Flexible electronic circuits, displays, and sensors based on organic active materials will enable future generations of electronics products that may eventually enter the mainstream electronics market. The motivations in using organic active materials come from their ease in tuning electronic and processing properties by chemical design and synthesis, low cost processing based on low temperature processes and reel-to-reel printing methods, mechanical flexibility, and compatibility with flexible substrates. Organic thin film transistors (OTFTs) are the basic building blocks for flexible integrated circuits and displays.
During the operation of the transistor, a gate electrode is used to control the current flow between the drain and source electrodes. Typically, a higher applied gate voltage leads to higher current flow between drain and source electrodes. The semiconductor material for a fast switching transistor should have high charge carrier mobility and on/off current ratio.
There are two types of organic semiconductors based on the type of majority charge carriers: p-type (holes as major charge carriers) and n-type (electrons as major charge carriers). To facilitate charge transport, the organic semiconductor layer usually consists of p-conjugated polymers, in which the p–p stacking direction should ideally be along the current flow direction. This requires the semiconductor molecules to self-assemble into a certain orientation upon either vapor or solution deposition. It is also important that the semiconductor thin film has large, densely packed and well-interconnected grains. Most small molecule, high performance organic semiconductors tend to have the long axes of the molecules oriented close to normal to the dielectric surface with the typical grain size in the order of at least a few micrometers. In case of solution processed semiconducting polymers, it is preferred for the p-conjugated plane to adapt an edge-on orientation on the surface.
The morphology of the semiconductor film is highly dependent on the chemical and physical nature of the dielectric surface. Patterning of dielectric surface can lead to selective patterning of the organic semiconductor in desired locations, which is important to reduce cross talk between devices. With proper control of the dielectric surface, arrays of organic semiconductor single crystals can be patterned over a large area for high performance transistors.
The dielectric layer for organic transistors should be as thin as possible, pinhole-free, and ideally with a high dielectric constant for low voltage operation. Inorganic, organic, and inorganic/organic hybrid materials have been investigated as the gate dielectric materials. Promising materials include poly(methy methacrylate) (PMMA), poly(styrene), poly(vinyl phenol), silsesquioxane (glass resin), and benzocyclobutene (BCB), etc. Crosslinked polymers generally are more robust as ultrathin dielectric materials. Even a well-ordered densely packed self-assembled monolayer (SAM) may be used as the thinnest possible high quality dielectric layer. Incorporation of high dielectric constant inorganic nanoparticles into a polymer matrix boosts the overall dielectric constant of the thin film.
For organic transistors to function properly, charge injection from the electrode needs to be efficient. This requires the work function of the electrode to match well with the energy level of the organic semiconductor such that the energy barrier for charge injection is low. Typically high work function electrodes (Au, Pd, or indium tin oxide) have been used for p-channel organic transistors. Electrode surface modification with a self-assembled monolayer can be used to improve the charge injection into the organic semiconductor. When the organic semiconductor is deposited onto the source and drain electrodes, the morphology of organic semiconductors is significantly different.
In summary, organic materials are promising candidates for flexible electronic devices. Significant progress has already been made in this field. Nevertheless, better understanding of the structure property relationship is still needed so that we can rationally design materials to achieve desired device performance parameters.
New n-Type Polymeric Semiconductors
The unifying basic requirement of most thin-film, organic electronic devices like OLEDs and OPVs is that they contain at least two semiconducting materials with offsets in their molecular orbital energy levels. In the organic semiconductor world, one can create such an energy offset by forming an interface between a more electron-rich (p-type) semiconductor and an electron-poor (n-type) material. It is at this interface that charge separation or recombination typically occurs. There are a number of available classes of relatively electron-rich, p-type semiconducting molecules and polymers. In contrast, there are few electron-poor, n-type semiconducting molecules, like metalloporphyrins and methanofullerenes. Even rarer are the n-type semiconducting, p-conjugated polymers like cyanoderivatives of poly(p-phenylenevinylenes).
High Efficiency In Organic Light-Emitting Devices
Hetero-structure OLEDs Electroluminescence of organic molecules has been a wellknown phenomenon for more than 50 years. Successful application of organic luminescence in light-emitting devices required device structures that overcame the problems associated with the high resistivity of organic materials, and achieved a well-balanced charge injection from the electrodes into organics. These two problems were solved by Tang and van Slyke3 with the thin film heterostructure concept for the organic LEDs (OLEDs).
OLEDs have promise to make a marked impact in full color displays and lighting applications. Both of these families of devices require high efficiency and long lifetime, as well as low-cost fabrication, a wide-gamut for sets of devices and high color saturation. OLEDs have demonstrated all of these properties; however, large area fabrication remains a significant challenge, making manufacturing costs quite high. Another technological challenge is the device lifetimes for deep blue devices. There are a large number of stable red and green phosphorescent emitters, giving device lifetimes approaching 106 hours. In contrast, the operational stability of the blue phosphor based OLEDs are typically markedly shorter, with the best values between 15K and 20K hours. The source of the enhanced instability of these blue devices is still an open question. While many fluorescent and phosphorescent OLEDs have been commercialized in small area mobile displays, there is still ample room for scientific investigation to better understand the parameters controlling and limiting organic electroluminescence.
Switching between doped and un-doped states induces changes in a number of Light Emitting Polymer (LEP) properties, such as polymer volume, absorption color, and reversible PL quenching. These controlled changes make LEPs promising for applications: an induced variation in absorption color may be exploited for electro-chromic displays while a change in volume may be utilized for electro-active artificial polymer muscles. The combination of semi-conductivity and intense PL results in LEP electro-luminescence and their use in polymer light emitting diodes (PLEDs). The high sensitivity of PL quenching to doping or charge transfer can be used to detect biological and explosive species. Therefore, the LEPs represent an important category of low-temperature processable materials useful for many scientific and technological explorations. PLEDs are currently under development for applications in flat panel displays and lighting with strong commercialization potential that depends on understanding and improvement of properties of the LEPs.
High-performance PLED requires the LEP layer to meet several stringent requirements:
1. Color purity, which is determined by the polymer band-gap and film morphology
2. Matching of ionization potentials and electron affinities between LEP and the different electrode materials
3. High PL quantum efficiency
4. Chemical and thermal stability
5. Processability which involves solubility, solution viscosity, and solvent-substrate compatibility.