Sn-Pb-mixtures for Perovskite Solar Cells
Abstract: Perovskite research has quickly gone from non-existent to one of the biggest reseach areas in Photo- voltaics. There are still many questions to be solved regarding the stability, environmental impact, reproducible large area devices, efficiency and module fabrication cost of Perovskite Solar Cells, PSCs, before commercial up-scaling. Scientists all over the world are working towards solutions and over 15000 scientific articles were published from 2009 up until the end of 2019. Perovskite show great promise to fill the gap where Silicon Solar Cells fail today, such as inside charging for Internet Of Things and as active material in tunable bandgap tandem cells . This master thesis focused on the fabrication of PSCs and the interchange of Lead, Pb, atoms with Tin, Sn, atoms in thin films and devices. Films with metal-organic-halide perovskite material Formamidinium(FA)LeadIodide, FAPbI3 or FAPI, and different percentage of Sn were fabricated by several methods and investigated for an extended time period. We showed that different percentage of Sn in FAPbI3 generated very different results regarding morphology, PhotoLuminescence(PL), photovoltaic performance, stablity and phase separation/segregation. The re- sults give reasonable indications that Sn compositional engineering can be used to stabilize the otherwise unstable FAPbI, probably by changing bond lenghts in the Pb-I-Pb lattice to accommodate for strain and the non-symmetrical FA cation. It is also possible that increased stability and PL performance can arise from the creation of separate phases that add in stabilizing the strain build up by creating micro- scopical 3D structures as could be seen when samples were thoroughly investigated by Scanning Electron Microscopy and Confocal Microscopy. Standard PSC in this report reached a maximum of 19.5% Power Conversion Efficiency, PCE, and pin-structure Sn-Pb-mixed PSC with 60% Sn reached over 7% PCE and a short circuit current density, Jsc, over 21 mA/cm2. Results indicated that the often used 50% Sn composition is not the most stable but that a choosing an alloy composition slightly off like 40% or 60% Sn for FAPbSnI3 can be favorable to achieve higher efficiency and longer stability. Changing ratios subsequently lead to rises and falls in the connectivity in morphology, stability, intensity of PL and X-ray Diffraction spectra depending on the composition, something that is also discussed in regard to phase separations, strain, unit cell lattice/size and morphology.
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