Excitons in direct band gap cubic semiconductors

Perturbation is applied to study of the Wannier-Mott excitons in direct band gap cubic semiconductors with a fourfold degenerate highest valence band. The fine structure of exciton energy levels is investigated. General formulae are derived for the matrix elements of the perturbation. From these expressions it is straightforward to obtain values of the fine structure splittings of the energy levels and the wave functions of corresponding staes in any order of perturbation theory. A comparison with results of previous works is made.     

Space-charge-limited magnetoresistance in organic semiconductors

We present a two-site model for large magnetic field effect on current of nonmagnetic organic semiconductor, based on bipolaron formation in the presence of both hyperfine field and an applied magnetic field. Under the framework of space-charge-limited current, we derived our current density and magnetoresistance expression. The magnetoresistance fit well with the Lorentzian empirical law. We discuss all parameters involved in, and elucidate the origin of positive and negative magnetoresistance under our model.     

Pressure-dependence of resistivity in organic semiconductors

Abstract

The electrical resistivities of seven organic semiconductors and two nickel organometallics were found to have pressure dependencies which can be expressed as very simple analytic forms. These dependencies suggest that simple theoretical models for the resistivity should be possible.     

Exciton binding energies in organic semiconductors

The exciton binding energy is one of the key parameters that govern the physics of many opto-electronic organic devices. It is shown that the previously reported values for the exciton binding energies in many organic semiconductors, which differ by more than an order of magnitude, can be consistently rationalized within the framework of the charging energy of the molecular units, with a simple dependence of the exciton binding energy on the length of these units. The implications of this result are discussed. PACS 71.35.-y; 78.20.-e; 78.66.Qn     

Trap-assisted recombination in disordered organic semiconductors

The trap-assisted recombination of electrons and holes in organic semiconductors is investigated. The extracted capture coefficients of the trap-assisted recombination process are thermally activated with an identical activation energy as measured for the hole mobility μ(p). We demonstrate that the rate limiting step for this mechanism is the diffusion of free holes towards trapped electrons in their mutual Coulomb field, with the capture coefficient given by (q/ε)μ(p). As a result, both the bimolecular and trap-assisted recombination processes in organic semiconductors are governed by the charge carrier mobilities, allowing predictive modeling of organic light-emitting diodes.     

Zener tunneling in one-dimensional organic semiconductors

Tunneling effect in one-dimensional organic semiconductors in the presence of an external electric field is studied within the framework of a tight-binding model and a nonadiabatic dynamical method. It is found that under a high electric field, electrons can transit from the valence band (VB) to the conduction band (CB), which is demonstrated to be Zener tunneling in organic semiconductors. The results also indicate a field-induced insulator-metal transition accompanied by the vanishing of the energy gap. It is found that, after the field is turned off, the Peierls phase cannot be recovered.     

Inversion of magnetoresistance in organic semiconductors

We report that organic semiconductors such as alpha-sexithiophene (alpha-6T) have magnetoresistance (MR) with unexpected sign changes; depending on applied voltage, temperature, and layer thickness, the resistance may either increase or decrease upon application of a small magnetic field (<100 mT). We propose that MR and the inversion of MR are due to the role of hyperfine interaction in a magnetic field, as illustrated by the recombination-limited regime.     

Space-charge-limited current fluctuations in organic semiconductors

Low-frequency current fluctuations are investigated over a bias range covering Ohmic, trap-filling, and space-charge-limited current regimes in polycrystalline polyacenes. The relative current noise power spectral density S(f) is constant in the Ohmic region, steeply increases at the trap-filling transition region, and decreases in the space-charge-limited-current region. The noise peak at the trap-filling transition is accounted for within a continuum percolation model. As the quasi-Fermi level crosses the trap level, intricate insulating paths nucleate within the Ohmic matrix, determining the onset of nonequilibrium conditions at the interface between the insulating and conducting phase. The noise peak is written in terms of the free and trapped charge carrier densities.     

Thermal-resistance properties of organic semiconductors

Conclusions These polymers may be used to produce thermal resistors. Thermal resistors based on polymers produced by condensation polymerization of paraphenylenediamine with tetrabromethane at 150 and 250 °C (whose I–V characteristics have a descending region) may be used in various types of automatic apparatus, control apparatus, protective apparatus, etc.Thermal resistors based on polymers produced by condensation polymerization of paraphenylenediamine with hexachlorethane at 150 and 350 ° C (whose nominal resistances decrease by four orders of magnitude as the temperature is increased from 20 to 300 ° C) can be used as temperature-sensitive pickups for temperature measurement and stabilization.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol.12, No. 3, pp. 90–93, March, 1969.     

Charge mobility measurement techniques in organic semiconductors

Organic electronics is currently experiencing a surge of activities worldwide prompted in part by recent advances in achieving high electronic mobility, light emission over a broad range of frequencies, demonstration of spin valve operation with giant magneto-resistance and other effects. Despite these encouraging promises, there are many roadblocks that hinder a broader proliferation of “plastic electronics” in contemporary technology. Such devices involve charge transport as a main process in their operation processes, and therefore, require high-performance charge-transporting materials. The charge-carrier mobility is the major determining factor for the speed of electronic devices. Arguably, the most significant of them is the challenge to achieve a comprehensive understanding of the fundamentals of charge injection and charge transport in organics. In spite of its simple definition, because of the difficulty of measuring velocity, accurate determination of the mobility of the carriers is not easy and indirect ways are used, each with its own advantages and disadvantages. The article discusses important mobility measurement techniques employed for organic optoelectronic devices.     

Einstein relation in chemically doped organic semiconductors

Based on the assumption of Gaussian energy distributions of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), analytical expressions of generalized Einstein relation in chemically doped organic semiconductor are developed, by approximation of Coulomb traps with a rectangle potential well. Numerical calculations show that traditional Einstein relations do not hold for chemically doped organic semiconductors. Similar to physical doping, the dependence of diffusion coefficient to mobility D/μ ratio on the carrier concentration has a maximum. An essential difference between chemical doping and physical doping is that, the D/μ ratio in chemically doped organic semiconductors depends not only on carrier concentration and doping concentration, but also on the applied electric field. PACS 71.20.Rv; 72.90.+y; 73.50.-h     

Recent advances in spin transport in organic semiconductors

The spin relaxation time is long in organic semiconductors because of the weak spin-orbit and hyperfine interactions,leading to intensive study on spin transport in organic semiconductors.The rapid progress towards utilizing spin degree of freedom in organic electronic devices is occurring.While the spin injection,transport and detection in organic semiconductors are demonstrated,the fundamental physics of these phenomena remains unclear.This paper highlights recent progress that has been made,focusing primarily on present experimental work.     

Causes of negative photoconductivity in liquid organic semiconductors

The authors analyze the applicability of the explanations of negative photoconductivity in solid semiconductors to liquid binary systems containing diethyl ether and an organic halogen derivative. Experiments confirm the explanation that the carrier concentration is reduced. The existing model is continued for the origin, drift, and recombination of the charge carriers, and it is used to derive equations for the equilibrium state, the conditions for occurrence of photoconductivity (positive or negative), and thermal and optical quenching. The representation of the short-range order and the equations agree qualitatively with previous measurements, which revealed regularities in the photoconductivities of certain liquids.     

Anisotropic charge transport in flavonoids as organic semiconductors

A quantum mechanical approach has been used to investigate on the potential for using two naturally occurring flavonoids: quercetin and luteolin as candidates for organic semiconductor. Selection of flavonoids enables to evaluate the effects of hydroxyl group structural features. The relationship between molecular packing and charge transport in flavonoids is presented. The calculated results indicate that quercetin should be an ideal candidate as high-performance p-type organic semiconductor material, while luteolin is predicted as n-type organic semiconductor material. The predicted maximum electron mobility value of quercetin is 0.075 cm² V^?1 s^?1, which appears at the orientation angle near 91°/271° of conducting channel on the reference planes b–c. Theoretical investigation of natural semiconductors is helpful for designing higher performance electronic materials used in biochemical and industrial field to replace expensive and rare organic materials.     

Polaron effect dependence of thermopower in organic semiconductors

A unified physical model for thermopower was presented in organic semiconductors, based on the Marcus theory and variable-range hopping theory. According to the proposed model, the characteristic of charge carrier thermoelectric transport in organic semiconductors has been investigated. In particular, polaron effects, energetic disorder, and carrier density dependence of the thermopower have been discussed in detailed. The calculation also shows a good agreement with the experimental data in organic semiconductors.     

Transport properties of orbitally hybridized organic semiconductors

A microscopic theory based on the orbital hybridization model via single orbital approximation is developed to calculate the current variation in organic semiconductors that are coupled to the external orbits from the environment. The charge transfer resulted from the orbital hybridization between the environment and the organic semiconductor rebuilds the energy levels and eventually alters the transport properties of the organic semiconductor. Two parameters in our theory, the orbital energy level of the environment relative to the energy level of organic semiconductor and the orbital hybridization interaction, dominate the current variation in the organic semiconductors. Our results show that the suppression of atomic dimerization due to orbital hybridization gives rise to an increase of electrical conduction in organic semiconductor. Also, after coupling with the environment, the charge-donating organic semiconductors are more conductive than the charge-accepting ones.     

Exciton dynamics at interfaces of organic semiconductors

We review recent progress of using time-resolved two-photon photoelectron spectroscopy (2PPE) to study the energetics and dynamics of excitons at surfaces and interfaces of two prototypical organic semiconductors: C₆₀ and pentacene. For C₆₀ thin films epitaxially grown on Au(1 1 1) and Cu(1 1 1) surfaces, we observe both charge transfer and exciton states. For excitons in C₆₀, the proximity of a metal surface leads to rapid, exciton band-mediated quenching. At the surface of pentacene thin films we observe a series of charge-transfer excitons where the electron and the photohole are bound across the interface. The ability of 2PPE to measure and directly relate exciton levels to single-electron levels is illustrated.     

Validity of the Einstein relation in disordered organic semiconductors

It is controversial whether energetic disorder in semiconductors is already sufficient to violate the classical Einstein relation, even in the case of thermal equilibrium. We demonstrate that the Einstein relation is violated only under nonequilibrium conditions due to deeply trapped carriers, as in diffusion-driven current measurements on organic single-carrier diodes. Removal of these deeply trapped carriers by recombination unambiguously proves the validity of the Einstein relation in disordered semiconductors in thermal (quasi)equilibrium.