Charge separation in semicrystalline polymeric semiconductors by photoexcitation: is the mechanism intrinsic or extrinsic?

We probe charge photogeneration and subsequent recombination dynamics in neat regioregular poly(3-hexylthiophene) films over six decades in time by means of time-resolved photoluminescence spectroscopy. Exciton dissociation at 10 K occurs extrinsically at interfaces between molecularly ordered and disordered domains. Polaron pairs thus produced recombine by tunneling with distributed rates governed by the distribution of electron-hole radii. Quantum-chemical calculations suggest that hot-exciton dissociation at such interfaces results from a high charge-transfer character.

Amylose-wrapped luminescent conjugated polymers.

Highly luminescent inclusion complexes consisting of poly(para-phenylene) (PPP) or poly(4,4'-diphenylene-vinylene) (PDV) in the helical cavity of amylose have been synthesised, structurally characterised by nuclear Overhauser spectroscopy and used to fabricate electroluminescent light-emitting diodes.

Tuning intrachain versus interchain photophysics via control of the threading ratio of conjugated polyrotaxanes.

Effective nanoscale control of intermolecular interactions in conjugated polymers is needed for the optimal development and exploitation of the latter in low-cost, large-area consumer electronics items, such as light-emitting and photovoltaic diodes, or transistors. Here we report our investigations on insulated molecular wires constituted by conjugated polymers threaded into cyclodextrin rings. Until now, there has been no detailed quantitative understanding of the role of progressive cyclodextrin encapsulation (quantifiable by the so-called "threading ratio", TR, or number of cyclodextrins per repeat unit) in tailoring the photophysics of the conjugated polymeric wires. We combine spectroscopic, electrical and surface analysis techniques to elucidate how the TR of cyclodextrin-threaded molecular wires controls formation of interchain species and related physical properties (0 < TR < or = 2.3; the maximum theoretical TR for close-packed CDs is 2.8). Increasing TR enhances the solid-state photoluminescence (PL) and electroluminescence quantum efficiency. To unravel the effect of progressive encapsulation on the intrachain decay kinetics of the polymer backbone, we added an electron-accepting quenching agent, methyl viologen (MV), to the polymer solutions. MV predominantly quenches the aggregate PL, thus enabling measurement of the decay kinetics of the intrinsic exciton even for low-TR polyrotaxanes, for which the different contributions are otherwise difficult to disentangle.

Modification of charge transport in triphenyldiamine films induced by acid oxidized single-walled carbon nanotube interlayers.

We report the modification of the transport, optical and electrical properties of 200 nm thick N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'biphenyl-4,4'diamine (TPD) films on indium-tin oxide (ITO) when a low density of acid oxidized single-walled carbon nanotubes (o-SWCNTs) are present at the ITO/TPD interface. Most significantly, current-voltage measurements show strong evidence for a change from contact to bulk limited transport. We postulate that these observations result from a change in the structure of the TPD film seeded by the presence of o-SWCNTs at the ITO surface.

Preparation and characterization of dense films of poly(amidoamine) dendrimers on indium tin oxide.

Indium tin oxide (ITO) substrates were modified with a layer of poly(amidoamine) (PAMAM) dendrimers to change their surface properties and, in particular, the substrates' work function. The functionalization procedure involved the electrostatic adsorption of positively charged PAMAM dendrimers of generation five onto negatively polarized ITO surfaces. Three different PAMAM dendrimers were used: PAMAM-NH2 and PAMAM-OH with terminal amine and hydroxyl groups, respectively, as well as Q-PAMAM-NH2, which had been prepared from PAMAM-NH2 by quaternization of the dendrimer's terminal and internal amine groups with methyl iodide. The resulting organic films were analyzed by contact angle goniometry, X-ray photoelectron spectroscopy, ellipsometry, and Kelvin probe force microscopy to confirm the presence of a dense layer. A Langmuir isotherm was derived from surface densities of fluorescence-labeled PAMAM-NH2 dendrimers from which we deduced an equilibrium binding constant, K(eq), of (1.3 +/- 0.3) x 10(5) M(-1). Kelvin probe measurements of the contact potential difference revealed a high reduction of the work function from 4.9 eV for bare ITO to 4.3 eV for ITO with a dense film of PAMAM-NH2 of generation five. PAMAM-OH and Q-PAMAM-NH2 resulted in slightly smaller work function changes. This study illustrates that the work function of ITO can be tuned by adlayers composed of PAMAM dendrimers.