Band Gap Regulation Of CIGS Absorber Layer And Its Optoelectronic Properties

With the rapid development of social economy,the demand for energy utilization continues to increase.The prominent energy shortage and environmental deterioration caused by the extensive use of traditional resources has become the main contradiction restricting the sustainable development of human society.It is urgent to seek and develop clean renewable energy resources.Solar energy has attracted worldwide attention for its advantages of clean,environmentally friendly and renewable.Among all kinds of solar cells,thin film solar cells have attracted much attention due to their advantages of low cost,large-scale production and fabrication of multi-junction series battery modules.CIGS thin film solar cells have the advantages of low pollution,good weak light absorption,high efficiency and non-fading features.Therefore,CIGS become the most promising type of thin film solar cells.Absorber layer is responsible for the absorption and utilization of sunlight.The quality of the CIGS absorber determines the final photoelectric conversion efficiency of the device.The fabrication methods of CIGS absorption layer based on vacuum technology mainly include co-evaporation method and magnetron sputtering method.As we know,the magnetron sputtering method has the advantages of relatively simple equipment,high repeatability,easy regulation of element proportion and high speed,continuous large-scale production.These features can better meet the needs of industrial production.For the preparing of CIGS absorber layer by magnetron sputtering,sputtering of metal precursor followed by post-selenization process is usually adopted.This preparation scheme has the characteristics of high yield and low material loss,which is more meet with the requirements of industrial production.Despite the achievements obtained by magnetron sputtering,there are still some efficiency gaps with that fabricated by co-evaporation.Based on detailed literature research and in-depth comparative analysis,we can be found that the open-circuit voltage of CIGS devices fabricated by magnetron sputtering is relatively low,which is one of the key factors for its low final photoelectric conversion efficiency.During the selenization process after magnetron sputtering,complicated high temperature selenization process are involved.In this process,controlling the formation of stable and homogeneous alloy phase,keeping the Se vapor pressure constant and appropriate heating rate determine the crystallization quality,the uniform distribution of components and the formation of impurity and defects of CIGS absorber layer.These factors will have a crucial role on the bandgap of the absorption layer,thus affecting the final open-circuit voltage of CIGS devices.In order to solve the above problems,the precursor of Cu/In/Ga layer was deposited by magnetron sputtering,and then CIGS absorber layer was prepared by post-selenization.The influence of magnetron sputtering power and selenium content on the structure and performance of devices was investigated.Ag doping was adopted to control the crystal quality and the band gap of CIGS absorber layer.Ag doping at different positions of CIGS was explored in detail.Based on the successful Ag doping,the influence of Ag-S co-doping on the band gap of CIGS absorption layer was also investigated to further optimize the band gap of CIGS absorber layer.The main reasearch contents and results of this paper are as follows:1.High quality CIGS absorber layer fabricated by magnetron sputtering and post-selenization.The influence of magnetron sputtering power and selenium content on the CIGS absorption layer was investigated in this part.The results show that the power of magnetron sputtering has an important influence on the quality of CIGS absorber layer,which directly determines its final quality.When the power of magnetron sputtering is 50 W,the device achieves a photoelectric conversion efficiency of 5.8%.In order to further improve the crystallinity of CIGS absorber layer,the effect of different selenium content on the performance of the absorption layer was studied.The results show that when the power of magnetron sputtering is 50 W and the content of selenium powder is 140 mg,CIGS absorber layer has good optical properties and better crystal quality.7.54%efficiency was achieved based on the high quality CIGS absorber layer.2.Influences of Ag doping on the band gap of CIGS absorption layer.In this chapter,Ag doping is introduced to control the crystal quality and band gap of CIGS absorption layer during sputtering.A layer of silver is sputtered on the top,middle and bottom of the Cu/In/Ga precursor,respectively.The influence of Ag doping on the performance of CIGS devices is investigated in detail.The effects on the crystal structure,surface morphology,chemical composition and optical band gap of CIGS absorber layer were investigated.The results show that Ag doping can effectively regualte the band gap of CIGS absorption layer.The band gap of all Ag doped CIGS absorber layer can be increased to more than 1eV.Ag doping at the bottom of the prefabricated layer can improve the crystallization quality of the absorption layer.Therefore,10.2%device efficiency was achieved,which is mainly due to the improvement of the open-circuit voltage and the crystallization quality of the CIGS absorber layer.3.The mechanism of Ag-S co-doping in CIGS absorber layer.Firstly,the effect of sulphur/selenium on the band gap of CIGS absorption layer is studied.The effects of S/?S+Se?ratio on the quality of the absorption layer were investigated.And the crystal structure and optical absorption characteristics were analyzed.The results show that the band gaps of the absorption layer are all improved to more than 1.2 eV after sulphur/selenization.When S/?S+Se?is 40%,the V_OC increases to 500mV,and the photoelectric conversion efficiency is 10.56%,but the short circuit current density decreases gradually.In order to solve this problem,based on 40%S/?S+Se?,the absorption layer was further optimized by Ag-S co-doping.It is found that the CIGS absorber layer prepared by co-doping has good crystallinity and wide band gap.Based on the optimized CIGS absorber,the open circuit voltage can be increased to 540mV,the short circuit current density can be increased to 36.68 mA/cm²,and the efficiency can be increased to 13.01%.