Positive polaron

Although it has been suggested55 that a similar species, the positive polaron, might have a sufficiently long lifetime in liquid water to undergo chemical reaction, no conclusive evidence for such reactions has yet been put forward. Positive ions formed in organic liquids probably decompose rapidly to free radicals with the elimination of a proton. 

After a photon has excited the conjugated polymer to form an exciton, the C60 accepts one electron due to its high electron affinity and establishes the anion Cg0. What is left on the polymer chain is a cation radical, i.e., a positive polaron, as depicted in Fig. 1.14, which is a mobile charge carrier that can move along the polymer backbone. This transfer is an exothermal reaction, where energy from the system is released. 

Fig. 1.21. (a) Light-induced ESR intensity as a function of the 3-factor in an MDMO-PPV/PCBM blend, = 9.5 GHz, T = 100 K, Aexc = 488 nm, P = 20 lW, 20 mW, and 200 mW. (b) A doubly integrated LESR signal of the prompt contribution as a function of the excitation power dependence. Squares correspond to the positive polaron signal and circles to Cg0... 

Figures 1.23a and b illustrate computer simulated ESR spectra of positive polaron P+ and C "0 signals in two frequency ranges, (a) 9.5 GHz and (b) 95 GHz with identical components of the -tensors, line width and amplitudes. The signals that overlap at 9.5 GHz can be clearly separated at 95 GHz, and coincide with the experimental ones in one important aspect the (/-anisotropy can be clearly resolved.

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.

Table 9.4 Carrier mobilities observed for conjugated polymers, units m2V sec 1. Carriers are positive polarons (holes) unless otherwise noted.

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... 

Photoinduced optical studies show that positive (P+) and negative (P ) polaron levels exist inside the n-n bandgap of EB associated with benzenoid and quinoid levels, respectively.17 They are believed to play an important role in charge injection and transport. We propose the following mechanism for the SCALE device operation. Under low bias voltages, electrons and holes can be injected from the electrodes into the quinoid and benzenoid levels of EB and form negative and positive polarons, respectively. These polarons transport to the EB/PPy interfaces via... 

Figure 1,59, Poly(para-phenylene sulphide) (PPS) (b) dibenzothiophene sulphide unit formed from (a) by an intramolecular phenyl-phenyl link (c) polybenzothiophene carrying a positive polaron.

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