D.D.C Bradley

Mobility enhancement in conjugated polymer field-effect transistors through chain alignment in a liquid-crystalline phase

A method is demonstrated by which liquid-crystalline self-organization in rigid-rod nematic conjugated polymers can be used to control the microstructure of the active semiconducting layer in solution-processed polymer thin-film transistors (TFTs). Enhanced charge carrier mobilities of 0.01–0.02 cm2/V s and good operating stability have been achieved in polyfluorene copolymer TFTs by preparing the polymer in a nematic glassy state and by aligning the polymer chains parallel to the transport direction with the help of an alignment layer. Mobility anisotropies of 5–8 for current flow parallel and perpendicular to the alignment direction have been observed that are of the same order of magnitude as optical dichroic ratios.

Highly polarized blue electroluminescence from homogeneously aligned films of poly(9,9-dioctylfluorene)

Homogeneous alignment of poly(9,9-dioctylfluorene) films on thin layers of rubbed precursor-route poly(p-phenylenevinylene) allows the construction of light-emitting diodes that emit highly polarized blue light (λem=458 nm). The rubbed poly(p-phenylenevinylene) acts as an effective hole-injecting alignment layer. Annealing of poly(9,9-dioctylfluorene) in its nematic phase followed by rapid quenching orients the polymer as a glassy monodomain on the alignment layer and gives devices with a polarization ratio of 25:1 and a luminance of up to 250 cd/m2.

Influence of film morphology on the vibrational spectra of dioctyl substituted polyfluorene (PFO)

Raman spectroscopy has been performed on samples of dioctyl substituted polyfluorene (PFO) with four morphologies: (i) as spin-coated, (ii) crystalline, (iii) glassy, (iv) prepared so as to contain a fraction of material with an intra-chain ordered conformation. The different morphologies were thermally induced in films spin-coated from solutions of PFO in tetrahydrofuran, chloroform and cyclohexane on aluminium, silicon and quartz substrates. There are no differences in the Raman spectra measurement for the glassy and spin-coated samples. The spectra of the samples with the intra-chain ordered state and in the crystalline phase show shifts in positions and linewidths of the Raman peaks. The vibrational modes are identified by reference to published spectra for both model compounds and related phenylene-based polymers.

Deep level transient spectroscopy (DLTS) of a poly(p-phenylene vinylene) Schottky diode

Abstract Deep level transient spectroscopy measurements have been carried out on ITO/poly( p -phenylenevinylene)/Al organic light emitting diodes that have a depletion region type Schottky barrier at the polymer/metal interface. The very long lived capacitance transients can be successfully described by the de-trapping of p-type majority carriers from a single energy trap level to a Gaussian distribution of transport states. The Gaussian width of 0.10±0.02 eV and trap depth of 0.75±0.05 eV are in excellent agreement with values measured from other unrelated experimental techniques.

Transient capacitance measurements of the transport and trap states distributions in a conjugated polymer

Abstract We have performed transient capacitance measurements on indium tin oxide/poly(p-phenylenevinylene)/Al diodes which have a depletion region type Schottky barrier at the polymer/Al interface. We show that both the transport and trap state distributions within the polymer can be determined from the results and present a fully consistent model that describes the observed behaviour in terms of a de-trapping of carriers from a discrete trap level to a Gaussian distribution of transport states.

High brightness conjugated polymer LEDs

Abstract We report on light emitting diodes (LEDs) using a green emission polymer system developed by Dow Chemical. The LEDs were tested with both AC and DC drive schemes. Optimised devices have a CW brightness in excess of 25,000 cd/m2 and a peak pulsed brightness exceeding 2,000,000 cd/m2. We describe the fabrication and processing steps required to reach this performance and highlight prospects for further development.

Intrachain ordered polyfluorene

From photophysical evidence, we suggest a structural model based on intrachain ordering that can account for the thermoreversible changes of the absorption spectrum of poly(9,9-dioctylfluorene) films under certain physicochemical treatment protocols. We correlate this model to the results of X-ray fibre diffraction experiments.

Optical studies of photoexcitations of poly(9,9-dioctyl fluorene)

We have studied the interplay between the morphology and photophysics of poly(9,9-dioctyl fluorene) (PFO) films. Spin-casting from solution produces a glassy film with a broad absorption band and a large Stokes shift due to exciton migration. Upon slowly warming a sample from 80 to 300 K, a fraction of the sample converts into a new phase with extended conjugation, which we refer to as the β-phase. Its absorption band exhibits a clear vibronic structure, and the fluorescence red-shifts by approximately 0.1 eV. The photoinduced absorption (PA) spectrum of the glassy phase is dominated by triplet excitons, whereas, both charged polarons and triplet excitons are seen in the PA spectrum of a sample containing both phases.

Influence of alkoxy substituents on the exciton binding energy of conjugated polymers

Abstract Scanning tunneling microscope (STM) spectroscopy and optical absorption measurements were used to determine the exciton binding energy (Eb) of poly(p-phenylenevinylene) (PPV). We find Eb=0.48±0.14 eV, which is significantly higher than the value of Eb reported previously for an alkoxy-PPV derivative. Furthermore, an analysis of photoluminescence (PL) and STM excited cathodoluminescence (CL) spectra, performed on PPV and on alkoxy-PPV derivatives, reveals that the energy separation between the dominant peaks in the vibronic progression is significantly smaller in the alkoxy-substituted compounds. The reduction of Eb appears to be related to the influence of the alkoxy side-chains, inducing a reduction of both the Coulomb interaction as well as the molecular structural relaxation energy of the exciton.

Conjugated reactive mesogens

As a novel approach to polarized electroluminescence, we report on the synthesis of the conjugated reactive mesogen 2-methyl-1,4-bis[2-(4-acryloylpropyloxyphenyl)ethenyl] benzene (chemical structure DSB MeO 3 O-acryl). DSB MeO 3 O-acryl was characterised with respect to its thermophysical and spectroscopic properties. Photoluminescent anisotropic networks from DSB MeO 3 O-acryl were demonstrated.