The optoelectronic diversity of layered halide perovskites
Keywords: Chemistry.

The path towards emergent photophysics and new technologically desirable physical properties lies through synthesis of new materials. A powerful framework for developing such materials is the idea of hybrids, which contain both organic and inorganic components in a single chemical compound. These two building blocks each offer advantages compared to the other. The ability to design organic molecules with exact precision over atomic connectivity and elemental specificity is unmatched, while inorganic building units contribute increased robustness and intriguing optoelectronic properties. Additionally, the differing reactivities and stabilities of these components dictates simple and mild synthetic conditions such as solution-state self-assembly, allowing easy access to a diverse phase space of possible materials. I have explored how organic-inorganic hybrid materials contain both striking similarities and drastic departures from the optoelectronic properties of their separate components, particularly evident in the family of layered lead-halide hybrids. Much of my focus centers on understanding and controlling the behavior of excitons in the inorganic layers of the two-dimensional lead-halide perovskites. Using a combination of in situ optical spectroscopy and X-ray diffraction techniques, I demonstrated how reversible intercalation of polarizable species such as the molecular dihalogens, such as iodine, can tune the degree of electronic confinement without altering the structure of the inorganic layers in which excitons are confined. More reactive halogens react with the perovskite to perform unusual chemistry with both the organic and inorganic layers. Room-temperature white-light photoluminescence is a rare phenomenon exhibited by certain layered lead-halide perovskites. I initially helped elucidate the photoluminescence mechanism, which I ascribed to the radiative recombination of self-trapped excitons. I further demonstrated that this broad emission is not restricted to a few members of the halide perovskite family, but rather is common to all layered lead-chloride and lead-bromide perovskites, although this emission is highly temperature dependent. With variable-temperature photoluminescence and single-crystal X-ray diffraction, I established correlations between the propensity to exhibit broad photoluminescence at a given temperature and particular octahedral tilts of the lead-bromide octahedra, and provided further support for the self-trapped exciton hypothesis. With additional both static and time-resolved temperature-dependent optical spectroscopies, I found that self-trapped excitons are pervasive in the lead-bromide perovskites, even when the broad photoluminescence is not visible, suggesting a persistent pathway for non-radiative recombination that limits quantum efficiency. Finally, I extended the phenomenon of broadband photoluminescence beyond the layered perovskites to the class of lead-halide hybrids generally. I synthesized and characterized both layered and zero-dimensional lead-bromide hybrids templated by sulfonium-based cations that exhibit room-temperature broad red emission.