9 research outputs found
Doubly stabilized perovskite nanocrystal luminescence downconverters
Halide perovskite nanocrystals (NCs) have emerged as a promising material for
applications ranging from light-emitting diodes (LEDs) to solar cells and
photodetectors. Still, several issues impede the realization of the
nanocrystals' full potential, most notably their susceptibility to degradation
from environmental stress. This work demonstrates highly stable perovskite
nanocrystals (NCs) with quantum yields as high as 95 % by exploiting a
ligand-assisted copolymer nanoreactor-based synthesis. The organic ligands
thereby serve a dual function by enhancing the uptake of precursors and
passivating the NCs. The polymer micelles and ligands thus form a double
protection system, shielding the encapsulated NCs from water-, heat- and
UV-light-induced degradation. We demonstrate the optoelectronic integrability
by incorporating the perovskite NCs as spectrally pure downconverters on top of
a deep-blue-emitting organic LED. These results establish a way of stabilizing
perovskite NCs for optoelectronics while retaining their excellent optical
properties
Porphyrin functionalization of CsPbBrI2/SiO2 core–shell nanocrystals enhances the stability and efficiency in electroluminescent devices
Zwitterionic carbazole ligands enhance the stability and performance of perovskite nanocrystals in light-emitting diodes
Zwitterionic Carbazole Ligands Enhance the Stability and Performance of Perovskite Nanocrystals in Light Emitting Diodes
We introduce a new carbazole-based zwitterionic ligand (DCzGPC) synthesized via Yamaguchi esterification which enhances the efficiency of lead halide perovskite (LHP) nanocrystals (NCs) in light emitting diodes (LED). A facile ligand exchange of the native ligand shell, monitored by nuclear magnetic resonance (NMR), ultraviolet-visible (UV-vis), and photoluminescence (PL) spectroscopy, enables more stable and efficient LHP NCs. The improved stability is demonstrated in solution and solid-state LEDs, where the NCs exhibit prolonged luminescence lifetimes and improved luminance, respectively. These results represent a promising strategy to enhance the stability of LHP NCs and to tune their optoelectronic properties for further application in LEDs or solar cells
What controls the orientation of TADF emitters?
Thermally-activated delayed fluorescence (TADF) emitters—just like phosphorescent ones—can in principle allow for 100% internal quantum efficiency of organic light-emitting diodes (OLEDs), because the initially formed electron-hole pairs in the non-emissive triplet state can be efficiently converted into emissive singlets by reverse intersystem crossing. However, as compared to phosphorescent emitter complexes with their bulky—often close to spherical—molecular structures, TADF emitters offer the advantage to align them such that their optical transition dipole moments (TDMs) lie preferentially in the film plane. In this report, we address the question which factors control the orientation of TADF emitters. Specifically, we discuss how guest-host interactions may be used to influence this parameter and propose an interplay of different factors being responsible. We infer that emitter orientation is mainly governed by the molecular shape of the TADF molecule itself and by the physical properties of the host—foremost, its glass transition temperature Tg and its tendency for alignment being expressed, e.g., as birefringence or the formation of a giant surface potential of the host. Electrostatic dipole-dipole interactions between host and emitter are not found to play an important role
Quantum efficiency enhancement of lead-halide Perovskite nanocrystal LEDs by organic lithium salt treatment
Surface-defect passivation is key to achieving high photoluminescence quantum
yield in lead halide perovskite nanocrystals. However, in perovskite
light-emitting diodes these surface ligands also have to enable balanced charge
injection into the nanocrystals to yield high efficiency and operational
lifetime. In this respect, alkaline halides have been reported to passivate
surface trap states and increase the overall stability of perovskite light
emitters. On the one side, the incorporation of alkaline ions into the lead
halide perovskite crystal structure is considered to counterbalance cation
vacancies, while, on the other side, the excess halides are believed to
stabilise the colloids. Here, we report an organic lithium salt, viz. LiTFSI,
as a halide-free surface passivation on perovskite nanocrystals. We show that
the treatment LiTFSI has multiple beneficial effects on lead halide perovskite
nanocrystals and LEDs derived from them. We obtain higher photoluminescence
quantum yield and longer exciton lifetime, and a radiation pattern that is more
favourable for light outcoupling. The ligand-induced dipoles on the nanocrystal
surface shift their energy levels toward lower hole-injection barrier. Overall,
these effects add up to a four- to seven-fold boost of the external quantum
efficiency in proof-of-concept LED structures, depending on the color of the
used lead halide perovskite nanocrystal emitters.Comment: 42 page
Corrigendum:What Controls the Orientation of TADF Emitters? (Front. Chem., (2020), 8, (750), 10.3389/fchem.2020.00750)
In the original article, there was a mistake in Supplementary Figure S8 of the Supplementary Material, and derived from that, in Table 1 as well as Figures 7A, 8A as published. The measured values of the GSP for two of the host materials, viz. BCPO and PO9, were interchanged by mistake. These values also resulted in a wrong calculation of the degree of PDMalignment (Λ). The corrected Table 1 as well as Figures 7A, 8A are attached below. (Figure presented.). The authors apologize for this error and state that this does not change the scientific conclusions of the article in any way. The original article and the Supporting Material have been updated.</p