| CZTS has a similar structure to CIGS thin film material.It has earth-abundant component elements,non-toxic,a high absorption coefficient(>104cm-1),and a suitable band gap energy（1.45eV）near the optimal band gap.The theoretical efficiency of CZTS is 32.2%（open circuit voltage:1.23V,short-circuit current:29mA/cm2,fill factor:90%）,which considered as one of the most promising solar cells.In this paper,Ag-doped CZTS was prepared via two methods:sputtering and sol-gel method.Systematic and in-depth research on material fabrication was carried out.This work mainly includes the following three aspects:1.On the soda-lime glass,the Mo electrode was sputtered with a DC power,followed by RF sputtering of Cu,ZnS,Sn,and Ag targets.The sputtering sequence of the Ag target was changed,and the effect of Ag at different positions on the CZTS characteristic was studied.The research showed that Ag does not stabilize at the deposition position and continuously diffuses during the sulfurization process.Diffusion does not make Ag uniformly distribute in the CZTS absorption layer,because the metal stack via sputtering is dense enough inhibit rapid diffusion of Ag.In the other hand,the diffusion rate of Ag is slowed after forming an alloy with other metals and the doping amount of Ag is lower which can’t provide enough driving force.Thus,different stacking order can cause uneven distribution of Ag in the absorption layer.The ZnS-Ag-Sn-Cu stacking sequence is benefited to facilitate the surface Ag-rich,increase the grain size and reduce the Sn-loss.The ACZTS device achieves a best efficiency of 2.9%,the open circuit voltage is 600 mV,and the short-circuit current density is 12.00 mA/cm2.2.Investigate the influencet of sulfur partial pressure on ACZTS:Put the optimized precursor in a graphite box with small holes,and control the sulfur partial pressure via changing the amount of sulfur powder.The results show that a lower sulfur partial pressure will cause the S/M ratio to be lower than 1,which proves that the sulfurization is insufficient,and there are exist lot of voids,which is caused by the Sn-loss.In contrast,under high sulfur partial pressure,the S/M ratio is over 1,and a thicker MoS2 is formed,which seriously damage the electrical performance of the device.Therefore,by adjusting the amount of sulfur powder,we found a suitable sulfur partial pressure,thereby obtaining an ACZTS absorption layer with no voids on the surface and a thin MoS2 layer,which significantly improved the photoelectric performance.3.A 2025 nm Ag was sputtered on a Mo-coated soda-lime glass using a RF power source.Then,using dimethylformamide as a solvent and copper acetate,zinc acetate,stannous chloride,and thiourea were sequentially dissolved.Then,the solution was titrated onto the Ag substrate,followed by spin coating,heating,and repeating 8 to 10 times to prepare an ACZTS precursor thin film.After high temperature sulfurization,an ACZTS thin film was obtained.The test results show that Ag significantly enhances the adhesion between the CZTS absorption layer and the Mo back electrode,improves the crystallinity of the CZTS,and enhances the device’s absorption in the long wave range.Finally,the ACZTS solar cell efficiency arrived 3.43%,the open-circuit voltage was 631 mV,and the short-circuit current density was 14.37 mA/cm2.Furthermore,we minute adjustment the composition of the sol-gel,improved the spin-coating-sulfurizaton craft,and used multiple sulfurizaton methods to fabricate an integral large-grain ACZTS film with an open circuit voltage of 674 mV and a short-circuit current density is 18.46 mA/cm2,and the photoelectric conversion efficiency is 3.5%.Among them,the open-circuit voltage and short-circuit current density values are very close to the current high-efficiency solar cells,but the fill factor is very low,which is a key factor limiting the improvement of efficiency.Improving the fill factor is the direction we need to improve and work hard in the future. |
