Characteristics of Cu2ZnSn(S,Se)4 thin film solar cells grown by thermal evaporation and thermal treatmentWednesday (28.09.2016) 11:15 - 11:30 Part of:
Cu2ZnSn(S,Se)4 kesterite type absorber thin films were deposited by thermal evaporation onto Mo coated glass substrates, which subsequently underwent a thermal treatment using various conditions. Upon evaporation the substrate temperature was set at 250 °C. As starting material well pre-characterized kesterite powder of different off-stoichiometry types has been employed [1, 2] which, in turn, was synthesized by solid state reaction [e.g. 2].
The thermal treatment was done both in a graphite box under Ar + S atmosphere as well as in evacuated silica ampoules, at which different temperatures, annealing times and heating rates have been applied. Prior to the fabrication of solar cell devices, including deposition of CdS buffer layer by chemical bath and sputtering of either ZnO/ITO or i-ZnO/AZO front electrodes, the superficial segregation of Sn(S,Se) secondary phase detected by grazing incidence X-ray diffraction (GIXRD) has been removed by chemical etching with KCN.
The effects both of compositionally different starting material as well as different annealing procedures on the properties of the thin films were carried out by GIXRD, Raman, EDX, SEM, XRF and GDOES in order to obtain information on microstructure, phase content and cross-sectional elemental distribution, respectively. The fast and simple process technique by thermal evaporation led to solar energy conversion efficiencies up to 2.5 %, though, is still well below the current record cell with 12.6 % . Higher amounts of tin at the back-interface as found by GDOES indicate the presence of additional Sn(S,Se) segregations between absorber layer and Mo back-contact, which is thought to cause the most detrimental effects on photovoltaic performance. However, since the formation of secondary phases elsewhere than at the surface cannot be removed anymore both the evaporation as well as the thermal treatment need to be improved such that a preferably single phase absorber layer is obtained.
 Lafond, A., et al., Crystal Structures of Photovoltaic Chalcogenides, an Intricate Puzzle to Solve: the Cases of CIGSe and CZTS Materials. Zeitschrift für Anorganische und Allgemeine Chemie, 2012. 638(15): p. 2571-2577.
 Rios, L.E.V., et al., Existence of off-stoichiometric single phase kesterite. Journal of Alloys and Compounds, 2015.
 Wang, W., et al., Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency. Advanced Energy Materials, 2014. 4(7).