Shih-Chun Lo

Green phosphorescent dendrimer for light-emitting diodes

Highly efficient organic LEDs made by solution processing are reported. It is shown that the dendritic architecture (see Figure) can be used to solubilize luminescent chromophores and form uniform films of blends. The simple device structures containing a light-emitting chromophore are amongst the most efficient solution-processed devices reported. Thanks to this technique, the inkjet printing of phosphorescent materials becomes feasible.

High-efficiency green phosphorescence from spin-coated single-layer dendrimer light-emitting diodes

We demonstrate very high-efficiency green phosphorescence from a single-layer dendrimer organic light-emitting diode formed by spin-coating. A first generation fac-tris(2-phenylpyridine) iridium cored dendrimer doped into a wide-gap 4,4′-bis(N-carbazole) biphenyl host displays a peak external quantum efficiency of 8.1% (28 Cd/A) at a brightness of 3450 Cd/m2 and a current density of 13.1 mA/cm2. A peak power efficiency of 6.9 lm/W was measured at 1475 Cd/m2 and 5 mA/cm2. We attribute this exceptionally high quantum efficiency for a single-layer device to the excellent film forming properties and high photoluminescence quantum yield of the dendrimer blend and efficient injection of charge into the emissive layer. These results suggest that dendrimers are an effective method for producing efficient phosphorescent devices by spin-coating.

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.

Charge transport in highly efficient iridium cored electrophosphorescent dendrimers

Electrophosphorescent dendrimers are promising materials for highly efficient light-emitting diodes. They consist of a phosphorescent core onto which dendritic groups are attached. Here, we present an investigation into the optical and electronic properties of highly efficient phosphorescent dendrimers. The effect of dendrimer structure on charge transport and optical properties is studied using temperature-dependent charge-generation-layer time-of-flight measurements and current voltage (I-V) analysis. A model is used to explain trends seen in the I-V characteristics. We demonstrate that fine tuning the mobility by chemical structure is possible in these dendrimers and show that this can lead to highly efficient bilayer dendrimer light-emitting diodes with neat emissive layers. Power efficiencies of 20 lm/W were measured for devices containing a second-generation (G2) Ir(ppy)(3) dendrimer with a 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene electron transport layer

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.

A rapid route to carbazole containing dendrons and phosphorescent dendrimers

A convergent strategy for the synthesis of three generations of dendrons comprised of carbazole moieties is described. The procedure to build the dendrons involves an iterative palladium catalysed amination–debenzylation sequence using N-benzyl-3,6-dibromocarbazole. The three carbazolyl focussed dendrons are then attached to a reactive fac-tris[2-phenylpyridyl]iridium(III) core by a palladium catalysed amination to give the dendrimers. The three generations of dendrons have one, three, and seven carbazole units leading to dendrimers with fac-tris[2-phenylpyridyl]iridium(III) cores and three, nine and twenty one carbazole units. The use of 9,9′-dialkylfluorenyl surface groups gave the dendrimers excellent solubility. The attachment of the carbazolyl-based dendrons did not change the emission colour significantly with the dendrimers emitting green phosphorescence. The dendrimers were highly luminescent with solution photoluminescence quantum yields of the order of 70%. Ground state molecular orbital calculations showed that while the “LUMO” was concentrated on the core iridium(III) complex the “HOMO” was delocalised across the core and each of the dendrons. This was reflected in the oxidation properties of the dendrimers whereby the increased carbazolyl character of the “HOMO” resulted in the first oxidation being moved to more positive potentials.

Effects of Fluorination on Iridium(III) Complex Phosphorescence: Magnetic Circular Dichroism and Relativistic Time-Dependent Density Functional Theory

We use a combination of low temperature, high field magnetic circular dichroism, absorption, and emission spectroscopy with relativistic time-dependent density functional calculations to reveal a subtle interplay between the effects of chemical substitution and spin-orbit coupling (SOC) in a family of iridium(III) complexes. Fluorination at the ortho and para positions of the phenyl group of fac-tris(1-methyl-5-phenyl-3-n-propyl-[1,2,4]triazolyl)iridium(III) cause changes that are independent of whether the other position is fluorinated or protonated. This is demonstrated by a simple linear relationship found for a range of measured and calculated properties of these complexes. Further, we show that the phosphorescent radiative rate, k(r), is determined by the degree to which SOC is able to hybridize T(1) to S(3) and that k(r) is proportional to the inverse fourth power of the energy gap between these excitations. We show that fluorination in the para position leads to a much larger increase of the energy gap than fluorination at the ortho position. Theory is used to trace this back to the fact that fluorination at the para position increases the difference in electron density between the phenyl and triazolyl groups, which distorts the complex further from octahedral symmetry, and increases the energy separation between the highest occupied molecular orbital (HOMO) and the HOMO-1. This provides a new design criterion for phosphorescent iridium(III) complexes for organic optoelectronic applications. In contrast, the nonradiative rate is greatly enhanced by fluorination at the ortho position. This may be connected to a significant redistribution of spectral weight. We also show that the lowest energy excitation, 1A, has almost no oscillator strength; therefore, the second lowest excitation, 2E, is the dominant emissive state at room temperature. Nevertheless the mirror image rule between absorption and emission is obeyed, as 2E is responsible for both absorption and emission at all but very low (<10 K) temperatures.

Phosphorescent Light-Emitting Transistors: Harvesting Triplet Excitons

Phosphorescent light-emitting transistors, in which light emission from singlet and triplet energy levels is harvested using solution-processed materials, are presented. While a green phosphorescent dendrimer exhibits an external quantum efficiency of 0.45% at 480 cd m(-2) , a red polymer/phosphorescent small-molecule blend produces a brightness exceeding 30 cd m(-2) with a relatively high hole mobility of 2.5 × 10(-2)  cm(2) V(-1) s(-1) .

High‐Performance, Fullerene‐Free Organic Photodiodes Based on a Solution‐Processable Indigo

A solution-processable dibromoindigo with an alkyoxyphenyl solubilizing group is developed and used as a new electron acceptor in organic photodiodes. The solution-processed fullerene-free organic photodiodes show an almost spectrally flat response with a high responsivity (0.4 A W(-1)) and a high detectivity (1 × 10(12) Jones). These values are comparable to silicon-based photodiodes.