Modification of Perovskite Polycrystalline Films by Molecular Additives and Ion Doping for the Improved Stability of Perovskite Solar Cells

Abstract

Organometal halide perovskite (OHP) exhibits superior optoelectronic properties so has been considered as a next-generation photovoltaic material. Owing to its excellent optoelectronic features as a photovoltaic material, such as an optical band gap in the range of 1.4 – 1.6 eV, long charge-carrier lifetime, a superb absorption coefficient, power conversion efficiencies (PCEs) of the associate perovskite solar cells (PSCs) over 25% was recorded recently. Compared to the intensive studies and remarkable PCEs of OHP solar cells, the stability of OHPs has attracted relatively little attention, and the resulting lack of understanding of this instability delays their commercial applications. OHP crystals can be degraded by various causes such as thermal energy, moisture, oxygen. At a high temperature, ions in OHP crystal escape from the crystal to destroy the crystal structure. Moisture easily reacts with OHP to form photo-inactive perovskite hydrates. The oxygen-induced degradation of OHPs, in particular, is critical in tin-based OHPs. Recently, various attempts to improve the stability of OHP crystals such as interfacial engineering, compositional engineering, and the addition of molecular additives have been tested; however, most of efforts to improve the stability of OHPs are mainly focused on interfacial or compositional engineering. In this thesis, OHP photoactive films were tuned to improve the stability of them. Simple and effective methods that tune the properties of 1) grain boundary of the crystal and 2) bulk crystal to improve the stability of OHP polycrystalline films were provided and their stabilizing mechanisms were investigated. Grain boundary of OHP was passivated by phenyl-C61-butyric acid methyl ester (PCBM) to improve thermal stability of OHP crystals, which also improved the PCEs of the associated solar cells. It was demonstrated that grain boundary has a significant influence on their thermal stability. Cells with smaller grain interface area (i.e., larger grain size) had higher thermal stability. The additive is located at grain boundaries and found to induce electron transfer reactions with halogens in the perovskite. The reaction products chemically passivate perovskite crystals and strongly bind halogen atoms at grain boundaries to their crystal lattice, preventing them from exiting from the crystal lattice, which improves thermal stability of perovskite crystals. Silver (Ag) ion was doped to a lead-based OHP crystals to tune the properties of the bulk crystal. It improved the stability of the OHP crystals against heat and humidity. The mechanisms of Ag doping were studied in morphological and thermodynamic veiwpoints. Ag doping increases the size of the OHP grains and reduces the size of the amorphous intergranular regions at the grain boundaries, and thereby hinders the infiltration of H2O molecules into the OHP films and their thermal degradation. The quantum mechanical simulation revealed that Ag doping increases both the energies of the adsorption of such detrimental molecules and heat-induced vacancy formation in OHP crystals. Ionic doping is also effective to improve the oxidative stability of tin-based OHPs. The improved oxidative stability of tin-based OHPs by Ag doping were investigated in the similar way. The value of this dissertation is that the methods provided in this thesis are focused on improving the stability of OHP photoactive films which has drawn relatively little attention, so these methods can be used in combination with other effective methods such as interfacial engineering to further improve the stability of OHP solar cells.