Research On Zn-based Buffer Layer Materials And Their Application In CIGS Solar Cells

Energy crisis and environmental pollution have forced mankind to seek renewable energy.Solar energy is the most important category of new energy.The Cu?In,Ga?Se₂?CIGS?solar cell is considered to be the most promising thin film solar cells due to its large absorption coefficient,high conversion efficiency and long-term stable performance.The highest reported power-conversion efficiency for a laboratory scale CIGS solar cell was 22.6%,which was very close to the efficiency of crystalline silicon solar cells.CdS prepared by chemical bath deposition method was prominent candidate to be used as the buffer layer material in the traditional structure of CIGS solar cells.However,cadmium is highly toxic and CdS has a band gap of only².4 eV,which not only go against environmental protection but also limit cell performance as its low transparency in the short-wavelength spectrum.Zn-based materials have been research focuses in Cd-free buffer layer due to its non-toxic,wider and adjustable band gap.The methods of preparing Zn-based buffer layers are mainly chemical bath deposition,magnetron sputtering and atomic layer deposition.In this paper,Zn(O_1-x,S_x)and Zn_1-xMg_xO buffer layer materials were prepared by sol-gel spin coating method.The Zn(O_1-x,S_x)thin films with different S/?S+O?were deposited by sol-gel spin coating method and Zn(O_1-x,S_x)/CIGS solar cells were investigated first.The minimum value for band gap of approximate 3.72 eV was obtained when the S/?S+O?was equal to 0.44.Efficiency of up to 7.28%was achieved for the Zn(O_1-x,S_x)/CIGS solar cell from S/?S+O?=0.18,which was attributed to the optimized conduction band offset of+0.45 eV at the Zn(O_0.82,S_0.18)/CIGS interface.The short circuit current density of Zn(O_0.82,S_0.18)/CIGS solar cell was 1.72 mA/cm² higher than that of CdS/CIGS.It was found that the efficiency of Zn(O_1-x,S_x)/CIGS solar cell after light-soaking treatment for 30 min was 11.6%higher than that of untreated sample.The effects of annealing atmosphere and annealing temperature on the properties of Zn(O_1-x,S_x)films were also studied.The crystallinity and photocurrent response of Zn(O_1-x,S_x)film annealed at 550?for 30 min under the atmosphere of Ar was the best.The performances of Zn_1-xMg_xO films prepared by sol-gel method and Zn_1-xMg_xO/CIGS solar cells were also investigated.It was found that the Mg addition not only changed the optical band gap of the Zn_1-xMg_xO thin film,but also optimized the band alignment between Zn_1-xMg_xO/CIGS.When Mg/?Mg+Zn?=0.18 and the number of spin coating layer is 4,the conduction band offset of+0.22 eV at the Zn_0.82Mg_0.18O/CIGS interface and the efficiency of 5.83%were obtained.The efficiency of Zn_0.82Mg_0.18O/CIGS solar cell with light soaking treatment for 30 min was 6.23%,which was 8%more than that.untreated sample.The efficiency of Zn_0.82Mg_0.18O/CIGS solar cell without i-ZnO layer was 6%higher than that of with i-ZnO layer.It indicated that Zn_1-xMg_xO can not only be used as a buffer layer,but also as a window layer.The structure of CIGS/Zn(O_1-x,S_x)/Zn_1-xMg_xO solar cell was simulated with SCAPS to verify the feasibility and the influences of the thickness of Zn(O_1-x,S_x)buffer layer and Zn_1-xMg_xO window layer on the performance of CIGS/Zn(O_1-x,S_x)/Zn_1-xMg_xO solar cell were investigated.For optimum buffer layer and window layer,the thickness of Zn(O_1-x,S_x)and Zn_1-xMg_xO was found to be in the range of70⁸0 nm and 10²0 nm.On this basis,the Zn(O_1-x,S_x)buffer layer of 80 nm and the Zn_1-xMg_xO window layer of 20 nm were prepared by sol-gel spin coating method respectively.The efficiency of CIGS/Zn(O_1-x,S_x)/Zn_1-xMg_xO solar cell is 3.37%.