Effects of recombination on trap-limited dispersive transport

The effects of monomolecular and bimolecular recombination on dispersive transport for a system with an exponential distribution of traps have been simulated using a Monte Carlo technique. Both the concentration of photo-excited carriers and the photocurrent fall off similarly for the two recombination modes, although the overall decay is somewhat slower for the bimolecular process. In each mode, the carrier concentration exhibits a power-law decay at long times, with the exponent proportional to the absolute temperature. We conclude that transient photoconductivity and photo-induced absorption measurements cannot determine the predominant recombination kinetics without a study of the dependence of the results on the incident light intensity. Furthermore, if the recombination mode is unknown, the conventional approach can give an erroneous result for the extent of the trap distribution.     

Picosecond decay kinetics of CdS luminescence studied by a streak camera

Using a picosecond laser and a streak camera we have observed the time dependence of the luminescence intensity of free excitons, bound excitons, and excitonic molecules in CdS. The observed kinetics show that the P band is due to bimolecular emission from free excitons and that bound excitons are generated from free excitons through monomelecular process and excitonic molecule through bimolecular process.     

Simulation study of the losses and influences of geminate and bimolecular recombination on the performances of bulk heterojunction organic solar cells

We use the method of device simulation to study the losses and influences of geminate and bimolecular recombinations on the performances and properties of the bulk heterojunction organic solar cells. We find that a fraction of electrons(holes)in the device are collected by anode(cathode). The direction of the corresponding current is opposite to the direction of photocurrent. And the current density increases with the bias increasing but decreases as bimolecular recombination(BR)or geminate recombination(GR) intensity increases. The maximum power, short circuit current, and fill factor display a stronger dependence on GR than on BR. While the influences of GR and BR on open circuit voltage are about the same.Our studies shed a new light on the loss mechanism and may provide a new way of improving the efficiency of bulk heterojunction organic solar cells.     

Features of charge-carrier recombination in amorphous molecular semiconductors doped with polymethine dyes

The photogeneration and recombination of charge carriers in poly-N-epoxypropylcarbazole films with additions of a polymethine dye are investigated irradiation of films with blocking contacts by light both within and outside the absorption range of the dye. The kinetics of the accumulation and relaxation of electron-hole pairs, whose lifetimes exceed tens and hundreds of seconds, are studied. It is postulated that an increase in the recombination luminescence intensity occurs in an electric field as a result of an increase in the efficiency of the bimolecular radiative recombination stimulated by trapped electrons from photogenerated excitons. Fiz. Tverd. Tela (St. Petersburg) 40, 629–635 (April 1998)     

Tunneling recombination luminescence under excitation of PbWO4:Mo crystals in the defect-related absorption region

Time-resolved emission and excitation spectra and luminescence decay kinetics were studied at 150-300 K for the green emission of PbWO₄:Mo crystals. It was found that the slow (μs-ms) decay component observed under excitation in the defect-related absorption region (around 3.8-3.9 eV) arises from the G(II) emission which appears at the tunneling recombination of optically created electron and hole centers. The study of the emission decay kinetics at different temperatures and excitation intensities allowed concluding that both the monomolecular and the bimolecular tunneling recombination process can be stimulated in the mentioned energy range. The monomolecular process takes place in the isolated spatially correlated pairs of electron and hole centers produced without release of electrons into the conduction band. The bimolecular process takes place in the pairs of randomly distributed centers created at the trapping of free electrons from the conduction band. The formation of electron centers under irradiation in the defect-related absorption region was investigated by the electron spin resonance (ESR) and thermally stimulated luminescence (TSL) methods. The possibility of various photo-thermally stimulated defects creation processes, which take place with and without release of free electrons into the conduction band, was confirmed.     

Dependence of the recombination kinetics of spatially separated electron-hole layers of the parallel magnetic field

The radiative recombination time of spatially separated electron-hole layers has been investigated as a function of the magnetic field parallel to the plane of a wide quantum well. The luminescence intensity has been found to decrease with increasing parallel magnetic field. This dependence is inconsistent with the theoretical predictions relating the decrease in the intensity to a decrease in the population of the energy levels allowed for radiative recombination. It has been shown that the intensity decrease is related to the exponential increase in the radiative recombination time with the magnetic field (due to a decrease in the overlapping of the electron and hole wavefunctions) and to the nonradiative recombination processes.     

Intensitive dependence of luminescence and photoconductivity for the electron-electron,electron-exciton and exciton-exciton interactions

Analytical solutions of the equations of kinetics were found for stationary states at specific external conditions, especially for high excitation intensities. This allowed us to establish analytical expressions of the lux-lux and lux-ampere characteristics and use them in analyzing the type of recombination mechanism, which corresponds to certain dependence of luminescence or photoconductivity on the excitation intensity.     

Absorption Spectral Studies Involving 4f–4f Transitions as Structural Probe in Chemical and Biochemical Reactions and Compositional Dependence of Intensity Parameters

Abstract

Absorption spectral studies on lanthanides are described, dealing with 4f–4f transitions as structural probe in chemical and biochemical reactions, and compositional dependence of intensity parameters in order to understand the structure and nature of lanthanide bimolecular interaction. This review is basically addressed to the suitable application of solution absorptional spectral spectroscopy: (i) in probing the binding characteristics and multimetal simultaneous coordination of biomolecules, (ii) following the different steps involved n complex sol-gel processes involving heterometallic alkoxides as precursors fore generation of suitable materials for future computing devices, and (iii) in pursuing the progress of some organic synthesis, specially Meerwin-Ponndorf-Verley reaction and Diels-Alder cycloaddition reaction, homogeneously catalysed by lanthanides. Solution kinetics of interactions of lanthanides involved in the reactions is presented.     

Bimolecular recombination coefficient as a sensitive testing parameter for low-mobility solar-cell materials

Bimolecular charge carrier recombination has been clarified in bulk-heterojunction solar cells based on a blend of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene using the time-of-flight method. We show how bimolecular recombination influences the charge carrier transport, how it limits the efficiency of low-mobility solar cells, and how to estimate the bimolecular recombination coefficient. We found that bimolecular recombination in these efficient photovoltaic materials is orders of magnitude slower as compared to Langevin recombination expected for low-mobility materials. This effect is inherent to the nanomorphology of the bicontinuous interpenetrating network creating separate pathways for electrons and holes, and paves the way for the fabrication of bulk-heterojunction solar cells where bimolecular recombination is not the limiting factor.     

Excitation Intensity Dependence of Free-exciton Transitions in GaN Grown by Low-pressure Metalorganic Chemical Vapor Deposition

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GaN深中心Zn的低温瞬态研究

本文研究了GaN:Zn的低温瞬态过程,同时测量了衰减中的时间分辨光谱,从两者测量的结果得出:2.89eV的光致发光寿命为300ns(2K).实验证实,Zn作为发光中心的同时也引起一些非辐射陷阱.2.89eV的瞬态曲线可分为两部分,在短时范围(1μs)内基本上是指数形式,可归结为导带中光激发的电子和束缚在ZnGa受主上空穴的复合.在长时范围(t>>1μs)内瞬态曲线则偏离了指数规律,相当好地符合Becqureal经验公式.     

Femtosecond dynamics of geminate recombination of radicals upon photodissociation of aromatic disulfides

The dynamics of the geminate recombination of thiyl radicals formed upon photodissociation of aromatic disulfides and the effect of the intramolecular relaxation on this recombination are studied using pico-and femtosecond kinetic spectroscopy. It is shown that, in terms of a phenomenological model, the geminate recombination of phenylthiyl radicals in neutral solvents can be satisfactorily described by a biexponential dependence. The model suggests the occurrence of primary geminate recombination in a solvent cage formed around an original molecule and secondary recombination controlled by the diffusional motion of the radicals of a pair. The primary geminate recombination, whose characteristic time (9 ps) is close to the characteristic times of intermolecular vibrational relaxation of complex molecules in solvents at room temperatures, masks the manifestation of thermalization processes in the time kinetics. The direct geminate recombination of aminophenylthiyl radicals with the formation of original molecules virtually does not occur because of the intramolecular charge transfer. In connection with this, the intermolecular vibrational relaxation manifests itself in the kinetics of the induced optical density in the region of the absorption band maximum of radicals as a growing component with a characteristic time of 6 ps.     

Photoconductivity decay in Te-Ge-Sb glasses

Photoconductivity kinetics in Te-Ge-Sb glasses in the region of bimolecular recombination is investigated as a function of temperature and time of illumination. An untypical shape of photoconductivity decay is interpreted in terms of Street and Mott model of recombination by tunnelling between randomly distributed D⁰ centers of concentration 10¹⁸–10¹⁹ cm^-3.     

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.     

Photopolymerization model for holographic gratings formation in photopolymers

This work describes the study of free-radical homopolymerization kinetics for a system based on acrylamide, triethanolamine, and methylene blue on a polyvinylalcohol matrix, by analyzing temporal variations of the diffraction efficiency. Due to the high viscosity of the material, it has been demonstrated that diffusion processes at the time of recording are negligible. So, the modulation index has been related to the parameters of the systems components, giving as a result, a method that can be used to estimate the chain-length of the polymer, the kinetic rate constants and absorption-scattering parameters. Depending on the termination step, two models of homopolymerization has been proposed – a bimolecular model (bimolecular termination) and a radicalic model (primary radical termination). Using these methods it was possible to compare the results obtained for the kinetic parameters of the photopolymerization process when the intensity and the concentration of each component were changed. Thus, holography can be used as holographic method to analyze the photopolymerization processes. PACS 42.70.Ln; 42.70.Jk; 42.40.Ht     

Exciton binding energies in II–VI compound strained layer superlattices

II–VI strained-layer superlattices are very efficient emitters of visible light. The dependence of the luminescence intensity on the excitation power density allows us to characterise the recombination processes involved in the emission. At low temperatures excitonic processes are dominant whereas electron-hole recombinations feature at room temperature. No special evidence of the dual nature of the emission is observed at intermediate temperatures because the optical transitions are broadened by well-width fluctuations. In spite of this we may estimate the exciton binding energy from the temperature dependence of the photoluminescence intensity, as long as the photoluminescence remains excitonic. This is the case for narrow wells in CdS---ZnS superlattices over the temperature range zero to room temperature. The estimated exciton binding energy measured in this way approaches the two dimensional limit but does not exceed it.     

High power non-linear magnetophotoconductivity in n-GaAs using the UCSB free electron laser

The kinetics of electrons bound to shallow donor impurities in n-GaAs was investigated by saturation spectroscopy using the University of California at Santa Barbara free electron laser. The resonant photothermal conductivity from 1s–2p⁺ transitions was measured at intensities greatly exceeding previous studies. Saturation of bound-to-free photoionization transitions was measured from 0 to 4 Tesla. The 1s–2p⁺ resonant photoconductive signal shows a distinct intensity dependence caused by the competing bound-to-free transitions which saturate differently. Evaluation of the electron recombination kinetics allows us to calculate the transition time of electrons from the 2p⁺ level to the ground state, the recombination time of free electrons, and the thermal ionization probability of the 2p⁺ state.     

Optical gain in photoconductors made of compensated and partially compensated GaAs

The dc gain behaviour of GaAs photoconductors realized utilizing a partially compensated buffer layer of an epitaxial MESFET structure as well as a Cr-doped semiinsulating substrate is studied. The light-power dependence of the gain hints to the dominant role of the bimolecular recombination process and trapmediated gain, and only a minor role of the surface photovoltaic effect. The possible correlation between dark current and gain mechanism in the MESFET-like device is pointed out.     

四苯基卟吩激发—重态衰变的“外部重原子效应”的研究

考查了在有芳烃卤代物RX(如氯苯、溴苯和碘苯)存在时,四苯基卟吩TPP的荧光衰变现象。基于荧光衰变动力学分析,发现:在RX和TPP之间生成复合物是引起重原子效应的一个基本步骤。荧光衰变的动力学参数则进一步表明:可观察的外部重原子效应主要是由于发生分子间电荷转移相互作用而引起的。     

双模自组织量子点光致发光的温度依赖性

采用稳态速率方程模型,对双模自组织量子点光致发光的温度依赖性进行了研究,模拟获得了不同温度下双模自组织量子点的光致发光光谱,并进一步研究了两组量子点分布的光致发光强度比的温度依赖性。研究表明:在低温下(<75K),两组量子点分布的发光强度比基本保持不变;随着温度的升高(75K相似文献