Triplet-state di(cation radicals) of 1,3-phenylene-bis(methylene phosphorane)

EPR spectroscopy is used to characterize a novel triplet ground state diradical obtained from oxidation of 1,3-phenylene-bis{[(4-t-butylphenyl)methylene]triphenyl phosphorane}.</p

Plastic solar cells : understanding the special additive

Solar cells use freely available sunlight to make electricity. At the present time, solar electricity does not come cheap, because solar panels are rather expensive. Now imagine that we could reduce costs by printing solar panels like we print newspapers! We can do just that with plastic solar cells. In this article, we explain the basic working principles of these novel plastic solar cells and then show how a stunning threefold increase in solar energy efficiency can be achieved by including a special additive to the printing ink. The function of such a special additive seems almost magical, but as scientists we know that true magic is really rare and we simply had to find out why and how it works. That was the subject of our recent investigations and in this article we describe how we divulged the secret of the special additive

Spatial resolution of methods for measuring the light-emission profile in organic light-emitting diodes

An analysis is presented of the resolution limits of two alternative methods for deducing the light-emission profile in organic light-emitting diodes (OLEDs) from the angular and polarization dependent emission spectra. The comparison includes the "fit-profile" (FP) method, within which the known physics of the recombination process is employed to describe the shape of the profile using a strongly reduced number of degrees of freedom, and the Tikhonov method, which provides a more general solution. First, the cases of a delta-function shaped emission profile and a broad single-peak emission profile are investigated. It is demonstrated that for these cases a ~1¿nm resolution of the peak position may be obtained, provided that the peak is positioned optimally in the OLED microcavity. Subsequently, an analysis is given for a double-peak emission profile and for a rectangular profile, as may be obtained in multilayer OLEDs, revealing a resolution of ~10¿nm for the cases studied. It is suggested that, in general, an optimal analysis should be based on a combined Tikhonov-FP approach

Simulation of electrical conductivity in a pi-conjugated polymeric conductor with infrared light

Irradn. with IR light is found to stimulate the elec. cond. of a film of an org. polymeric conductor [poly(3,4-ethylenedioxythiopene) with polystyrene sulfonate]. The change in cond. is found to be linear in the intensity of the irradn. (4-400 mW/cm2). Both frequency and time domain measurements reveal that the change in resistance induced by irradn., relaxes according to DR(t) ~ (1/t)0.6, with t as the time after excitation. As a possible mechanism for this relaxation, the authors model the diffusion of heat from the polymer film to the supporting glass substrate. By assuming that the change in resistance is linear with the raise in temp. caused by the IR irradn., one predicts a DR(t) ~ (1/t)0.5 dependence. The similarity between the model and exptl. behavior is taken as an indication that the relaxation is limited by heat transport from the polymer film and that the thermalization of the charge carriers occurs on a shorter time scale. Elec. characterization is complemented with optical measurements. These show IR-induced transient absorption of the polymer film with practically the same relaxation behavior as the change in resistance. This suggests that the optical transients are also due to thermal excitations. In the sub-ps time domain, measurements of the change in optical transmission (DT/T) induced by the IR pulse show a very short-lived component with a lifetime close to the instrumental resoln. (.apprx.500 fs). The rapid response is followed by a slow component that decays according to (DT/T)(t) ~ (1/t)0.65. This is interpreted in terms of cooling of the excited charge carriers limited by heat transport, indicating that the thermalization of the carriers occurs on the sub-ps time scal

Contactless charge carrier mobility measurement in organic field-effect transistors

With the increasing performance of organic semiconductors, contact resistances become an almost fundamental problem, obstructing the accurate measurement of charge carrier mobilities. Here, a generally applicable method is presented to determine the true charge carrier mobility in an organic field-effect transistor (OFET). The method uses two additional finger-shaped gates that capacitively generate and probe an alternating current in the OFET channel. The time lag between drive and probe can directly be related to the mobility, as is shown experimentally and numerically. As the scheme does not require the injection or uptake of charges it is fundamentally insensitive to contact resistances. Particularly for ambipolar materials the true mobilities are found to be substantially larger than determined by conventional (direct current) schemes

Electron transport in polyfluorene-based sandwich-type devices: Quantitative analysis of the effects of disorder and electron traps

Results of a combined experimental and modeling study of electron transport in a blue-emitting polyfluorene-based copolymer in sandwich-type devices are presented. We show how, for wide temperature and layer thickness ranges, an accurate and internally consistent drift-diffusion model description of the voltage-dependent current density can be obtained. We employ an adapted form of the "extended Gaussian disorder model," within which the density of states (DOS) is described as a superposition of a Gaussian DOS and an exponential DOS ("trap states"), characterized by only a small set of physically meaningful parameters. A comparison is made with the hole mobility reported for related polymers

The interfaces of poly(p-phenylene vinylene) and fullerene derivatives with Al, LiF, and Al/LiF studied by secondary ion mass spectroscopy and x-ray photoelectron spectroscopy: Formation of AlF3 disproved

Two mutually exclusive mechanisms have been proposed to explain the improved electron injection by the insertion of a LiF layer between the metal cathode and the active organic layer of organic photoelectronic devices: the dipole and the doping mechanism. The possibility of the doping mechanism was studied by investigating the interface of poly[2-methoxy-5-(3',7'dimethyl-octyloxyl)-1,4-phenylenevinylene] (MDMO-PPV) or 1-(3-(methoxycarbonyl)propyl)-1-phenyl[6,6]C-61 (PCBM) with Al, LiF, or Al/LiF. In this mechanism, Li dopes the organic layer, after liberation via the reaction Al+3LiF--&gt;AlF3+3Li. If this reaction takes place, AlF3 should be detectable at the surface. However, SIMS measurements showed that AlF3 is not present at the Al/LiF/MDMO-PPV and Al/LiF/PCBM interfaces. This is evidence that the proposed reaction does not occur. Other evidence that the doping mechanism cannot be the general mechanism to explain the enhanced electron injection comes from the presence of LiF on both organic surfaces. XPS measurements indicate that there is a reaction of Al with the carboxylic oxygen of PCBM, and that a LiF layer between PCBM and Al prevents this reaction. (C) 2002 American Institute of Physic