Jason D. Slinker

Solid-state electroluminescent devices based on transition metal complexes

Transition metal complexes have emerged as promising candidates for applications in solid-state electroluminescent devices. These materials serve as multifunctional chromophores, into which electrons and holes can be injected, migrate and recombine to produce light emission. Their device characteristics are dominated by the presence of mobile ions that redistribute under an applied field and assist charge injection. As a result, an efficiency of 10 lm/W--among the highest efficiencies reported in a single layer electroluminescent device--was recently demonstrated. In this article we review the history of electroluminescence in transition metal complexes and discuss the issues that need to be addressed for these materials to succeed in display and lighting applications.

DNA charge transport over 34 nm

Molecular wires show promise in nanoscale electronics, but the synthesis of uniform, long conductive molecules is a significant challenge. Deoxyribonucleic acid (DNA) of precise length, by contrast, is synthesized easily, but its conductivity over the distances required for nanoscale devices has not been explored. Here we demonstrate DNA charge transport (CT) over 34 nm in 100-mer monolayers on gold. Multiplexed gold electrodes modified with 100-mer DNA yield sizable electrochemical signals from a distal, covalent Nile Blue redox probe. Significant signal attenuation upon incorporation of a single base-pair mismatch demonstrates that CT is DNA-mediated. Efficient cleavage of these 100-mers by a restriction enzyme indicates that the DNA adopts a native conformation accessible to protein binding. Similar electron-transfer rates measured through 100-mer and 17-mer monolayers are consistent with rate-limiting electron tunnelling through the saturated carbon linker. This DNA-mediated CT distance of 34 nm surpasses that of most reports of molecular wires.

Orientation of pentacene films using surface alignment layers and its influence on thin-film transistor characteristics

We have investigated the effect of surface order on the orientation and mobility of pentacene. The surface order was created using monolayers and polymers that are normally used to align liquid crystals. Rubbed polyvinylalcohol layers were found to align approximately 27% of the pentacene grains within a 30° range. When introduced in a thin-film transistor, they were found to enhance the saturation current by a factor of 2.5. A mechanism for this enhancement is proposed.

Green electroluminescence from an ionic iridium complex

We report green emission from a single-layer device based on the ionic transition metal complex [Ir(F‐mppy)2(dtb‐bpy)]+(PF6−), where F-mppy is 2-(4′-fluorophenyl)-5-methylpyridine and dtb-bpy is 4,4′-di-tert-butyl-2,2′bipyridine. External quantum efficiencies of up to 1.1% are achieved with air-stable contacts, and up to 1.8% with a CsF∕Al top contact. Addition of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate was found to improve the device response time and cause a bias-dependent shift in the emission spectrum. As a result, electroluminescence was observed at 531 nm (CIE coordinates: 0.3230 and 0.5886), the lowest wavelength reported to date for a device based on ionic transition metal complexes.

Multiplexed DNA-Modified Electrodes

We report the use of silicon chips with 16 DNA-modified electrodes (DME chips) utilizing DNA-mediated charge transport for multiplexed detection of DNA and DNA-binding protein targets. Four DNA sequences were simultaneously distinguished on a single DME chip with 4-fold redundancy, including one incorporating a single base mismatch. These chips also enabled investigation of the sequence-specific activity of the restriction enzyme Alu1. DME chips supported dense DNA monolayer formation with high reproducibility, as confirmed by statistical comparison to commercially available rod electrodes. The working electrode areas on the chips were reduced to 10 microm in diameter, revealing microelectrode behavior that is beneficial for high sensitivity and rapid kinetic analysis. These results illustrate how DME chips facilitate sensitive and selective detection of DNA and DNA-binding protein targets in a robust and internally standardized multiplexed format.

Organic light-emitting devices with laminated top contacts

We demonstrate the fabrication of organic light-emitting devices based on a ruthenium complex with indium tin oxide anodes and laminated Au cathodes. Light emission was uniform over the whole device area, indicating a high-quality mechanical and electrical contact. The devices showed no rectification, indicating that the laminated contact was ohmic and caused no damage to the ruthenium complex. Comparison with devices using evaporated Au cathodes confirmed the quality of the lamination process.

Photophysical properties of tris(bipyridyl)ruthenium(II) thin films and devices

Absorption and luminescence spectra as well as luminescence lifetimes have been measured for Ru(bpy)2+3 in solution and in thin films of varying thicknesses, and these properties have been correlated with the efficiency of organic light emitting devices (OLEDs) made of the films. The lifetimes decrease for films below about 50 nm in thickness but are relatively constant for thicknesses above about 100 nm. This behavior is consistent with a model in which quenching is caused both by intrinsic properties of the molecules and by Forster energy transfer between chromophores that carries the excitation to surface layers, where the excitation is more efficiently quenched. The external quantum efficiency of the OLEDs is also found to increase with thickness, approaching 1% for thicknesses near 200 nm.

High stability light-emitting electrochemical cells from cationic iridium complexes with bulky 5,5′ substituents

We explore the photophysical, electrochemical, and electroluminescent properties of the ionic transition metal complex [(ppy)2Ir(bpy*)](PF6) where ppyH is 2-phenylpyridine and bpy* is 5,5′-diaryl-2,2′-bipyridine. Single layer devices of the structure ITO/[(ppy)2Ir(bpy*)](PF6)/Au exhibited high stability, with half-lives on the order of 100 h at a bias of −4 V. Long lifetimes are achieved through the bulky nature of the aryl substituents, which serves to limit chromophore–chromophore self-quenching, and 5,5′ positioning of these bulky groups is clearly advantageous for device performance.

Blue light emitting electrochemical cells incorporating triazole-based luminophores

We report the electrochemical, photoluminescence, and electroluminescence properties of four fluorinated cationic iridium complexes bearing pyridyltriazole ancillary ligands. All the complexes display unstructured emission in the true blue region at 298 K with photoluminescent λem ranging from 452 to 487 nm in acetonitrile solution, in powder and in PMMA doped thin films. The nature of the emission is a mixed metal-to-ligand/ligand-to-ligand charge transfer state. Photoluminescence (PL) quantum efficiencies both in solution and in the solid state were low while excited state decay kinetics were found to be multiexponential. Each complex undergoes quasi-reversible oxidation and irreversible reduction with large HOMO–LUMO gaps. A detailed computational investigation corroborates the spectroscopic assignments. Additionally, light-emitting electrochemical cells (LEECs) were fabricated for each of the four complexes. The electroluminescence (EL) spectra of all complexes were red-shifted relative to the PL spectra. The LEEC containing 2a is the bluest emitter (λmax = 487 nm) of the family of complexes.

Improving light-emitting electrochemical cells with ionic additives

Light-emitting electrochemical cells with ionic transition metal complexes (iTMCs) comprising the active layer have great potential for lighting applications, but to date high luminance and fast turn on times have been challenging to attain without significant loss of lifetime. Here, we demonstrate iridium iTMC devices with high luminances of 3000–5000 cd/m2, reduced turn on times, and long lifetimes through incorporation of ionic additives. Furthermore, we show additional reduction of turn on time to seconds through heat processing. We rationalize these results as the enhancement of carrier injection through improved double-layer formation at the electrodes.