Materials Science Research Lecture
Abstract: Metal halide perovskites have been intensively studied as promising materials for use in photovoltaics and light emitting diodes (LEDs) in view of their excellent semiconducting properties, broad spectral range of light absorption, and narrow bandwidth emission. In this talk, I will present use of low dimensional perovskite to increase stability, concentrate charge, improve light emission property, and create optical sources.
Low-dimensional (quasi-2D) perovskite films exhibit improved stability while retaining high solar cell performance of conventional three-dimensional perovskites. The dimensionality of this material is tuned by varying the ratio of methylammonium halides to phenylethylammonium halides. This creates a distribution of domains with varying numbers of perovskite layers centered on an average value <n>. An impressive photoluminescence quantum yield enhancement is achieved in the low-<n> regime. This is attributed to an inhomogeneous energy landscape that favors funneling of energy into domains with lower bandgap (higher n).
I have reported that a mixed-perovskite material, one comprised of a population of differently quantum-size-tuned grains, funnels photoexcitations to the lowest bandgap light-emitter in the mixture. The materials function as charge carrier concentrators, ensuring that radiative recombination successfully outcompetes trapping (non-radiative recombination).
To advance perovskite LEDs towards commercial relevance, the issue of stability needs to be addressed. We pinpointed a chief cause of the dramatic degradation of halide perovskites in LEDs, finding that photogenerated charges accumulating at exposed perovskite edge sites activate physisorbed oxygen, converting it into reactive superoxide and triggering perovskite degradation. We have therefore developed a strategy to protect edge sites in low-dimensional perovskites. At an applied level, we achieve perovskite films that exhibit a near-perfect passivation, with their photoluminescence quantum yields closely approaching unity. These films are stable under continuous illumination in ambient conditions for over 100 hours. They are bright, stable, efficient and easily solution-processible, which renders them useful for light emitting applications. Finally, I will also introduce highly luminescence, solid-state, zero-dimensional perovskites, and their promise as next-generation optical sources.
More about the Speaker: Li Na Quan is currently a postdoctoral researcher at the University of California, Berkeley, where she works in the research group of Prof. Peidong Yang on photophysical investigation of low dimensional nanomaterials. She earned her Ph.D in Chemistry at Ewha Womans University in Korea, advised by Prof. Dong Ha Kim and Prof. Ted Sargent at University of Toronto.