Ralph H. Young

E-Type Delayed Fluorescence of a Phosphine-Supported Cu2(μ-NAr2)2 Diamond Core: Harvesting Singlet and Triplet Excitons in OLEDs∥

A highly emissive bis(phosphine)diarylamido dinuclear copper(I) complex (quantum yield = 57%) was shown to exhibit E-type delayed fluorescence by variable temperature emission spectroscopy and photoluminescence decay measurement of doped vapor-deposited films. The lowest energy singlet and triplet excited states were assigned as charge transfer states on the basis of theoretical calculations and the small observed S(1)-T(1) energy gap. Vapor-deposited OLEDs doped with the complex in the emissive layer gave a maximum external quantum efficiency of 16.1%, demonstrating that triplet excitons can be harvested very efficiently through the delayed fluorescence channel. The function of the emissive dopant in OLEDs was further probed by several physical methods, including electrically detected EPR, cyclic voltammetry, and photoluminescence in the presence of applied current.

Nonradiative recombination centers and electrical aging of organic light-emitting diodes: Direct connection between accumulation of trapped charge and luminance loss

Organic light-emitting diodes (OLEDs) are attractive for display applications because of their high brightness, low driving voltage, and tunable color. Their operating lifetimes, hundreds or thousands of hours, are sufficient for only a limited range of applications. The luminance efficiency decreases gradually as the device is operated (electrically aged), for reasons that are poorly understood. A prototypical OLED has the structure anode|HTL|ETL|cathode, where the HTL and ETL are hole- and electron-transporting layers, and the recombination and emission occur at or near the HTL|ETL interface. We find that the decreasing luminance efficiency is linearly correlated with an accumulation of immobile positive charge at the HTL|ETL interface, and the magnitude of the charge is comparable to the total charge at that interface when an unaged device is operated. A natural explanation of the connection between the two phenomena is that electrical aging either generates hole traps (and trapped holes) or drives met...

Radiative and nonradiative electron transfer in contact radical-ion pairs

Abstract The relationship between radiative and nonradiative electron transfer is explored for return electron transfer processes in the contact radical-ion pairs formed by excitation of ground state CT complexes. Using a conventional nonadiabatic theory of electron transfer, absolute rate constants for nonradiative return electron transfer, varying over more than two orders of magnitude, can be predicted from information obtained from analyses of the corresponding radiative processes. The effects of solvent polarity, driving force and molecular dimension on the rates of nonradiative return electron transfer are studied.

High-efficiency, low-voltage phosphorescent organic light-emitting diode devices with mixed host

We report high-efficiency, low-voltage phosphorescent green and blue organic light-emitting diode (PHOLED) devices using mixed-host materials in the light-emitting layer (LEL) and various combinations of electron-injecting and electron-transporting layers. The low voltage does not rely on doping of the charge-transport layers. The mixed LEL architecture offers significantly improved efficiency and voltage compared to conventional PHOLEDs with neat hosts, in part by loosening the connection between the electrical band gap and the triplet energy. Bulk recombination in the LEL occurs within ∼10 nm of the interface with an electron-blocking layer. A “hole-blocking layer” need not have hole- or triplet-exciton-blocking properties. Optical microcavity effects on the spectrum and efficiency were used to locate the recombination zone. The effect of layer thickness on drive voltage was used to determine the voltage budget of a typical device. The behavior of undoped devices was investigated, and the electrolumines...

Quadrupole effects in the magic‐angle‐spinning spectra of spin‐1/2 nuclei

Magic‐angle spinning fails to eliminate the effect of dipolar coupling on the spectrum of a spin‐1/2 nucleus (e.g., 13C ) coupled to a quadrupolar nucleus (e.g., 14N). A quantitative theory of this phenomenon, based on an adiabatic approximation, is presented, together with numerical simulations of spectra. The spectrum is sensitive to the sign and magnitude of the electric‐field gradient at the quadrupolar nucleus, the angles between principal axes of the dipolar and quadrupolar interaction tensors, and the orientation of the spinning axis with respect to the external magnetic field.

Current-induced fluorescence quenching in organic light-emitting diodes

The electroluminescence quantum efficiency of organic light-emitting diodes with a doped Alq [tris(8-quinolinolato)aluminum] emitting layer is found to decrease markedly with increasing current density. This phenomenon was investigated using multilayer device structures permitting bipolar or unipolar carrier transport, and luminescence measurements with simultaneous optical and electrical excitation. The loss of electroluminescence quantum efficiency is attributed to the quenching of the singlet-excited state of the dopant by a cationic species.

Highly efficient fluorescent-phosphorescent triplet-harvesting hybrid organic light-emitting diodes

We demonstrate highly efficient white and nonwhite hybrid organic light-emitting diodes (OLEDs) in which singlet and triplet excited states, generated in the recombination zone, are utilized by fluorescence and phosphorescence, respectively. The excited states are formed at a blue fluorescent light-emitting layer (LEL), and the triplets diffuse through a spacer layer to one or more phosphorescent LEL(s). A key feature enabling the triplet diffusion in such OLEDs is the use of a blue fluorescent emitter with triplet energy above, or not much below, that of the fluorescent host. Additional material properties required for triplet harvesting are outlined. At 1000 cd/m2 a blue and yellow harvesting OLED shows 13.6% external quantum efficiency, 3.8 V, 30.1 lm/W, and color characteristics suitable for display application. High-efficiency harvesting R+G+B white, and B+G and B+R nonwhite OLEDs are also demonstrated. The triplet-harvesting mechanism was verified in all devices by physical methods including spectra...

Dipolar lattice model of disorder in random media analytical evaluation of the gaussian disorder model

Abstract One contribution to the distribution of site energies (DOS) in a random medium with localized charge-transport sites is the electrostatic potential of randomly placed and randomly oriented dipolar species nearby. This contribution has recently been modelled using a lattice on which a certain fraction (f) of the points is occupied by dipoles with random orientations. Simulations indicated that the DOS is close to a Gaussian distribution as long as f is not too small. This fact was taken to justify the application of the Gaussian disorder model (GDM) of charge transport in situations where dipolar contributions to the disorder are important. Analytical results on this model are presented here, including an expression for the r.m.s. width of the DOS that differs considerably from the previous result. While the central portion of the DOS is well approximated by a Gaussian distribution in many cases, the low- and high-energy tails generally deviate from this distribution. A simple principle is introdu...

Transient photocurrents across organic–organic interfaces

Hole photocurrent transients in organic–organic bilayers are described. Transitions in the photocurrents for holes moving across the organic–organic interfaces are observed. The magnitude of the photocurrent increases (or decreases) when holes transfer from a lower (or higher) mobility material into a higher (or lower) one. These results demonstrate a novel technique for studying energetic barriers and hole injection dynamics at organic–organic interfaces.

Nuclear spin-lattice relaxation via paramagnetic centers in solids. 13C NMR of diamonds

Abstract Diamonds of gem quality give narrow 13C NMR signals (about 200 Hz width at half height) even in a spectrometer designed for use with liquids. Industrial diamond powders require magic-angle spinning to give narrow NMR resonances. Spin-lattice relaxation in the industrial powders depends on time (t) exponentially in t 1 2 . A theory is developed to explain this behavior in terms of relaxation by paramagnetic centers in the absence of nuclear spin diffusion.