75 research outputs found
Fast electrochemical doping due to front instability in organic semiconductors
The electrochemical doping transformation in organic semiconductor devices is
studied in application to light-emitting cells. It is shown that the device
performance can be significantly improved by utilizing new fundamental
properties of the doping process. We obtain an instability, which distorts the
doping fronts and increases the doping rate considerably. We explain the
physical mechanism of the instability, develop theory, provide experimental
evidence, and perform numerical simulations. We further show how improved
device design can amplify the instability thus leading to a much faster doping
process and device kinetics.Comment: 4 pages, 4 figure
Ambient fabrication of flexible and large-area organic light-emitting devices using slot-die coating
The grand vision of manufacturing large-area emissive devices with low-cost roll-to-roll coating methods, akin to how newspapers are produced, appeared with the emergence of the organic light-emitting diode about 20 years ago. Today, small organic light-emitting diode displays are commercially available in smartphones, but the promise of a continuous ambient fabrication has unfortunately not materialized yet, as organic light-emitting diodes invariably depend on the use of one or more time- and energy-consuming process steps under vacuum. Here we report an all-solution-based fabrication of an alternative emissive device, a light-emitting electrochemical cell, using a slot-die roll-coating apparatus. The fabricated flexible sheets exhibit bidirectional and uniform light emission, and feature a fault-tolerant >1-μm-thick active material that is doped in situ during operation. It is notable that the initial preparation of inks, the subsequent coating of the constituent layers and the final device operation all could be executed under ambient air
Time- and angle-resolved photoemission spectroscopy with optimized high-harmonic pulses using frequency-doubled Ti:Sapphire lasers
Time- and angle-resolved photoemission spectroscopy (trARPES) using femtosecond extreme ultraviolet high harmonics has recently emerged as a powerful tool for investigating ultrafast quasiparticle dynamics in correlated-electron materials. However, the full potential of this approach has not yet been achieved because, to date, high harmonics generated by 800 nm wavelength Ti:Sapphire lasers required a trade-off between photon flux, energy and time resolution. Photoemission spectroscopy requires a quasi-monochromatic output, but dispersive optical elements that select a single harmonic can significantly reduce the photon flux and time resolution. Here we show that 400 nm driven high harmonic extreme-ultraviolet trARPES is superior to using 800 nm laser drivers since it eliminates the need for any spectral selection, thereby increasing photon flux and energy resolution to < 150 meV while preserving excellent time resolution of about 30 fs. © 2014 The Authors
Self-amplified photo-induced gap quenching in a correlated electron material.
Capturing the dynamic electronic band structure of a correlated material presents a powerful capability for uncovering the complex couplings between the electronic and structural degrees of freedom. When combined with ultrafast laser excitation, new phases of matter can result, since far-from-equilibrium excited states are instantaneously populated. Here, we elucidate a general relation between ultrafast non-equilibrium electron dynamics and the size of the characteristic energy gap in a correlated electron material. We show that carrier multiplication via impact ionization can be one of the most important processes in a gapped material, and that the speed of carrier multiplication critically depends on the size of the energy gap. In the case of the charge-density wave material 1T-TiSe2, our data indicate that carrier multiplication and gap dynamics mutually amplify each other, which explains-on a microscopic level-the extremely fast response of this material to ultrafast optical excitation
A model for the dynamics and internal structure of planar doping fronts in organic semiconductors
The dynamics and internal structure of doping fronts in organic
semiconductors are investigated theoretically using an extended drift-diffusion
model for ions, electrons and holes. The model also involves the injection
barriers for electrons and holes in the partially doped regions in the form of
the Nernst equation, together with a strong dependence of the electron and hole
mobility on concentrations. Closed expressions for the front velocities and the
ion concentrations in the doped regions are obtained. The analytical theory is
employed to describe the acceleration of the p- and n-fronts towards each
other. The analytical results show very good agreement with the experimental
data. Furthermore, it is shown that the internal structure of the doping fronts
is determined by the diffusion and mobility processes. The asymptotic behavior
of the concentrations and the electric field is studied analytically inside the
doping fronts. The numerical solution for the front structure confirms the most
important predictions of the analytical theory: a sharp head of the front in
the undoped region, a smooth relaxation tail in the doped region, and a plateau
at the critical point of transition from doped to undoped regions.Comment: 13 pages, 11 figure
Graphene Photonics and Optoelectronics
The richness of optical and electronic properties of graphene attracts
enormous interest. Graphene has high mobility and optical transparency, in
addition to flexibility, robustness and environmental stability. So far, the
main focus has been on fundamental physics and electronic devices. However, we
believe its true potential to be in photonics and optoelectronics, where the
combination of its unique optical and electronic properties can be fully
exploited, even in the absence of a bandgap, and the linear dispersion of the
Dirac electrons enables ultra-wide-band tunability. The rise of graphene in
photonics and optoelectronics is shown by several recent results, ranging from
solar cells and light emitting devices, to touch screens, photodetectors and
ultrafast lasers. Here we review the state of the art in this emerging field.Comment: Review Nature Photonics, in pres
On the desired properties of a conjugated polymer-electrolyte blend in a light-emitting electrochemical cell
We present results from a systematic study on the influence of the conjugated polymer (CP) on the performance of planar light-emitting electrochemical cells (LECs) with a device structure of Au/{CP + poly(ethylene oxide) (PEO) + KCF 3 SO 3 }/Au. We have employed six different CPs, and we demonstrate that in order to attain a fast turn-on time and a strong light emission intensity, it is critical that the p-type doping (oxidation) potential of the CP is situated within the electrochemical stability window of the {PEO + KCF 3 SO 3 } electrolyte. We also find that a high ionic conductivity of the active material correlates with a small phase separation between the CP and the {PEO + KCF 3 SO 3 } electrolyte, and that a doping concentration of ∼0.1 dopants/CP repeat unit is a generic feature of the progressing doping fronts in all investigated devices. Finally we report the first observation of a light emission zone positioned in close proximity to the positive anode in a CP-based LEC. \ua9 2008 Elsevier B.V. All rights reserved
Investigating DNA Double Strand Breaks (DSB) in mammalian cells by novel fluorescent reporters
An efficient DNA damage response is critical for maintaining the integrity of the mammalian genome, and ensuring the accurate transfer of genetic information between generations. Of particular biological relevance are DNA double strand breaks (DSB), which if repaired incorrectly may contribute to carcinogenesis. Review of contemporary literature has led to the identification of protein interactions and transcriptional events, tightly associated with the mammalian DSB response. Characteristics of selected events have been manipulated, with the notion of developing a reporter system that offers a sensitive and rapid method of detecting DSB in living mammalian cell models. Work presented here provides a quantitative evaluation of DSB generation in various mammalian cell lines, following chemical and irradiation treatment, and highlights the limitations of currently used markers. A series of recombinant proteins comprising peptide interacting domains, which exhibit altered spatio-temporal dynamics in relation with each other following DSB induction, are proposed as potential reporters of damage in mammalian cells. Novel gene constructs have been engineered that encode these peptide interacting domains, sandwiched between fluorescence-resonance-energy transfer (FRET) capable proteins. DSB specific events are predicted to induce peptide interactions that may be tracked in real time, by monitoring alterations in the fluorescent properties of such a recombinant protein. In an alternative approach, the transcriptional up-regulation of RAD52 mRNA following DSB induction was extended to whole cells. Optimisation of a fluorescent molecular beacon probe complementary to mammalian RAD52 mRNA is described, and data obtained in mammalian cells following DSB induction supports the notion that RAD52 is actively transcribed as part of the DSB response.EThOS - Electronic Theses Online ServiceBiotechnology and Biological Sciences Research Council (BBSRC) and GlaxoSmithKline (GSK).GBUnited Kingdo
Salt concentration effects in planar light-emitting electrochemical cells
Incorporation of ions in the active layer of organic semiconductor devices may lead to attractive device properties like enhanced injection and improved carrier transport. In this paper, we investigate the effect of the salt concentration on the operation of light-emitting electrochemical cells, using experiments and numerical calculations. The current density and light emission are shown to increase linearly with increasing ion concentration over a wide range of concentrations. The increasing current is accompanied by an ion redistribution, leading to a narrowing of the recombination zone. Hence, in absence of detrimental side reactions and doping-related luminescence quenching, the ion concentration should be as high as possible
A unifying model for the operation of light-emitting electrochemical cells
The application of doping in semiconductors plays a major role in the high performances achieved to date in inorganic devices. In contrast, doping has yet to make such an impact in organic electronics. One organic device that does make extensive use of doping is the light-emitting electrochemical cell (LEC), where the presence of mobile ions enables dynamic doping, which enhances carrier injection and facilitates relatively large current densities. The mechanism and effects of doping in LECs are, however, still far from being fully understood, as evidenced by the existence of two competing models that seem physically distinct: the electrochemical doping model and the electrodynamic model. Both models are supported by experimental data and numerical modeling. Here, we show that these models are essentially limits of one master model, separated by different rates of carrier injection. For ohmic nonlimited injection, a dynamic p-n junction is formed, which is absent in injection-limited devices. This unification is demonstrated by both numerical calculations and measured surface potentials as well as light emission and doping profiles in operational devices. An analytical analysis yields an upper limit for the ratio of drift and diffusion currents, having major consequences on the maximum current density through this type of device
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