Negative polaron

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. 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...

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). 

Fig. 1.10. Structure, spin configuration and optical transitions for the positive and negative polarons. Note that only two transitions are optically allowed...

Fig. 5.1. Charge generation process in a single-layer conjugated polymer device under short-circuit conditions in the MIM model. VB valence band, CB conduction band, Eg bandgap, P+, P positive and negative polarons...

Fig. 9.12 Energy level schemes of solitons and polarons in polyacetylene (a) soliton, (b) anti-soliton, (c) negative soliton, (d) positive soliton, (e) negative polaron and (f) positive polaron.

This leads to typical values at room temperature of 5xlO-6m2V-1sec-1 for films of soluble PPV polymers cast from solution. At 30 K a value of 3 x 10-4 m2V-lsec-1 was obtained for poly(2,3-dibutoxy-l,4-phenylenevinylene). Comparison with ESR data shows that in PAni the intra-chain motion of negative and positive polarons is similar but that negative polarons are more mobile between chains. j... 

FIGURE 5.4. Electronic transitions (dotted arrows) due to positive and negative polarons. Occupation of electronic energy levels is denoted by solid arrows. The ground state is shown as a reference. 

Other interchain excited states are possible in PPV, including bound polaron pairs, where the positive and negative polarons reside on neighboring chains. 

From picosecond transient photoconductivity measurements on PPP films,22 we know that mobile charged states decay within 110 ps. In conventional routes to PPPs, defects like branched chains and large torsion angles of neighboring rings are known to occur. These defects act as shallow or deep traps for positive and negative polarons,38,39 which limit the mobility of charge carriers.40 The synthetic route toward the PPP-type ladder-polymers prevents the described defects and leads to a trap concentration of less than 1 trap per 1000 monomer units,28 whereas substi-... 

---