Charge States in Polymers

A Structural characteristic of conducting organic polymers is the conjugation of the chain-linked electroactive monomeric units, i.e. the monomers interact via a 7t-electron system. In this respect they are fundamentally different from redox polymers. Although redox polymers also contain electroactive groups, the polymer backbone is not conjugated. Consequently, and irrespective of their charge state, redox polymers are nonconductors. Their importance for electrochemistry lies mainly in their use as materials for modified el trodes. Redox polymers have been discussed in depth in the literature and will not be included in this review. 

Tvingstedt K, Vandewal K, Gadisa A, Zhang FL, Manca J, Inganas O (2009) Electroluminescence from charge transfer states in polymer solar cells. J Am Chem Soc 131 11819... 

We mentioned the main models for generation, transfer, and recombination of the charge carriers in polymers. Very often, these models are interwoven. For example, the photogeneration can be considered in the frame of the exciton model and transport in the frame of the hopping one. The concrete nature of the impurity centers, deep and shallow traps, intermediate neutral and charged states are specific for different types of polymers. We will try to take into account these perculiarities for different classes of the macro-molecules materials in the next sections. 

In bulk heterojunction solar cells, the metal/semiconductor interface is even more complex. Now the metal comes into contact with two semiconductors, one p-type (typically the polymer) and one n-type (typically the fullerene) semiconductor. A classical electrical characterization technique for studying the occurrence of charged states in the bulk or at the interface of a solar cell is admittance spectroscopy. If a solar cell is considered as a capacitor with capacitance C, the complex admittance Y is given by... 

The purpose of this paper is the presentation of a brief overview of recent literature in which new models of electronic states in polymers and molecular solids have been proposed (, 2, 5-16). Since localized (e.g., molecular-ion) states seem prevalent in these materials, I indicate in Sec. II the physical phenomena which lead to localization. Sec. Ill is devoted to the description of a model which permits the quantitative analysis of the localized-extended character of electronic states and to the indication of the results of spectroscopic determinations of the parameters in this model for various classes of polymeric and molecular materials. I conclude with the mention in Sec. IV of an important practical application of these concepts and models The contact charge exchange properties of insulating polymers ( 7, 17, 18, 19). 

The existence of exciton states in polymers such as CuPA has not been considered in detail in the literature, but it seems reasonable to suppose that efficient energy transfer can occur along the polymer chains. The absorption transition has been attributed to the formation of a charge transfer state and it is therefore possible that exciton dissociation is enhanced by the local electric field. Alternatively, exciton dissociation may occur at the polymer surface, with the electron being transferred to an acceptor such as molecular oxygen. The reaction scheme... 

The principal characteristics of the triboelectret state in polymers recorded experimentally are i) the efficient surface charge density (ESCD) value and ii) the thermally stimulated depolarization (TSD) current spectrum, i.e. the discharge current dependence of the electret on its heating temperature. The analysis of TSD spectra helped to estimate the parameters of the triboelectret state, including the homo- to heterocharge relation in a dielectric, activation energy of the charge relaxation processes, relaxation time and others. 

In a Type I supercapadtor, J x(anode) = ).ed(cathode), meaning that Type I supercapacitor voltages are limited by the overoxidation of the polymer, which usually occurs around 0.5-0.75 V [4j. Type I supercapacitors have a charged state in which one polymer layer is fully oxidized and the other layer is completely neutral. In the discharged state, both layers are 50% oxidized—Whence at most only 50% of the total polymer s p-doping capacity is used [7,143,144]. 

Both polarons and bipolarons are mobile and can move along the polymer chain by the rearrangement of double and single bonds in the conjugated system that occurs in an electric field. Conduction by polarons and bipolarons is the dominant mechanism of charge transport in polymers with nondegenerate ground states. 

---