Thin film solar cells comprised of Cu(In,Ga)Se2 (CIGS) have efficiencies directly competitive with that of silicon devices1. Much research has focused on how to increase CIGS efficiency further and perhaps the most promising approach thus far comes from the incorporation of alkali elements either during or after the absorber layer deposition. Both techniques can be incorporated in commercial fabrication of CIGS devices at little extra cost, suggesting their industrial relevance.
To date, sodium and potassium are the most heavily researched alkali elements. Many studies have shown that adding sodium and/or potassium to the CIGS layer results in performance increases, although the mechanisms causing this are still greatly in question2,3,4. Additionally, the method used for incorporating the alkali elements, most commonly through post-deposition treatment (PDT) or substrates containing the elements of interest, influences the effect that the same element may have on the device performance5.
Using photoluminescence (PL) to map areas of full devices with a resolution of < 1 µm allows for the acquisition of pixel-by-pixel information of sub-grainsize variations in bandgap and radiative recombination. We simultaneously map the performance by observing the laser beam induced current and voltage (LBIC and LBIV, respectively). Furthermore, we measure Raman spectra at the same resolution to map different phases that may exist in the CIGS layer. From this, we correlate how bandgap, radiative recombination, and phase existence each correlate separately to electrical performance, as well as to each other.
The samples analyzed were deposited on either soda-lime glass (sodium containing) and sapphire substrates (sodium free). Samples were then either exposed or not to a NaF PDT. This work presents a correlative analysis of absorber inhomogeneity within PDT CIGS against the performance to determine the influence that deposition method and sodium content have on device efficiency. Additionally, we explain the effect the different incorporation pathways have on micrometer scale performance. Preliminary results show that increasing sodium content consistently narrows the bandgap distribution in the absorber layer. Additionally, results show that sodium introduction via the substrate results in the elimination of a Raman defect peak at approximately 150 meV. This suggests that sodium incorporation via the substrate better supports defect passivation, although the mechanism for why this is not yet identified.
Spring 2018 MRS Abstract – CM04
1Green, M. A., et al. Prog. Photovoltaics Res. Appl. 2017. 2Reinhard, P., et al. IEEE J. Photovoltaics, 2015. 3Reinhard, P., et al. A. N. Chem. Mater. 2015. 4Aguiar, J. A., et al. M. Adv. Mater. Interfaces 2016. 5Hsu, C.-H., et al. Prog. Photovolt Res. Appl. 2015.