72 research outputs found
Charge mobility determination by current extraction under linear increasing voltages: the case of non-equilibrium charges and field-dependent mobilities
The method of current extraction under linear increasing voltages (CELIV)
allows for the simultaneous determination of charge mobilities and charge
densities directly in thin films as used in organic photovoltaic cells (OPV).
In the past, it has been specifically applied to investigate the interrelation
of microstructure and charge transport properties in such systems. Numerical
and analytical calculations presented in this work show that the evaluation of
CELIV transients with the commonly used analysis scheme is error prone once
charge recombination and, possibly, field dependent charge mobilities are taken
into account. The most important effects are an apparent time-dependence of
charge mobilities and errors in the determined field dependencies. Our results
implicate that reports on time-dependent mobility relaxation in OPV materials
obtained by the CELIV technique should be carefully revisited and confirmed by
other measurement methods.Comment: 15 pages, 9 figure
Direct Detection of Singlet-Triplet Interconversion in OLED Magnetoelectroluminescence with a Metal-Free Fluorescence-Phosphorescence Dual Emitter
We demonstrate that a simple phenazine derivative can serve as a dual emitter for organic light-emitting diodes, showing simultaneous luminescence from the singlet and triplet excited states at room temperature without the need of heavy-atom substituents. Although devices made with this emitter achieve only low quantum efficiencies of < 0.2%, changes in fluorescence and phosphorescence intensity on the subpercent scale caused by an external magnetic field of up to 30 mT are clearly resolved with an ultra-low-noise optical imaging technique. The results demonstrate the concept of using simple reporter molecules, available commercially, to optically detect the spin of excited states formed in an organic light-emitting diode and thereby probe the underlying spin statistics of recombining electron-hole pairs. A clear anticorrelation of the magnetic-field dependence of singlet and triplet emission shows that it is the spin interconversion between singlet and triplet which dominates the magnetoluminescence response: the phosphorescence intensity decreases by the same amount as the fluorescence intensity increases. The concurrent detection of singlet and triplet emission as well as device resistance at cryogenic and room temperature constitute a useful tool to disentangle the effects of spin-dependent recombination from spin-dependent transport mechanisms
Spontaneous fluctuations of transition dipole moment orientation in OLED triplet emitters
The efficiency of an organic light-emitting diode (OLED) depends on the
microscopic orientation of transition dipole moments of the molecular emitters.
The most effective materials used for light generation have threefold symmetry,
which prohibit a priori determination of dipole orientation due to the
degeneracy of the fundamental transition. Single-molecule spectroscopy reveals
that the model triplet emitter tris(2-phenylisoquinoline)iridium(III)
(Ir(piq)3) does not behave as a linear dipole, radiating with lower
polarization anisotropy than expected. Spontaneous symmetry breaking occurs in
the excited state, leading to a random selection of one of the three ligands to
form a charge transfer state with the metal. This non-deterministic
localization is revealed in switching of the degree of linear polarization of
phosphorescence. Polarization scrambling likely raises out-coupling efficiency
and should be taken into account when deriving molecular orientation of the
guest emitter within the OLED host from ensemble angular emission profiles
Interplay between J- and H-type coupling in aggregates of π-conjugated polymers: a single-molecule perspective
Strong dipole–dipole coupling within and between π‐conjugated segments shifts electronic transitions, and modifies vibronic coupling and excited‐state lifetimes. Since J‐type coupling between monomers along the conjugated‐polymer (CP) chain and H‐type coupling of chromophores between chains of a CP compete, a superposition of the spectral modifications arising from each type of coupling emerges, making the two couplings hard to discern in the ensemble. We introduce a single‐molecule H‐type aggregate of fixed spacing and variable length of up to 10 nm. HJ‐type aggregate formation is visualized intuitively in the scatter of single‐molecule spectra
Temperature and current dependence of the magnetoresistive behavior of poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS)
We investigate the magnetic field effects in thin-film diodes made of the conducting polymer poly(styrene-sulfonate)-doped poly(3,4-ethylenedioxythiophene) as a function of temperature and electrical current. Magnetoresistance of these devices can be measured to high precision on two distinct magnetic field scales: <3 mT, where a pronounced nonmonotonic magnetoresistance response can be resolved, owing to weak hyperfine coupling, and at intermediate magnetic fields, ranging between 3 and 10 mT, where strong monotonic magnetoresistance is seen. The detailed examination of the magnetoresistance effects in both regimes allows one to scrutinize the accuracy of the underlying models for the behavior of these kinds of materials. (C) The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License
Quantum interference in second-harmonic generation from monolayer WSe2
A hallmark of wave-matter duality is the emergence of quantum-interference phenomena when an electronic transition follows different trajectories. This type of interference results in asymmetric absorption lines such as Fano resonances(1), and gives rise to secondary effects such as electromagnetically induced transparency when multiple optical transitions are pumped(2-5). Few solid-state systems show quantum interference and electromagnetically induced transparency(5-11), with quantum-well intersubband transitions in the infrared region(12,13) offering the most promising avenue to date to devices exploiting optical gain without inversion(14,15). Quantum interference is usually hampered by inhomogeneous broadening of electronic transitions, making it challenging to achieve in solids at visible wavelengths and elevated temperatures. However, disorder effects can be mitigated by raising the oscillator strength of atom-like electronic transitions-excitons-that arise in monolayers of transition-metal dichalcogenides(16,17). Quantum interference, probed by second-harmonic generation(18,19), emerges in monolayer WSe2, without a cavity, to split the frequency-doubled laser spectrum. The splitting exhibits spectral anticrossing behaviour, and is related to the number of Rabi flops the strongly driven system undergoes. The second-harmonic generation power-law exponent deviates strongly from the canonical value of 2, showing a Fano-like wavelength dependence that is retained at room temperature. The work opens opportunities in solid-state quantum-nonlinear optics for optical mixing, gain without inversion and quantum-information processing
Time-Domain Interferometry of Surface Plasmons at Nonlinear Continuum Hot Spots in Films of Silver Nanoparticles
Nonlinear continuum generation from diffraction-limited hot spots in rough silver films exhibits striking narrow-band intensity resonances in excitation wavelength. Time-domain Fourier spectroscopy uncovers how these resonances arise due to the formation of a “plasmon staircase”, a discreteness in the fundamental oscillation of the plasmon excitations responsible for generating the white-light continuum. Whereas multiple scattering from discrete antennas can be invoked to explain hot spot formation in random assemblies of isolated particles, hot spots in films of fused nanoparticles are excited by interfering propagating surface plasmons, launched by scattering from individual nanoparticle antennas. For closed films, discrete propagating plasmons interact coherently over distances of tens of microns to pump the hot spot
Nanotesla magnetoresistance in π-conjugated polymer devices
We demonstrate submicrotesla sensitivity of organic magnetoresistance in thin-film diodes made of the conducting polymer poly(styrene sulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS). The magnetoresistance sensitivity is shown to be better than 20 parts per billion (ppb). As for other conjugated polymers, magnetoresistance can be separated into two regimes of field strength: the nonmonotonic ultrasmall magnetic field effect on magnetic field scales below 2 mT, and the monotonic intermediate magnetic field effect on scales over several tens of mT. The former gives the PEDOT:PSS magnetoresistance curve a characteristic W-shaped functionality, with inverted turning points compared to those found in conventional organic light-emitting diode (OLED) devices. We succeed in resolving the ultrasmall magnetic field effect of the PEDOT: PSS layer incorporated within an OLED structure, which is responsible for an additional magnetoresistive feature on the ppm scale. Such a device shows unprecedented complexity in magnetoresistance with a total of four extrema within a field range of +/- 1 mT. We propose that these unique characteristics arise from spin-spin interactions in the weakly bound carrier pairs responsible for the spin-dependent recombination probed in magnetoresistance
Electrical detection of ferromagnetic resonances with an organic light-emitting diode
Organic semiconductors show strong magnetic-field effects in transport and luminescence because of inherently spin-dependent recombination. We explore whether paramagnetic resonance features can be enhanced in a hybrid structure comprising a thin yttrium iron garnet (YIG) film, undergoing ferromagnetic resonance (FMR) and an organic light-emitting diode (OLED). We investigate the effect of radio-frequency (RF) driving of this hybrid structure in a magnetic field. Under these conditions, an indirect bolometric effect enables the detection of FMR driven in the YIG film in the DC resistance of the OLED. The increased RF power absorption of the YIG film under resonance gives rise to a heating of the magnetic film. Subsequent heat transfer to the OLED causes a change in transport characteristics of the device. Good agreement of this electrically detected signal is found with a direct measurement of the RF power absorption. Using temperature dependent measurements, the thermal nature of the resistance signal is confirmed
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