| Cu(In,Ga)Se2(CIGS) thin film solar cell has attracted more and more attention due to its outstanding stability, high conversion efficiency, and excellent performance under low light intensity. It is regarded as one of the most promising photovoltaic devices in the world. The great potential in industrialization of CIGS thin film solar cells is increasingly obvious because of the capacity developments of some big companies, such as Solar Frontier in Japan and Hanergy in China, etc. So far, commercial Solar Frontier has surpassed 3 GW glass modules with an average efficiency of 13.8%. Hanergy has developed very quickly and its capacity has reached 400 MW of glass modules and 100 MW of flexible modules since acquiring Solibro and Miasole. On the other hand, several research groups, like Solar Frontier in Japan, ZSW and Solibro in Germany, have been able to achieve efficiencies of 21% or above, using CIGS absorbers grown on Mo-coated substrates. The Optimization of all function layers has promoted efficiency breakthroughs, in which the deep investigation of CIGS preparation process and doping with alkali metals played important roles.Nowadays, high-efficiency CIGS thin film solar cells were usually obtained by “3-stage” co-evaporation method, which contributes to a larger grain size and a gradient band gap to reduce the internal defects in the material and lead to a higher short-circuit current density and open-circuit voltage at the same time. In this paper, CIGS thin films were prepared under different Se fluxes by varying the heating temperature of Se effusion cells(Tsec) from 150? to 190 ?. The influence of Se flux on different properties of CIGS thin films and corresponding solar cells were systematically investigated. It is found that the thickness(or the deposition rate) decreases with the increasing Tsec. The Cu content in the CIGS decreases with the increasing Tsec while In content increases and Se content keeps relatively constant. The CIGS thin films deposited at lower Tsec show larger size grains from the cross-sectional scanning electron microscopy(SEM) images, which is related to the higher CGI during the deposition process. All CIGS thin films deposited at high Tsec show A1 ordered defect compound(ODC) phase at 153 cm-1 from the Raman measurements. The CIGS thin film solar cells fabricated at 160? shows best efficiency of 11.8% in this series due to the relatively higher open circuit voltage(Voc), fill factor(FF), and short circuit current density(Jsc).It is well-known that the incorporation of even a small amount of alkaline elements(Na, K) into the CIGS absorber will significantly improve the solar cells efficiency. For sodium, diffusion from soda-lime glass substrate was regarded as the most effective way for the doping. In general, the doping of Na results in enhanced efficiency of the solar cells due to the increased hole concentration and conductivity in absorber, improved morphology, increased grain size and reduced defect centers. In contrast, the impact of K doping was not so clear until now. This work will focus on the impact of KF doing by PDT to the CIGS absorbers and especially to the CIGS thin film solar cells. The KF doping on CIGS absorber layers and thin film solar cells were performed by PDT process. The influence of KF source temperatures, substrate temperatures and evaporation time were investigated systemically. XRD and Raman measurement revealed that K element doping has no obvious effect on the crystal structures of CIGS thin films. Surface and cross-sectional SEM images displayed that the surface morphologies of CIGS thin films become more compact and smooth while the grain sizes remain unchanged after KF PDF treatment. C-V measurement revealed the net carrier density increased after doped with KF, but the J-V analysis revealed that the quality of pn junction at CdS/CIGS interface were not improved. As a result, the conversion efficiency of the KF-doped CIGS was slightly improved from 13% to 14.3%. |
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