Tuning the Luminescence of Layered Halide Perovskites.
Layered halide perovskites offer a versatile platform for manipulating light through synthetic design. Although most layered perovskites absorb strongly in the ultraviolet (UV) or near-UV region, their emission can range from the UV to the infrared region of the electromagnetic spectrum. This emission can be very narrow, displaying high color purity, or it can be extremely broad, spanning the entire visible spectrum and providing high color rendition (or accurately reproducing illuminated colors). The origin of the photoluminescence can vary enormously. Strongly correlated electron-hole pairs, permanent lattice defects, transient light-induced defects, and ligand-field transitions in the inorganic layers and molecular chromophores in the organic layers can be involved in the emission mechanism. In this review, we highlight the different types of photoluminescence that may be attained from layered halide perovskites, with an emphasis on how the emission may be systematically tuned through changes to the bulk crystalline lattice: changes in composition, structure, and dimensionality.
View details for PubMedID 30689364
Layered Halide Double Perovskites: Dimensional Reduction of Cs2AgBiBr6
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
2018; 140 (15): 5235–40
We investigate the consequences of dimensional confinement on halide double perovskites by synthesizing two-dimensional analogues of the recently reported three-dimensional double perovskite Cs2AgBiBr6. The layered perovskites (BA)4AgBiBr8 (1) and (BA)2CsAgBiBr7 (2) (BA = CH3(CH2)3NH3+) feature metal-halide sheets of mono and bilayer thickness, respectively, where the ordered double-perovskite lattice is partitioned by organic cations. Electronic structure calculations indicate that the indirect bandgap of Cs2AgBiBr6 becomes direct when the infinitely thick inorganic lattice is reduced to monolayer thickness. Calculations on model systems allow us to separate the effects of dimensional reduction from those of the accompanying structural distortions in the inorganic sublattice. Detailed optical characterization shows that the photophysical properties of 1 and 2 are markedly different than those of their well-studied lead-halide analogs. Hybrid layered derivatives of double perovskites substantially expand on the substitutional flexibility of halide perovskites to encompass greater compositional and electronic diversity.
View details for PubMedID 29575889
Charge Carrier Dynamics in Cs2AgBiBr6 Double Perovskite
JOURNAL OF PHYSICAL CHEMISTRY C
2018; 122 (9): 4809–16
Double perovskites, comprising two different cations, are potential nontoxic alternatives to lead halide perovskites. Here, we characterized thin films and crystals of Cs2AgBiBr6 by time-resolved microwave conductance (TRMC), which probes formation and decay of mobile charges upon pulsed irradiation. Optical excitation of films results in the formation of charges with a yield times mobility product, φΣμ > 1 cm2/Vs. On excitation of millimeter-sized crystals, the TRMC signals show, apart from a fast decay, a long-lived tail. Interestingly, this tail is dominant when exciting close to the bandgap, implying the presence of mobile charges with microsecond lifetimes. From the temperature and intensity dependence of the TRMC signals, we deduce a shallow trap state density of around 1016/cm3 in the bulk of the crystal. Despite this high concentration, trap-assisted recombination of charges in the bulk appears to be slow, which is promising for photovoltaic applications.
View details for PubMedID 29545908