S.C. Moratti

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.

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.

High electron affinity polymers for LEDs

Summary form only given. A new series of alternating copolymers based on poly(p-phenylenevinylene) (PPV) was synthesized. The electron affinity of the polymers is substantially increased through the use of electron-withdrawing substituents. Optimum conditions for the condensation polymerization were explored, and polymers with a M/sub n/ in excess of 30,000 were produced. The synthesis is extremely flexible and allows a large variety of conjugated polymers with different emission properties to be synthesized. Cyclic voltammetry was used to estimate the position of the HOMO and LUMO relative to poly(2,5-dialkoxy-1,4-phenylenevinylene) analogues. Fabrication of efficient bilayer polymer LEDs using PPVin conjunction with the new polymers and stable metal negative contacts such as aluminum and gold is reported.

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.

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.

Conjugated polymer light‐emitting diodes on silicon substrates

We report the fabrication of polymer electroluminescent devices on doped silicon substrates. An electron‐injecting aluminum electrode is thermally evaporated onto the silicon substrate, and a layer of a high electron affinity conjugated polymer, poly(cyanoterephthalylidene), CN‐PPV, is then spin coated onto this. A hole‐transporting layer of poly(p‐phenylene vinylene), PPV, is formed on top of the CN‐PPV layer by thermal conversion of a spin‐coated layer of the sulphonium precursor polymer. Finally, a transparent hole‐injecting top electrode of indium‐tin oxide is formed by rf sputtering. These devices show efficient red electroluminescence, with emission from the CN‐PPV layer.

Light-emitting diodes based on poly(methacrylates) with distyrylbenzene and oxadiazole side chains

Abstract We report investigations on the use of singly and doubly functionalised poly(methacrylates) (PMA) for the fabrication of polymeric light-emitting diodes (LEDs). The functional groups we used were distyrylbenzene (DSB) as blue chromophore and 2-phenyl-5-phenyl-1,3,4-oxadiazole (PPD) or 2-phenyl-5-biphenyl-1,3,4-oxadiazole (PBD) as charge-transport groups to produce three homopolymers (with one functional group only) and a copolymer bearing both DSB and PBD groups (PMA-DSB-PBD). The polymers have been used alone or blended together in single-layer devices or in double-layer devices, for which a thin poly( p -phenylene vinylene) (PPV) layer was used as a hole-transporting material. Despite the high photoluminescence yield of these DSB-based materials, successful fabrication of blue-emitting LEDs has only been achieved by using the copolymer, both in single- and double-layer devices.

Polymer electroluminesence in the near infra-red

Abstract We report the fabrication of polymer light emitting diodes from a cyano-substituted thienylene phenylene vinylene copolymer. Photoluminescence and electroluminescence studies of these devices show a broad emission spectrum which peaks in the near infra-red and extends out to a wavelength of 1000nm. Bilayer structures using poly(p-phenylene vinylene) as a hole transport layer and the cyano-copolymer as the emissive layer showed improved electroluminescence internal quantum efficiency of up to 0.2%..

Chromism and luminescence in regioregular poly(3-dodecylthiophene)

Abstract We report photoluminescence studies of poly(3-dodecylthiophene) (P3DT) in solution. In a good solvent the polymer exhibits luminescence with a high quantum efficiency and a decay time of 500 ps. In a poor solvent the emission is red shifted with a 20-fold reduction in quantum efficiency and a decay profile that is non-monoexponential, but has an average lifetime that is very similar to the good solvent environment. The data indicate a large increase in the natural radiative lifetime from approximately 1 ns in a good solvent to 20 ns in a poor solvent, which implies an emitting state that is different in the two situations. In the poor solvent the spectrum is almost identical to that of the thin film, suggesting that the polymer aggregates in the solution and the emitting species is the same in both environments. The data are consistent with the formation of an excited state that is not localized on a single chain but is delocalized over more than one chain.