J. C. de Mello

Experimental determination of the rate law for charge carrier decay in a polythiophene: Fullerene solar cell

We use transient photovoltage and differential charging experiments, complemented by transient absorption data, to determine charge carrier lifetimes and densities in a poly(3-hexylthiophene): methanofullerene solar cell at Voc as a function of white light-bias intensity. For a typical device, the charge carrier decay dynamics are observed to exhibit an approximately third order dependence on charge density (dn∕dt∝n3).

Charge extraction analysis of charge carrier densities in a polythiophene/fullerene solar cell: Analysis of the origin of the device dark current

We demonstrate the use of a simple charge extraction measurement to determine the charge carrier densities n in annealed poly(3-hexylthiophene):methanofullerene solar cells under operating conditions. By applying charge extraction to the device under forward bias in the dark (Jdark), we find Jdark∝n2.6. This dependence on charge density is the same as that we find for bimolecular recombination losses observed in such devices under irradiation at open circuit, suggesting that the dark current originates from bimolecular recombination at the polymer/fullerene interface.

Influence of thermal treatment on the conductivity and morphology of PEDOT/PSS films

Abstract The electrical properties of conducting polymers are strongly dependent on their film morphology and chemical and physical structure, which in turn can be strongly modified via a variety of post-deposition treatments. The aqueous dispersion of poly(3,4-ethylenedioxythiophene) with polyelectrolyte, poly(styrene sulfonic acid) (PEDOT/PSS) was used as the conducting polymer in our study. We report an investigation of the conductivity and morphology of spin-coated films and their dependence on heat treatment procedures in both air and N 2 atmospheres.

Patterning of organic devices by interlayer lithography

We report a new lithographic procedure that enables the patterning of as-received semiconducting polymers and small molecules at the near micron level without causing discernible degradation of the patterned material. The method involves a minimum of processing steps, requires no modification of the active layer, and is compatible with both rigid and flexible substrates. The technique makes use of an intermediate resist layer between the substrate and the active layer, i.e.underneath the active layer, and involves the simultaneous patterning of the resist and active layers in a single expose/develop step. The technique has been successfully applied to the fabrication of flexible ITO-free light-emitting diodes and photodiodes, yielding peak quantum efficiencies of 8.8 cd A−1 and 57% respectively comparable to similar devices fabricated on ITO-coated glass. It is also readily extendible to the patterning on a single substrate of multiple devices incorporating different component materials, e.g. the red, green and blue pixels of a colour display.

Photostable phosphorescent polymer nanospheres for high sensitivity detection

Time-gated interrogation of phosphorescent labels allows high sensitivity detection of analytes by excluding background fluorescence and stray excitation light. Here we report a method for preparing phosphorescent polymer nanospheres with optical properties specifically designed to enable time-gated detection with both organic and inorganic light-emitting diodes and photodiodes. The nanospheres were formed by co-precipitating the phosphorescent dye ruthenium diphenyl phenanthroline with the amphiphilic statistical co-polymer poly(styrene-co-maleic acid). To minimize aggregation and ensure effective encapsulation against water and oxygen ingress, the polymer chains were cross-linked with the small-molecule cross-linker bisphenol A diglycidyl ether, resulting in near-monodisperse nanospheres with excellent resilience against aggregation and quenching. The nanospheres exhibited long phosphorescence lifetimes of 5.3 ± 0.1 μs, high photoluminescence quantum efficiencies of 33 ± 5%, and excellent stability under UV irradiation. Using a blue light-emitting diode as a light-source and a silicon photodiode as a detector, a low detection limit of 4 μg ml−1 was achieved. The suitability of the nanospheres for bioanalysis was demonstrated by surface functionalising them with biotin to yield a simple biolabel for streptavidin.

Direct dispersion of SWNTs in highly conductive solvent-enhanced PEDOT:PSS films

Abstract Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) is shown to be an effective dispersant for single-wall carbon nanotubes (SWNTs), enabling uniform aqueous suspensions to be obtained at weight loadings of up to 0.23 mg/ml (>1% by weight relative to PEDOT:PSS) without recourse to additional surfactants. Thin films spin-coated from PEDOT:PSS/SWNT suspensions exhibited sheet resistances of 90 Ω/sq. at 80 % transmittance, slightly higher than equivalent films of pure PEDOT:PSS which exhibited sheet resistances of 70 Ω/sq. at the same transmittance.

Integrated sensors for point-of-care detection

We report a low cost device for performing chemiluminescent (CL) assays in a miniaturised format. The device comprises a poly(dimethylesiloxane) microfluidic chip for performing the CL assay coupled to a polymer photodiode based on a 1:1 blend by weight of poly(3-hexylthiophene) [P3HT] and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 [PCBM]. The integration of organic photodiodes with microfluidic chips offers a promising route to low cost fully integrated diagnostic devices for point-of-care applications.