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Browsing by Author "Sadewasser, Sascha"

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  • Optical and structural investigation of Cu2ZnSnS4 based solar cells
    Publication . Teixeira, Jennifer P.; Salomé, Pedro M. P.; Sousa, Marta G.; Fernandes, Paulo A.; Sadewasser, Sascha; Cunha, António F. da; Leitão, Joaquim P.
    The structural and optical properties of two solar cells in which the Cu2ZnSnS4 absorber layer was sulphurized by two different methods (S flux and graphite box), were studied. The grain sizes are dependent on the sulphurization method, the larger ones being obtained for the sulphurization in a S flux. The optical properties were investigated by photoluminescence (PL). A broad and asymmetric band was observed for the sample with the larger grains, whereas for the other one a very broad emission was obtained, mostly influenced by the CdS buffer layer. The dependence on the excitation power revealed the influence of fluctuating potentials created by strong doping and high compensation of the absorber layer. Radiative recombination channels are quite different from the ones typical of semiconductor materials with flat bands. A relationship between the PL intensity from the absorber layer measured at low temperatures, and the final PV performance is established. Thus, we propose that PL can be used as an evaluation experimental technique in order to decide if a certain absorber should be processed into a full solar cell or not.
    2016Journal article Restricted access Show more
  • Voids in Kesterites and the Influence of Lamellae Preparation by Focused Ion Beam for Transmission Electron Microscopy Analyses
    Publication . Ribeiro-Andrade, Rodrigo; Sahayaraj, Sylvester; Vermang, Bart; Correia, M. Rosario; Sadewasser, Sascha; Gonzalez, Juan Carlos; Fernandes, P. A.; Salome, Pedro M. P.
    Kesterite solar cells based on Cu 2 ZnSnS 4 and Cu 2 ZnSnSe 4 (CZTSe) are potential future candidates to be used in thin-film solar cells. The technology still has to be developed to a great extent and for this to happen, high levels of confidence in the characterization methods are required, so that improvements can be made on solid interpretations. In this study, we show that the interpretations of one of the most used characterization techniques in kesterites, scanning transmission electron microscopy (STEM), might be affected by its specimen preparation when using focused ion beam (FIB). Using complementary measurements based on scanning electron microscopy and Raman scattering spectroscopy, compelling evidence shows that secondary phases of ZnSe mixed in the bulk of CZTSe are the likely cause of the appearance of voids in STEM lamellae. Sputtering simulations support this interpretation by showing that Zn in a ZnSe matrix is preferentially sputtered compared with any metal atom in a CZTSe matrix.
    2019Journal article Restricted access Show more