Research On Doping And Structure Optimization Of Copper Indium Gallium Selenide Solar Cell

Copper indium gallium selenide（CIGS）thin film solar cells have long attracted much attention because of their high photoelectric conversion efficiency,good operating stability,high light absorption coefficient of the absorber layer and adjustable band gap.However,the photovoltaic conversion efficiency of CIGS cells is still far from its theoretical limit efficiency,and the conventional buffer layer using Cd S will pollute the environment.Therefore,in this thesis,elemental regulation of the absorber layer and alkali metal doping mechanism in the absorber layer are studied for cell efficiency enhancement;optimization of Cd-free buffer layer and Si doping in the absorber layer of buffer-free CIGS cells are carried out for the environmental problems caused by the conventional Cd S buffer layer.The main research contents and conclusions are as follows:（1）The CIGS bandgap versus Ga content（Ga/Ga+In,GGI）relationship,which is highly consistent with the experimental results,is obtained by optimizing the first-principles generalized function,and the effect law of composition-regulated bandgap on the performance of the absorber layer of the cell is further elucidated by device simulations.The results show that the energy band structure of CIGS can be accurately calculated using the hybrid density functional,and the difference between the calculated band gap and the experimental value is<7%.When GGI=0.1 and S/S+Se=0.6 in the absorber layer of the cell,corresponding to a band gap of 1.4 e V,the conversion efficiency of the cell can reach 19.86%,which is significantly higher than that of the CIGS solar cell with a band gap of 1.2 e V（18.2%）commonly prepared experimentally at present.Considering that the current experimental process cannot perfectly solve the negative impact caused by high Ga and S content,the cell with an efficiency of 19.02%was obtained by optimizing the double gradient distribution of elements inside the absorber layer.（2）Through device simulation,the influence law of different mechanisms of alkali metal doping in the absorber layer on the performance of CIGS cells is revealed.It is shown that the defect passivation effect of alkali metal doping can significantly improve the CIGS solar cell performance,among which the use of post deposition treatment（PDT）is the most effective for improving the device carrier complex and the cell performance is most obviously improved.The use of PDT method to dope alkali metal induces the formation of the second phase on the absorber surface will have a more serious negative impact on the cell performance,which is due to the high position of the bottom of the conduction band of the second phase and hinders the electron transport.（3）By optimizing the Cd-free buffer layer,the mechanism of the energy band matching between the absorber layer and the buffer layer on the performance of the CIGS cell was investigated.The results show that the cell efficiencies of 18.17%and17.61%can be achieved using In₂S₃ and Zn(O_0.5S_0.5)as the buffer layer,respectively,which are very close to that of the cell using conventional Cd S as the buffer layer（18.2%）.When the conduction band offset of the absorber layer and the buffer layer is0.1～0.3 e V,the cell can show excellent performance,and using(Zn_0.85Mg_0.15)O as the second buffer layer can further improve the cell efficiency,but the improvement is very small.（4）The effect of Si doping on the performance of the CIGS absorber layer was investigated using a magnetron sputtering CIGS quaternary ceramic target post-annealing process.The results show that Si doping has a significant effect on the surface morphology of the film,with trace Si improving the film denseness but causing tiny particles on the grain surface,and large amounts of Si leading to blurred morphology,as well as the appearance of white particles on the film surface,presumably thought to be Cu₂Se,and the presence of Si also leads to significant refinement of the grains at the bottom of the film.The Si doping has a small effect on the optical properties of CIGS,while the electrical properties vary considerably,and the carrier concentration can be increased up to 10¹⁹ cm^-3,which is analyzed to be caused by the presence of highly conductive Cu₂Se.The calculations show that it is difficult for Si to enter into the Cu In Se₂ lattice to form the corresponding substitution defects.