Preparation And Photovoltaic Properties Of Cu₂ZnSnS₄ Thin Films Based On Sputtering

Seeking for sustainable green energies is a key issue of economic development in 21^st century.Solar energy will certainly obtain long-term development and wide application prospect as it is a kind of inexhaustible green energy.Though the conversion efficiency of commercial silicon solar cell is high and its production technology is mature,the great amount of raw material used,huge waste in processing and complex fabrication workmanship will prevent it from further reducing cost.Thus thin film solar cells with a few micrometer thickness are getting more and more attention.Cu₂ZnSnS₄?CZTS?,a new thin film solar cell absorber material,has many advantages like abundant sources and non-toxicity.So the problems in Cu?In,Ga?Se₂?CIGS?and CdTe solar cells are no longer existed in CZTS.Its optical band gap is 1.5eV,which matches well with solar spectrum.The theoretical conversion efficiency limit for CZTS is 32.4%,so it has the potential in future application.In this paper,CZTS thin film was prepared by sulfurizing the precursor deposited by co-sputtering CZTS and Cu targets.The usage of Cu target solved the Cu-poor problem in precursor,which happened under the condition of single CZTS target sputtering.The increase of substrate temperature could affect the growth mechanism of CZTS grains.Small CZTS grains formed in precursor during sputtering process,which lowered the crystallization temperature of CZTS thin film and promoted homogeneous element distribution and grain growth.The efficiency of solar cell based on this CZTS thin film was 1.33%.Due to the different vertical diffusion preference of Cu,Zn,Sn and S in post selenization process,a three-layer structure precursor was introduced with element gradient in vertical direction.The upper and bottom layers were prepared by co-sputtering CZTS and Zn targets to obtain a Cu-poor and Zn-rich layer,while the middle layer was prepared by co-sputtering CZTS and Cu targets to obtain a stoichiometric layer.The excess Zn in top layer could promote CZTS grain to grow up,while the excess Zn in bottom layer would balance element gradient.The effect of Zn sputtering power,sulfurization time and sulfur powder mass on the property of CZTS thin films and soalr cells was studied.The solar cell's efficiency improved to 3.35%under optimal condition.Selenization process was introduced to help grains further grow up.Selenization temperature,time and selenium particle mass all could affect the morphology and structure of CZTSSe thin films.A CZTSSe solar cell with an efficiency of 4.52%was prepared under 580?selenization temperature,10min selenization time and 10mg selenium mass.Though an obvious improvement of conversion efficiency was obtained by preparing CZTSSe solar cells,the decrease of open-circuit voltage limited its further enhancement,which was mainly due to the decrease of the band gap of CZTSSe thin film.We found that with larger amount of Ge substitution to Sn in CZTSSe thin film,the grain size and film band gap increased.The electrical property of thin film also got improved.However,the volatilization of Ge element caused serious problems like pinholes and cracks on the film surface,which harmed the performance of solar cells.A max conversion efficiency of CZGeTSSe solar cell with a value of only 1.45%was obtained.To avoid this problem,Ge doping in CZTSSe thin film was achieved by depositing a thin Ge layer with sereval nanometer thick above or under the precursor.Ge doping was found that it could improve film's crystallization,leading to the improvement of open-circuit voltage in solar cell.The position of Ge layer would affect the inner stress in CZTSSe thin film.When it was deposited above the precursor,the situation was beneficial to the post solar cell preparation process.With the increase of Ge layer's thickness,its morphology transformed from seperated islands to continous thin film,which promoted Ge doping.With a continous Ge layer above the precursor,Se gas firstly reacted with Ge and then Ge substitution was realized.However,excess Ge amount would also precipitate on the surface.The efficiency of Ge doped CZTSSe solar cell got improved to 5.38%when Ge layer was proper 10nm thick.In order to expand the application area of CZTSSe solar cells,flexible CZTSSe solar cells was prepared.The large difference of thermal expansion coefficient between substrate and Mo/CZTSSe layer would cause a large thermal stress formed in the interface and thin film during the annealing process,leading to the peeling-off problem.Ti foil was chosen as the substrate in flexible CZTSSe solar cell.With the introduction of an extra 10nm thick Ge buffer layer between Ti foil and Mo layer,the stress in the interface was reduced.And diffused Ge element improved CZTSSe grains'crystallization.The efficiency of solar cell with Ge transition layer was 1.35%larger than that without Ge buffer layer.The porous structure on single-crystalline silicon wafer prepared by metal assisted chemical etching could also reduce the stress in the interface of substrate/Mo layer or in CZTSSe thin film,which helped improve the device performance.