We demonstrate that nuclear spin fluctuations lead to the electric current noise in the mesoscopic samples of organic semiconductors showing the pronounced magnetoresistance in weak fields. For the bipolaron and electron-hole mechanisms of organic magnetoresistance, the current noise spectrum consists of the high frequency peak related to the nuclear spin precession in the Knight field of the charge carriers and the low frequency peak related to the nuclear spin relaxation. The shape of the spectrum depends on the external magnetic and radiofrequency fields, which allows one to prove the role of nuclei in magnetoresistance experimentally.
In organic light emitting diodes with small area the current may be dominated by a finite number, N of sites in which the electron-hole recombination occurs. As a result, averaging over the hyperfine magnetic fields, b_h, that are generated in these sites by the environment nuclei is incomplete. This creates a random (mesoscopic) current component, ΔI(B), at field B having relative magnitude ~ N^(-1/2). To quantify the statistical properties of ΔI(B) we calculate the correlator K(B, ΔB)= <δI(B - ΔB/2)δI(B + ΔB/2)> for parallel and perpendicular orientations of ΔB. We demonstrate that mesoscopi…
The charge transport in some organic semiconductors demonstrates nonlinear properties and further universal power-law scaling with both bias and temperature. The physical origin of this behavior is investigated here using variable range hopping theory. The results shows, this universal power-law scaling can be well explained by variable range hopping theory . Relation to the recent experimental data is also discussed.
A theory is introduced for spin relaxation and spin diffusion of hopping carriers in a disordered system. For disorder described by a distribution of waiting times between hops (e.g. from multiple traps, site-energy disorder and/or positional disorder) the dominant spin relaxation mechanisms in organic semiconductors (hyperfine, hopping-induced spin-orbit, and intra-site spin relaxation) each produce different characteristic spin relaxation and spin diffusion dependences on temperature. The resulting unique experimental signatures predicted by the theory for each mechanism in organic semicondu…
Wetzelaer, Koster, and Blom [PRL 107, 066605] recently observed that the classic Einstein relation $\frac{D}μ=\frac{kT}{q}$ is still valid in disordered semiconductors in thermal(quasi)equilibrium by studying the diffusion-driven currents of single-carrier diodes. This is in contrast to the previous theoretical prediction [2] and experiment observation from Leo et.al. [3] where the Einstein relation increases with 1/T. We will discuss the reason for this discrepancy here and suggest that one-carrier diodes experiment might nor be suitable for verifying Einstein relation in organic semiconducto…
A recent theory of magnetoresistance in positionally disordered organic semiconductors is extended to include exchange and dipolar couplings between polarons. Analytic results are discovered when the hyperfine, exchange, and dipolar interactions have little time to operate between hopping events. We find an angle-of-field dependence of the magnetoresistance that agrees with previous experiments and numerical simulations. In addition we report new magnetoresistive behavior that critically depends upon the amount of anisotropy in the dipolar interaction.
High charge mobility in active layers of organic electronic devices is often necessary for their efficient operation. As a result, search for high-mobility materials among the plethora of synthesizable organic semiconductors is of paramount importance for organic electronics. However, a model for rapid but reliable prediction of charge mobility in various organic semiconductors is still lacking. To solve this issue, we propose a simple analytical model that considers the most essential factors governing the charge transport in these materials: intermolecular electronic coupling, charge delocal…
In this article we briefly review the current state of the experimental research on spin polarized transport in organic semiconductors. These systems, which include small molecular weight compounds and polymers, are central in the rapidly maturing area of organic electronics. A great deal of effort has been invested in the last eight years toward understanding spin injection and transport in organics. These developments have opened up the possibility of realizing a new family of organic spintronic devices which will blend the chemical versatility of organic materials with spintronic functional…
We study persistent currents in semiconductor ballistic rings with spin-orbit Rashba interaction. We use as a working model the multicomponent Tomonaga-Luttinger liquid which arises due to the nonparabolic dispersion relations of electrons in the rings with rather strong spin-orbit coupling. This approach predicts some new characteristic features of persistent currents, which may be observed in experimental studies of semiconductor ballistic rings.
In this paper, we demonstrate the principle of operation of a metal-insulator-semiconductor (MIS) capacitor based on undoped organic semiconductor. In spite of low charge concentration within the semiconductor, this device exhibits a capacitance variation with respect to applied gate voltage $V_g$, resembling the capacitance-voltage $C$-$V$ characteristics of a traditional doped semiconductor based MIS capacitor. A physics based model is developed to derive charge concentration, surface potential $ψ_s$ and the capacitance of organic MIS capacitor. The model is validated with TCAD simulation re…
We report charge transport measurements in nanoscale vertical pillar structures incorporating ultrathin layers of the organic semiconductor poly(3-hexylthiophene)(P3HT). P3HT layers with thickness down to 5 nm are gently top-contacted using wedging transfer, yielding highly reproducible, robust nanoscale junctions carrying high current densities (up to $10^6$ A/m$^2$). Current-voltage data modeling demonstrates excellent hole injection. This work opens up the pathway towards nanoscale, ultrashort-channel organic transistors for high-frequency and high-current-density operation.
In this work electronic structures and optical properties of organic-inorganic exciton and polaritons in two-dimensional heterostructures combining both organic and semiconductor materials are studied. In those systems, Wannier-Frenkel hybrid exciton has unique and interesting properties that can improve the efficiency of optical materials. When an organic-semiconductor combined heterostructure is illuminated by high-intensity electromagnetic radiation with the frequency of the photons at or near the resonance frequency of the Wannier-Frenkel exciton, we obtain a macroscopically occupied syste…
By combining quantum simulations of electron transport and scanning-gate microscopy, we have shown that the current transmitted through a semiconductor two-path rectangular network in the ballistic and coherent regimes of transport can be paradoxically degraded by adding a third path to the network. This is analogous to the Braess paradox occurring in classical networks. Simulations reported here enlighten the role played by congestion in the network.
Despite the recent interest in "organic spintronics", the dominant spin relaxation mechanism of electrons or holes in an organic compound semiconductor has not been conclusively identified. There have been sporadic suggestions that it might be hyperfine interaction caused by background nuclear spins, but no confirmatory evidence to support this has ever been presented. Here, we report the electric-field dependence of the spin diffusion length in an organic spin-valve structure consisting of an Alq3 spacer layer, and argue that this data, as well as available data on the temperature dependence …
We discuss the possibility of diffusive conduction and thus of suppression of shot noise by a factor 1/3 in mesoscopic semiconductor devices with two-dimensional and one-dimensional potential disorder, for which existing experimental results do not provide a conclusive result. On the basis of our numerical analysis, we conclude that it is quite difficult to achieve diffusive transport over a reasonably wide parameter range, unless the device dimensions are increased up to the macroscopic scale. In addition, in the case of one-dimensional disorder, some mechanism capable of mode-mixing has to b…
An inherent difficulty in studying mesoscopic effects in semiconductor--superconductor hybrid structures is the large Schottky barrier which often forms at the interface. A large technological effort has been invested in in improving the contact between the superconductor and the two-dimensional electron gas (2DEG) of a semiconductor heterostructure, and in recent years this has become possible for e.g. GaAs-Al, GaAs-In, and InAs-Nb junctions. This development motivates quantitative theoretical modeling of sample-specific transport properties. The aim of our work is to model the conducting pro…
The perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) and 1,4,5,8-naphthalene-tetracaboxylic-dianhydride (NTCDA) are planar pi-stacking organic molecules that have been shown to be excellent model compounds for studying the growth and optoelectronic properties of organic semiconductor thin films, particularly organic diodes. Some observations have shown that this molecules, particularly PTCDA a brick-like shaped molecule easily forms well-ordered films on various substrates due to its unique crystal structure which is characterized by flat lying molecules In this work we will explore some …
We show that the performance of pentacene transistors can be significantly improved by maximizing the interfacial area at single walled carbon nanotube (SWCNT)/pentacene. The interfacial areas are varied by anchoring short SWCNTs of different densities (0-30/μm) to the Pd electrodes. The average mobility is increased three, six and nine times for low, medium and high SWCNT densities, respectively, compared to the devices with zero SWCNT. The current on-off ratio and on-current are increased up to 40 times and 20 times with increasing the SWCNT density. We explain the improved device performanc…
Taking into account the spin precession caused by the spin-orbit splitting of the conduction band in semiconductor quantum wells, we have calculated the Fourier spectra of conductance and state-density correlators in a 2D ring, in order to investigate the structure of the main peak corresponding to Aharonov-Bohm oscillations. In narrow rings the peak structure is determined by the competition between the spin-orbit and the Zeeman couplings. The latter leads to a peak broadening, and produces the peak splitting in the state-density Fourier spectrum. We have found an oscillation of the peak inte…
We report on the strong coupling between the Bloch surface wave supported by an inorganic multilayer structure and $J$-aggregate excitons in an organic semiconductor. The dispersion curves of the resulting polariton modes are investigated by means of angle-resolved attenuated total reflection as well as photoluminescence experiments. The measured Rabi splitting is 290 meV. These results are in good agreement with those obtained from our theoretical model.
We present an investigation of the microscopic interplay between excitons and charge carriers by means of combined photoluminescence (PL) and charge carrier transport measurements on organic thin film transistors (OTFT). For this purpose, the prototypical organic semiconductor Diindenoperylene (DIP) was utilized as active material. The OTFT accumulation layer provides a spatially defined interaction zone for charges and photo-generated excitons leading to a PL intensity reduction of up to 4.5%. This effect correlates with the accumulated hole carrier density and provides a lower estimate of ab…
A small magnetic field (~30 mT) can effectively modulate the electroluminescence, conductance and/or photocurrent of organic semiconductor based devices, up to 10% at room temperature. This organic magnetic field effect (OMFE) is one of the most unusual phenomena of both organic electronics and, more basically, magnetism, since all device components are nonmagnetic. However, in spite of latest surge of research interest, its underlying mechanism is still hotly debated. Here we experimentally identify that the magnetic field induced increase of intersystem crossing rate (between either excitons…
Charge transport properties in organic semiconductors are determined by two kinds of microscopic disorders, namely energetic disorder related to the distribution of localized states and the spatial disorder related to the morphological features of the material. From a semi-classical picture, the charge transport properties are crucially determined by both the carrier mobility and the electrostatic field distribution in the material. Although the effect of disorders on carrier mobility has been widely studied, how electrostatic field distribution is distorted by the presence of disorders and it…
Understanding charge transport in organic semiconductors in large electric fields is relevant to many applications. We present transport measurements in organic field-effect transistors based on poly(3-hexylthiophene) and 6,13-bis(triisopropyl-silylethynyl) pentacene with short channels, from room temperature down to 4.2 K. Near 300 K transport in both systems is well described by thermally assisted hopping with Poole-Frenkel-like enhancement of the mobility. At low temperatures and large gate voltages, transport in both materials becomes nearly temperature independent, crossing over into fiel…
Over the last two decades organic spintronics has developed into a striving field with exciting reports of long spin diffusion lengths and spin relaxation times in organic semiconductors (OSCs). Easily processed and inexpensive, OSCs are considered a potential alternative to inorganic materials for use in spintronic applications. Spin currents have been detected in a wide range of materials, however, there is still uncertainty over the origin of the signals. Recently, we explored spin transport through an organic semiconductor with lateral spin injection and detection architectures, where the …
We investigate the role of the nuclear spin quantum dynamics in hyperfine-induced spin relaxation of hopping carriers in organic semiconductors. The fast hopping regime with a small carrier spin precession during a waiting time between hops is typical for organic semiconductors possessing long spin coherence times. We consider this regime and focus on a carrier random walk diffusion in one dimension, where the effect of the nuclear spin dynamics is expected to be the strongest. Exact numerical simulations of spin systems with up to 25 nuclear spins are performed using the Suzuki-Trotter decomp…
We consider the non-radiative resonant energy transfer from a two-dimensional Wannier exciton (donor) to a Frenkel exciton of a molecular crystal overlayer (acceptor). We characterize the effect of the optical anisotropy of the organic subsystem on this process. Using realistic values of material parameters, we show that it is possible to change the transfer rate within typically a factor of two depending on the orientation of the crystalline overlayer. The resonant matching of donor and acceptor energies is also partly tunable via the organic crystal orientation.
We photo-excite slab polymer waveguides doped with J-aggregating dye molecules and measure the leaky emission from strongly coupled waveguide exciton polariton modes at room temperature. We show that the momentum of the waveguide exciton polaritons can be controlled by modifying the thickness of the excitonic waveguide. Non-resonantly pumped excitons in the slab excitonic waveguide decay into transverse electric and transverse magnetic strongly coupled exciton waveguide modes with radial symmetry. These leak to cones of light with radial and azimuthal polarizations.
We present a theory of organic magnetoresistance (OMR) based on the quenching of the quantum correlation between the carrier's spin and its local environment when the incoherent hopping takes place. We prove that this process contributes a spin-dependent prefactor to the attempt-to-escape frequency in the hopping rate, with its value modulated by the magnetic field. The resulting OMR exhibits a positive Lorentzian saturation component and a negative small-field component, which are independent of model parameters. These behaviors, with their isotope effects, are in good agreement with experime…
The magneto-electronic field effects in organic semiconductors at high magnetic fields are described by field-dependent mixing between singlet and triplet states of weakly bound charge carrier pairs due to small differences in their Landé g-factors that arise from the weak spin-orbit coupling in the material. In this work, we corroborate theoretical models for the high-field magnetoresistance of organic semiconductors, in particular of diodes made of the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) at low temperatures, by conducting magnetoresistance …
The complex admittance of metal/oxide/pentacene thin film junctions is investigated under ambient conditions. At low frequencies, a contribution attributed to proton diffusion through the oxide is seen. This diffusion is shown to be anomalous and is believed to be also at the origin of the bias stress effect observed in organic field effect transistors. At higher frequencies, two dipolar contributions are evidenced, attributed to defects located one at the organic/oxide interface or within the organic, and the other in the bulk of the oxide. These two dipolar responses show different dynamic p…
Magnetoelectroluminescence (MEL) of organic semiconductor has been experimentally tuned by adopting blended emitting layer consisting of both hole and electron transporting materials. A theoretical model considering intermolecular quantum correlation is proposed to demonstrate two fundamental issues: (1) two mechanisms, spin scattering and spin mixing, dominate the two different steps respectively in the process of the magnetic field modulated generation of exciton; (2) the hopping rate of carriers determines the intensity of MEL. Calculation successfully predicts the increase of singlet excit…
The theoretical effects of phase separation on encounter-limited charge carrier recombination in organic semiconductor blends are investigated using kinetic Monte Carlo (KMC) simulations of pump-probe experiments. Using model bulk heterojunction morphologies, the dependence of the recombination rate on domain size and charge carrier mobility are quantified. Unifying competing models and simulation results, we show that the mobility dependence of the recombination rate can be described using the power mean of the electron and hole mobilities with a domain size dependent exponent. Additionally, …
Inducing an inversion layer in organic semiconductors is a highly nontrivial, but critical, achievement for producing organic field-effect transistor (OFET) devices, which rely on the generation of inversion, accumulation, and depletion regimes for successful operation. In recent years, a major milestone was reached: an OFET was made to successfully operate in the inversion-mode for the first time. Here, we develop a pulsed bias technique to characterize the dopant type of any organic material system, without prior knowledge or characterization of the material in question. We use this techniqu…
We explore the generation of anti-Stokes fluorescence from strongly coupled organic dye microcavities following resonant ground-state excitation. We observe polariton emission along the lower polariton branch, with our results indicating that this process involves a return to the exciton reservoir and the absorption of thermal energy from molecules in a vibrationally excited ground-state. We speculate that the generation of a population of "hot" polaritons is enhanced by the fact that the cavity supresses the emission of Stokes-shifted fluorescence, as it is located energetically below the cut…
Time-resolved and temperature-dependent photoluminescence measurements on one-dimensional sexithiophene lattices reveal intrinsic branching of photoexcitations to two distinct species: self-trapped excitons and dark charge-transfer excitons (CTX; > 5% yield), with radii spanning 2-3 sites. The significant CTX yield results from the strong charge-transfer character of the Frenkel exciton band due to the large free exciton bandwidth (~400 meV) in these supramolecular nanostructures.
A model of charge hopping transport that accounts for anisotropy of localized states and Coulomb interaction between charges is proposed. For the anisotropic localized states the degree of orientation relates exponentially to the ratio of conductivities in parallel and perpendicular directions, while the ratio of Seebeck coefficients stays nearly unaffected. However, the ratio of Seebeck coefficients increases if Coulomb interaction is screened stronger in a direction parallel to the predominant orientation of the localized states. This implies two different physical mechanisms responsible for…
Whereas spintronics brings the spin degree of freedom to electronic devices, molecular/organic electronics adds the opportunity to play with the chemical versatility. Here we show how, as a contender to commonly used inorganic materials, organic/molecular based spintronics devices can exhibit very large magnetoresistance and lead to tailored spin polarizations. We report on giant tunnel magnetoresistance of up to 300% in a (La,Sr)MnO3/Alq3/Co nanometer size magnetic tunnel junction. Moreover, we propose a spin dependent transport model giving a new understanding of spin injection into organic …
The electron-hole pair created via photon absorption in organic photoconversion systems must overcome the Coulomb attraction to achieve long-range charge separation. We show that this process is facilitated through the formation of excited, delocalized band states. In our experiments on organic photovoltaic cells, these states were accessed for a short time (<1ps) via IR optical excitation of electron-hole pairs bound at the heterojunction. Atomistic modelling showed that the IR photons promote bound charge pairs to delocalised band states, similar to those formed just after singlet exciton di…
We have experimentally tested the hypothesis of free charge carrier mediated spin-transport in the small molecule organic semiconductor Alq3 at room temperature. A spin current was pumped into this material by pulsed ferromagnetic resonance of an adjacent NiFe layer, while a charge current resulting from this spin current via the inverse spin-Hall effect (ISHE) was detected in a Pt layer adjacent on the other side of the Alq3 layer, confirming a pure spin current through the Alq3 layer. Charge carrier spin states in Alq3, were then randomized by simultaneous application of electron paramagneti…
Organic light emitting devices and solar cells are machines that create, manipulate and destroy excited states in organic semiconductors. It is crucial to characterize these excited states, or excitons, to optimize device performance in applications like displays and solar energy harvesting. This is complicated if the excited state is a triplet because the electronic transition is dark with a vanishing oscillator strength. As a consequence, triplet state spectroscopy must usually be performed at cryogenic temperatures to reduce competition from non-radiative rates. Here, we control non-radiati…
One of the basic assumptions in organic field-effect transistors, the most fundamental device unit in organic electronics, is that charge transport occurs two-dimensionally in the first few molecular layers near the dielectric interface. Although the mobility of bulk organic semiconductors has increased dramatically, direct probing of intrinsic charge transport in the two-dimensional limit has not been possible due to excessive disorders and traps in ultrathin organic thin films. Here, highly ordered mono- to tetra-layer pentacene crystals are realized by van der Waals (vdW) epitaxy on hexagon…
Separating the influence of hyperfine from spin-orbit interactions in spin-dependent carrier recombination and dissociation processes necessitates magnetic resonance spectroscopy over a wide range of frequencies. We have designed compact and versatile coplanar waveguide resonators for continuous-wave electrically detected magnetic resonance, and tested these on organic light-emitting diodes. By exploiting both the fundamental and higher-harmonic modes of the resonators we cover almost five octaves in resonance frequency within a single setup. The measurements with a common pi-conjugated polyme…
In organic semiconductors working under ambient circumstance, there are remarkable quantum effects lack of comprehensive understanding, and an exotic composite particle named charge transfer (CT) exciton is normally regarded as the key ingredient. Another essential substance is the phonons stemming from intra- and inter-molecular vibrations, and the relevant diagonal electron-phonon couplings give rise to spatial localization and the off-diagonal couplings refer to the dispersion of electron wavefunctions. In this work, we first propose a toy model based upon the fracton physics to phenomenolo…
The transport properties at finite temperature of crystalline organic semiconductors are investigated, within the Su-Schrieffer-Heeger model, by combining exact diagonalization technique, Monte Carlo approaches, and maximum entropy method. The temperature-dependent mobility data measured in single crystals of rubrene are successfully reproduced: a crossover from super- to sub-diffusive motion occurs in the range $150 \leq T \leq 200$ K, where the mean free path becomes of the order of the lattice parameter and strong memory effects start to appear. We provide an effective model which can succe…
Highly-anisotropic in-plane magneto-resistance (MR) in graphite (HOPG) samples has been recently observed (Y. Kopelevich et al., arXiv:1202.5642) which is negative and linear in low fields in some current direction while it is giant, super-linear and positive in the perpendicular direction. In the framework of the hopping conductance theory via non-zero angular momentum orbitals we link extraordinary MRs in graphite and in organic insulators (OMAR) observed in about the same magnetic fields. The theory predicts quadratic negative MR (NMR) when there is a time-reversal symmetry (TRS), and linea…
A theoretical proposal for testing Bell's inequality in mesoscopic systems is presented. We show that the entanglement of two electron spins can be detected in the spin filter effect in the mesoscopic semiconductor / ferromagnetic semiconductor / semiconductor junction. The current-current correlation function is calculated by use of the quantum scattering theory and we compare it with the local hidden variable theory. We also discuss the influence of an imperfect spin filter and derive the condition to see the violation of Bell's inequality experimentally.
Exploration of spin-currents in organic semiconductors (OSECs) induced by resonant microwave absorption in ferromagnetic substrates has been of great interest for potential spintronics applications. Due to the inherently weak spin-orbit coupling (SOC) of OSECs, their inverse spin Hall effect (ISHE) response is very subtle; limited by the microwave power applicable under continuous-wave (cw) excitation. Here we introduce a novel approach for generating significant ISHE signals using pulsed ferromagnetic resonance, where the ISHE is ~2-3 orders of magnitude larger compared to cw excitation. This…
We consider the resonant energy transfer from a two-dimensional Wannier exciton (donor) to a Frenkel exciton of a molecular crystal overlayer (acceptor) when the active medias are separated by a metallic layer, possibly an electrode. We characterize the effect of the surface plasmon on this process. Using realistic values of material parameters, we show that it is possible to change the transfer rate within typically a factor of 5 (up to 44 according to geometrical configuration). We then take into account the quenching of the organic luminescence due to the proximity with the metal. This latt…
