OLED and PLED Devices Employing Electrogenerated, Intramolecular Charge‐Transfer Fluorescence

Abstract

The light generating mechanism of a series of light emitting diodes with electron donor-bridge-acceptor systems (D-b-A) as the emitting species was examined by constructing model diodes based on small organic molecules (OLEDs) as well as on molecularly doped electroactive (poly-N-vinylcarbazole, PVK) and insulating (polystyrene, PS) polymers (PLEDs). The direct electrogeneration of an intramolecular charge-transfer (CT) fluorescence of the donor-bridge-acceptor systems occurred readily in OLED devices with a D-b-A system as the emissive layer. In diodes with PS as the host matrix, hole-injection and electron-injection occurred directly in the D-b-A molecules residing close to the anode and the cathode, respectively. In the PVK diodes, hole-injection occurred primarily into PVK and the positive charge carrier was subsequently trapped on the D-b-A molecule, whereas electron-injection at the cathode side occurred directly into the D-b-A molecules. Charge-hopping between neighboring molecules then occurred until a hole and electron resided on the same molecule, which is equivalent to the formation of the CT excited state, and which finally relaxed by intramolecular charge recombination under the emission of CT fluorescence.

Parametric Studies of Pulsed Laser Deposition of Indium Tin Oxide and Ultra-thin Diamond-like Carbon for Organic Light-emitting Devices

“…The effect of highly insulating DLCUV as a buffer layer in our OLEDs is reflected in the lower VI-on and VL-on. In addition, the hole transporting capability in PVK may assist in the OLED operation where hole injection and in this case, tunneling occurs directly into the PVK layer [47]. The internal barrier height (IBH) [44] where DLC was inserted, is thus 0.9 eV for ITO and PVK interface.…”

High‐Performance Organic Light‐Emitting Diodes Based on Intramolecular Charge‐Transfer Emission from Donor–Acceptor Molecules: Significance of Electron‐ Donor Strength and Molecular Geometry

“…In addition, PVK probably acts as a buffer layer between PEDOT and BPQ-MPT, minimizing the quenching of the ICT fluorescence of BPQ-MPT by PEDOT, as has been previously reported for emissive D-A molecules. [12] Further improvements were achieved by incorporating a 20 nm thick TPBI layer as a hole-blocking layer, resulting in a maximum brightness of 31 370 cd m -2 and a maximum EQE of 5.78 % at a brightness of 1140 cd m -2 (6.3 V, 5.2 mA cm -2 ), with a luminous efficiency of 21.9 cd A -1 and power efficiency of 10.8 lm W -1 . Although higher brightness values have been reported for green OLEDs, these observed maximum luminous-and power-efficiency values are superior to most reported nondoped fluorescent green OLEDs, including recently reported green OLEDs based on tris(8-quinolato)aluminum (Alq 3 ) [2d,25a] , polyfluorene copolymers, [3a,b,25b] and even some doped Alq 3 OLEDs.…”

“…The emission from such D-A molecules, in principle, can originate from intramolecular charge-transfer (ICT) excited states formed by the annihilation reaction between the donor radical cations and the acceptor radical anions. [6,11,12] Thus, a judicious choice of the D and A units could facilitate simultaneous manipulation of the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels and the emission color of the D-A molecule.…”

“…[7][8][9][10][11][12] Systematic studies of the effects of factors such as the electron-donating strength of the donor moiety, the electron-accepting strength of the acceptor moiety, and the molecular geometry on the EL of D-A molecules have been lacking but are essential for a rational molecular design of next-generation emissive materials for OLEDs. We note that in the context of charge separation in photoelectrochemical and photovoltaic devices, extensive studies of such structural factors on electrontransfer processes in D-A molecules exist.…”

Photoinduced Charge-Transfer Association of Tetracyanoquinodimethane with Aminobiphenyls

“…[1][2][3][4] The charge-transfer systems in molecular optoelectronic devices such as molecular-based transistors, wires, and rectifiers, or in the doping of organic semiconducting polymers, are currently important issues in materials science. [5][6][7][8] For instance, 7,7',8,8'-tetracyanoquinodimethane (TCNQ) has been widely used as a strong electron acceptor to form highly conducting charge-transfer complexes. [9][10][11][12] Since the first report of the tetrathiafulvalene/TCNQ complex as an "organic metal" in 1973, a number of subsequent studies related to charge-transfer complexes have been reported.…”