Effect Of Heavy Alkali Metal Elements On CIGS Thin Films And Solar Cells

Copper indium gallium selenium solar cell(CIGS)is one of the most promising thin film solar cells in the world due to their low cost,high conversion efficiency,good weak light response,stable performance and flexibility.Recently,the post deposition treatment of heavy alkali elements(KF-PDT,Rb F-PDT and CsF-PDT)has played an important role in improving the performance of CIGS solar cells.The world record of CIGS solar cell efficiency of 23.35% is achieved by CsF-PDT.The heavy alkali elements post deposition treatment(Alkali-PDT)is of great importance in improving the performance of CIGS solar cells,but the mechanism is not fully understood.This work focus on the mechanism of CsF-PDT pushing the device efficiency.However,there are few studies on it,while there are many experimental data on KF-PDT.Therefore,based on the data of KF-PDT reported in literature,SCAPS simulation is conducted to analyze the key factors that improve the performance of Cu-poor(conventional)CIGS solar cells by heavy alkali elements post deposition treatment(Alkali-PDT).Then,the theoretical analysis results above are applied to investigate of CsF-PDT.Finally,based on the understanding of the mechanism of CsF-PDT,the modified CsF-PDT(CsInSe2-PDT)is applied to the Cu-rich(unconventional)CIGS solar cells with severe interfacial recombination,and the device performance is improved.The specific contents of this paper are as follows:In this paper,SCAPS simulation is used to study the key factors of KF-PDT to improve the performance of Cu-poor CIGS solar cells.It is found that KInSe2 is formed in the surface of the absorber after KF-PDT,which can help to reduce the valence band energy of the absorber surface.This is beneficial to lower the interfacial recombination of hetero-junction and enhance the device performance.Especially,the KF-PDT is more suitable for enhancing the performance of high-Ga Cu-poor CIGS solar cells with severe interfacial recombination.Based on the investigation of the mechanism of CsF-PDT,it is found that CsInSe2 compound with wide band gap is formed in the absorber surface after CsF-PDT.It contributes to decreasing the valence band in the absorber surface and increasing the hole potential barrier in its surface,which is beneficial to reduce the interfacial recombination of the hetero-junction and improve the device performance.However,when Cs+ is over doped,new defects(such as 240 me V deep level defects)will form in the film,thereby deteriorating the device performance.Besides,the conduction band in the absorber surface after CsF-PDT is affected not only by the amount of CsInSe2,but also by the content of Ga in the absorber surface.Low Ga content in the absorber surface and high CsInSe2 can easily lead to the upward movement of the conduction band in the absorber surface,which can cause the formation of electron barrier in its surface.This can result in the increase of the recombination in the film,thus deteriorating the device performance.It is demonstrated that CsF-PDT is similar to KF-PDT,which is more obvious to advance the performance of the high Ga Cu-poor CIGS solar cells with high interfacial recombination.According to the above studies,we find that the CsF-PDT should be more suitable for enhancing the device performance of CIGS solar cells with high interfacial recombination.In this paper,CsInSe2 compounds are evaporated in the surface of Cu-rich CIGS absorber(CsInSe2-PDT)to reduce the serious interfacial recombination of Cu-rich(non-conventional)CIGS cells and improve the device performance.The results show that after CsInSe2-PDT,the work function,valence band energy and grain boundary recombination in the surface of the Cu-rich CIGS absorber are all decreased,which can effectively inhibit the interfacial recombination.Moreover,the formation of CsInSe2 compound in the absorber surface can also effectively prevent the diffusion of Cu+ from the interior to the surface of the absorber,so that the interior of the absorber can still maintain the advantages of Cu-rich thin film.Therefore,CsInSe2-PDT is conducive to decreasing the interfacial recombination of the hetero-junction while maintaining the Cu-rich character in the absorber.After CsInSe2-PDT,the VOC of Cu-rich CIGS cells increases by about 90 m V,with an efficiency increase of 26.3%.