Polarons organic materials

In the last few years the first theoretical models describing spin injection at hybrid organic-inorganic interfaces and spin transport in organic materials were proposed. Most models take into account the polaronic nature of carriers in organic semiconductors [17, 18]. The interface role was strongly underlined by, among others. Smith and coworkers [19]. 

This low density of surface defects is the major factor that determines the record performance of single-crystal OFETs and enables exploration of the fundamental limits of charge carrier transport in organic materials, hi addition, these devices provide an efficient tool for studying the polaron-defect interactions. This section focuses on defects that can be formed in the process of crystal growth, OFET fabrication, and as a result of the interaction with ambient environment. 

The situation is drastically different in organic materials. Here, the electronic polaron has long enough time to form, so the energy levels of a charged molecule are significantly shifted with respect to that of a neutral molecule, as shown in Figure... 

This concept of polarons and, in particular, excitonic polarons has been used to explain observed features of the one-dimensional conducting organic materials based on 7,7,8,8-tetracyano-p-quinodimethane, TCNQ (85). It indicates that a way to reduce the Coulomb repulsion between electrons in the chain is to surround each chain by a highly polarizable medium. However, a limit may be reached beyond which, if the surrounding medium were made more polari zable, the effects due to band narrowing would outweigh the benefits of reduced Coulomb repulsion (85). 

C. W. Spangler, L. S. Sapochak, and B. D. Gates, Polaron and bipolaron formation in model extended TT-electron systems potential nonlinear optics applications, in Organic Materials for Non-Linear Optics, Vol. I (R. A. Hann and D. Bloor, eds.). Roy. Soc. Chem., London, 1989, pp. 57-63. 

Despite the success of the disorder model concerning the interpretation of data on the temperature and field dependence of the mobility, one has to recognize that the temperature regime available for data analysis is quite restricted. Therefore it is often difficult to decide if a In p vs or rather a In p vs representation is more appropriate. This ambiguity is an inherent conceptual problem because in organic semiconductors there is, inevitably, a superposition of disorder and polaron effects whose mutual contributions depend on the kind of material. A few representative studies may suffice to illustrate the intricacies involved when analyzing experimental results. They deal with polyfluorene copolymers, arylamine-containing polyfluorene copolymers, and c-bonded polysilanes. 

Fig. 4 Schematic illustration of the processes leading to photocurrent generation in organic solar cells, (a) Photon absorption in Step 1 leads to excitons that may diffuse in Step 2 to the donor/ acceptor (D/A) interface. Quenching of the exciton at the D/A interface in Step 3 leads to formation of the charge-transfer (CT) state. Note that processes analogous to Steps 1-3 may also occur in the acceptor material, (b) Charge separation in Step 4 leads to free polarons that are transported through the organic layers and collected at the electrodes in Steps 5 and 6, respectively, (c) The equilibria involved in Steps 1-4- strongly influence device efficiency...

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