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The large polyanions ionically associate with PANI, thereby eliminating issues of dopant volatility and providing long-term conductivity. Excess acid groups along the polyanion can hydrogen bond with water, rendering the complex water dispersible and processable from aqueous media. The conductivity of films cast from these dispersions can reach 2.
Organic electronics research has thrived and expanded over recent years due to its promise to encourage development of products which can complement silicon-based applications with novel functionalities that cannot be addressed by silicon technologies. Plastic electronics may perhaps even replace silicon when this is economically feasible, in applications that operate under modest performance requirements e. Organic semiconductors promise to impact the industry by reduced complexity processing, compatibility with arbitrary substrates and great chemical tunability.
The seemingly infinite variety of organic compounds presents the prospect of tailoring materials to have any desired properties. Organic-based applications can thus potentially address a broad market, which includes flexible displays, electronic papers, sensors, disposable or wearable electronics, medical applications, and more. There is a tremendous excitement in both academia and industry towards the study of organic field-effect transistors OFETs , organic photovoltaics OPVs , organic light-emitting diodes OLEDs , and sensors.
Mobility is a complex property, as it is dependent on both the molecular structure of the organic semiconductor and its microstructure, but also the quality of the contacts, interfaces, and other factors of intrinsic or extrinsic nature. The operation frequency is directly proportional with the transconductance, which in turn is determined by the mobility value, as we will show in Section This chapter will focus on two major techniques frequently used to estimate the charge carrier mobility in organic semiconductors.
Mobility calculations using FET structures are able to provide details about material properties at the interface with gate dielectrics. The bulk-mobility can be extracted from space-charge-limited current SCLC measurements. The values of mobility obtained from these two methods are often very different due to anisotropy arising from molecular packing along various crystallographic directions, or as a result of different structure or chemistry of the surface versus the bulk ,.
For this reason, these techniques address different aspects of the organic solid and thus are complementary. Organic electronics have found their way from basic academic research to industrial applications over recent years in a quickly growing market.
This market covers especially application fields in light-emitting diodes LEDs , field-effect transistors FETs and solar cells which open the window for a novel type of technologies. The main element are the semiconducting, conjugated organic materials, which can be processed into devices from solution, allowing large area and low-cost fabrication. Moreover, many organic semiconductors are mechanically flexible and therefore applicable to bendable electronic elements.
The mass fabrication of the devices occurs by high-speed and inexpensive methods at low temperatures and is realized by well-known continuous in-line roll-to-roll technologies, such as inkjet, offset, rotary screen printing and others, which are adapted to the specific requirements of the organic materials. Due to the low cost processing, one-way applications of electronic elements are realizable such as radio-frequency identification RFID tags and sensors in which FETs play a major role.
Thereby, the performance of a transistor is mainly determined by the speed of the charge carrier transport from one electrode to the other, which in turn depends on various factors such as molecular design, supramolecular organization, thin film microstructure and charge carrier transport. Therefore, the device operation can be controlled by chemical design of the molecules.
Substituents also lead to the formation of a distinct superstructure driven by the local phase separation between the flexible aliphatic part and rigid aromatic fraction. Additionally, the molecular packing is strongly influenced by using such molecular units. For instance, the steric demand of the substituents can modulate the packing distance of neighboring molecules and the molecular arrangement of the building blocks to each other. Changes in the crystal unit cell due to chemical modifications of small molecules have a great impact on the device performance, while in most cases polymeric backbones with alkyl side chains with so-called lamellae superstructures are less sensitive.
But the modification of the macromolecular structure has a strong effect on the microstructure of polymers, which includes the organization of the molecules into domains within a range from few tens of nanometers to few micrometers. Most small molecules processed from solution form only highly crystalline thin layers, while the microstructure of polymers is more complex and in many cases dependent on the polymer design, such as alkyl position and molecular weight. The most prominent example in this context is poly 3hexylthiophene P3HT for which the polymer regularity determines the thin film topography and order.
This is an important point since the charge carrier transport is a function of density of structural defects and, domain boundaries, and also of the molecular orientation.
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Defects and boundaries are trapping sites for charge carriers, while molecules oriented in the transport direction promote the charge carrier mobility. All three factors can be tailored by the processing conditions, such as concentration, type of solvent, temperature and surface energy.
Interestingly, few conjugated polymers form only low ordered thin surface layers, but reveal at the same time FET mobilities far above 0. Such materials are of great interest for mass production. Rapid roll-to-roll techniques generally yield poorly ordered films on the substrate surface and lead, for most organic semiconductors, to mobilities at least two orders of magnitude lower than those obtained in the laboratory. Polymers with less order-dependent device performance would overcome this problem.
To ensure an identical device performance after mass production, new processing approaches are required which can induce a higher molecular order on the mesoand macroscopic scale, or alternatively orient the molecules in the desired direction of charge carrier transport. This chapter discusses various processing methods of prominent organic semiconductors, small crystalline molecules and polymers, for FET applications which are separated in two groups: In general, organic electronics has created immense academic interest due to its unlimited and flexible molecular engineering possibilities, allowing new organic materials with tailored physical properties to be synthesized [ 1, 2 ].
Understanding of the basic transport mechanisms in this special class of electronic materials is not only of fundamental academic interest, but also carries great technical relevance in that more professional knowledge may help to increase transport efficiency with the consequences of increased device speed, reduced power and heat losses in such promising fields as organic light-emitting diode OLED displays, organic field-effect transistor OFET used in integrated circuits, or photovoltaic cells [ 4 ]. Electron and hole mobilities are the fundamental parameters determining the charge transport phenomena in semiconductors.
This market covers especially application fields in light-emitting diodes LEDs , field-effect transistors FETs and solar cells which open the window for a novel type of technologies. Goodreads is the world's largest site for readers with over 50 million reviews. OTFTs can be divided in two main classes: Other than that, the book is pretty good and would recommend for the content. Shopbop Designer Fashion Brands. CPD consists of any educational activity which helps to maintain and develop knowledge, problem-solving, and technical skills with the aim to provide better health care through higher standards.
Typically, hopping-type transport within localized density of states DOS is observed in disordered organic semiconductors [ 5 ]. In hopping transport, mobility should be thermally activated, whereas in band transport in inorganic crystals, in contrast, carrier mobility follows the opposite temperature dependence, e. The most popular techniques used in the past to experimentally measure the charge carrier mobility in various semiconductors are:.
Each of the above methods has its advantages and disadvantages. The Hall effect is a classic technique used to study the charge transport and film conductivity of inorganic semiconductors; however, it is not suitable for low conductivity as well as low-mobility disordered organic materials due to experimentally difficult to register low Hall voltages. TOF and XTOF techniques have been widely used to study various semiconductors, while special attention has been paid to organic materials suitable for xerography. However, this technique has a number of limitations and requirements for studied films as discussed in the following section which often render this technique inapplicable to low-mobility semiconductors with strongly pronounced dispersive time-dependent transport.
CELIV, as a relatively novel technique has shown great promise in being able to reliably characterize charge transport and recombination processes in a very broad range of semiconductors; however, lately it has been shown that the interpretation of CELIV transient signals is not straightforward, requiring adequate correction factors and experimental conditions to be taken into account.
The FET technique is very widely used to measure the carrier mobility at the interface between the semiconductor and dielectric, where the interface properties often determine the charge transport, in contrast to bulk transport properties of the films [ 28 ]. Since it is a steady-state measurement technique, the amount of trapped charge must be taken into account in order to correctly estimate the carrier mobility.
Many other techniques have been reported in the literature that are able to determine the charge carrier mobility, but they fall outside the scope of this chapter.
The advantages and disadvantages of discussed techniques are specified. As an outcome of each presented technique, typical experimental results of charge transport in disordered organic materials are highlighted. The strength of organic electronics is based on the near endless number of different organic compounds. In particular, organic emitters span the whole visible spectrum and monochrome OLEDs in all colors are available. Most emitter systems consist of a phosphorescent emitted molecule embedded in a matrix with suitable triplet energy.
In blue-emitting devices, FIrpic is probably the most widely used emitter. Green emission is provided by Ir ppy 3 and Ir ppy 2 acac. In the red, Ir MDQ 2 acac is widely used. Phosphorescent emitter materials are thus dominated by irid-ium-based complexes with platinum and osmium-based materials playing a relatively minor role. However, in particular for generation of blue light, fluorescent emitter molecules are still frequently used due to their often more favorable longevity and more saturated blue emission.
We have thus listed a few particularly interesting fluorescent systems as well, albeit they generally reach lower efficiencies due to their inability to harvest triplet excitons. To increase the outcoupling efficiency, a detailed understanding of the optical properties of the OLED cavity is essential. The results of such studies as published by Meerheim et al. The simulation results for a monochrome red OLED, i. The simulations are based on an optical model treating the emitting molecule as forced damped harmonic oscillator.
The normalized radiated power peaks at several in-plane wave vectors, which correspond to the various resonant modes of the cavity. For in-plane wave vectors below 1, the out-of-plane wave vector is real and the corresponding waves are plane waves. In particular, coupling to the plasmonic mode is strong and thus much power is lost at the cathode. However, coupling to these evanescent modes is a near-field effect, i.
If the distance between the emitter and the cathode is increased, coupling to the plasmonic mode is strongly reduced. This effect is shown in Fig. The optical simulation is repeated for varying ETL thickness, i. At the peaks of the plot of the fraction of outcoupled photons vs. Shi Polymer Light-Emitting Diodes: Devices and Materials, X. Request an e-inspection copy.
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