Xiao-Chang Li

Bis(dithienothiophene) organic field-effect transistors with a high ON/OFF ratio

A conjugated, fused-thiophene oligomer, bis(dithienothiophene) (BDT), has been synthesized and deposited by vacuum sublimation as the active layer in organic thin film transistors (TFTs). The TFTs show exceptionally high ON/OFF ratios up to 108 between accumulation and depletion with sharp turn-on characteristics comparable to that of amorphous silicon TFTs (subthreshold slope S=0.6 V/decade). Field-effect mobilities are 0.02–0.05 cm2/V s. The good performance is explained by the relatively high π-π* gap of the short-chain BDT molecule and the favorable coplanar π-π stacking in BDT, differing from the herringbone stacking in the oligothiophenes.

Light-emitting diodes based on poly(methacrylates) with distyrylbenzene and oxadiazole side chains

Abstract We report investigations on the use of singly and doubly functionalised poly(methacrylates) (PMA) for the fabrication of polymeric light-emitting diodes (LEDs). The functional groups we used were distyrylbenzene (DSB) as blue chromophore and 2-phenyl-5-phenyl-1,3,4-oxadiazole (PPD) or 2-phenyl-5-biphenyl-1,3,4-oxadiazole (PBD) as charge-transport groups to produce three homopolymers (with one functional group only) and a copolymer bearing both DSB and PBD groups (PMA-DSB-PBD). The polymers have been used alone or blended together in single-layer devices or in double-layer devices, for which a thin poly( p -phenylene vinylene) (PPV) layer was used as a hole-transporting material. Despite the high photoluminescence yield of these DSB-based materials, successful fabrication of blue-emitting LEDs has only been achieved by using the copolymer, both in single- and double-layer devices.

A blue light emitting copolymer with charge transporting and photo-crosslinkable functional units

The synthesis of a new poly(methacrylate) copolymer bearing an efficient blue light emitting chromophore, a charge transporting aromatic oxadiazole and a UV-sensitive crosslinkable unit is reported. The statistical copolymer was synthesized via a multi-step route and has been characterised by NMR, FT-IR, and DSC. The copolymer has excellent solution processibility and exhibits strong luminescence. Upon cross-linking via UV-treatment, the copolymer becomes insoluble, while gaining both mechanical strength and thermal stability without loss of luminescence. The structure and properties of this novel copolymer and its application in light emitting devices are discussed.

The synthesis, optical and charge transport properties of poly(aromatic oxadiazole)s

In this work we report the synthesis and characterization of processable poly(aromatic oxadiazole)s. The polymers are rendered processable by either dissolving the polymer in strongly acidic organic solvent or by the thermal conversion of a soluble precursor polyhydrazide. The optical and charge transporting properties of the polymers have been investigated by UV-Vis absorption, photoluminescence spectroscopy and cyclic voltammetry. All the polymers have been found to exhibit strong purple-blue fluorescence and good hole-blocking and electron-injecting properties.

Preparation of bis(dithienothiophene) derivatives for organic thin film transistors

Abstract The synthesis of two α,α′-disubstituted bis(dithienothiophene) (BDT) derivatives is described. These have been deposited by vacuum sublimation as the active layers in organic thin film transistors (TFTs). The devices exhibit high ON/OFF ratios and preliminary studies show mobilities of 1 X 10−3 and 2 x 10−2 cm2 V−1 s−1 respectively for dithiohexyl (DTH-BDT) and dioctyl (DO-BDT) substituted bis(dithienothiophene). DTH-BDT is found to be considerably more soluble than the unsubstituted BDT.

Novel poly(arylene vinylene)s carrying donor and acceptor substituents

New electroluminescent poly(arylene vinylene)s 1 containing electron-withdrawing/donating substituents have been synthesized by the precursor route outlined below. The synthesis of two novel polymers and the evaluation of their photoluminescent and electroluminescent properties in polymer light-emitting diodes (LEDs) are reported.

Ionic space-charge assisted current injection in organic light emitting diodes

Abstract We have fabricated organic LEDs with mobile ions incorporated into the active polymer layer, similar in structure to the electrochemical cells (LECs) reported by Pei et al. We report characteristics for devices with ionic concentrations 17 cm −3 , which show rectifying characteristics We show that device behaviour can be understood solely in terms of an electrostatic model. The accumulation of ionic space-charge in the vicinity of the two electrodes is shown to permit easier injection of electronic carriers than is possible in a conventional LED. The device characteristics are strongly dependent on ionic concentration. Rectification is predicted at low ionic concentrations, switching to symmetrical behaviour at higher concentrations

Synthesis and optoelectronic properties of aromatic oxadiazole polymers

A one-pot polycondensation of 4,4′-(hexafluoroisopropylidene)bis(benzoic acid)1 with terephthalic dihydrazide 2 and/or hydrazine sulfate 3 affords aromatic oxadiazole polymers 4, 5 and 6, respectively, the optical and electrochemical properties of which have been investigated for applications as charge transporting layers in electroluminescent devices.

Chemical interactions at polymer interfaces: poly(p-xylylene-α-tetrahydrothiophene-bromide, chloride) on indium–tin-oxide

Abstract Chemical interactions, at the interface between a precursor polymer for poly( p -phenylenevinylene), namely poly( p -xylylene-α-tetrahydrothiophene-chloride), and an indium–tin-oxide transparent electrode, are identified using X-ray photoelectron spectroscopy. The HCl eliminated in the conversion process interacts with the surface of the indium–tin-oxide substrate leading to the formation of indium chloride which then diffuses into the polymer. It appears that indium–tin-oxide, or possibly the indium chloride generated at the interface, acts as a catalyst for the thermal elimination reaction. The results show unequivocally that interfacial interactions are of major importance in understanding, and ultimately improving, the performance of polymer-based electro-luminescent devices.

Precursor route chemistry and optoelectronic properties of poly(pyridine vinylene)

Several soluble precursor routes, involving sulfonium-, methoxy-, and chloro-substituted polymers, have been investigated for the synthesis of poly(pyridine vinylene)(PPyV). The thermal conversions of these precursors to fully conjugated polymer PPyV have been characterised by TGA, FT-IR and UV-VIS spectroscopy in conjunction with microanalysis. Of all the precursors, the chloro-substituted polymer is the most convenient for forming good quality thin films. PPyV fluoresces orange red and has a HOMO-LUMO band gap of 2.2 eV. PPyV has a lower luminescent quantum efficiency compared with its analogue, poly(phenylene vinylene) (PPV), but a higher electron affinity according to CV measurements (with reduction potential of -1.3 V). PPyV was used as an orange red emissive layer in a LED using aluminium as cathode.