Hon Hang Fong

Tetrathienoacene Copolymers As High Mobility, Soluble Organic Semiconductors

Increasing the rigidity of the thiophene monomer through the use of an alkyl-substituted core that consists of four fused thiophene rings is shown to be a promising route toward high-performance organic semiconductors. We report on a dialkylated tetrathienoacene copolymer that can be deposited from solution to yield ordered films with a short pi-pi distance of 3.76 A and with a field-effect hole mobility that exceeds 0.3 cm2/V.s. This polymer enables simple transistor fabrication at relatively low temperatures, which is particularly important for the realization of large-area, mechanically flexible electronics.

Alkylsubstituted Thienothiophene Semiconducting Materials: Structure−Property Relationships

A family of conjugated polymers with fused structures consisting of three to five thiophene rings and with the same alkyl side chains has been synthesized as a means to understand structure-property relationships. All three polymers showed well-extended conjugation through the polymer backbone. Ionization potentials (IP) ranged from 5.15 to 5.21 eV; these large values are indicative of their excellent oxidative stability. X-ray diffraction and AFM studies suggest that the polymer with the even number of fused thiophene rings forms a tight crystalline structure due to its tilted side chain arrangement. On the other hand, the polymers with the odd number of fused thiophene rings packed more loosely. Characterization in a field-effect transistor configuration showed that the mobility of the polymer with the even number of rings is 1 order of magnitude higher than its odd-numbered counterparts. Through this structure-property study, we demonstrate that proper design of the molecules and properly arranged side chain positions on the polymer backbone can greatly enhance polymer electronic properties.

Hole transports in molecularly doped triphenylamine derivative

Hole mobilities of N 0 ; N 0 -bis(3-methylphenyl)-(1; 1 0 -biphenyl)-4; 4 0 -diamine (TPD), TPD doped with 5, 6, 11, 12-tetraphenylnathacene (rubrene), and TPD doped with 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryle) 4H-pyran (DCM1) were measured by a time-of-flight (TOF) technique between 200 and 300 K. Pristine TPD has a hole mobility of about 2 � 10 � 3 cm 2 V � 1 s � 1 at room temperature. The corresponding TOF time transients were non-dispersive. Upon doping, the hole mobility decreased slightly for rubrene-doped TPD, and sharply for DCM1-doped TPD. The TOF time transients remain non-dispersive for both kind of doped samples. The dependence of the mobility on electric field and temperature for undoped and doped TPD was investigated. 2002 Elsevier Science B.V. All rights reserved.

Electron transit time and reliable mobility measurements from thick film hydroxyquinoline-based organic light-emitting diode

The time delay (τd) in the transient electroluminescence (EL) signal of a bilayer organic light-emitting diode with a structure of indium-tin oxide /N,N′-diphenyl-N,N′-bis(3methylphenyl)-(1,1′-biphenyl)-4,4′-diamine /tris(8-hydroxyquinoline) aluminum (Alq3)/Al has been measured and analyzed as a function of the thickness (D) of the Alq3 layer. For a thin layer of Alq3 (D 200 nm), τd approaches the intrinsic electron transit time through Alq3. Electron mobility of Alq3 can be evaluated for the thick-film devices and the results are in excellent agreement with independent time-of-flight measurements. The application of transient EL in mobility measurement for C540-doped Alq3 is discussed.

Acid-sensitive semiperfluoroalkyl resorcinarene: an imaging material for organic electronics.

An acid-sensitive semiperfluoroalkyl resorcinarene was synthesized, and its lithographic properties were evaluated. Its solubility in segregated hydrofluoroether solvents enables the patterning of delicate organic electronic materials.

Orthogonal processing: A new strategy for organic electronics

The concept of chemical orthogonality has long been practiced in the field of inorganic semiconductor fabrication, where it is necessary to deposit and remove a layer of photoresist without damaging the underlying layers. However, these processes involving light sensitive polymers often damage organic materials, preventing the use of photolithography to pattern organic electronic devices. In this article we show that new photoresist materials that are orthogonal to organics allow the fabrication of complex devices, such as hybrid organic/inorganic circuitry and full-colour organic displays. The examples demonstrate that properly designed photoresists enable the fabrication of organic electronic devices using existing infrastructure.

Hole transporting properties of tris(8-hydroxyquinoline) aluminum (Alq3)

The hole transporting properties of tris (8-hydroxyquinoline) aluminum (Alq3) were investigated by time-of-flight (TOF) technique between 278 and 373K, and under an applied field range of 0.6–1.3MV∕cm. At room temperature, the hole mobility has a value between 10−9 and 10−8cm2V−1s−1. The hole mobility is at least two orders of magnitude less than electron under identical preparation and measurement conditions. Generally, all hole TOF transients of Alq3 exhibit a nondispersive behavior, with a clear plateau region and a dispersion tail. Two disorder transport models, namely, the Gaussian disorder model (GDM) and the correlated disorder model (CDM), were applied to analyze the temperature and field dependent hole mobility data. The GDM, however, is found to be invalid because it fails to produce a meaningful positional disorder parameter. The CDM gives a better fit to the data, yet the model is still not satisfactory.

Organic light-emitting diodes based on a cohost electron transporting composite

The efficiency of green organic electroluminescent devices have been improved by cohosting the electron dominant complex, 4,7-diphenyl-1,10-phenanthroline into the traditional electron transporting layer of tris (8-hydroxyquinoline) aluminum. In this cohost strategy, we demonstrate that the luminous efficiency is enhanced by >20% while the driving voltage can be reduced by ∼30% in a uniformly mixed composition as compared to the traditional device configuration. The corresponding device lifetime under atmospheric condition is extended by a factor of ∼1.8, attributed to the reduction of the accumulated positive charges near the electron-hole recombination regime. Results indicate that the knowledge of bulk conductivity engineering of organic n-type transporters is essential in enhancing organic light-emitting devices.

Dry photolithographic patterning process for organic electronic devices using supercritical carbon dioxide as a solvent

The particular challenge of micropatterning organic materials has stimulated numerous approaches for making effective and repeatable patterned structures with fine features. Among all the micropatterning techniques photolithography, being the preferred method for the inorganic semiconductor industry, did not create much impact due to its incompatibility with the majority of organic electronic materials. Here we introduce a novel, chemically benign approach to dry photolithographic patterning of organic materials using super-critical carbon dioxide (scCO2) as a green developing solvent. We illustrate the possible applications of the new technique by patterning conducting polymers and light emitting polymers for organic light emitting diodes.

Nondispersive hole transport in a polyfluorene copolymer with a mobility of 0.01cm2V−1s−1

The hole mobility in the fluorene copolymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)) diphenylamine)] (TFB) was measured using the time-of-flight technique. Transport was found to be nondispersive throughout the temperature range between 220 and 350K, indicating the absence of intrinsic traps in this material. At room temperature, TFB shows a hole mobility of 0.01cm2V−1s−1, with a weak field dependence. The hole mobility is independent of sample thickness in the range between 0.9 and 6.4μm. These results are in agreement with a narrow transport manifold, with a width of 65.9±0.5meV.