Estimation of Electrical Parameters of OD Organic Semiconductor Diode from Measured I‐V Characteristics

In this paper the effect of temperature on the electrical properties of organic semiconductor disperse orange dye 25 (OD) have been examined. Thin films of OD have been deposited on In₂O₃ substrates using a centrifugal machine. DC current‐voltage (I‐V) characteristics of the fabricated devices (Al/OD/In2O3) have been evaluated at varying temperatures ranging from 40 to 60°C. A rectification behavior in these devices has been observed such that the rectifying ratio increases as a function of temperature. I‐V characteristics observed in Al/OD/In2O3 devices have been classified as low temperature (≤ 50°C) and high temperature characteristics (approximately 60°C). Low temperature characteristics have been explained on the basis of the charge transport mechanism associated with free carriers available in OD, whereas high temperature characteristics have been explained on the basis of the trapped space‐charge‐limited current. Different electrical parameters such as traps factor, free carrier density, trapped carrier density, trap density of states, and effective mobility have been determined from the observed temperature dependent I‐V characteristics. It has been shown that the traps factor, effective mobility, and free carrier density increase with increasing values of temperature, whilst no significant change has been observed in the trap density of states.     

Charge‐Carrier Density Independent Mobility in Amorphous Fluorene‐Triarylamine Copolymers

A charge‐carrier density dependent mobility has been predicted for amorphous, glassy energetically disordered semiconducting polymers, which would have considerable impact on their performance in devices. However, previous observations of a density dependent mobility are complicated by the polycrystalline materials studied. Here charge transport in field‐effect transistors and diodes of two amorphous, glassy fluorene‐triarylamine copolymers is investigated, and the results explored in terms of a charge‐carrier density dependent mobility model. The nondispersive nature of the time‐of‐flight (TOF) transients and analysis of dark injection transient results and transistor transfer characteristics indicate a charge‐carrier density independent mobility in both the low‐density diode and the high‐density transistor regimes. The mobility values for optimized transistors are in good agreement with the TOF values at the same field, and both have the same temperature dependency. The measured transistor mobility falls two to three orders of magnitude below that predicted from the charge‐carrier density dependent model, and does not follow the expected power‐law relationship. The experimental results for these two amorphous polymers are therefore consistent with a charge‐carrier density independent mobility, and this is discussed in terms of polaron‐dominated hopping and interchain correlated disorder.     

Tunneling Current in Polycrystalline Organic Thin‐Film Transistors

Several recent papers have demonstrated that charge‐carrier mobility in organic field‐effect transistors made of vacuum‐evaporated films may become temperature‐independent at low temperature. To account for this behavior, we developed a model based on the polycrystalline nature of these films, where charge transport is mostly limited by grain boundaries. The free‐carrier density in the intergrain regions is controlled by traps, which leads to the formation of back‐to‐back Schottky barriers at each side of the grain boundaries. The height and width of these barriers is estimated from solving Poisson’s equation using the graded‐channel approximation. It is shown that in most cases the barrier width is negligibly small as compared to the physical size of the grain boundaries. In the high‐temperature regime, the conducting channel can be simply described by grains and grain boundaries connected in series, so that the overall resistance reduces to that of the grain boundaries. At low temperatures, tunneling through the barrier becomes predominant, leading to temperature‐independent mobility. A complete two‐dimensional model for charge tunneling through the barriers is developed. A quantitative check of the model is made by least‐squares fitting of the gate voltage‐dependent current measured on an octithiophene transistor at low temperature, which gives a reasonable determination of the trap density and size of the grain boundaries.     

Improved conversion efficiency of a‐Si:H/µc‐Si:H thin‐film solar cells by using annealed Al‐doped zinc oxide as front electrode material

In recent years, zinc oxide has been investigated as a front electrode material in hydrogenated amorphous silicon/hydrogenated microcrystalline silicon (a‐Si:H/µc‐Si:H) tandem solar cells. Such as for other transparent conducting oxide materials and applications, a proper balancing of transparency and conductivity is necessary. The latter is directly related to the density and the mobility of charge carriers. A high density of charge carriers increases conductivity but leads to a higher absorption of light in the near‐infrared part of the spectrum due to increased free‐carrier absorption. Hence, the only way to achieve high conductivity while keeping the transparency as high as possible relies on an increase of carrier mobility. The carrier density and the mobility of sputtered Al‐doped zinc oxide (ZnO:Al) can be tailored by a sequence of different annealing steps. In this work, we implemented such annealed ZnO:Al films as a front electrode in a‐Si:H/µc‐Si:H tandem solar cells and compared the results with those of reference cells grown on as‐deposited ZnO:Al. We observed an improvement of short‐circuit current density as well as open‐circuit voltage and fill factor. The gain in current density could be attributed to a reduction of both sub‐band‐gap absorption and free‐carrier absorption in the ZnO:Al. The higher open‐circuit voltage and fill factor are indicators of a better device quality of the silicon for cells grown on annealed ZnO:Al. Altogether, the annealing led to an improved initial conversion efficiency of 12.1%, which was a gain of +0.7% in absolute terms. Copyright © 2013 John Wiley & Sons, Ltd.     

Dependence of charge carrier mobility of 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine on doping concentration of tetrafluoro-tetracyano-quinodimethane

Electrical transport of pure and tetrafluoro-tetracyano-quinodimethane doped 4,4′,4″-tris(N-3-methylphenyl-N-phenylamino)triphenylamine (m-MTDATA) films have been studied at various temperatures and doping concentrations. Pure films show space charge limited conduction with field and temperature dependent mobility. The J-V characteristics of doped m-MTDATA were ohmic at low voltages due to thermally released carriers from dopant states. At higher voltages the current density increases nonlinearly due to field dependent mobility and carrier concentration thereby filling of tail states of HOMO of the host. The conductivity of doped films were analysed using the Unified Gaussian Disorder Model (UGDM). The carrier concentration obtained from the fitting show a non-linear dependence on doping concentration which may be due to a combined effect of thermally activated carrier generation and increased carrier mobility.     

Electrical characterization of controlled and unintentional modified metal–organic contacts

A method to characterize metal–organic contacts subjected to controlled technological treatments or unintentional degradation processes is proposed. The procedure is useful to characterize different fundamental aspects of a metal–organic structure such as the height of the interface energy barrier, the presence of impurities or trapping effects and the carrier mobility. Current–voltage curves in organic diodes are analyzed and the value of the free carrier density at the metal–organic interface is extracted and discussed. The charge carrier density is chosen for this analysis as this is one of the key physical parameters in the understanding of the physical processes involved in the device operation. The extracted charge, when described as a function of the current density, gives information about the doping and impurity concentration, the effective barrier seen by the carriers at the metal–organic interface or effective barriers seen by the carriers in the hopping processes across the organic material. An important advantage of the proposed procedure is the low computational time. Also, the procedure aims to provide a quick analysis for researchers on how the physical properties of the devices are evolving when they are technologically altered or degraded.     

Measurement of charge-carrier behaviour in PIN diodes using a laser technique

A measurement system for the determination of charge-carrier densities in operating power devices is described. The system is computer-controlled and uses the free carrier absorption at 3.39 μm wavelength to deduce charge densities as a function of space and time. Measurements of carrier distribution in PIN power diodes have enabled derivation of the ambipolar diffusion length, the carrier lifetime and the carrier mobility.     

Electric potential mapping by thickness variation: A new method for model-free mobility determination in organic semiconductor thin films

Charge transport, with charge carrier mobility as main parameter, is one of the fundamental properties of semiconductors. In disordered systems like most organic semiconductors, the effective mobility is a function of the electric field, the charge carrier density, and temperature. Transport is often investigated in a space-charge limited current (SCLC) regime in thin film single carrier devices, where an electric current is driven in the direction perpendicular to the surface. Direct evaluation of the current–voltage characteristics, however, is problematic, because parasitic contributions from injection or extraction barriers can falsify results.     

Local charge carrier mobility in disordered organic field-effect transistors

In conventional field-effect transistors, the extracted mobility does not take into account the distribution of charge carriers. However, in disordered organic field-effect transistors, the local charge carrier mobility decreases from the semiconductor/insulator interface into the bulk, due to its dependence on the charge carrier density. It is demonstrated that the conventional field-effect mobility is a good approximation for the local mobility of the charge carriers at the interface.     

The role of the density of states on the hole mobility of disordered organic semiconductors

It was recently demonstrated that the hopping mobility in disordered organic semiconductors depends both on the charge carrier concentration and on the density of states but their relative influence is still an open question. The mobility is almost constant below a certain concentration and increases at large concentration where the density of states is conventionally assumed exponential. Hence the experimental hole mobility in polymer FETs is at least one order of magnitude larger than the hole mobility in LEDs based on the same materials. In order to investigate this issue, the mobility of disordered organic semiconductors is calculated by numerically solving the variable range hopping (VRH) equations in a wide range of carrier concentrations ranging from low concentrations, typical of hole-only diodes, to high concentrations, typical of field effect transistors. The exact mobility is numerically calculated for various density of states (DOS) and it is compared with the experimental data in order to investigate the dependence of the hole mobility on the DOS. Our calculations show that the strong dependence of the hole mobility on the charge carrier density is strictly correlated to the shape of the DOS and that a single gaussian, in general does not explain the mobility behavior in the whole range of concentrations; depending on the material, it could be accurately approximated by a single gaussian, an exponential, or by a combination of both functions.     

Restraints in low dimensional organic semiconductor devices at high current densities

The understanding of the charge carrier transport in electronic materials is of crucial interest for the design of efficient devices including especially the restraints that arise from device miniaturization. In this work the performance of organic thin-film and single crystal field-effect transistors with the same active material was studied in detail focusing on the high current density regime, where a pronounced non-hysteretic maximum in the transconductance was found. Interestingly, in this operation mode for both, thin films and single crystals, comparable densities of free and gate-induced charge carriers were estimated. Kelvin probe microscopy was used to measure the contact potential difference and the electrical field along the transistor channel during device operation exhibiting the formation of local space charges in the high current density regime.     

Modeling the transition from ohmic to space charge limited current in organic semiconductors

This paper presents a model for charge transport in organic and polymeric diodes that provides a physical explanation of the transition from ohmic to space-charge limited current (SCLC) regimes. The proposed model is based on two established models: a unified model for the injection and transport of charge in organic diodes, including a proper boundary condition for the free charge density at the metal–organic interface; and a temperature and electric-field dependent mobility model. The model reproduces published experimental current–voltage characteristics (for different temperatures and device lengths). The modeling results highlight the importance of the boundary condition at the interface that is used to explain different trends and their transitions in the current–voltage characteristics: linear, quadratic and a higher than quadratic trend at high electric fields. The model uses a finite charge at the interface to eliminate any dependence of the parameters of the mobility model with the length of the organic layer. Importantly, this result differs from others that consider an infinite charge at the interface, and make use of a length-dependent mobility, or other more complex models that incorporate a dependence with the free charge density in order to avoid such a dependence with the device length.     

Defect Modulation Doping

The doping of semiconductor materials is a fundamental part of modern technology, but the classical approaches have in many cases reached their limits both in regard to achievable charge carrier density as well as mobility. Modulation doping, a mechanism that exploits the energy band alignment at an interface between two materials to induce free charge carriers in one of them, is shown to circumvent the mobility restriction. Due to an alignment of doping limits by intrinsic defects, however, the carrier density limit cannot be lifted using this approach. Here, a novel doping strategy using defects in a wide bandgap material to dope the surface of a second semiconductor layer of dissimilar nature is presented. It is shown that by depositing an insulator on a semiconductor material, the conductivity of the layer stack can be increased by 7 orders of magnitude, without the necessity of high‐temperature processes or epitaxial growth. This approach has the potential to circumvent limits to both carrier mobility and density, opening up new possibilities in semiconductor device fabrication, particularly for the emerging field of oxide thin film electronics.     

Boundary condition model for the simulation of organic solar cells

Organic solar cells (OSCs) are promising photovoltaic devices to convert solar energy into electrical energy. Their many advantages such as lightweight, flexibility and low manufacturing costs are intrinsic to the organic/polymeric technology. However, because the performance of OSCs is still not competitive with inorganic solar cells, there is urgent need to improve the device performance using better designs, technologies and models. In this work, we focus on developing an accurate physics-based model that relates the charge carrier density at the metal-organic boundaries to the current density in OSCs. This analysis is based on our previous studies on single-carrier and bipolar diodes. The model for the boundary condition of the charge carrier density at the interfaces of OSCs follows a power-law function with the current density, both in dark and under illumination. Simulated current-voltage characteristics are verified with experimental results. The numerical simulations of the current-voltage characteristics of OSCs consider well-established models for the main physical and optical processes that take place in the device: light absorption and generation of excitons, dissociation of excitons into free charge carriers, charge transport, recombination and injection-extraction of free carriers. Our analysis provides important insights on the influence of the metal-organic interfaces on the overall performance of OSCs. The model is also used to explain the anomalous S-shape current-voltage curves found in some experimental data.     

Numerical Analysis on Current Transport Characteristics in Single Layer Organic Electroluminescent Devices

A new model to describe I-V characteristics of organic light-emitting devices (OLEDs) is developed based on experimental results. The dependence of I-V characteristics on energy barrier, trap density and carrier mobility is analyzed. The result shows that this model combines the Fowler-Nordheim tunnel theory and the trap charge limited current theory with exponential trap distribution (TCL), and it describes the current transport characteristics of OLEDs more comprehensively. The I-V characteristics follow Fowler-Nordheim theory when the energy barrier is high, the trap density is small and the carrier mobility is large.In other cases they follow the TCL theory.     

Two-dimensional carrier flow in a transistor structure under nonisothermal conditions

A two-dimensional mathematical model is developed to predict the internal behavior of power transistors operating under steady-state conditions. This model includes the internal self-heating effects in power transistors and is applicable to predict the transistor behavior under high-current and high-voltage operating conditions. The complete set of partial differential equations governing the bipolar semiconductor device behavior under nonisothermal conditions is solved by numerical techniques without assuming internal junctions and other conventional approximations. Input parameters for this model are the dimension of the device, doping profile, mobility expressions, generation-recombination model, and the boundary conditions for external contacts. Computer results of the analysis of a typical power transistor design are presented for specified operating conditions. The current density, electrostatic potential, carrier charge density, and temperature distribution plots within the transistor structure illustrate the combined effect of the electrothermal interaction, base conductivity modulation, current crowding, base pushout, space charge layer widening, and current spreading phenomena in power transistors.     

双层有机电致发光器件有机层厚度优化的数值研究

在陷阱电荷限制电流传导理论的基础上,提出了双层有机电致发光器件的数值模型,研究了结构为“阳极/空穴输运层( HTL) /发光层( EML) /阴极”的器件中电流密度和量子效率随有机层的特征陷阱能量、陷阱密度和载流子迁移率的依赖关系.研究发现,对于给定的HTL 和EML 的特征陷阱能量、陷阱密度和载流子迁移率,存在一个最优的HTL 和EML 之间的厚度比率,在此最优厚度比下,器件的电流密度和量子效率达到最大.通过有机层厚度的优化,器件的电流密度和量子效率可提高多达两个数量级.另外,还研究了最优厚度比随有机层特征陷阱能量、总陷阱密度和载流子迁移率之间的     

双层有机电致发光器件有机层厚度优化的数值研究

在陷阱电荷限制电流传导理论的基础上,提出了双层有机电致发光器件的数值模型,研究了结构为"阳极/空穴输运层(HTL)/发光层(EML)/阴极"的器件中电流密度和量子效率随有机层的特征陷阱能量、陷阱密度和载流子迁移率的依赖关系. 研究发现,对于给定的HTL和EML的特征陷阱能量、陷阱密度和载流子迁移率,存在一个最优的HTL和EML之间的厚度比率,在此最优厚度比下,器件的电流密度和量子效率达到最大.通过有机层厚度的优化,器件的电流密度和量子效率可提高多达两个数量级.另外,还研究了最优厚度比随有机层特征陷阱能量、总陷阱密度和载流子迁移率之间的定量关系.     

Thermal equilibrium noise of space charge limited current in silicon for holes with field-dependent mobility

Measurements of the limiting white noise of space charge limited hole current in silicon were made at d.c. voltages where the hole mobility becomes field-dependent. The surprising result is established that the hole temperature is equal to the lattice temperature at 110°K within 10 per cent at voltages where the deviations from constant mobility reach 50 per cent. This result contradicts accepted theories relating carrier temperature to field-dependent mobility.     

Multiband Mobility in Semiconducting Carbon Nanotubes

We present new data and a compact mobility model for semiconducting single-wall carbon nanotubes, with only two adjustable parameters, the elastic and inelastic collision mean free paths at 300 K. The mobility increases with diameter, decreases with temperature, and has a more complex dependence on charge density. The model and data suggest that the room temperature mobility does not exceed 10 000 cm²/Vmiddots at high carrier density (n > 0.5 nm^-1) for typical single-wall nanotube diameters, due to the strong scattering effect of the second subband.