Doping small molecule materials

Based on their thermal properties, polymeric and glassy molecular PR materials can be classified as high-7 and low-7), materials. The Tg values of polymer composites can be lowered by doping small molecules as plasticizer. Introduction of a long alkyl side chain can effectively reduce the Tg of fully functionalized polymers. The glassy molecular materials usually possess low Tg if they are amorphous. For the high-7 and low-7), materials, the figures of merit of the NLO chromophores are defined by Eqs. (35) and (40), respectively. The latter are obviously enhanced by contributions from the polarizability anisotropy of the NLO chromophore. 

Most practical polymer-based electrophotographic photoreceptor systems now in use are in fact solid solutions of an active species in a binder host polymer 11). This system concept embodies the notion of full chemical control of the transport process. Thus, the concentration of dopant molecules directly controls the drift mobility, which is in turn controlled by the overlap of wave functions between active sites. The host polymer binder is then specialized for its mechanical and adhesive properties. Understanding the key features of small-molecule transport provides guidelines for the optimization of injected-carrier range. The key point is to understand how chemically induced traps arise in such systems and how molecularly doped materials containing various contaminants in substantial quantities can still efficiently transport charge. (Even part-per-million concentrations of con-... 

In the final part of this section, the starting materials are small molecules or monomers, and CMs are synthesized through polymerization. For route 7, this is achieved with hexachlorobuta-1,3-diene, which is cathodically polymerized in LiBFV CH3CN to form a cation-doped black polymer [440]. 

Small organic molecules doped into polymer binders have also been employed in DEED devices. Coating is done using inexpensive solution techniques. A commonly used polymer is polyvinylcarbazole [50-52], which is a hole transport material with moderate mobility. Polysilanes have also been used as hole transport agents [53]. Common laser dyes may be employed as the emissive molecular dopant, for example coumarins [50, 51], rhodamines [50] or pyrans [51, 53]. Similar considerations of energetics and concentration apply to doped polymers as to small molecule dopant-host systems the dopant should provide the lowest energy site for the exciton and concentration quenching should be minimized. 

As a consequence of the strong interrelationship between PANI s optical properties and its redox chemistry, PANl possesses polyelectrochromic abilities. In strongly acidic electrolytes, PANl that is doped with small-molecule acids can stably and reversibly switch between its green ES and transparent LB forms at higher pH, PANI s ES state is dedoped to EB and the material s electroactivity is lost. Transitions from ES to the purple PB form of PANl involve the loss of protons and thus their electrochemistry is pH dependent. In the acidic media required to stabilize switching between ES and LB, however, the oxidation potential required to access PB is beyond the PANI s degradation... 

In conducting polymers, the extra carriers added upon doping are able to drift under an applied electrical field. In semiconducting polymers, no carriers are available except those thermally excited across the gap. However, negative (positive) carriers can be injected into the material by metallic contacts when the barrier between the metal work function and the LUMO (HOMO) molecular levels is overcome. Then, the injected carriers can move inside the semiconductor if a bias field is applied. Injection of carriers and their transport is a fundamental issue for all electronic devices and transistors in particular. In the following, main transport properties of organic semiconductors (both small molecules and polymers-based) used as active materials in transistors will be reviewed. 

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