Currents and photocurrents in organic materials determined by the interface phenomena

The role of interface between molecular material and electrode on currents and photocurrents is considered. Mechanisms of charge carrier injection, electrode recombination and transport are discussed. Particularly thermal, excitonic, photo and tunneling injection of charge carriers, diffusion in presence of image force, interface barrier between electrode and organic materials and two organic materials, non-uniformity of electrodes and other phenomena on charge carrier injection are considered. The data presented in the review which complete theoretical considerations have been taken from previous as well as current literature. The considered phenomena are very important for the analysis of many practical problems for molecular electronic devices such as rectification of current, organic transistors, electroluminescence, photovoltaic effects and some similar problems.     

Poly(silylene)s: charge carrier photogeneration and transport

An Erratum has been published for this article in Polymers for Advanced Technologies 12(10)2001, 603. Poly(diorganylsilylene)s can be excited within the framework of Si bonds in the polymer backbone as well as within a side group skeleton. An electron transfer from the main chain to a side group or an interchain electron transfer is necessary to form a quasi‐stable ion‐pair. Dissociation of the ion‐pair in the external electric field can be described in terms of the Onsager theory of geminate recombination. The charge carrier transport proceeds predominantly along σ‐delocalized Si backbone with participation of interchain hopping and polaron formation. A model of disordered polarons seems to be adequate to describe the charge carrier transport properties. Copyright © 2001 John Wiley & Sons, Ltd.     

Electrical field-effect of polyvinylidenefluoride (PVF2)

Summary By means of modified field-effect measurements the properties of space charge distribution in polymers at the polymer-metal interface have been investigated. In general, these properties are due to excess charge and/or dipole orientation. The superposition of both effects is investigated in PVF₂ (phase-II) in order to show whether the conformational PVF₂ properties in the crystalline or in the noncrystalline regions control the space charge distribution. In the temperature region 60 °C <T < 90 °C and for times greater than 60 sec the space charge properties are significant influenced by additional dipole relaxations which result from the beginning chain mobility within the crystallites of PVF₂. In the time interval 1 sec <t < 60 sec charge carrier injection at the PVF₂-gold contact can be described by the Richardson-Schottky theory. Based on this theory, a plot of reduced current versus reduced field is presented displaying all experimental data obtained at various temperatures and field-strength on a single curve. No clear distinction can be given in this case between bulk-limited and electrode-limited processes.With 6 figures     

电极材料对NPB/Alq3有机电致发光器件性能的影响

采用不同材料作为有机电致发光器件(OELDs)的电极, 制备了基本结构为[阳极/NPB(40 nm)]/Alq3(50 nm)/阴极]的异质结双层器件, 并通过改变OELDs器件的阴极或阳极来研究电极材料对器件光电性能的影响. 研究结果表明, 各器件电流-电压(I-V)关系的基本特征与陷阱电荷限制电流(TCLC)机制的拟合情况相符. 由于有机材料本身能级的无序性以及载流子迁移率对温度和电场的依赖性, 不同电极的载流子注入能力与其功函数并无直接关系. 双层器件中由于空穴传输层的引入, 使得载流子复合区域位于有机层异质结界面处, 降低了金属阴极对激子的猝灭作用, 从而大大提高了器件性能. 此外, 金属电极OLEDs器件结构具有的微腔效应会导致发射光谱的位移和谱峰宽度变窄, 这表明通过对金属电极的表面改性和优化可使器件性能超过常规结构的器件.     

Device physics of polymer light-emitting diodes

This article reviews a device model for the current and light generation of polymer light-emitting diodes (PLEDs). The model is based on experiments carried out on poly(dialkoxy-p-phenylene vinylene) (PPV) devices. The transport properties of holes in PPV have been investigated with indium tin oxide (ITO)/PPV/Au hole-only devices. The hole current is dominated by bulk conduction properties of the PPV, in contrast to previous reports. As the hole current is space-charge limited, the hole mobility as a function of electric field E and temperature T can be directly determined. The hole mobility exhibits a field dependence ln(μ) ∼ ✓E as also has been observed from time-of-flight experiments in many molecularly doped polymers and amorphous glasses. For the zero-field hole mobility an activation energy of 0.48 eV is obtained. The electron conduction in PPV has been studied by using Ca/PPV/Ca electron-only devices. It appears that the electron current is strongly reduced by the presence of traps with a total density of 10¹⁸ cm⁻³. Combining the results of electron- and hole-only devices a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. It is calculated that the unbalanced electron and hole transport gives rise to a bias-dependent efficiency. By comparison with experiment it is found that the recombination process in PPV is for 95% nonradiative. Furthermore, the experiments reveal that the bimolecular recombination process is thermally activated with an identical activation energy as measured for the charge carrier mobility. This demonstrates that the recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes towards each other. The occurrence of Langevin recombination explains why the conversion efficiency (photon/carrier) of a PLED is temperature independent. © 1998 John Wiley & Sons, Ltd.     

Influence of Extraction Barrier on the Loss Process of Photogenerated Charge Carrier in Polymer Bulk Heterojunction Solar Cells

Based on a full device model adopting three-dimensional Pauli master equation approach, the charge carrier loss process due to poor extraction channels between electrode and active layer in polymer bulk heterojunction(BHJ) solar cells was studied. The influence of barrier height on device performance was simulated to reveal the importance of electrode improvement. It was found that relatively large extraction barrier height(over 0.40 eV) can lead to the significant diminishing of the overall charge collection efficiency, since bimolecular recombination rate would increase to a high level due to enhanced space charge accumulation effect near electrodes. In contrast, the percentage of charge carrier annihilated due to geminate recombination did not change significantly with barrier height variation. Our simulation results may provide the basis for a more accurate model and potential direction of polymer BHJ solar cells improvement.     

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。     

Space charge influenced photocurrent transients in fluid solution

The dependence of space charge influenced current transients on the spatial distribution of photogenerated carriers in the bulk of a conductivity cell was investigated with the aim of correlating the observed time evolution of the currents with carrier properties such as mobilities and recombination constant. Approximate analytical solutions for the limiting cases of sheet and full interelectrode illumination show how experimental transients result from the competition between the dynamically interdependent processes: interionic recombination, charge carrier migration and discharge at the electrodes, and space charge buil-up. The voltage dependence of the time at which the secondary photocurrent maxima are observed yield mobilities of 2.0 × 10^?4 and 2.8 × 10^?3 cm² V^?1 s^?1 respectively for the pyrene cation and the solvated electron in tetrahydrofuran at room temperature. A bimolecular recombination rate constant of 2.3 × 10^?9 cm³ s^?1 is shown to be consistent with the space charge densities present after total separation of the positive and negative carriers for various periods of charge carrier recombination.     

The Origin of the Improved Efficiency and Stability of Triphenylamine‐Substituted Anthracene Derivatives for OLEDs: A Theoretical Investigation

Herein, we describe the molecular electronic structure, optical, and charge‐transport properties of anthracene derivatives computationally using density functional theory to understand the factors responsible for the improved efficiency and stability of organic light‐emitting diodes (OLEDs) with triphenylamine (TPA)‐substituted anthracene derivatives. The high performance of OLEDs with TPA‐substituted anthracene is revealed to derive from three original features in comparison with aryl‐substituted anthracene derivatives: 1) the HOMO and LUMO are localized separately on TPA and anthracene moieties, respectively, which leads to better stability of the OLEDs due to the more stable cation of TPA under a hole majority‐carrier environment; 2) the more balanceable hole and electron transport together with the easier hole injection leads to a larger rate of hole–electron recombination, which corresponds to the higher electroluminescence efficiency; and 3) the increasing reorganization energy for both hole and electron transport and the higher HOMO energy level provide a stable potential well for hole trapping, and then trapped holes induce a built‐in electric field to prompt the balance of charge‐carrier injection.     

How To Make Ohmic Contacts to Organic Semiconductors

The process of charge injection plays an important role in organic semiconductor devices. We review various experimental techniques that allow injection to be separated from other competing processes, and quantify the injection efficiency of a contact. We discuss the dependence of the injection efficiency on parameters such as the energy barrier at the interface, the carrier mobility of the organic semiconductor, its carrier density (doping level), the presence of mobile ions, and the sample geometry. Based on these findings, we outline guidelines for forming ohmic contacts and present examples of contact engineering in organic semiconductor devices.     

Electrical Methods to Elucidate Charge Transport in Hybrid Perovskites Thin Films and Devices

The panchromatic light absorption and excellent charge carrier transport properties in organo lead halide perovskites allowed to achieve an unprecedented power conversion efficiency in excess of 25 % for thin film photovoltaics fabrication. To understand the underlying phenomena, various comprehensive set of optical and electrical techniques have been employed to investigate the charge carrier dynamics in such devices. In this perspective, we aim to summarize the electrical transport properties of perovskite thin films by using (i) impedance spectroscopy (IS), (ii) space charge limited current (SCLC), (iii) field‐effect transistors (FETs) and (iv) time‐of‐flight (TOF) methods. We have deliberated various equivalent circuit used to model the perovskite solar cells by means of IS. The SCLC technique provide vital electrical parameters such as mobility, activation energy, traps density and distribution, carrier concentration, density of states, etc. The TOF technique measures mobility as a primary parameter while the FETs configuration provide a valuable insight into the in‐plane charge transport in perovskites thin films. We believe that these notable understanding will provide insights into charge carrier dynamics in perovskite materials and devices.     

OLED and PLED Devices Employing Electrogenerated,Intramolecular Charge‐Transfer Fluorescence

The light generating mechanism of a series of light emitting diodes with electron donor–bridge–acceptor systems (D–b–A) as the emitting species was examined by constructing model diodes based on small organic molecules (OLEDs) as well as on molecularly doped electroactive (poly‐N‐vinylcarbazole, PVK) and insulating (polystyrene, PS) polymers (PLEDs). The direct electrogeneration of an intramolecular charge‐transfer (CT) fluorescence of the donor–bridge–acceptor systems occurred readily in OLED devices with a D–b–A system as the emissive layer. In diodes with PS as the host matrix, hole‐injection and electron‐injection occurred directly in the D–b–A molecules residing close to the anode and the cathode, respectively. In the PVK diodes, hole‐injection occurred primarily into PVK and the positive charge carrier was subsequently trapped on the D–b–A molecule, whereas electron‐injection at the cathode side occurred directly into the D–b–A molecules. Charge‐hopping between neighboring molecules then occurred until a hole and electron resided on the same molecule, which is equivalent to the formation of the CT excited state, and which finally relaxed by intramolecular charge recombination under the emission of CT fluorescence.     

Quantum dot solar cells. harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films

By using bifunctional surface modifiers (SH-R-COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO(2) films. Upon visible light excitation, CdSe QDs inject electrons into TiO(2) nanocrystallites. Femtosecond transient absorption as well as emission quenching experiments confirm the injection from the excited state of CdSe QDs into TiO(2) nanoparticles. Electron transfer from the thermally relaxed s-state occurs over a wide range of rate constant values between 7.3 x 10(9) and 1.95 x 10(11) s(-1). The injected charge carriers in a CdSe-modified TiO(2) film can be collected at a conducting electrode to generate a photocurrent. The TiO(2)-CdSe composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a photon-to-charge carrier generation efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO(2)/CdSe interfaces and internal TiO(2) grain boundaries.     

Current-voltage characteristics of thin poly(biphenyl-4-ylphthalide) films

The paper considers the features of the charge transport near to the threshold of the transition of polymer films to the high conductive state, induced by a small uniaxial pressure. The problem has not been solved so far, how the energy structure of a wide-band-gap organic dielectric varies near this threshold. The current-voltage characteristics of poly(biphenyl-4-ylphthalide) films at different uniaxial pressures were measured and analyzed. The interpretation of the obtained results is carried out within the framework of the space charge limited conduction model. The estimation of the injection model of transport parameters such as the charge carrier mobility and concentration, trapping state concentration and others are carried out. The analysis of the obtained results allows to make the following preliminary conclusion. Pressure increase promotes formation of a narrow trap band near the quasi-Fermi level resulting from the increase in the injection. This can give rise to a sharp magnification of the charge carrier mobility and even transition to the metallic state.     

Efficient Enhancement of Electron Transport and Collection Capability in PTB7:PC71BM‐based Solar Cells Enabled by Sulforhodamine Cathode Interlayers

Development of low‐cost water‐/alcohol‐soluble interfacial materials is a crucial issue to promote the commercialization of polymer solar cells (PSCs). Herein, two derivatives of low‐cost rhodamine, called sulforhodamine 101 (SR101) and sulforhodamine B (SRB), are applied as cathode interfacial layers (CILs) to effectively improve the charge‐carrier transportation and collection, reduce the work function (WF) of Al counter electrode, and decrease the series resistance and charge recombination in the PSCs. As a result, SR101‐based devices show excellent performance with the highest power conversion efficiency (PCE) of 9.10 %, superior to that of both the control devices with MeOH/Al and Ca/Al. Notably, sulforhodamine is commercially available with low cost and great solution‐processability. This work demonstrates that sulforhodamine has a great potential as a CIL material,which is suitable for the large‐area fabrication process and commercialization of highly efficient PSCs.     

C_(60)与MoO_3混合材料做空穴注入层的单层有机电致发光器件

我们制备研究了基于结构为氧化铟锡(ITO)/C_(60)(1.2nm):MoO_3(0.4nm)/1,3,5-三(1-苯基-1H-苯并咪唑-2-基)苯(TPBi):三(2-苯基吡啶)铱[Ir(ppy)_3](33%,90 nm)/LiF (0.7 nm)/Al (120 nm)的高效绿色磷光单层有机发光二极管(OLED)。分别将C_(60),MoO_3与C_(60):MoO_3混合物作为空穴注入层(HIL)作为对比。TPBi在发光层中起着主体以及电子传输材料的双重作用。在使用电子传输型主体的单层OLED中,空穴注入层性质对于调节电子/空穴注入以获得电荷载流子传输平衡起重要作用。因此,适当调节空穴注入层是实现高效单层OLED的关键因素。由于MoO_3较大的电子亲和能(6.37 eV)会诱导电子从C_(60)的最高占据分子轨道(HOMO)能级转移至MoO_3,从而形成C_(60)阳离子,并使得Mo元素的价态从+6降至+5,C_(60):MoO_3混合就可以较好的调节空穴注入性质。最终实现最大电流效率为35.88 cd·A~(-1)的单层有机发光器件。     

Dynamic theory of the alternating current response of semiconductor electrodes under illumination

A photoinduced admittance enhancement has been observed on n-GaAs and n-GaP electrodes in the potential range between flatband and stationary photocurrent onset. In order to provide a theoretical evaluation, the alternating current response of a semiconductor electrode under illumination has been investigated on the basis of non-equilibrium treatment of the carrier balance in the semiconductor and of the interfacial charge transfer kinetics. Superposition of an irreversible stationary and small-amplitude periodic rate has been treated for the following cases of charge transfer at the interface: (a) one-step electrochemical process; (b) two-step electrochemical process including an adsorbed intermediate and partial charge transfers; (c) parallel couple of one-step electrochemical and partial charge transfer chemisorption process. Empirical criteria for preference of charge transfer over surface recombination have been considered. In connection with the present development, the general equivalent circuit of a semiconductor electrode has been briefly derived from the dynamical charge balance. The theoretical approach of the stationary photocurrent-voltage curve has been discussed and refined.     

Master equation approach to charge injection and transport in organic insulators

We develop a master equation model of a disordered organic insulator sandwiched between metallic electrodes by treating as rate processes both the injection and the internal transport. We show how the master equation model allows for the inclusion of crucial correlation effects in the charge transport, particularly of the Pauli exclusion principle and of space-charge effects, besides, being dependent on just the microscopic form of the transfer rate between the localized electronic states, it allows for the investigation of different microscopic scenarios in the organic, such as polaronic hopping, correlated energy levels, interaction with image charge, etc. The model allows for a separate analysis of the injection and the recombination currents. We find that the disorder, besides increasing the injection current, eliminates the possibility of observation of a Fowler-Nordheim injection current at zero temperature, and that it does not alter the Schottky barrier size of the zero-field thermionic injection current from the value based on the energy difference between the electrode Fermi level and the highest occupied molecular orbital/lowest unoccupied molecular orbital levels in the organic, but it makes the Arrhenius temperature dependence appear at larger temperatures. We investigate how the I(V) characteristics of a device is affected by the presence of correlations in the site energy distribution and by the form of the internal hopping rate, specifically the Miller-Abrahams rate and the Marcus or small-polaron rate. We show that the disorder does not modify significantly the ebeta square root E field dependence of the net current due to the Schottky barrier lowering caused by the attraction between the charge and its image in the electrode.     

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.     

Coating Polymeric Carbon Nitride Photoanodes on Conductive Y:ZnO Nanorod Arrays for Overall Water Splitting

Polymeric carbon nitride (PCN) photosensitizers are proposed replacements for their inorganic counterparts in solar‐to‐fuel conversion via photoelectrochemical water splitting. However, intense charge recombination, primarily because of surface defects, limits the use of PCN in PEC systems. Now, photoanodes are designed by coating PCN films onto highly conductive yttrium‐doped zinc oxide (Y:ZnO) nanorods (NRs) serving as charge collectors. The generation of charge carriers can therefore be promoted by this type II alignment. The charge collectors would be kept nearby for charge separation and transport to be used in the interfacial redox reactions. The photocurrent density of the polymer electrode is improved to 0.4 mA cm^?2 at 1.23 V vs. the reversible hydrogen electrode in a Na₂SO₄ electrolyte solution under AM 1.5 illumination. The result reveals a more than 50‐fold enhancement over the PCN films achieved by powder; the efficiency can be preserved at 95 % for 160 minutes.