Magnus Granström

Laminated fabrication of polymeric photovoltaic diodes

Photoexcited electron transfer between donor and acceptor molecular semiconductors provides a method of efficient charge generation following photoabsorption, which can be exploited in photovoltaic diodes. But efficient charge separation and transport to collection electrodes is problematic, because the absorbed photons must be close to the donor–acceptor heterojunction, while at the same time good connectivity of the donor and acceptor materials to their respective electrodes is required. Mixtures of acceptor and donor semiconducting polymers, (or macromolecules) can provide phase-separated structures which go some way to meeting this requirement, providing high photoconductive efficiencies. Here we describe two-layer polymer diodes, fabricated by a lamination technique followed by controlled annealing. The resulting structures provide good connectivity to the collection electrodes, and we achieve a short-circuit photovoltaic quantum efficiency of up to 29% at optimum wavelength, and an overall power conversion efficiency of 1.9% under a simulated solar spectrum. Given the convenience of polymer processing, these results indicate a promising avenue towards practical applications for such devices.

Indium–tin oxide treatments for single- and double-layer polymeric light-emitting diodes: The relation between the anode physical, chemical, and morphological properties and the device performance

We report combined studies of the influence of chemical and physical treatments on the properties of indium–tin oxide (ITO) thin films. The ITO films were also used as transparent anodes of polymeric light-emitting diodes (LEDs) incorporating poly(p-phenylene vinylene) (PPV) as the emitter material, with, or without, doped poly(3,4-ethylene dioxythiophene) (PEDOT) as a hole-injection/transport layer. Structures based on a soluble green derivative of PPV, poly(4,4′-diphenylene diphenylvinylene) were also tested. We studied chemical (aquaregia, degreasing, RCA protocol) and physical (oxygen and argon plasmas, Teflon, and paper rubbing) treatments and, in contrast to recently published work, we find that for Balzer Baltracon ITO, oxygen plasma and not aquaregia yields the highest efficiencies and luminances and the lowest drive voltages. For oxygen-plasma-treated anodes, the device efficiency clearly correlates with the value of the ITO surface work function, which in turn depends on the time of treatment. I...

White light emission from a polymer blend light emitting diode

A new type of polymer light emitting diodes that emit white light is reported. In these diodes, several electroluminescent substituted polythiophenes have been combined to give the necessary components of the visible spectrum. These emitting polymers are then mixed with an insulating polymer to diminish the energy transfer from high‐band‐gap polymers to low‐band‐gap polymers. The resulting emission at 20 V is shown to be close to the equienergy white point as defined by the CIE (Commission Internationale de l’Eclairage).

Micrometer- and Nanometer-Sized Polymeric Light-Emitting Diodes

A method for the fabrication of micrometer-and submicrometer-sized polymeric light-emitting diodes is presented. Such diodes have a variety of applications. Light sources of dimensions around 100 nanometers are required for subwavelength, near-field optical microscopy. Another possible application is patterning on the micrometer and nanometer scale. The diodes have been made in the form of a sandwich structure, with the conductive polymer poly(3,4-ethylene-dioxythiophene) polymerized in the pores of commercially available microfiltration membranes defining the hole-injecting contacts, poly[3-(4-octylphenyl)-2,2;-bithiophene] as the light-emitting layer, and a thin film of calcium-aluminum as the electron injector.

Ultrathin Self‐Assembled Layers at the ITO Interface to Control Charge Injection and Electroluminescence Efficiency in Polymer Light‐Emitting Diodes

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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.

A polythiophene microcavity laser

We report photopumped lasing in a microcavity device with a polythiophene layer as the emitter. These microcavity devices are built by joining two polymer coated dielectric mirrors at elevated temperature. When photopumping the film, a lasing threshold is observed at 120 nJ/cm(2). Comparative studies with fast pump-probe spectroscopy of thin polythiophene films and the same polymer in photopumped lasing studies, indicate that the gain coefficient is 80 +/- 20 cm(-1), and that the exciton concentration is 2 X 10(17) cm(-3) at the lasing transition, well below the exciton-exciton recombination level

Polythiophene polymers in light emitting diodes: making multicolour devices

Summary form only given. Polythiophenes with alkyl and alkylphenyl substituents, with bandgaps from 1.8 to 3 eV, have been synthesised and used in polymer light emitting diodes. The bandgap is systematically tuned by the pattern and character of substituents on the polythiophene main chain. Steric repulsion between substituents and the main chain forces the polymer out of planarity, increasing the bandgap. By using these soluble polymers in light emitting diodes we have obtained the colours blue, green, red and infrared, with quantum efficiencies reaching the 0. I- I % range. Using multilayers we have obtained multicolour emission in these diodes. Special geometries allows the use of these polymer LEDs as /spl les/100 nm diameter light sources.

Organic photodiodes using polymeric anodes

We have fabricated photovoltaic devices using poly (p-phenylene vinylene) (PPV) as the active layer and poly (3,4 ethylenedioxythiophene) (PEDOT) doped with polystyrene sulfonic acid (PSS) and aluminium as electrodes. We have compared the performance of these devices with a device where indium-tin oxide (ITO) was used as electrode. Using PEDOT the spectrum of the external quantum efficiency is constant in a broad range of energy, while devices made with ITO show only a very narrow peak at the onset of the absorption. When PPV is converted on top of PEDOT the photoluminescence is more quenched than on top of ITO, suggesting interfacial reactions between PPV and PEDOT:PSS.

Polymeric light-emitting diodes of submicron size — structures and developments

Micron- and submicron-sized light-emitting diodes (LEDs) made using conjugated polymers as electroluminescent layers and contact materials are presented. Two different routes to make arrays of such small light sources have been developed. The benefits and drawbacks of the use of the conjugated polymer poly(2,3-ethylene-dioxythiophene) (PEDOT) as hole injector in polymer LEDs are also discussed.