Showing papers by "Franco Cacialli published in 1996"

Emission enhancement in single-layer conjugated polymer microcavities

Abstract: We have studied the luminescence properties of microcavities which are formed by a single layer of poly(para‐phenylenevinylene) sandwiched between a dielectric mirror coated with a conducting indium tin oxide layer and a semitransparent metal electrode. Compared with a device without cavity structure, the spectral and spatial emission are significantly narrowed, and the forward emission intensity is enhanced. We measure a spectral linewidth (full width at half maximum) of the cavity modes of about 4 nm in photoluminescence and 20 nm in electroluminescence and an enhancement of luminescence intensity in the forward direction of more than an order of magnitude. The implications of the narrowing of the emission and possible transfer mechanisms for excitation energy are discussed.

Efficient green light‐emitting diodes from a phenylated derivative of poly(p‐phenylene–vinylene)

Abstract: We have used polyvinylcarbazole, PVK, in combination with poly(p‐phenylene vinylene), PPV and a green‐emitting, soluble derivative, poly(4,4′‐diphenylene diphenylvinylene) or briefly PDPV, to fabricate triple‐layer light‐emitting diodes PPV/PVK/PDPV/metal, with indium tin oxide as the anode. We report luminances in excess of 1300 cd/m2 with Al cathodes, turn‐on voltages of ∼8 V in structures 300 nm thick, and internal efficiencies in the range 0.4%–0.8% for Al and Ca cathodes; at least ten times higher than for single‐layer PDPV devices. Quantum efficiencies for Ca or Al electrodes are only a factor 1.5 different. The improvements over single‐layer PDPV devices are considered to be due to better hole injection by PPV and to the formation of a hole barrier at the PPV/PVK interface, which forces a better balance of the electron and hole currents.

Site‐selective chemical‐vapor‐deposition of submicron‐wide conducting polypyrrole films: Morphological investigations with the scanning electron and the atomic force microscope

Abstract: We report morphological investigations of polypyrrole thin films deposited by means of a self‐aligning vapor phase technique onto glass, silicon and silicon dioxide substrates, coated with an oxidizing precursor. The variation of the deposition parameters allows the control of the film microstructure which can be fibrillar and strongly anisotropic or globular and tendentially isotropic. Patterning of the precursor by electron‐beam lithography allows the production of submicron wide lines as shown by both the scanning electron microscope and the atomic force microscope.

Light-emitting diodes fabricated with conjugated polymers

Abstract: Processible conjugated polymers can be used to fabricate a range of thin-film diodes which can be designed to show good characteristics, both as electroluminescent diodes and also as photoconductive diodes. We discuss the present understanding of the operation of light-emitting diodes which use conjugated polymers for both charge transport and emission. We highlight the improvement to the electroluminescence efficiency that can be produced by the use of two polymer layers selected so that the heterojunction between the two layers is able to confine charge and thus, bring about electron-hole capture to generate excitons at this interface. The nature of these diodes allows fabrication of microcavity structures in which the emission characteristics of the polymer are strongly modified by the allowed cavity modes. This can be used to control the colour of emission and sharpness of the emission bands.

Using Microcavities to Manipulate Luminescence in Conjugated Polymers

Abstract: Photoluminescence and electroluminescence measurements of microcavities containing the conjugated polymer poly(p-phenylene vinylene), PPV, are reported. Modification of the emission wavelength, linewidth, and spatial output are observed, and are related to the cavity design.