Study On The Synthesis Of Copper-zinc-tin-sulfur And Iron-doped Copper-zinc-tin-sulfur Powders

Cu2ZnSnS4(CZTS)is one of the most promising candidates for low cost solar cell applications.However,the currently reported non-doped pure chalcogenide Cu2ZnSnS4 thin film solar cells have a maximum power conversion efficiency(PCE)of only 8.4%,much lower than the ideal efficiency(32.2%)for CZTS thin film solar cells.This is mainly due to the defects of the CZTS material itself(secondary phase ZnS,Cu2+ and Zn2+ anti-occupancy defects,short carrier life,etc.)restricts the efficiency of CZTS thin film solar cells,the researchers found that cationic partial substitution CZTS can reduce material defects and improve battery efficiency.Meanwhile,theoretical and experimental research indicated that there are three different crystal structures of CZTS:kesterite(K),stannite(S)and wurtzite(W)phase.The emerging metastable wurtzite phase CZTS has a distinctive dense hexagonal structure,higher carrier concentration,lower resistivity,strong light absorption and a suitable energy band(1.4-1.6 eV).However,the wurtzite phase CZTS is more difficult to synthesize than the traditional thermally stable kesterite phase CZTS and the stannite phase CZTS.Therefore,this paper mainly studies three problems:the effective synthesis of pure kesterite CZTS powder;the selective synthesis of kesterite and wurtzite CZTS nanocrystalline;synthesis of Fe substitution CZTS nanocrystalline to achieve controllable kesterite structure,stannite structure and wurtzite structure transformation.The main results are as follows:1.CZTS crystals were successfully synthesized using hot injection method.XRD and Ramna studies show that the optimum reaction conditions for the synthesis of CZTS crystals by hot injection method is:injection temperature 250 °C,incubation for 1 hour.The CZTS particles exhibit strong optical absorption and a suitable optical band gap of 1.35 eV in the visible region.SEM analysis shows that the CZTS crystals synthesized by the hot injection method are irregularly shaped,and the agglomeration phenomenon is obvious,the dispersibility is very poor,and the individual crystals are growing abnormally.The improved hot injection method successfully synthesized pure kesterite phase CZTS crystal at the injection temperature of 210 °C.At the same time,the CZTS crystal synthesized by the improved hot injection method is spherical,which has greatly improved.2.When the CZTS nanocrystals were synthesized by heating-up method,the CZTS nanocrystals were transformed from kesterite structure to the wurtzite structure with the increase of the first reaction temperature.XRD and Raman tests show that the optimum reaction conditions for the synthesis of wurtzite CZTS nanocrystals by heating-up method are as follows:the first reaction temperature is 180 0C,time is 1h,and the second reaction temperature is 250 ?,time is 30min.The samples exhibited strong optical absorption properties and suitable optical bandgap(1.51 eV)under visible light.The formation mechanism of the obtained wurtzite CZTS nanocrystals was studied.We found that the synthesis of the wurtzite phase CZTS in the experiment is the first formation of amorphous CZTS particles(the first step of the heating reaction),and then the amorphous particles crystallize at high temperatures to form wurtzite phase CZTS nanocrystals(second step reaction).3.Cu2FexZn1-xSnS4(x = 0?0.1 and 0.2?1)nanoparticles were successfully prepared under the air condition by the heating-up method.First,the structural phase transition of the CFZTS(x = 0?0.1)nanocrystals with the Fe content increased from the wurtzite phase to the kesterite phase.Then,the structural transformation of CFZTS(x = 0.2?1)nanocrystals from kesterite to stannite phase is studied.It is found that the exact phase transition from kesterite to stannite is x = 0.6.Finally,the optical properties of CFZTS were studied by UV-Vis spectroscopy.We found that the addition of Fe could effectively improve the optical properties of CZTS.The obtained CFZTS nanoparticles showed strong light absorption capacity and exhibited adjusted bandgap of CFZTS in the range of bandgap energy from 1.5 eV to 1.1 eV with different Fe content.