Ebinazar B. Namdas

Highly efficient single-layer dendrimer light-emitting diodes with balanced charge transport

High-efficiency single-layer-solution-processed green light-emitting diodes based on a phosphorescent dendrimer are demonstrated. A peak external quantum efficiency of 10.4% (35 cd/A) was measured for a first generation fac-tris(2-phenylpyridine) iridium cored dendrimer when blended with 4,4′-bis(N-carbazolyl)biphenyl and electron transporting 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene at 8.1 V. A maximum power efficiency of 12.8 lm/W was measured also at 8.1 V and 550 cd/m2. These results indicate that, by simple blending of bipolar and electron-transporting molecules, highly efficient light-emitting diodes can be made employing a very simple device structure.

Triplet exciton diffusion in fac-tris(2-phenylpyridine) iridium(III)-cored electroluminescent dendrimers

We have studied triplet-triplet annihilation in neat films of electrophosphorescent fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)3]-cored dendrimers containing phenylene- and carbazole-based dendrons with 2-ethylhexyloxy surface groups using time-resolved photoluminescence. From measured annihilation rates, the limiting current densities above which annihilation would dominate in dendrimer light-emitting devices are found to be >1A∕cm2. The triplet exciton diffusion length varies in the range of 2–10 nm depending on the dendron size. The distance dependence of the nearest-neighbor hopping rate shows that energy transfer is dominated by the exchange mechanism.

High performance light emitting transistors

Solution processed light emitting field-effect transistors (LEFETs) with peak brightness exceeding 2500cd∕m2 and external quantum efficiency of 0.15% are demonstrated. The devices utilized a bilayer film comprising a hole transporting polymer, poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b] thiophene) and a light emitting polymer, Super Yellow, a polyphenylenevinylene derivative. The LEFETs were fabricated in the bottom gate architecture with top-contact Ca∕Ag as source/drain electrodes. Light emission was controlled by the gate voltage which controls the hole current. These results indicate that high brightness LEFETs can be made by using the bilayer film (hole transporting layer and a light emitting polymer).

Control of efficiency, brightness, and recombination zone in light-emitting field effect transistors.

The split-gate light emitting field effect transistors (SG-LEFETs) demonstrate a new strategy for ambipolar LEFETs to achieve high brightness and efficiency simultaneously. The SG architecture forces largest quantity of opposite charges on Gate 1 and Gate 2 area to meet in the center of the channel. By actively and independently controlling current injection from separated gate electrodes within transporting channel, high brightness can be obtained in the largest injection current regime with highest efficiency.

Tuning of emission color for blue dendrimer blend light-emitting diodes

We demonstrate efficient tunable blue electroluminescence from blends of two solution-processible light-emitting dendrimers. These materials can be blended to form optical quality thin films with no phase-separation effects, irrespective of the blend ratio. External quantum efficiencies of 1% have been measured for the blend systems and the emission color can be tuned from deep blue (emission peak 401nm) to blue green (477nm) by blend composition. A power efficiency of 1.5lm∕W (at 200Cd∕m2 and 5.4V) is measured for a single layer, first-generation blue-green fluorene-thiophene dendrimer. These results show that by choice of a dendrimer structure with common branching units and surface groups, dissimilar cores can be blended with excellent miscibility. This provides a simple way of tuning the color of organic light-emitting diodes.

Simultaneous enhancement of brightness, efficiency, and switching in RGB organic light emitting transistors.

An innovative design strategy for light emitting field effect transistors (LEFETs) to harvest higher luminance and switching is presented. The strategy uses a non-planar electrode geometry in tri-layer LEFETs for simultaneous enhancement of the key parameters of quantum efficiency, brightness, switching, and mobility across the RGB color gamut.

Solution-Processed n-Type Organic Field-Effect Transistors With High on / off Current Ratios Based on Fullerene Derivatives

Solution-processed n-type organic field-effect transistors (OFETs) based on the fullerene derivative {6}-1-(3-(2- thienylethoxycarbonyl)-propyl)-{5}-l-phenyl-[5,6]-C61 (TEPP) and phenyl-C61-butyric acid methyl ester (PCBM) in a multiring source/drain structure are reported. Devices with TEPP show high electron mobility up to 7.8 x 10-2 cm2/Vs in the saturation regime for bottom-contact OFETs with Au S/D electrodes with a solution-processed fullerene derivative. The ON/OFF ratios reported in this letter, which are in the range of 105 -106, are among the highest values reported for such devices. This mobility is always higher compared to PCBM devices prepared in identical conditions. The mobility of TEPP and PCBM increased with increasing temperatures in the range of 100-300 K with activation energy of 78 and 113 meV, respectively, which suggests that the thermally activated hopping of electrons is dominant in TEPP.

The synthesis and properties of solution processable red-emitting phosphorescent dendrimers

We report methodology for the preparation of symmetric and asymmetric solution processable phosphorescent dendrimers that are comprised of 2-ethylhexyloxy surface groups, biphenyl based dendrons, and iridium(III) complex cores. The symmetric dendrimer has three dendritic 2-benzo[b]thiophene-2′-ylpyridyl (BTP) ligands with the dendritic ligands responsible for red emission. The asymmetric dendrimer has two dendritic 2-phenylpyridyl ligands and one unsubstituted BTP ligand. Iridium(III) complexes comprised of 2-phenylpyridyl ligands are normally associated with green emission whereas those containing BTP ligands emit red light. Red emission is observed from the asymmetric dendrimer demonstrating that emission occurs primarily from the metal-to-ligand charge transfer state associated with the ligand with the lowest HOMO–LUMO energy gap. The photoluminescence quantum yields (PLQYs) of the symmetric and asymmetric dendrimers were strongly dependent on the dendrimer structure. In solution the PLQYs of the asymmetric and symmetric dendrimers were 47 ± 5% and 29 ± 3% respectively. The photoluminescence lifetime of the emissive state of both dendrimers in solution was 7.3 ± 0.1 µs. In the solid state the comparative PLQYs were reversed with the symmetric dendrimer having a PLQY of 10 ± 1% and the asymmetric dendrimer a PLQY of 7 ± 1%. The comparatively larger decrease in PLQY for the asymmetric dendrimer in the solid state is attributed to increased core–core interactions. The intermolecular interactions are greater in the asymmetric dendrimer because there is no dendron on the BTP ligand. Electrochemical analysis shows that charge is injected directly into the cores of the dendrimers.

Conjugated polyelectrolytes for organic light emitting transistors

We report on solution-processed light emitting field-effect transistors (LEFETs) that incorporate symmetric high work function (WF) source and drain metal electrodes. A key architectural design is the incorporation of a conjugated polyelectrolyte (CPE) electron injection layer atop the emissive layer. The device structure also comprises a hole-transporting layer underneath the emissive layer. Both holes and electrons are injected from stable, high WF metal though the CPE layer leading to electroluminescence near the electron-injecting electrode. With the benefits of the simplicity in device fabrication, the LEFETs incorporating CPEs are interesting structures for integrated organic optoelectronic devices.

All solution-processed, hybrid light emitting field-effect transistors

All solution-processed, high performance hybrid light emitting transistors (HLETs) are realized. Using a novel combination of device architecture and materials a bilayer device comprised of an inorganic and organic semiconducting layer is fabricated and the optoelectronic properties are presented.