytical TEM Using Scanning Transmission Electron Microscopy (STEM) 29
• a) High Angle Annular Dark Field (HAADF) 29
• b) Energy Dispersive X-ray spectroscopy (EDS) 31
• 5) TEM sample preparation 32
• B. Scanning Electron Microscopy (SEM) 33
• C. Raman Spectroscopy 33
• D. X-ray Diffraction (XRD) 33
• Reference 34
• 4. Phase Evolution of Cu2ZnSnS4 (CZTS) Kesterite Thin Films during the Sulfurization Process 35
• A. Introduction 35
• B. Experimental Details 37
• C. Results and Discussion 38
• 1) Characterization of Sulfurized Thin Films 38
• 2) Discussion of the phase evolution behavior during the sulfurization process 47
• D. Conclusion 48
• References 50
• 5. Evolution of Detrimental Secondary Phases in Unstable Cu2ZnSnS4 Films during Annealing 53
• A. Introduction 53
• B. Experimental details 55
• C. Results and Discussion 56
• D. Conclusion 69
• Reference 70
• 6. Phase Evolution Pathways of Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Thin Films by Annealing the Sputtered Cu-Sn-Zn Metallic Precursor 73
• A. Introduction 73
• B. Experimental Details 75
• C. Results and Discussion 77
• 1) Characterization of the Sulfurized Thin Films 77
• 2) Characterization of the Selenized Thin Films 79
• 3) Phase Evolution Pathways for Formation of Kesterite Films 86
• D. Summary and Conclusion 93
• Reference 95
• 7. I-V characteristics of CZTS-Se TFSCs 99
• A. Experimental details 99
• 1) CZTS-Se Absorber Layer by Annealing of Metal Precursor 99
• 2) CdS Thin Fim by Chemical Bath Deposition Method(CBD) 100
• 3) i-ZnO and AZO Thin Films by RF Magnetron Sputtering 100
• 4) Electrical Measurements 100
• B. Results and Discussion 100
• C. Summary and Conclusion 101
• 8. Summary and Conclusion 104
• Summary in Korean 106