共轭聚合物薄膜中光生载流子的产生及输运机制

对共轭聚合物光生载流子的产生机制进行了初步探讨,分析了由最初产生的电子 空穴对经过晶格驰豫之后形成极化子 激子的热离化过程,认为同一链上的激子会迅速复合,只有链间激子对光电流作出贡献.研究了共轭聚合物中载流子的输运机制,导出了共聚物的电导率公式,其计算值与实验结果符合,我们认为是极化子的链间跃迁实现了整个共聚物的电导和光致发光,较好地解释了实验事实.     

Charge carrier photogeneration in polymers

The charge carrier photogeneration in polymers can be depicted as a multistage process that involves i) photoexcitation to a neutral molecular electronic state, ii) autoionization of the excited state, iii) thermalization of the hot electron, leading to the formation of a Coulomb field-bound geminate electron-hole pair, and iv) thermal dissociation of the electron-hole pair into free carriers. This model is proposed for all types of polymers with a large ocnjugated pendant group and both the saturated and non-saturated main chain. The separation distance of the electron-hole pairs ranges from 2 to 3 nm, and the primary quantum efficiency of the electron-hole pairs formation assumes values of several tenths of charge per photon. In intrinsic materials like substituted polyacetylenes both the interchain and intrachain charge transfer excitons can be formed. Onsager's theory can provide a satisfactory explanation of the dissociation step. In polymers with the saturated main chain and large conjugated pendant groups the charge-transfer excitons are very often stabilized by self-trapping in dimer deep traps. Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday     

Electron injection in amorphous 3-methylpentane

The main characteristics of the current obtained when photo-injecting electrons into glassy 3-methylpentane have been investigated: decrease of the thermalization range during glass configurational relaxation, space charge build-up inside the sample due to deeply trapped electrons in the glass and effect of the excitation light wavelength. The thermalization range is found to be ≈ 80Å.     

Time-dependent quantum efficiency in Onsager model

The time-dependent analytical solution of the Onsager problem with a Coulomb potential is used to calculate the quantum efficiency corresponding to a delayed application of the electric field after the creation of charge carriers. This experiment would measure the time-dependence of neutralization, and give information about the initial distribution of thermalization lengths.     

Generation of charge carrier pairs in tetracene layers

The mechanism of negative and positive charge carrier generation by light absorption in tetracene layers has been studied. We conclude that there are different processes determining electron and hole production. Positive charge carriers are produced without recombination while the negative charge carrier generation depends strongly on the recombination process. The experimental data for charge carrier generation in tetracene layers are treated theoretically taking into account photogeneration, recombination of charge carriers, trapping and transport processes inside the sample.     

Intrinsic intermediate gap states of TiO2 materials and their roles in charge carrier kinetics

Titanium dioxide (TiO₂) is regarded as an important prototype photocatalytic material for several decades. The charge carrier kinetics determines the photocatalytic properties of TiO₂ materials; this is found to be greatly dependent on electronic structures. It has been revealed that the intrinsic intermediate gap states (intrinsic GSs) play a significant role in charge carrier kinetics that drive the photocatalytic processes of TiO₂ materials, which are not well summarized until now. Motivated by this thought, the purpose of this review focuses on physiochemical science of the intrinsic GSs of TiO₂ materials and their important role in charge carrier kinetics. We first give a summary on the chemical resources of the intrinsic GSs in TiO₂ and their physiochemical nature. Their general energy distribution, charge carrier population, and the associated thermodynamic properties are also elaborated from an overall viewpoint. We further carefully summarize and compare the experimental studies on the energy and the density distribution of the intrinsic GSs and discuss the associated chemical resources and charge carrier localizations. Trapping is the dominant function of intrinsic GSs in the charge carrier kinetics of TiO₂ materials. The significant effect of trapping on the transport, recombination, and interfacial transfer of charge carriers are also comprehensive summarized. Furthermore, the effects of charge carrier kinetics on photocatalytic performances are also discussed to some extents. Because of the importance of intrinsic GSs in modulating charge carrier kinetics, it is expected to increase the photocatalytic activity by engineering the intrinsic GSs, not only for TiO₂ materials, but also for the other semiconductor photocatalysts.     

Measurement and energy correction due to charge carrier lifetimes in large HPGe spectrometers

This work addresses the energy spectrum correction due to increased charge carrier collection times in larger HPGe spectrometers. The energy of the radiation interaction is expected to be proportional to the total collected charge. This is increasingly not true with larger HPGe spectrometers. Some charge is lost as the total charge travels from the interaction location to the collection electrode. This path dependent loss of charge results in decreased energy resolution. In HPGe spectrometers, this process is characterized by the charge carrier lifetime constant and is given as an exponential function of the charge carrier collection time divided by this constant. Thus large detectors can experience exponential decrease in energy resolution as charge carrier collection time increases. We studied the effect of charge carrier lifetime on energy resolution for a p-type point contact HPGe spectrometers using pulse shape analysis. We present a method using the rise time to correct for the charge carrier lifetime on a pulse by pulse basis for a given HPGe spectrometer.     

Quasiresonance charge exchange of hydrogen isotopes mesic atoms in excited states

Processes of charge exchange of hydrogen isotopes mesic atoms in excited states at low collision energies 10^?2?E?1 eV are studied. The cross sections calculated depend on energy like ～E ^?1 and are of an order of the atomic cross sections (～10^?16 cm²). It is shown that the high rates (～10¹² s^?1) of charge exchange and thermalization of mesic atoms in excited states at the liquid hydrogen density are comparable with the rates of cascade transitions in mesic atoms.     

Dependence of charge transfer reorganization energy on carrier localisation in organic molecular crystals

Taking the organic molecular material dithiophene-tetrathiafulvalene (DT-TTF) as an example of a high mobility organic molecular material, we use density functional calculations to calculate the dependency of the reorganization energy associated with charge carrier transport on: (i) the geometric and electronic responsiveness of the local molecular crystal environment, and, (ii) the local spatial extent of the charge carrier. We find that in our most realistic extended models the charge transfer reorganization energy is strongly dependent on carrier localization. In particular, whereas highly localized carriers are found to be highly susceptible to their charge transfer efficiency being affected by changes in the local crystal environment, more delocalized carriers are better able to maintain their low reorganization energies. Considering that maintaining a relatively small charge transfer reorganization energy magnitude is an important factor in achieving high carrier mobilities, we suggest that those materials better able to sustain carriers with short-range thermally resistant intermolecular delocalisation should be sought for device applications.     

Efficient charge transport along phenylene-vinylene molecular wires

We have studied the motion of charge carriers along isolated phenylene-vinylene (PV) chains using a combination of experimental and theoretical methods. The conductive properties of positive charges along PV chains in dilute solution were studied by using the pulse-radiolysis time-resolved microwave conductivity (TRMC) technique. This technique enables the measurement of high-frequency (tens of GHz) charge carrier mobilities along isolated PV chains without the use of electrodes. The charge carrier mobility along PV chains with finite and infinite length was studied theoretically by charge transport simulations with parameters from density functional theory (DFT) calculations. The high-frequency charge carrier mobility is found to depend strongly on the conjugation length of the PV chains and is found to increase both with increasing length of the PV chain and with increasing conjugation fraction. The experimental results are in good agreement with the calculated results. On the basis of this combined experimental and theoretical study an intrachain charge carrier mobility of a few tens of cm2/Vs is expected for an infinitely long PV chain without conjugation breaks.     

Charge carrier mobility and concentration as a function of composition in AgPO3-AgI glasses

Conductivity data of the xAgI(1 - x)AgPO(3) system (0 ≤ x ≤ 0.5) were collected in the liquid and glassy states. The difference in the dependence of ionic conductivity on temperature below and above their glass transition temperatures (T(g)) is interpreted by a discontinuity in the charge carrier's mobility mechanisms. Charge carrier displacement occurs through an activated mechanism below T(g) and through a Vogel-Fulcher-Tammann-Hesse mechanism above this temperature. Fitting conductivity data with the proposed model allows one to determine separately the enthalpies of charge carrier formation and migration. For the five investigated compositions, the enthalpy of charge carrier formation is found to decrease, with x, from 0.86 to 0.2 eV, while the migration enthalpy remains constant at ≈0.14 eV. Based on these values, the charge carrier mobility and concentration in the glassy state can then be calculated. Mobility values at room temperature (≈10(-4) cm(2) V(-1) s(-1)) do not vary significantly with the AgI content and are in good agreement with those previously measured by the Hall-effect technique. The observed increase in ionic conductivity with x would thus only be due to an increase in the effective charge carrier concentration. Considering AgI as a weak electrolyte, the change in the effective charge carrier concentration is justified and is correlated to the partial free energy of silver iodide forming a regular solution with AgPO(3).     

有机半导体中载流子迁移率的测量方法*

本文简要地介绍了有机半导体中载流子迁移率的几种模型,着重阐述了测量有机半导体中载流子迁移率的各种方法的测试原理。主要有如下几种：稳态（CW）直流电流-电压特性法（steady-state DC J-V）,飞行时间法（time of flight, TOF）,瞬态电致发光法（transient electroluminescence,transient EL）,瞬态电致发光法的修正方法即双脉冲方波法和线性增压载流子瞬态法（carrier extraction by linearly increasing voltage,CELIV）,暗注入空间电荷限制电流（dark injection space charge limited current, DI SCLC）,场效应晶体管方法（field-effect transistor,FET）,时间分辨微波传导技术（time-resolved microwave conductivity technique,TRMC）,电压调制毫米波谱（voltage-modulated millimeter-wave spectroscopy,VMS）光诱导瞬态斯塔克谱方法（photoinduced transient Stark spectroscopy）,阻抗（导纳）谱法（impedance（admittance）spectroscopy）。说明了各种实验方法的应用范围、使用条件和优缺点。     

Charge carrier transport in poly[methyl(phenyl)silylene]: the effect of additives

Poly[methyl(phenyl)silylene], PMPSi, was doped with compounds of the electron acceptor type. The charge carrier mobility increases with increasing electron affinity of the acceptors having zero dipole moments. At the same time the energy distribution of hopping states narrows. On the other hand, the hole drift mobility is influenced by the dipole moment of the dopand. The electrostatic charge carrier‐dipole interaction causes a broadening of the energy distribution of transport states which results in a decrease in the charge carrier mobility. The charge carrier transport can be explained by the disordered polaronic theory, according to which the activation energy of charge carrier mobility has contributions based on the dynamic disorder, i.e. the polaronic barrier, and on the static disorder, i.e. the variation of the energy of transport states as a result of the environment. Copyright © 2001 John Wiley & Sons, Ltd.     

Interfacial charge transfer in carbon nitride heterojunctions monitored by optical methods

Solar energy conversion is inciting tremendous research efforts in many fields due to the vast potential of sunlight as a sustainable energy source. For solar energy to become widely used and become a major component of our energy mix, energy storage on large scales must be addressed and the components used must be abundant. Artificial photosynthesis to produce solar fuels holds promise as a way to convert solar energy into storable energy. Organic photocatalysts have rapidly established themselves as a viable alternative to inorganic systems. Organic photocatalyst can be prepared from inexpensive precursors and offer a synthetic versatility and tunability that can be exploited to improve efficiencies. Carbon nitride (CN_x) has emerged as a leading organic photocatalyst with advantageous chemical and photo stabilities. Recombination of photogenerated electrons and holes limit the efficiency of CN_x materials below levels necessary to become a viable energy production system. To improve the efficiency and key characteristics such as light harvesting, charge carrier lifetime, and interfacial rate of charge transfer, a second material is put in contact with CN_x to form a heterojunction. While there are many examples of heterojunctions improving the photocatalytic activity beyond that of the isolated CN_x, we are still lacking the deep understanding of charge carrier dynamics necessary to rationalize the improvements and design optimal junctions. This review covers the studies of CN_x heterojunctions that have used optical methods to monitor the charge carrier dynamics. Time-resolved photoluminescence (TRPL) is the most common technique used and there are many examples that have used transient absorption spectroscopy (TAS) to probe the charge carrier dynamics. However, attempting to link the lifetime change to the activity differences does not yield a clear trend. It is likely that the reactive charges are not consistently being monitored and is obscuring the expected correlations. Both shorter and longer charge carrier lifetimes can be observed with both TRPL and TAS techniques and can be interpreted as arising from interfacial charge separation. Even when the same materials are used in the junction there is no consistency in observing a shorter or longer lifetime. The holistic view of charge carrier dynamics in CN_x heterojunctions presented here intends to identify overarching themes from a wide range of CN_x-containing systems and help take stock of where our current understanding stands. More specific spectral assignments and linking the observed lifetimes to certain photophysical or photochemical processes are needed to build models to help us understand the links between the charge carrier dynamics and the activity. These are crucial to develop general strategies that will lead to optimal CN_x heterojunctions.     

Photoinduced charge carrier generation in a poly(3-hexylthiophene) and methanofullerene bulk heterojunction investigated by time-resolved terahertz spectroscopy

We report on the ultrafast photoinduced charge separation processes in varying compositions of poly(3-hexylthiophene) (P3HT) blended with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Through the use of time-resolved terahertz spectroscopy, the time- and frequency-dependent complex photoconductivity is measured for samples with PCBM weight fractions (WPCBM) of 0, 0.2, 0.5, and 0.8. By analysis of the frequency-dependent complex conductivity, both the charge carrier yield and the average charge carrier mobility have been determined analytically and indicate a short (<0.2 nm) carrier mean free path and a suppressed long-range transport that is characteristic of carrier localization. Studies on pure films of P3HT demonstrate that charge carrier generation is an intrinsic feature of the polymer that occurs on the time scale of the excitation light, and this is attributed to the dissociation of bound polaron pairs that reside on adjacent polymer chains due to interchain charge transfer. Both interchain and interfacial charge transfer contribute to the measured photoconductivity from the blended samples; interfacial charge transfer increases as a function of increasing PCBM. The addition of PCBM to the polymer films surprisingly does not dramatically increase the production of charge carriers within the first 2 ps. However, charge carriers in the 0.2 and 0.5 blended films survive to much longer times than those in the P3HT and 0.8 films.     

Gate Tunable Organic Light Emitting Diodes: Principles and Prospects

This record summarizes our recent developments on gate‐tunable organic light‐emitting diodes (OLEDs). The key point is to modulate the charge carrier injection barrier by the applied gate potential. One way is to electrochemically dope charge carrier injection layer through porous electrodes. The electrochemically doped charge carrier layer thus form gate‐tunable contact with porous electrodes. Another way is to modulate the work‐function of electrodes that can have varied charge carrier injection barriers following the applied gate potential. Gate‐tunable OLEDs based on these two working principles have been fabricated, characterized and demonstrated for displaying simple digitals and letters. New materials including dielectric, porous electrodes, work function tunable electrodes, and charge carrier injection materials have been further explored for performance improvement.     

Femtosecond time-resolved two-photon photoemission studies of electron dynamics in metals

Electron-hole excitation and relaxation in the bulk, at interfaces, and surfaces of solid state materials play a key role in a variety of physical and chemical phenomena that are important for surface photochemistry, particle-surface interactions, and device physics. Information on charge carrier relaxation in metals can be obtained through analysis of linewidths measured by photoemission and related techniques, which give an estimate of the upper limit for electron and hole relaxation; however, many factors can contribute to spectral broadening, thus it is difficult to extract specific information on electronic relaxation processes. With femtosecond lasers it is possible to probe directly in a time-resolved fashion the charge carrier dynamics in metals by a variety of linear and nonlinear optical techniques. Femtosecond time-resolved two-photon photoemission has attracted particularly strong interest because it incorporates many of the surface analytical capabilities of photoemission and inverse photoemission — the traditional probes for surface and bulk band structures of solid state materials — with time-resolution that is approaching the fundamental response of electrons to optical excitation. Advances in the direct measurements of electron-hole excitation, charge carrier relaxation, and dynamics of intrinsic and adsorbate induced surface states are reviewed. With femtosecond lasers it also is possible to probe a variety of coherent phenomena, and even to control the charge carrier dynamics in metals through the optical phase of the excitation light. Pioneering experiments in this new field also are discussed.     

Influence of thermostats and carrier gas on simulations of nucleation

We investigate the influence of carrier gas and thermostat on molecular dynamics (MD) simulations of nucleation. The task of keeping the temperature constant in MD simulations is not trivial and an inefficient thermalization may have a strong influence on the results. Different thermostating mechanisms have been proposed and used in the past. In particular, we analyze the efficiency of velocity rescaling, Nose-Hoover, and a carrier gas (mimicking the experimental situation) by extensive MD simulations. Since nucleation is highly sensitive to temperature, one would expect that small variations in temperature might lead to differences in nucleation rates of up to several orders of magnitude. Surprisingly, the results indicate that the choice of the thermostating method in a simulation does not have--at least in the case of Lennard-Jones argon--a very significant influence on the nucleation rate. These findings are interpreted in the context of the classical theory of Feder et al. [Adv. Phys. 15, 111 (1966)] by analyzing the temperature distribution of the nucleating clusters. We find that the distribution of cluster temperatures is non-Gaussian and that subcritically sized clusters are colder while postcritically sized clusters are warmer than the bath temperature. However, the average temperature of all clusters is found to be always higher than the bath temperature.     

Advantages of synthetic multi-element reference material with pseudo-biological matrix in activation analysis

Recent developments in the studies of low-energy electron attachment to molecules have been reviewed in light of rate constants and cross sections obtained for various halogen-containing compounds. A rational mechanism for the initial attachment is proposed. A method for the investigation of electron thermalization in gases containing an electron-attaching compound has been presented. The electron-energy-loss rate constants and the thermalization times for some electron scavengers are derived.