K. Pichler

Optical spectroscopy of highly ordered poly(p-phenylene vinylene)

The authors report a study of the photophysical properties of poly(p-phenylene vinylene), PPV, prepared in a way that gives an especially high degree of intrachain order. Optical absorption, photoluminescence, photoinduced absorption, and photoconductivity excitation spectra are presented and compared to data reported for less well ordered PPV. Spectral red shifts, sharpening of spectral lines, and a transfer of oscillator strength into the vibronic ground states of the electronic transitions are observed. Photoinduced absorption due to long-lived charged excitations, previously reported for less ordered PPV, could not be detected in this material. Photoconductivity excitation spectra show a steep rise at the absorption edge with no appreciable offset between the onsets for photoconduction and absorption. A very slow photocurrent component is observed, which the authors associated with the trapping and subsequent thermal release of photocarriers.

Optical spectroscopy of triplet excitons and charged excitations in poly (p-phenylenevinylene) light-emitting diodes

Abstract Sub-gap absorption is measured under forward drive conditions in electroluminescent devices fabricated from thin films of poly( p -phenylenevinylene) sandwiched between electrodes of indium/tin oxide and calcium. Quantitative concentrations of excitations are obtained. Absorption seen at 1.37 eV is due to an allowed transition to a higher-lying state of the excitons formed by capture of positive and negative charges in the polymer. Absorption bands at 0.65 and 1.55 eV are assigned to charged bipolarons which do not directly participate in the electroluminescence process.

PHOTOLUMINESCENCE AND ELECTROLUMINESCENCE IN CONJUGATED POLYMERIC SYSTEMS

The basic working principles and construction of electroluminescent polymer devices are described. The opportunities for combining creative synthetic chemistry and imaginative device physics to address issues of colour, efficiency, and control of processing of semiconducting polymeric materials are reviewed. New advances in the construction of multilayer devices and controlled syntheses of poly(p-phenylenevinylene) are reported.

Electronic excitations in luminescent conjugated polymers

Abstract We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved and single-layer EL diodes achieve luminous efficiencies above 2 Lumen W−1 and peak brightnesses up to 90 000 cd m−2. We discuss measurements of photoconductivity, photovoltaic response, photoluminescence excitation spectra and stimulated emission in films of PPV. We consider that the photoexcited state in these films of PPV is the intrachain singlet exciton. We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments and can be used as the active lasing medium when incorporated in suitable microcavity structures.

Electronic processes of conjugated polymers in semiconductor device structures

Abstract We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved, and single-layer EL diodes achieve luminous efficiencies above 2 Lumens/W and peak brightnesses up to 90,000 cd/m 2 . We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments, and can be used as the active lasing medium when incorporated in suitable microcavity structures.

Exciton dissociation at a poly(p-phenylenevinylene)/C60 heterojunction

Abstract We report measurements of the photovoltaic effect in heterojunction structures consisting of a layer of C 60 thermally evaporated onto a film of the conjugated polymer poly( p -phenylenevinylene) (PPV) sandwiched between electrodes of indium-tin oxide and Al. Peak quantum efficiencies of up to about 9% (electrons collected per incident photon) were measured under short-circuit conditions, with corresponding open-circuit voltages approaching 0.9 V. Increased quantum yields were obtained under forward and reverse biases. We model the response as arising from the dissociation of photogenerated excitons at the PPV/C 60 interface, with electron transfer to the C 60 . The C 60 layer acts as a dielectric spacer, providing a constructive enhancement of the optical fields at the PPV/C 60 interface. We have applied this model to estimate an exciton diffusion range of 60 to 80 A.

Transport studies in C60 and C60/C70 thin films using metal-insulator-semiconductor field-effect transistors

Abstract We present a study of transport in C 60 and C 60 /C 70 (98% C 60 ) thin films using metal-insulator-semiconductor field-effect transistors (MISFETs). The devices are used to measure the field-effect mobility for charges at a fullerene/insulator interface between 300 and 473 K. We infer from our results that C 60 operates as an n-type semiconductor. We observe that, on exposure to oxygen, the conductivity falls by up to five orders of magnitude but that this process is reversible by annealing the devices in vacuum at 473 K for at least 10 h. We also show that it is possible to obtain working devices in air at elevated temperatures. The highest value measured here for the field-effect mobility, at room temperature, is 2 × 10 −3 cm 2 V −1 s −1 . However, even for films of nominally the same composition, the measured values varied widely. This indicates that sample preparation plays an important role in device performance. All devices remained working at temperatures in excess of 473 K, demonstrating the good thermal stability of the C 60 films. We observed a change in the conductivity and field-effect mobility in one device around room temperature. We discuss this anomaly with reference to the structural phase transition of C 60 single crystals at 260 K.

Conjugated Polymer Light-emitting Diodes

It is now established that conjugated polymers can be used to provide charge transport and to act as the emissive layer in thin-film light-emitting diodes (LEDs). The operation of these devices provides important information about the semiconductor physics of these materials. We discuss here the progress made in the design, fabrication and measurement of these devices, and in the understanding of the basic properties that determine device performance.

Field‐effect transistors based on poly(p‐phenylene vinylene) doped by ion implantation

We have fabricated metal‐insulator‐semiconductor field‐effect transistors (MISFETs), with thin films of polycrystalline poly(p‐phenylene vinylene) (PPV) as the semiconducting layer and report here the successful operation of a PPV MISFET based on the p‐type doping of the polymer layer by ion implantation of iodine. The measured field‐effect mobility of the charge carriers in this ion‐ implanted PPV is in the range of 10−7 to 10−8 cm2/V s. These values are in the same range as those obtained from a PPV MISFET in which the PPV was doped from the gas phase.

Chain alignment in poly(p-phenylene vinylene) on oriented substrates

Abstract We show that conjugated polymers processed via soluble precursor polymers can be oriented by forming them on highly-oriented poly(tetrafluoroethylene), PTFE. The oriented PTFE induces preferential orientation of the precursor polymers which is preserved and possible enhanced during the thermal conversion to the final conjugated polymer. Orientation is substantially better if precursor polymers are used which have a some content of stiff segments in their backbone. We characterise the state of alignment of the polymer by polarised optical absorption measruements. We consider that orientation is confined to a thin layer adjacent to the substrate, and find from the form of the interband absorption that the extent of conjugation is enhanced in this oriented region.