Polarons negative

Figure 8-16. A picture of the pholoexcilalion scenario in m-LPPP, see text for a discussion. Pv is a positively charged chain (polaron), while X- can be either a negatively charged chain or an electron acceptor, such as oxygen.

Figure 9-21. EL process in PLEDs. VB... valence band LB. ..conducting band V... potential M,M2... Mclal electrodes, U... bias voltage Z X2 —Interface luyers tK...bandgap P and Pr... positive and negative polarons /. Fermi energy, and 0... work I unclion.

The electrical conductivity in the solid state is determined by the product of the carrier concentration and the carrier mobility. In conjugated polymers both entities are material dependent and, i.e., are different for electrons and holes. Electrons or holes placed on a conjugated polymer lead to a relaxation of the surrounding lattice, forming so-called polarons which can be positive or negative. Therefore, the conductivity, o, is the sum of both the conductivity of positive (P+) and negative polarons (P ) ... 

The efficient formation of singlet excitons from the positive and negative charge carriers, which are injected via the metallic contacts and transported as positive and negative polarons (P+ and P ) in the layer, and the efficient radiative recombination of these singlet excitons formed are crucial processes for the function of efficient electroluminescence devices. 

Fig. 4. Energy level diagrams showing possible electronic configurations for positively-charged polaron (a) and bipolaron (b) defects and (c) a schematic bipolaron band model. The negatively-charged polaron would carry three electrons and the bipolaron four. Also shown is the neutral polaron-exciton (d) which would decay to restore the chain structure.

Fig. 9 Illustration highlighting the possible importance of entropy in the charge separation process. Positive and negative polarons experience a strong coulombic attraction that may be offset by entropy to achieve charge separation. Reproduced with permission from [54]. Copyright 2010 American Chemical Society...

The energy E will necessarily have this minimum, but its value at this point can be positive or negative only in the latter case will a stable self-trapped particle (i.e. a small polaron) form. This is most likely to occur for large effective mass, and thus for holes in a narrow valence band or for carriers in d-bands. If the polaron is unstable then there is practically no change in the effective mass of an electron or hole in equilibrium in the conduction or valence band. 

A rather large negative MR, with a substantial anisotropy, has been observed in reentrant insulating samples with high x (Oiwa et al. 1998b Katsumoto et al. 1998). A destruction of bound magnetic polarons, as invoked in an earlier work on (In.Mn)As... 

Fig. 1. Three THz-scans at 10K The pulse transmitted through air (Efree(tJ), the unexcited sample (E,ram(t)), and the photoexcited sample ( , (/)). The 45ps delay between and E,rani is caused by the large real part of the dielectric function (see inset of Fig. 2). A less obvious phase shift also exists between E,mns and associated with the real part of e. Inset Left - the (001) face of rutile. Right - the lattice distortion when an electron is placed in the polar lattice results in polaron formation (partly) positively charged Ti-atoms are attracted, and (partly) negative O-atoms repelled (see text).

However, while the evidence for the existence of polarons was extremely convincing, that for bipolarons was rather more problematical in that it was largely effectively negative in nature the absence of an absorption peak in the optical spectrum, the absence of a signal in epr studies on the decline of the observed signal. In essence, bipolarons had not been actually observed. This fact was remedied by the work of Christensen and Hamnett (1991) who employed ellipsometry and FTIR to study the growth and electrochemical cycling of polypyrrole in situ in aqueous solution. 

Recent pulse radiolysis experiments (20, 30) have fully established the presence of the negative polarons (hydrated electrons) in irradiated water, and also have demonstrated the occurrence of Reaction 39 (15). 

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