Lihua Zhu

Chronocoulometric determination of doping levels of polythiophenes : influences of overoxidation and capacitive processes

Quasi-stationary determination of doping levels of polythiophenes has been successfully achieved in a wide potential window by using a modified potential-step chronocoulometric measurement of the charges consumed in oxidation and reduction processes of the polymers. Influences of experimental conditions, capacitive current-producing process and overoxidation process are discussed on the estimation of the doping levels of the polymers.

Enhanced hole injection in organic light-emitting diodes consisting of self-assembled monolayer of tripod-shaped π-conjugated thiols

Self-assembled monolayers (SAMs) of two tripod-shaped π-conjugated thiols and a disulfide have been grafted onto Au(111)/mica substrates. These SAMs exhibit oxidation and reduction peaks in the cyclic voltammograms arising from the electrochemical responses of the conjugated oligothiophene parts in the molecules. Organic light-emitting devices have been fabricated by using the SAM-coated Au electrode as the anode. In comparison with the device consisting of Au/TPD/Alq3/Mg–Ag, the electroluminescence performance of the device using the SAM grafted Au electrode is much improved for the SAM of the tripod-shaped thiols and worsened for the SAM of the disulfide. The different influences of the SAMs on the performances are explained by considering structures of the molecules and the SAMs, and the energy level alignment at the related interfaces in the devices.

A relationship between driving voltage and the highest occupied molecular orbital level of hole-transporting metallophthalocyanine layer for organic electroluminescence devices

Abstract Effects of metallophthalocyanines (MPcs) are systematically investigated on electroluminescence (EL) characteristics of ITO/MPc/TPD/Alq3/Mg–Ag devices. A linear relationship is found between driving voltages of the devices and energies of the highest occupied molecular orbital (HOMO) level of MPcs, showing that the driving voltage of the device is substantially determined by the energy barrier for hole injection at the ITO/MPc interface. When an MPc having a higher HOMO level, such as CuPc or ZnPc, is selected as a hole transport material, the driving voltage of the device at a luminance of 100 cd m−2 significantly reaches as low as 5.5 V compared with 7.8 V for the device without an MPc layer. The insertion of MPc layer leads to increasing of the EL output, but does not affect the EL efficiency of the devices.

Mobilities of charge carriers hopping between π-conjugated polymer chains

Mobilities of charge carriers in a copolymer film where a possible charge transport is restricted to an interchain path are measured by electrochemical means with doping level as a parameter. The mobility in the low doping regime, corresponding to that of polarons, is 10−6xa0cm2xa0V−1xa0s−1, comparable with polaron mobilities in polythiophene films where hopping transports via interchain and intrachain routes are both feasible. The coincidence suggests strongly a potential role of the interchain hopping transport in the conduction mechanism of common conducting polymers. As the doping level increases, the mobility falls down to 0.4xa0×xa010−6xa0cm2xa0V−1xa0s−1 and then rises up to 6xa0×xa010−6xa0cm2xa0V−1xa0s−1. nBased on spectroelectrochemical measurements of the copolymer film, the decrease of the mobility is interpreted in terms of the generation of cation radical π-dimers that are inactive in charge transport. The mobility increase in a high doping region is ascribed to some structural change of a polymer network, leading to enhancement of the interchain hopping probability.

Electrochemical generation of cation radical π-dimers in polymer film composed of pentathiophenes bridged by monosilanylene units

Electrochemical oxidation of poly[(ethoxypropylsilanylene)penta(2,5-thienylene)] films is investigated in acetonitrile containing tetraethylammonium perchlorate. A cyclic voltammogram of the polymer film exhibits two pairs of redox waves with oxidation/reduction peak potentials at 0.64/0.46 and 0.92/0.77 V. Measurements of mass changes on the polymer electrode using a quartz crystal microbalance indicate that during the first redox process a doping/dedoping of perchlorate ions takes place in a reversible manner. The maximum doped charge in the potential range up to 0.7 V compares well with a theoretical value for the one-electron oxidation of the pentathiophene unit acting as an independent redox site. A spectroelectrochemical study reveals that two sorts of chemical species are generated in the first oxidation step: one is a cation radical (polaron) with absorption band maxima at 0.95 and 1.68 eV and the other a π-dimer with band maxima at 1.23 and 2.05 eV. The π-dimer becomes dominant as the applied potential increases. The second oxidation step, resulting in a dicationic state of the pentathiophene unit, is found to be accompanied by degradation of the polymer.

Electrochemistry and spectroelectrochemistry of poly[(tetraethyldisilanylene)quinque(2,5-thienylene)]

Electrochemistry of poly[(tetraethyldisilanylene)quinque(2,5-thienylene)] (DS5T) has been investigated in acetonitrile with cyclic voltammetry, potentiostatic oxidation and spectroelectrochemical measurements. In multicycle voltammograms of a DS5T film there are a pair of redox peaks attributable to anion-doping and dedoping. During potential cycling or electrolyzing at certain positive potentials, the polymer is subject to decomposition to some extent. An irreversible oxidation subpeak for the decomposition of the polymer is observed in the first-cycle voltammogram with low scan rates. Spectroelectrochemical measurements demonstrate that the slow decomposition process tends to be inhibited by further oxidation of its decomposed product, followed by redepositing of a solid film onto the DS5T film. The further oxidation of the decomposed product also leads to some unexpected observations in the spectroelectrochemistry of DS5T. Moreover, if the forward potential scanning is returned at more positive potentials, two pairs of oxidation/reduction peaks occur in the cyclic voltammogram of DS5T and the polymer film becomes much more unstable at the potential of the second oxidation peak.

Electrochemical cleavage of a Si-Si bond in poly[(tetraethyldisilanylene)oligo(2,5-thienylene)] films

Abstract Electrochemical stability of poly[(tetraethyldisilanylene)oligo(2,5-thienylene)] (DS m T; m refers to the number of thienylene units) films is investigated by means of in situ UV–vis–NIR spectroscopy, in situ and ex situ fluorescence spectroscopy, gel-permeation chromatography, and FT-IR spectroscopy. It is found that a Si–Si bond in DSmT film with m =3 to 5 is cleaved at potentials as low as 0.5 V versus Ag/Ag + in acetonitrile, resulting in dissolution of oligothiophene-like species. The decomposed products are oxidized to form another polymer film on the surface of an original DS m T film and a doping reaction observed earlier takes place on the composite polymer film.

Metallophthalocyanine films as hole-transport layer in organic light-emitting devices

Abstract Electroluminescence (EL) characteristics of the double-MPc-layered devices consisting of ITO/MPc1/MPc2/Alq 3 /Mg–Ag were investigated, where MPc1 and MPc2 represent two metallophthalocyanines and Alq 3 stands for tris(8-quinolinolato)aluminum(III). A compatible double-MPc-layered structure led to a decreased hole-injection barrier at the ITO/MPc1 interface and an increased electron-blocking ability of the MPc2 layer in the device. The EL efficiency of the device was improved further by using a 4,4′-bis( m -tolyphenylamino) biphenyl (TPD) layer as an additional hole-transport layer due to its good electron-blocking ability. For the double-MPc-layered devices with a TPD layer, the EL performances were dependent primarily on the electronic structure of ITO/MPc1 interface rather than that of MPc1/MPc2 or MPc2/TPD interface.

Photophysical properties of σ–π-conjugated alternating oligothienylene–oligosilylene polymers

UV-visible absorption and fluorescence properties of three series of σ–π-conjugated polymers (copolymers of alternative oligothienylene and oligosilylene units) have been studied in dioxane solution. The energies of the absorption maximum, fluorescence maximum, and the 0–0 transition are found to be linearly dependent on the reciprocal of the number of thiophene rings in the repeating unit of the polymer chain, but almost independent of the silicon atom number. The σ–π-conjugation in the polymers results in red shift in the absorption and fluorescence maxima, higher fluorescence quantum yields, and longer fluorescence lifetimes of the polymers, with respect to their corresponding analogous α-oligothiophenes.

Electrochemical oxidation of poly[(hexamethyltrisilanylene)oligo(2,5-thienylene)] films

Abstract Electrochemistry of σ–π-conjugated polymers, poly[(hexamethyltrisilanylene)oligo(2,5-thienylene)] has been investigated in acetonitrile by using cyclic voltammetric and spectroelectrochemical techniques. Cyclic voltammograms of these polymers films display generally two pairs of main redox peaks, indicating the doping and dedoping processes of the polymers. The electrochemical oxidation of the polymer films results in their partial decomposition, besides the doping, which is shown by electrochemical quartz crystal microbalance and spectroelectrochemical measurements. The decomposition products are characterized by UV-visible absorption spectroscopy, fluorescence spectroscopy, FT-IR, and GPC measurements. It is concluded that the decomposition originates mainly from electrochemical cleavage of silicon–silicon bonds in the polymer chains, and the electrochemical stability of the σ–π-conjugated polymers may be improved by decreasing the oligosilanylene block size in the polymer chains.