| Recently,thin film solar cells including amorphous silicon,CdTe and Cu(In,Ga)Se2-xSx(CIGS),began to set foot in the market due to its low price.However,the preparation of these solar cells has suffered from either severe preparation condition with high vacuum,and/or the limited reserve of inorganic materials on the earth.Aside from this,its toxicity also limits their large production.As an excellent candidate of the third generation solar cells,copper zinc tin sulfur Cu2ZnSnS4(CZTS)has attracted much attention,giving the credit to its environmentally friendly features,rich content in the earth’s crust and excellent photoelectric properties.It is well known that the band gap of CZTS is ~1.5 eV,which matches well with the other semiconductor materials in solar cell.Apart from these,its great light absorption coefficient is favorable to the light absorbance in UV-vis range.The photoelectric conversion efficiency of theory is over 30%.CZTS films will become another promising material as a replacement of CIGS series materials in the near future with continual improvement in preparation technology.At present,CZTS thin films can be obtained by various approaches including vacuum and non-vacuum method.However,these approaches are neither versatile nor particularly low-cost for the achievement of large area devices.So far CZTS solar cell prepared with by hydrazine solution has acquired the highest efficiency of 12.6%.In thin paper,a low-cost and non-toxic method based on the thermal decomposition and reaction of metal-thiourea-oxygen sol-gel complexes was used to synthesize CZTS thin film.The low-dimensional ZnO@CdS heterojunction nano-arrays coupling with the as-prepared CZTS thin film were employed to fabricate a novel solar cell with inverted structure.The vertically aligned nanowires(NWs)allow facilitating the charge carrier collection/transfer with large interface areas.In the meantime,we adopt multiple spin on Mo substrate to obtain a certain thickness of CZTS precursor film.High quality CZTS thin film was obtained by annealing in sulfur atmosphere.However,CZTS can decompose and produce quadratic phase zinc sulfide(ZnS)due to the high temperature.In order to remove ZnS phase on the surface of CZTS thin film,a certain concentration of hydrochloric acid solution to etch CZTS thin film.XRD,Raman,Abs,SEM,IV were employed to investigate the photoelectric properties characteristics of film material and the corresponding solar cell.A low dimensional Zn O@CdS nano heterojunction arrays were synthetized by aqueous solution method as the window layer and buffer layer in the solar cell,respectively.A low-cost and non-toxic method based on the thermal decomposition and reaction of metal-thiourea-oxygen sol-gel complexes was developed to synthesize CZTS film.The inverted structure of as-obtained solar cell is FTO/ZnO@CdS NWs/CZTS/Ag.By optimizing the parameters including the annealing temperature of CZTS absorber,the thickness of CdS buffer layer and the morphology of ZnO NWs,the solar photovoltaic conversion efficiency as high as 2.27% was obtained by using such solar cell with inverted structure.Precursor CZTS thin films are prepared by using sol-gel method for Mo substrate,and annealed in the sulfur atmosphere to get high quality crystal film.We set a proper pressure in the process of annealing in order to suppress the volatilization of tin element and get a better crystalline thin film.CZTS thin films and the corresponding solar cell with different annealing temperatures were prepared and characterized.Further increasing the annealing temperature shows properties improvement of thin film crystalline.It was found that the photoelectric conversion efficiency was firstly increased and then progressively decreased as the function of annealed temperature.The annealed temperature of 580 oC was demonstrated to be the best annealed temperature toward the highest photoelectric conversion efficiency.Finally,the Raman spectrum measured from the as-grown sample with the 325nm(UV)excitation wavelength is dominated by three intense peaks at 348,696,and 1044 cm-1,which are identified as the first-,second-,and third-order peaks characteristic of ZnS phase.After chemical etching with a solution of hydrochloric acid(HCl),the intensity of the ZnS Raman peaks is drastically reduced.By optimizing the parameters including the concentration of HCl and the etching time,we obtained the best photoelectric property by etching the film in 5% v/v HCl solution for 5mins,and eventually the efficiency is promoted to as high as 4.72%. |
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