Date of Award
Doctor of Philosophy
Halide perovskites and germanium semiconductors are promising materials for many optical applications such as solar cells and LEDs due to their unique photophysical properties. Compositional substitution and dimensional manipulation can enhance physical or chemical properties of perovskite and germanium semiconductors which in turn promotes their performance in optoelectronic devices. In this thesis, we report the synthetic exploration of composition-control and dimensionality-control of organometal halide perovskite crystals by tuning halide-incorporation and exploiting bulky alkylammonium cations as capping ligands. We also demonstrate a systematic synthesis of all the series of mixed halide perovskite polycrystals and their low dimensional analogues. By optimizing synthetic conditions, we are able to inhibit the appearance of a reversible photoinduced PL peak derived from surface traps.
We also synthesize lead-free perovskites for the environmental concerns. Lead is a heavy metal element and its potential toxicity raises concerns for environmental compatibility. To address this problem, we developed a synthetic route to antimony perovskites and germanium perovskites. Surface-bound (CH3)3Sb2I9 layers restrict the growth of CH3NH3PbI3, resulting in CH3NH3PbI3 nanocrystals. Compared to the bulk perovskites, the antimony-capped nanocrystals show stronger photoluminescence. With a direct bandgap of 1.6 eV and a corner-sharing octahedral network crystal structure that are comparable to CH3NH3PbI3, CsGeI3 is potentially promising for photovoltaic applications. To manipulate the optoelectronic properties, we doped high-spin, divalent manganese ions (Mn2+) into the octahedral Ge2+ sites of CsGeI3. Electron paramagnetic resonance (EPR) helps us better understand the local ion environment and composition of both CsGeI3 and its doped analogue (CsGe1-xMnxI3). Our results expand the lead-free halide perovskite family and set the stage for their application beyond photovoltaics to spintronics and magnetic data storage.
Finally, we fabricated and characterized Ge1-xSnx alloy nanocrystals and Ge1-xSnx core/shell nanocrystals. Germanium has an indirect bandgap of 0.66 eV, which is too narrow for ideal solar cell light harvester materials and limits their absorption efficiency. By tin incorporation and quantum confinement effect, we could enhance their efficiency of solar absorption and in turn their quantum yield. We synthesized Ge1-xSnx and Ge1-xSnx/CdS core/shells in solution phase. Inclusion of tin is confirmed by X-ray diffraction and Raman peak shift. Tin alone does not result in enhanced photoluminescence intensity, however, adding an epitaxial CdS shell onto the Ge1-xSnx nanocrystals does enhance the photoluminescence up to 15ÃÂ over Ge/CdS nanocrystals with a pure Ge core. More effective passivation of surface defects—and a consequent decrease in surface oxidation—by the CdS shell as a result of improved epitaxy (smaller lattice mismatch) is the most likely explanation for the increased photoluminescence observed for the Ge1-xSnx/CdS materials. With enhanced photoluminescence in the near-infrared, Ge1-xSnx core/shell nanocrystals might be useful alternatives to other materials for energy capture and conversion applications and as imaging probes.
Men, Long, "Synthetic exploration of halide perovskites and germanium semiconductors" (2017). Graduate Theses and Dissertations. 16411.