Preparation Of Copper Zinc Tin Sulfide Thin-films By Sol-gel Sulfurization Method And Its Solar Cells Performance

Copper zinc tin sulfide?Cu₂ZnSnS₄,CZTS?thin films have been regarded as a promising light absorber material for thin film solar cells because they are composed of earth-abundant and non-toxic elements,and exhibit an optimal optical band gap?1.5 e V?,a high optical absorption coefficient?>104 cm-1?as well as a theoretical limit efficiency of 32.8%.In this thesis,CZTS thin films were prepared by sol-gel sulfurization method,and the effects of preparation process for the precursor thin films,sulfurization variable,interfacial modification and element doping on the structural,morphological and compositional properties of the CZTS thin films were studied.The photoelectric properties of CZTS thin film solar cells using the as-prepared CZTS thin films were measured,which were fabricated according to the device structure of glass/Mo/CZTS/Cd S/i-ZnO/ITO/Ni:Al.Finally,the relationships between the structures,morphologies,compositions and the photoelectric properties were revealed.Results provided important guiding values and practical applications for the preparation of high-quality CZTS thin films and the improvement of CZTS thin film solar cells conversion efficiency.?1?CZTS sol-gel was prepared by using copper?II?acetate monohydrate,zinc acetate dihydrate and tin?II?chloride as metal sources,thiourea as sulfur source and 2-methoxyethanol as solvent.Through gravitational thermal and infrared spectrum analysis,the optimal pre-heating temperature of the CZTS sol-gel was 300°C.By optimizing the preparation process for the deposition of the precursor thin films and revealing the relationships between the preparation parameters,the CZTS thin films were successfully prepared showing low-carbon,voids free,no cracks and compact morphologies.It exhibited a single phases of kesterite structure with the preferential orientation along?112?direction,and had Cu-poor,Zn-rich and S-rich compositions with the optical band gap of 1.52 e V.?2?The effects of sulfurization variables on the structural,morphological,compositional and photoelectric properties of the CZTS thin films were investigated.As the sulfurization temperature increased,recrystallization and Cu S were observed in the CZTS thin films.When the sulfurization temperature was 580oC,the CZTS thin films showed compact and uniform morphology,better crystallinity and no Cu S secondary phase.As the sulfurization time increased,the grain size and crystallinity of the CZTS thin films as well as the thickness of Mo S2 layer were all increased.When the sulfurization time was over 60 min,the surface of the CZTS thin films started to decompose due to the excess losses of Sn and S compositions.The CZTS thin film solar cells fabricated by using the CZTS thin films sulfurized for 60 min exhibited the best photoelectric properties showing an efficiency of 4.42%.As the sulfur partial pressure increased,the losses of Sn and S at the surface of CZTS thin films could be effectively controlled,and the thickness of Mo S2 layer decreased at the same time,but more Zn S were observed on surface of CZTS thin films.The decomposition at the back surface of the CZTS thin films was found due to the diffusion of Zn atoms into the Mo S2 layer.When the sulfur partial pressure was 0.15 bar,the CZTS thin film solar cells showed the best photoelectric properties with an efficiency of 6.15%.?3?The effects of ZnO barrier layer between the CZTS thin films and Mo back contact,and chemical etching of Zn S at the surface of the CZTS thin films by using HCl solution on the structural,morphological,compositional and photoelectric properties of the CZTS thin films were studied.Results showed that the ZnO barrier layer effectively inhibited the diffusion of Zn atoms in the CZTS thin films towards the Mo back contact and prevented the thermal decomposition at the back surface of CZTS thin films.In addition,it reduced the effect of secondary phases which had the low band gap on the photoelectric properties of CZTS thin film solar cells,as a result exhibited an improved open circuit voltage and fill factor.The conversion efficiency of the CZTS thin film solar cells was improved to 6.33%.The removal of Zn S effectively improved the spectral response of CZTS thin film solar cells in short wavelength region,resulted in an increase of short circuit current density and fill factor.The conversion efficiency of the CZTS thin film solar cells was improved to 6.68%.?4?Using NaOH and the Sb?Ac?3 as doping sources,the effects of Na+ and Sb3+ doping on the structural,morphological,compositional and photoelectric properties of the CZTS thin films were studied.As the doping mole ratio of Na+ ions increased form 0 to 1.0%,the grain size,crystallinity,morphology,carbon impurities contents,carrier concentrations of the CZTS thin films as well as the built-in-potential at the space charge region were all improved.As a result,the conversion efficiency of the corresponding CZTS thin film solar cells was improved to 7.25%.When the doping mole ratio of Na+ was beyond 1.0%,all the photoelectric performance parameters of the CZTS thin film solar cells decreased.As the doping mole ratio of Sb3+ increased form 0 to 0.5%,the morphology,carbon impurities content,carrier concentrations of the CZTS thin films as well as the built-in-potential at the space charge region were all further improved.As a result,the conversion efficiency of the corresponding CZTS thin film solar cells was improved to 7.79%.When the doping mole ratio of Sb3+ was beyond 0.5%,all the photoelectric performance parameters of the CZTS thin film solar cells decreased.