Field-Driven Ion Migration and Color Instability in Red-Emitting Mixed Halide Perovskite Nanocrystal Light-Emitting Diodes

Perovskite nanocrystals have shown great promise as the basis of a new family of nanocrystal light-emitting diodes (LEDs). However, the external quantum efficiency and color stability of these materials still lag behind those of well-established technologies. Producing stable efficient red emitters with electroluminescence (EL) in the “pure” red range of 620–650 nm is a particular challenge. Here we present mixed halide CsPbBr_3–<i>x</i>X_<i>x</i> (X = I or Cl) peNC organic LEDs using peNC emitters with photoluminescence across the visible region to produce LEDs displaying EL across the visible spectrum. By focusing on the yellow-orange to deep red (560–680 nm) visible regime, we present evidence that field-driven halide separation in CsPbBr_3–<i>x</i>I_<i>x</i> peNCs is responsible for the observed red-shifting and splitting of the EL peaks. Greater compositional stability is demonstrated to be the key to higher efficiency, long-lived devices for deep red-emitting mixed halide peNCs with higher compositional concentrations of iodide

Shape‑, Size‑, and Composition-Controlled Thallium Lead Halide Perovskite Nanowires and Nanocrystals with Tunable Band Gaps

Perovskite nanocrystals have shown themselves to be useful for both absorption- and emission-based applications, including solar cells, photodetectors, and LEDs. Here we present a new class of size-, composition-, and shape-tunable nanocrystals made from Tl₃PbX₅ (X= Cl, Br, I). These can be synthesized via colloidal methods to produce faceted spheroidal nanocrystals, and perovskite TlPbI₃ nanowires. Crystal structures for the orthorhombic and tetragonal phase materials, for both pure and mixed halide species, are compared to the literature and also calculated from first-principles in VASP. We show the ability to tune the band gap by halide substitution to create materials that can absorb strongly between 250 and 450 nm. In addition, we show evidence of the confinement effect in pure halide Tl₃PbBr₅ nanocrystals suggesting size-tuning is possible as well. By tuning the band gap we can create materials with specific absorption spectra suitable for photodetection that display conduction and photoresponse properties similar to previously observed perovskite nanocrystals. We also observe weak emission consistent with indirect band-gap materials. Finally, we are able to demonstrate shape control in these materials, to give some insight into observable phase changes with varying reaction conditions, and to demonstrate the utility of the TlPbI₃ perovskite nanowires as wide-band-gap photoconductors. These novel perovskite nanocrystalline materials can be expected to find applications in photodetectors, X-ray detectors, and piezoelectrics

The Evolution of Quantum Confinement in CsPbBr₃ Perovskite Nanocrystals

Colloidal nanocrystals (NCs) of lead halide perovskites are considered highly promising materials that combine the exceptional optoelectronic properties of lead halide perovskites with tunability from quantum confinement. But can we assume that these materials are in the strong confinement regime? Here, we report an ultrafast transient absorption study of cubic CsPbBr₃ NCs as a function of size, compared with the bulk material. For NCs above ∼7 nm edge length, spectral signatures are similar to the bulk material–characterized by state-filling with uncorrelated charges–but discrete new kinetic components emerge at high fluence due to bimolecular recombination occurring in a discrete volume. Only for the smallest NCs (∼4 nm edge length) are strong quantum confinement effects manifest in TA spectral dynamics; focusing toward discrete energy states, enhanced bandgap renormalization energy, and departure from a Boltzmann statistical carrier cooling. At high fluence, we find that a hot-phonon bottleneck effect slows carrier cooling, but this appears to be intrinsic to the material, rather than size dependent. Overall, we find that the smallest NCs are understood in the framework of quantum confinement, however for the widely used NCs with edge lengths >7 nm the photophysics of bulk lead halide perovskites are a better point of reference