A.B. Holmes

Electroluminescence in conjugated polymers

Research in the use of organic polymers as the active semiconductors in light-emitting diodes has advanced rapidly, and prototype devices now meet realistic specifications for applications. These achievements have provided insight into many aspects of the background science, from design and synthesis of materials, through materials fabrication issues, to the semiconductor physics of these polymers.

Exciton diffusion and dissociation in a poly(p‐phenylenevinylene)/C60 heterojunction photovoltaic cell

We report measurements of the photovoltaic response of two‐layer photocells formed with layers of the conjugated polymer poly(phenylenevinylene), PPV and fullerene, C60, formed between indium‐tin oxide and aluminum electrodes. Peak quantum efficiencies of up to ∼9% (electrons collected per incident photon) were measured under short‐circuit conditions. We model the photovoltaic response as arising from excitons photogenerated in the PPV layer which are able to diffuse to the interface with the C60 layer where they are ionized. We obtain a value for the exciton diffusion range of 7±1 nm, both from the spectral response and from the absolute efficiency. We demonstrate that the branching ratio for the creation of singlet excitons from absorbed photons is close to unity.

MEASUREMENT OF ABSOLUTE PHOTOLUMINESCENCE QUANTUM EFFICIENCIES IN CONJUGATED POLYMERS

Abstract Measurements of absolute photoluminescence (PL) efficiencies have been performed for solid films of several conjugated polymers commonly used for electroluminescence. In poly( p -phenylenevinylene) (PPV), a PL efficiency of 0.27 is measured in samples which show an initial PL decay time-constant of 320 ps. These values indicate that photoexcitation in PPV produces intra-chain singlet excitons with a high quantum yield. The PL efficiencies of derivatives of PPV have been investigated, and efficiencies in excess of 0.4 have been measured for cyano-substituted PPVs.

Poly(p‐phenylenevinylene) light‐emitting diodes: Enhanced electroluminescent efficiency through charge carrier confinement

We have fabricated light‐emitting diodes with poly(p‐phenylenevinylene) as the emissive layer, and with an electron‐transporting layer formed from a solid state dispersion of 2‐(4‐biphenylyl)‐5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole in poly(methyl methacrylate), placed between this and the negative electrode. These structures show typically a tenfold improvement in efficiency in the low‐voltage regime and an eightfold improvement in the high‐voltage regime over devices without the electron‐transporting layer. Typical efficiencies are about 0.8% photons/electron. We consider that the role of the electron‐transport layer is to confine holes to the emissive layer.

Electrochemical and optical studies of PPV derivatives and poly(aromatic oxadiazoles)

The cyclic voltammograms and optical properties for PPV, MEH-PPV, CN-PPV and poly(aromatic oxadiazoles) have been measured on Au/Pd and ITO substrates. The ITO is a convenient substrate for preparing films of PPV under high temperature and vacuum. Furthermore the CV data obtained with polymer films on ITO more closely resemble the working EL device.

Polymer band alignment at the interface with indium tin oxide: consequences for light emitting devices

The influence of the ITO work function on the band alignment in semiconducting polymer over-layers was studied using UPS. Ultra-thin films of poly(bis-(2-dimethyloctylsilyl)-1,4-phenylenevinylene) were studied directly on ITO, as well as with an intermediate layer of an electrically conducting polymer (poly(3,4-ethylenedioxythiophene) doped with poly(4-styrene sulfonate)). For the polymer spin-coated directly on ITO the vacuum levels are aligned. Correspondingly, with the intermediate conducting polymer, the bands in the semiconducting polymer align to the vacuum level of the conducting polymer layer. Thus, the barrier to hole injection is determined by the work function of the conducting polymer instead of the ITO.

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.

Crystal network formation in organic solar cells

We have studied the effects of annealing on performance and morphology of photovoltaic devices using blends of two organic semiconductors: a conjugated polymer and a soluble perylene derivative. The efficiency of such photovoltaic cells has been determined. The effect of temperature on blend morphology has been investigated for actual device films. Annealing leads to the formation of micron size perylene crystals and an enhancement of the quantum efficiency. This enhancement has been attributed to the formation of an electron conducting perylene crystal network.

Dye-based donor/acceptor solar cells

We have fabricated organic donor/acceptor solar cells with three different architectures using soluble derivatives (dyes) of the molecular semiconductors phthalocyanine as electron donor (D), i.e. hole transport material and perylene as electron acceptor (A), i.e. electron transport material. These architectures comprise a blend and a double layer structure as well as the only recently reported laminated-device structure. The organic semiconducting films were deposited at room temperature via spin coating from solution. Current–voltage (I/V) characteristics and external quantum efficiency spectra will be discussed. The measured quantum efficiencies reach values between 0.3% and 1.1% with a photoresponse covering the entire spectrum of visible light. Our results show that together with insoluble small molecules (pigments) and conjugated polymers, dye molecules represent a new class of organic semiconducting materials that can be used to manufacture D/A solar cells.

Electroluminescence from multilayer conjugated polymer devices: Spatial control of exciton formation and emission

Abstract We have constructed electroluminescent diodes using several layers of conjugated polymers with differing band gaps; these provide a range of different colour light-emitting layers and can be used to control charge injection and transport. Poly(1,4-phenylenevinylene, PPV, and derivatives have been used, with indium/tin oxide as hole-injecting layer and calcium as electron-injecting contact layer. For this selection of materials, we show that the ordering of the polymer layers allows control of the colour of device emission. Emission can be produced in more than one layer.