Theoretical Calculation Of Phase Stability And Electrical Properties Of Copper-zinc-tin-sulfur Compounds

With the demand of energy increasing,researching and developing renewable energy have become a worldwide theme.Although Cu2ZnSnS4(CZTS)compounds have the advantages of abundance,friendly environment,and low cost,there are still many questions such as second phases and low photoelectric conversion efficiency in experiment.Therefore,the research of the relationship between componential change and phase stability or electrical property in CZTS compound plays a critical role in the regulation of structure and property of CZTS compound,and further promotes their application in solar cells.In this thesis,the crystal structures of CZTS with stoichiometric ratio and off stoichiometric ratio are constructed and optimized based on first principles.Phase stability and electrical property are studied.On one hand,the relationship between the ratio of Cu/(Zn+Sn)and phase stability is explored through formation energy,which can provide a theoretical component range of CZTS pure phase.On the other hand,the relationship between composition changes in CZTS compound and the eleetrical properties such as carrier mobility,carrler concentration and conductivity are researched by calculating band structures.The main results are listed as following:(1)Using 16-atom CZTS cell as a basic model structure,three types of CZTS structures(kesterite,stannite,primitive-mixed CuAu)with stoichiometric ratio are constructed and optimized.Moreover,the band structures of three types of CZTS are refined by the GGA+U method,and all three types of CZTS have good light absorption coefficients.(2)Using 64-atom kesterite supercell as a basic model,and common phase structures in the system are constructed and optimized.Besides,CZTS supercell structures with off stoichiometric ratio are constructed and optimized.By comparing the enthalpy of formation,specific conclusions are found:?When the component of CZTS is stoichiometric,kesterite phase is the stable phase in thermodynamics.? When the component of CZTS is off stoichiometric,especially with a slight deviation in component,kesterite phase can still maintain the stability.Calculated results prove that:If Cu/(Zn+Sn)ratio is in the range of 1.000 to 0.882,CZTS pure phase is the stable phase in phase or the combination of second phases can be the stable phase.While Cu/(Zn+Sn)range is less than 0.778,the combination of second phases is the stable one which means there must be second phases co-existing.And All experimental results we could find are in accordance with the calculated kesterite phase stable range.Such a range can provide a profitable guidance for the composition design of Cu-Zn-Sn-S alloys for thin film compound solar cells.Furtherly,the model and the calculation method can be extended for the prediction of the phase stability in other alloy systems.(3)Using 64-atom kesterite supercells with ZnCu substitution defects as basic models,the electrical properties are calculated.With the component deviating,the carrier concentration decrease and increase,while the carrier mobility changes in the opposite.Moreover,calculation results of conductivity show that conductivity increases firstly,decreases then and increases finally with the deviation increasing.