Yunhui Zhu

Bright white electroluminescence from a single polymer containing a thermally activated delayed fluorescence unit and a solution-processed orange OLED approaching 20% external quantum efficiency

Developing a high-performance orange/red thermally activated delayed fluorescence (TADF) emitter, which is of great importance for white light emission and full-color displays, is still challenging due to the large non-radiative internal conversion rates governed by the energy-gap law in the purely organic emissive materials. In this study, a series of conjugated polymers with a backbone-donor/pendant-acceptor architecture was designed and synthesized, in which a twisted donor/acceptor structural unit was covalently bonded into the backbone via its donor fragment. The polymers excellently inherited the intrinsic features of the TADF small molecule used as a monomer in copolymerization. The non-doped organic light-emitting diodes (OLEDs) exhibited white emission, with CIE coordinates of (0.32, 0.31), by balancing the emission from the backbone and the TADF unit. Moreover, a near 10% external quantum efficiency (EQE) with warm white emission and an EQE of up to 19.4% with bright orange emission were achieved in the doped TADF polymer-based devices with controlled content of the TADF unit.

Mechanism of the electro-optic effect in the perovskite-type ferroelectric KNbO3 and LiNbO3

Based on the theoretical model published by Chen et al. [Chem. Phys. Lett. 397, 222 (2004)], we present an efficient scheme for studying the mechanism of the electro-optic (EO) effect in perovskite-type ferroelectrics employing the electronic density-functional theory and the generalized gradient approximation. The results show that the main contribution to the EO effect in KNbO3 and LiNbO3 crystals comes from the relative displacements of the central metal ion and oxygen in the NbO6 oxygen octahedron induced by a low frequency applied electric field. These displacements change the band wave functions of the electrons in the crystals through electron-phonon coupling, which then affects the electronic response of the lattice to the incident optical wave (i.e., induces a change of refractive index). Furthermore, the relationship between the microstructure of perovskite-type ferroelectrics and the clamped EO effect and the physical meaning of the parameter βk(ij) published by Chen et al. are explained.

Defect-induced charge-order melting in thin films of Pr0.5Ca0.5MnO3

We have investigated the relation between defect structure and charge order melting in thin films of epitaxial Pr0.5Ca0.5MnO3 (PCMO), grown under strain on SrTiO3. We compared the behavior of an 80 nm film grown in one deposition step at 840 degrees C with the behavior of a film grown in two steps. In the two-step case, a thin PCMO layer of 10 nm was deposited at 120 degrees C, followed by 70 nm deposited at 840 degrees C. The increase of the growth temperature leads to complete crystallization of the first layer and the lattice constants of the two-step grown film indicate that tensile strain is still present. On the other hand, a magnetic field of only 5 T is required to melt the charge-order state in the two-step grown film, which is a much lower than the value for the normally grown film. This appears to be connected to a larger amount of threading dislocations present in the first (recrystallized) layer. (c) 2007 American Institute of Physics.

Efficient non-doped yellow OLEDs based on thermally activated delayed fluorescence conjugated polymers with an acridine/carbazole donor backbone and triphenyltriazine acceptor pendant

Polymers with thermally activated delayed fluorescence (TADF) are very suitable and promising emitters for use in solution-processed organic light-emitting diodes (OLEDs). However, only a few efficient TADF polymers have been reported to date. In this work, a series of polymers with acridine/carbazole donor backbones and triphenyltriazine acceptor pendant are synthesized. The polymers show remarkable TADF due to the intrinsic twisted donor/acceptor structure derived from the copolymerization monomer and have a high photoluminescence quantum yield of up to 90%, in neat films, through management of the monomer ratios. Non-doped OLEDs based on the polymers display a yellow emission and achieve maximum external quantum and power efficiencies of up to 15.5% and 50.5 lm W−1. More importantly, the non-doped devices possess low turn-on and driving voltages of 2.53 V and 4.05 V (1000 cd m−2) with an extremely small efficiency roll-off of as low as 0.5%. Compared with doped devices, non-doped devices exhibit superior electroluminescent performances, with similar efficiencies but a lower efficiency roll-off and driving voltage, verifying that the designed conjugated polymers are superior emitters for non-doped TADF OLEDs by simple solution-processing.

Highly Efficient TADF Polymer Electroluminescence with Reduced Efficiency Roll-off via Interfacial Exciplex Host Strategy

Solution-processed organic light-emitting diodes (s-OLED) consisting of TAPC/TmPyPB interfacial exciplex host and polymer PAPTC TADF emitter are prepared, simultaneously displaying ultralow voltages (2.50/2.91/3.51/4.91 V at luminance of 1/100/1000/1000 cd m-2), high efficiencies (14.9%, 50.1 lm W-1), and extremely low roll-off rates (J50 of 63.16 mA cm-2, L50 of ca. 15000 cd m-2). Such performance is distinctly higher than that of pure-PAPTC s-OLED. Compared to pure-PAPTC, the advanced emissive layer structure of TAPC:PAPTC/TmPyPB is unique in much higher PL quantum yield (79.5 vs 36.3%) and nearly 4-fold enhancement in kRISC of the PAPTC emitter to 1.48 × 107 s-1.

Photorefractive parameters and light-induced absorption in BaTiO3

The light-induced absorption coefficient Δα and the light-intensity dependence of the photorefractive parameters known as the effective trap density NE, the mobility-recombination product μτr and the photoconductivity σph have been measured for four nominally pure BaTiO3 crystals. The qualitative correlation between Δα, NE, Δα,NE, (μτr)−1 and σph has been revealed. The results are explained by the two-center model of deep and shallow traps.

Erasure of photorefractive gratings in barium titanate

An analytical solution of the light‐induced decay of the photorefractive gratings from the charge transport equations with shallow and deep traps is presented. It is predicted that the light‐induced decay curve is biexponential, and a superposition of the shallow‐ and deep‐trap decays in the special case is suggested. The prediction is verified in the light‐induced photorefractive grating erasure experiments. The erasing intensity dependence of photoconductivity of the BaTiO3 sample is also presented.

Magnetization reversal and coercivity of Fe3Se4 nanowire arrays

The microstructure and magnetic properties of Fe3Se4 nanowire (NW) arrays in anodic aluminum oxide (AAO) porous membrane are studied. Cross-sectional SEM and plane-view TEM images show that the mean wire diameter (dw) and the center-to-center spacing (D) of Fe3Se4 nanowires are about 220 nm and 330 nm, respectively. The field-cooled magnetization dependent on the temperature indicates a Curie temperature around 334 K for the Fe3Se4 nanowires. The coercivities of Fe3Se4 nanowires at 10 K, obtained from the in-plane and out-of-plane hysteresis loops, are as high as 22.4 kOe and 23.3 kOe, which can be understood from the magnetocrystalline anisotropy and the magnetization reversal process.