J. Grüner

Light-emitting diodes fabricated with conjugated polymers — recent progress

Abstract We consider 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 efficient electron—hole capture to generate excitons. We also report recent results on the fabrication of diodes on top of non-transparent substrates such as silicon. This requires the deposition of a transparent top electrode, and we have used indiumtin oxide in this role.

Emission enhancement in single-layer conjugated polymer microcavities

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.

Photocurrent measurements on aggregates in ladder-type poly(p-phenylene)

Abstract We report measurements of the photovoltaic effect on single layer devices made with a ladder-type poly-(paraphenylene) (LPPP). Besides the photocurrent in the blue spectral range associated with π to π ∗ intra-chain absorption we observe an additional response in the yellow part of the spectrum centered at 2.25 eV. We also observe a weak absorption feature at the same energy, and we consider that both arise from absorption into π electron states formed over several aggregated polymer chains. The photocurrent due to excitation of these states is enhanced by two orders of magnitude compared to blue excitation and vanishes under reverse bias (i.e. ITO negative). We find that the photocurrent response due to the aggregate states is more sensitive to oxygen exposure than the response due to intra-chain absorption.

A blue light emitting copolymer with charge transporting and photo-crosslinkable functional units

The synthesis of a new poly(methacrylate) copolymer bearing an efficient blue light emitting chromophore, a charge transporting aromatic oxadiazole and a UV-sensitive crosslinkable unit is reported. The statistical copolymer was synthesized via a multi-step route and has been characterised by NMR, FT-IR, and DSC. The copolymer has excellent solution processibility and exhibits strong luminescence. Upon cross-linking via UV-treatment, the copolymer becomes insoluble, while gaining both mechanical strength and thermal stability without loss of luminescence. The structure and properties of this novel copolymer and its application in light emitting devices are discussed.

ELECTROLUMINESCENCE AND PHOTOLUMINESCENCE INVESTIGATIONS OF THE YELLOW EMISSION OF DEVICES BASED ON LADDER-TYPE OLIGO(PARA-PHENYLENE)S

Abstract We report investigations of electroluminescence and photoluminescence in several soluble ladder-type oligo( para -phenylene)s which we have used in a range of single- and double-layer electroluminescence devices. In double-layer devices which incorporate a thin film of poly(phenylenevinylene) (PPV) as a hole transport layer, yellow emission with 0.6% internal quantum efficiency can be achieved. We find that pure blue emission can be obtained by diluting the luminescent polymer into a poly(9-vinylene carbazole) (PVK) matrix. This gives a blue-emitting electroluminescent device with more than 0.1% internal quantum efficiency.

4-AcNI—a new polymer for light-emitting diodes

Abstract We report investigations on a 4-acetamido-substituted naphthalimide polymer (4-AcNI) as a component for light-emitting diodes (LEDs). Devices prepared with 4-AcNI as the active material are relatively inefficient (4 × 10−4%). Higher internal quantum efficiency values of 0.4% are achieved if 4-AcNI is used as an electron-transporting layer (ETL) in devices which use poly(p-phenylene vinylene) (PPV) as a hole transport layer. For Al cathodes, this material proves to be superior to similar devices which use 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) as the ETL. The results are used to shed some light on the function of ETLs in organic LEDs.

Novel poly(arylene vinylene)s carrying donor and acceptor substituents

New electroluminescent poly(arylene vinylene)s 1 containing electron-withdrawing/donating substituents have been synthesized by the precursor route outlined below. The synthesis of two novel polymers and the evaluation of their photoluminescent and electroluminescent properties in polymer light-emitting diodes (LEDs) are reported.

Synthesis of a polyphenylene light-emitting polymer

Abstract A new poly(phenylene- co -dialkoxyphenylene) derivative was synthesized by the Suzuki coupling of aromatic bromoboronic acids. This showed strong violet photoluminescence (peak 415 nm) in the solid state and good solubility in organic solvents. A multilayer light-emitting diode was constructed with the structure indium-tin oxide/poly( p -phenylenevinylene)/polyphenylene/calcium. Efficient light emission was observed with a peak maximum in the blue region of the visible spectrum.

New CF3-substituted PPV-type oligomers and polymers for use as hole blocking layers in LEDs

Abstract New CF 3 -substituted PPV derivatives have been synthesised by Wittig-Horner polycondensation. To obtain a better understanding of the relationship between absorption and luminescence properties and structure a single crystal X-ray analysis of a model oligomer has been performed. The effect of electron withdrawing trifluoromethyl groups at vinylidene linkages in PPVs on absorption, luminescence, hole blocking and electron injecting properties has been investigated.

Luminescence properties of poly(p-phenylenevinylene): Role of the conversion temperature on the photoluminescence and electroluminescence efficiencies

We have investigated the luminescence properties of poly(p-phenylenevinylene) (PPV) prepared via the standard precursor route as a function of the conversion temperature in the range 170–270 °C. In particular, we have determined the absolute photoluminescence (PL) efficiencies of PPV thin films prepared on quartz or indium–tin oxide (ITO) coated glass substrates and found that the dependence on conversion temperature is different, depending on the type of substrate. The optical data show that heating at 170 °C for 10 h is sufficient to achieve full conversion. For PPV on quartz, a further increase of the temperature induces a decrease of the PL efficiency, whereas for PPV on ITO the PL efficiency shows a nonmonotonic dependence on the conversion temperature, with a maximum for conversion at about 205 °C. We discuss this behavior with reference to the interplay between the decrease in concentration of PL-quenching impurities (formed by reaction of the conversion byproducts and ITO) and an increase of excit...