K. Petritsch

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.

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.

Workfunction of purified and oxidised carbon nanotubes

We report for the first time the ultraviolet photoelectron spectroscopy (UPS) of purified multi-wall carbon nanotubes. Purification was carried out by successive sonication, centrifuge, sedimentation, and filtration processes of nanotubes-water solution dispersed by the non-ionic surfactant. Purified nanotubes showed the workfunction of 4.3 eV, very close to that of graphite (4.4 eV). It was found that purified nanotubes possess a finite density of states (DOS) at Fermi energy (EF), suggesting the existence of metallic nanotubes as predicted theoretically. The oxygen-plasma treatment which opens the end-cap and generates defects in the outer layers increased the workfunction dramatically up to 4.8 eV and removed the DOS at EF, indicating a new way to modify the electronic properties of nanotubes.

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.

Liquid crystalline phthalocyanines in organic solar cells

Small exciton diffusion lengths, insufficient charge transport and low absorption prevent organic materials from achieving commercially useful power conversion efficiencies. The recently reported long exciton diffusion range along the columnar axes in the mesophase of disc shaped liquid crystals might represent a way to circumvent these limitations. In this paper we investigate a double-layer photovoltaic device comprising a discotic phthalocyanine derivative as electron-donor and a perylene derivative as electron-acceptor material. We fabricated one device with a pristine phthalocyanine layer and compare it with a device that was thermally treated to obtain a higher ordered mesophase.

Photovoltaic properties of MEH-PPV/PPEI blend devices

Abstract We report on the photophysical properties of blends of poly(2-methoxy-5-(2′-ethyl-hexyloxy)- p -phenylene vinylene), MEH-PPV, and perylene bis(phenethylimide), PPEI. Highly efficient quenching of photoluminescence (PL) efficiency was observed. We have measured the photocurrent response of an MEH-PPV/PPEI blend in a sandwich cell between indium tin oxide (ITO) and aluminium electrodes. The external quantum efficiency (EQE) is enhanced considerably with regard to single layer MEH-PPV devices.

High efficiency polymer photodiodes

Organic polymer photodiodes have been fabricated using different device designs, and by the use of blends between derivatives of poly(thiophene) and poly(phenylene vinylene), a broad wavelength response is accomplished. For single layer devices, an external quantum yield of 4.5% is achieved under short-circuit conditions, increasing to 28% at 2V negative bias. Even better results are found for double layer devices, having a spectral response covering the range from 350 nm to 750 nm, and exhibiting 28% external quantum yield at short circuit and reaching almost 60% at -2V bias.

Ultrathin organic photovoltaic devices

Abstract We fabricated ITO/poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene(MEH-PPV)/Al photodiodes with different thickness of the MEH-PPV layer ranging between 10 and 250nm. Although efficient radiationless exciton quenching has been found within 20nm of a metal electrode we obtained the highest photon to current conversion efficiencies for films of about this thickness. These ultrathin devices have their peak response virtually at the wavelength with the peak absorption suggesting that the whole bulk is contributing efficiently to the photocurrent and most excitons in this thin layer can be dissociated within their diffusion range.

A series of novel liquid crystalline octakis(alkylthio)-substituted phthalocyanines

Abstract Liquid crystalline phthalocyanines (Pcs) are promising materials for opto-electronic applications (e.g. solar cells, electrophotography) due to their high absorption coefficients in the long wavelength region of the optical spectrum and high charge carrier mobilities in the columnar mesophase. We report here the synthesis and properties, in particular the thermotropic phase behaviour, of a series of new octakis(alkylthio)-substituted pcs.