| Abstract | Metal halide perovskites (MHPs) are an emerging class of semiconductors with advantageous properties for use in applications such as light emitting diodes1 and photovoltaics.2 MHPs are made up of by three components in an ABX3 formula where A is a +1 cation such as methylammonium (MA), B is a +2 cation such as lead, and X is a halide anion such as bromide. MHPs take the perovskite crystal form which in the case of MAPbBr3 is cubic, with lead-halide octahedra [PbBr6]4- at the corners of the cube and the MA+ in the center. MHPs have a tunable bandgap,3 are solution processable,4 defect tolerant,5 and have shown high power conversion efficiency in solar cell devices.6
In this dissertation, high-pressure spectroscopy is used as a tool to investigate the effect of growing varying sizes of nanocrystalline MHP (7 nm and 4 nm) in a porous silica (pSiO2) matrix. As the crystal size decreases, the ratio of the number of atoms at the surface of the nanocrystal increases compared to the total number of atoms in the crystal, so if surface defects play a major role in MHP non-radiative recombination, the pressure-induced effects will be more apparent in the smaller nanocrystal. The photoluminescence (PL) and in selected measurement, the visible light absorption of the MHPs in pSiO2 were monitored as a function of pressure. MAPbBr3 grown within a 7 nm pore size pSiO2 showed a novel blue shift to higher energy light emission under high-pressure when compared with the bulk MAPbBr3. The MAPbBr3 grown in 4 nm pore size pSiO2 was relatively insensitive to increasing pressure. It is apparent that the size and shape of the host material largely impacts the pressure induced shift in PL emission.
Energy/charge transfer between porous silicon (pSi) and prefabricated ligand passivated MHP quantum dots (QDs) was investigated using PL quenching experiments. The PL intensity and lifetime of the QDs was measured as the concentration of the pSi is added and a Stern-Volmer model is used to elucidate the mechanism of interaction. The Stern-Volmer plots showed a mixture of both static (irreversible complex forming) and dynamic (collisional) mechanisms in solution.7 The surface area, surface chemistry and pore size of the pSi has a large impact on the PL lifetime and intensity quenching due to accessibility to the surface of the pSi by the QDs and the interaction of the QD with the surface. It is inferred that a Förster resonance energy transfer mechanism is occurring due to the collisional nature of the interaction between pSi and MHP with the ligand passivation preventing intimate interaction of the MHP and pSi.
Large triazine based organic macrocycles were investigated for defect passivation in MHP thin films. Lewis bases are common defect passivating agents in MHP synthesis with oleylamine being commonly used in colloidal nanocrystal formation.8 Triazine based Lewis bases can occupy defect sites and improve the optoelectronic properties of the MHP. Our hypothesis is that large organic macrocycles can ideally add stability to a given MHP film and be tailored to have different passivating moieties, in this case a 24-member triazine macrocycle with a butyl amine functionality. The addition of the triazine based macrocycle containing the amine functionality to MHP thin films showed a fivefold increase in the PL intensity and significant narrowing of the emission linewidth of the MHP thin film in contrast to a macrocycle with a valine moiety that does not provide passivating effects.
Overall, physiochemical manipulation of charge transfer, defect passivation, and high-pressure probing of surface defects in MHPs were shown in this dissertation. It is apparent that the MHP semiconductor is influenced greatly by the interfacial environment and that it can be optimized for use in LED’s and PV devices with novel bandgap tunability, a better understanding of charge transfer between pSi and MHPs, and tailored defect passivation of thin films, many challenges remain however. |
