Polaronic and bipolaronic charge states

Polaronic and bipolaronic charge states are well known in electroactive polymers, and can be observed in model oligomers with overall delocalization lengths as small as 16 atoms. It has been suggested that localized charge states may be involved in oligomers and polymers having enhanced proper-... 

MODELING ELECTRQACTIVE SEGMENTS FORMATION AND STABILIZATION OF POLARONIC AND BIPOLARONIC CHARGE STATES... 

There is no doubt that polaronic and bipolaronic charge states can be supported in stable form in small oligomers in solution, or incorporated as part of a copolymer sequence. Enhanced x properties can derive from either N, P or BP states as a function of increased delocalization length, and we anticipate several families of copolymers based on the modeling studies discussed in this paper to become available in the near future to test these proposals. 

Figure 9.1. Formation of polaronic and bipolaronic charge states during oxidative doping of PTV.

A revisal of the equilibrium between polarons and doubly charged states of the polymer chains shows that the previous assumptions on the extension of polaron, bipolarons and polaron pairs were not justified. A more general formulation is presented for the equilibrium concentrations and the kinetics. But the differences are almost negligible up to the maximum charge concentrations that can be achieved in accumulation layers. The resulting rate constants for formation and dissociation of (immobile) bipolarons can be estimated using a rate constant for the bipolaron formation determined recently by Salleo and Street, and indicate that these processes can cause the hysteresis on the time scale of the measurements. 

Spangler, C. W. and Havelka, K. O., Design of new nonlinear optic-active polymers. Use of delocalized polaronic or bipolaronic charge states, in Materials for Nonlinear Optics Chemical Perspectives (ACS Symp. Ser., 455), Marder, S. R., Sohn, J. E., and Stucky, G. D., Eds., American Chemical Society, Washington, D.C., 1991, 661. 

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

The charged quasiparticles can be probed by electrical dc conductivity measurements (for polarons), magnetic susceptibility (for polarons and bipolarons), electron-spin resonance (ESR) (for polarons) and optical measurements (for polarons and bipolarons). As ESR is well suited for studying spin-carrying polarons, optical modification of the ESR (optically detected magnetic resonance ODMR) can be applied to link the emissive or absorbing properties of the polymer with its spin state. 

Band Structure Calculations and Experimental Results The spectroscopic properties discussed above are related primarily to intrachain electronic structure. One exception is the stability of gap states (e.g., polarons) versus the three-dimensional interaction effects mentioned in Chapter 11, Section IV.D. Energy and charge transport are, of course, dependent on interchain transfers. So while there are only a few three-dimensional band structure calculations (e.g., for PA [184] and PPV [185]), there are many theoretical calculations concerning infinite perfectly periodic one-dimensinal chains, the effects of local perturbations, and the elementary excitations of these chains solitons, polarons, and bipolarons. Only a few hints of that work will be given here. It has been discussed and reviewed several times (see, e.g., Refs. 186 to 188). 

Nature of charge carriers. Once injected, an electron and a hole must be brought at small enough distance to allow their recombination. This will depend on the nature and transport properties of the charge carriers. All CPs used in EL are nondegenerate ground-state polymers. The relevant notions of polaron and bipolaron and their relative stability were introduced in Chapter 11, Section IV.C. Some characteristic times pertinent to polaron or bipolaron formation will be discussed first, then the influence of traps. 

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