Carrier Recombination Coefficient Research Articles

Numerical calculation of transient electric field considering space charge in oil-paper insulation of converter transformers

PurposeThe purpose of this paper is to develop a numerical simulation method based on the transient upstream finite element method (FEM) and Schottky emission theory to reveal the distribution characteristics of space charge in oil-paper insulation.Design/methodology/approachThe main insulation medium of the converter transformer in high voltage direct current transmission is oil-paper insulation. However, the influence of space charge is difficult to be fully considered in the insulation design and simulation of converter transformers. To reveal the influence characteristics of the space charge, this paper proposes a numerical simulation method based on Schottky emission theory and the transient upstream FEM. This method considers the influence of factors, such as carrier mobility, carrier recombination coefficient, trap capture coefficient and diffusion coefficient on the basis of multi-physics field coupling calculation of the electric field and fluid field.FindingsA numerical simulation method considering multiple charge states is proposed for the space charge problem in oil-paper insulation. Meanwhile, a space charge measurement platform based on the electrostatic capacitance probe method for oil-paper insulation structure is built, and the effectiveness and accuracy of the numerical simulation method is verified.Originality/valueA variety of models are calculated and analyzed by the numerical simulation method in this paper, and the distribution characteristics of the space charge and total electric field in oil-paper insulation medium with single-layer, polarity reversal of plate voltage and double-layer are obtained. The research results of this paper have the guiding significance for the engineering application of oil-paper insulation and the optimal design of converter transformer insulation.

A comprehensive analyses of aging characteristics of oil-paper insulation system in HVDC converter transformers

This paper investigates the dissolved gases evolution in transformer oil under combined DC/AC electrical-thermal stress. Dissolved gases detected in transformer aged insulation oil reveal that oil under AC electric field combined with thermal stress can produce more dissolved gases than oil under DC electric field with thermal stress but less than the gases produced in oil under distorted AC or combined AC/DC voltage stress. This is attributed to the divergent migration properties of the charged components under different types of electric fields. To further understand this behavior, carrier recombination coefficient is proposed to explain the oil DGA results under various aging stresses. Results show that diagnostic parameters such as breakdown voltage, oil interfacial tension, and moisture content in pressboard should be used along with DGA results in order to accurately diagnose the insulation condition within converter transformers that impose a combined AC/DC voltage stress on the insulation system.

Impact of AC/DC electric field in thermally generated dissolved gases in transformer oil

Here we report the combined effect of electric field with thermal stress on dissolved gas analysis (DGA). Dissolved gases detected from accelerated aging experiments show that AC electric field combined with thermal stress leads to higher gas generation than DC electric field with thermal stress or by thermal stress alone. Research reveals that the differences are caused by the divergent migration properties of the charged components under AC and DC electric fields. A mathematical explanation correlates the DGA results by proposing a carrier recombination coefficient parameter. The study suggests that dissolved gas content does not reflect the insulation performance of transformers by simply employing DGA without considering the operating condition of the transformer. It is essential that a comprehensive evaluation criterion should accompany DGA in order to accurately determine the insulation status.

Space Charge Transport Characteristic Considering the Non-Uniform Electric Effect of Ion Mobility

Oil-paper insulation is the main insulating medium of converter transformers. Space charge can result in the distortion of electric field in oil-paper insulation, hence it's necessary to analyze the space charge transport characteristic and its influence on the oil-paper insulation. In the paper, the space charge transport characteristic considering the electric field is investigated, and an upstream finite element method is presented to calculate the space charge distribution in oil insulation of converter transformer considering the influence of injection barrier, ion mobility, trapped coefficient and carrier recombination coefficient. Finally, the insulation state of tortuous oil-path is evaluated. Experiments were conducted to analyze the relationship between ion mobility and electric field intensity, the results indicate that the influence of space charge on the electric field intensity in the tortuous oil-path can not be ignored for the design of converter transformer.

A comparative study of photoluminescence internal quantum efficiency determination method in InGaN/GaN multi-quantum-wells

The photoluminescence (PL) measurement is considered to be a powerful and convenient method to determine the internal quantum efficiency (IQE) of InGaN/GaN multi-quantum-wells. However, as different calculation models are developed for different PL experiments, the calculated IQE values can sometimes differ a lot, mainly due to different experimental conditions. In this paper, a comparative study is presented among the temperature-dependent PL (TDPL) measurement and power-dependent PL (PDPL) measurement and time-resolved PL (TRPL) measurement, and the relationship between different IQE calculation models is analyzed both theoretically and experimentally. It is revealed that despite distinct IQE values, different PL methods are actually compatible with each other. The discrepancy in TDPL and TRPL results comes from the temperature dependent relationship between the carrier concentration and carrier generation rate. Through a careful conversion of the excitation condition, both TDPL and TRPL results are correspondent to an IQE value under a specific excitation power in the PDPL measurement. Finally, through the combination of different PL measurements, an accurate measurement of IQE and carrier recombination coefficients can be achieved at different temperatures and excitation conditions.

Explanation of low efficiency droop in semipolar (202¯1¯) InGaN/GaN LEDs through evaluation of carrier recombination coefficients.

We report the carrier dynamics and recombination coefficients in single-quantum-well semipolar (202¯1¯) InGaN/GaN light-emitting diodes emitting at 440 nm with 93% peak internal quantum efficiency. The differential carrier lifetime is analyzed for various injection current densities from 5 A/cm2 to 10 kA/cm2, and the corresponding carrier densities are obtained. The coupling of internal quantum efficiency and differential carrier lifetime vs injected carrier density (n) enables the separation of the radiative and nonradiative recombination lifetimes and the extraction of the Shockley-Read-Hall (SRH) nonradiative (A), radiative (B), and Auger (C) recombination coefficients and their n-dependency considering the saturation of the SRH recombination rate and phase-space filling. The results indicate a three to four-fold higher A and a nearly two-fold higher B0 for this semipolar orientation compared to that of c-plane reported using a similar approach [A. David and M. J. Grundmann, Appl. Phys. Lett. 96, 103504 (2010)]. In addition, the carrier density in semipolar (202¯1¯) is found to be lower than the carrier density in c-plane for a given current density, which is important for suppressing efficiency droop. The semipolar LED also shows a two-fold lower C0 compared to c-plane, which is consistent with the lower relative efficiency droop for the semipolar LED (57% vs. 69%). The lower carrier density, higher B0 coefficient, and lower C0 (Auger) coefficient are directly responsible for the high efficiency and low efficiency droop reported in semipolar (202¯1¯) LEDs.

Photogeneration and recombination of charge carrier pairs and free charge carriers in polymer/fullerene bulk heterojunction films

AbstractPhoto‐generation and recombination of free charge carriers in poly‐3 (hexylthiophene) (P3HT) and [6,6]‐phenyl C61 butyric acid methyl ester (PCBM) blend films has been studied at different PCBM concentrations by means of fluorescence spectroscopy and transient photocurrent methods. We show that more than 80% of excitons form charge transfer (CT) states at PCBM concentrations above 4%. Efficiency of the CT state dissociation into free charge carries strongly depends on the PCBM concentration; the dissociation efficiency increases more than 30 times when PCBM concentration increases from 1 to 32%. We attribute the strong concentration dependence to formation of PCBM clusters facilitating electron migration and/or delocalization. Reduced charge carrier recombination coefficient has also been observed at high PCBM concentrations. We suggest that this may be partly caused by the reduced stability of reformed Coulombicaly bound charge pairs.

Recombination coefficients of GaN-based laser diodes

We measure the charge carrier recombination coefficients of InGaN quantum wells by analyzing the dynamical properties of (Al,In)GaN laser diodes emitting in the violet spectral range. Relaxation oscillations and turn-on delays are fitted to a rate equation model including a charge carrier density dependent recombination rate. Using optical gain spectroscopy we can directly determine the injection efficiency of the devices and thereby separate the effect of charge carrier leakage from that of carrier recombination. We find a third-order recombination coefficient of (4.5±0.9)×10-31cm6s-1 which is in agreement with theoretical predictions for phonon- and alloy-disorder-assisted Auger scattering.

Luminescence decay in highly excited GaN grown by hydride vapor-phase epitaxy

Carrier recombination dynamics in GaN grown by hydride vapor-phase epitaxy has been studied by means of transient photoluminescence under high photoexcitation conditions that are close to stimulated emission regime. The luminescence transient featured an exponential decay with the time constant of 205 ps at room temperature. The transient was shown to be in good agreement with a model of saturated centers of nonradiative recombination with the trap density of ∼1017 cm−3 and carrier recombination coefficients of ∼10−8 cm3/s. In such a regime, the lifetimes of electrons and holes have a common value of 410 ps.

Influence of separate confinement heterostructures on the effective carrier recombination coefficient in quantum well laser structures

We analyzed the carrier recombination coefficient of quantum well laser structures by combining two-level ambipolar rate equations and a carrier diffusion equation for the separate confinement heterostructure (SCH) layer. We derived a new analytical expression for the effective carrier recombination coefficient (Beff) in quantum well laser structures. From our analysis, we found out that the dominant factor that determines Beff is not the diffusion across the SCH layer but the ratio of confined carriers in the well to the total carriers in both the well and the SCH layers. Our new expression well explained the measured results of 1.3 μm strained-layer quantum well lasers with SCH layers of different thicknesses.

Separate-confinement heterostructure dependence of the effective carrier recombination coefficient of strained InGaAs/InGaAsP multiple quantum well lasers

We experimentally investigated the separate-confinement heterostructure (SCH) layer thickness and SCH band-gap wavelength dependence of the effective carrier recombination coefficient of strained InGaAs/InGaAsP multiple quantum well lasers. The dependence is explained by the carrier transport between the SCH layers and the wells.

High-speed operation of strained InGaAs/InGaAsP MQW lasers under zero-bias condition

5 High-speed zero-bias operation of 1.5- mu m In/sub 0.62/Ga/sub 0.38/As/In compressively-strained-MQW (multi-quantum-well) lasers at high temperatures is reported. This is achieved by optimizing the SCH (separate confinement heterostructure) composition to minimize the lasing delay time. Using a laser with an optimized SCH composition, zero bias 1-Gb/s modulation at 70 degrees C with a large eye opening time of 700 ps is demonstrated. In the optimization, strong carrier lifetime dependence on the SCH composition is observed. To see this dependence more precisely, the SCH structure dependence of effective carrier recombination coefficient B, which is an essential parameter for the carrier lifetime, has been experimentally investigated. >

Motion and recombination of charge carriers in a polydiacetylene single crystal

From photocurrent transients excited by a ruby laser γ/μ = (6.3±1.5) × 10 –6 V cm is derived for the ratio of carrier recombination coefficient to mobility in polydiacetylene-bis (toluenesulfonate) single crystals. The sum of electron and hole mobility in chain direction is 2.8 cm 2/V s with a uncertainly of a factor of 2 in both directions.