Polaron recombination

The authors postulated that the conductivity is proportional to the number of carriers in the film and that the mobility of polarons is equal to that of bipolarons, hence the conductivity is independent of carrier type. Thus, the conductivity increases steadily as polarons, and then both polarons and bipolarons, are generated but should attain a steady value when polaron recombination to give half as many bipolarons becomes important. 

Aida and coworkers developed poly(pyrrole)-containing mesoporous silica films in both hexagonal and lamellar phases.34 The polypyrrole chains are highly constrained and insulated when incorporated within hexagonal mesoscopic channels and the possibility of the polarons recombining into bipolarons is significantly suppressed. In contrast, the two-dimensional lamellar phase affords spatial freedom for electron recombination. 

The foregoing section attempted to provide an introduction to the dynamics of singlet excitons, generated either by photoexcitation or by polaron recombination, and the effects of polarons and TEs on the SE dynamics. We now turn to the basic structure and dynamics of OLEDs, which obviously reflect the basic processes described above. 

Since the polaron resonance is also observed in dilute methylene dichloride solutions, one may be tempted to assign it to intrachain polaron recombination. Yet this assignment may be incorrect on two counts (i) it is not clear that the polymer chains ate completely separated from each other by the solvent (ii) even if the chains are tnily isolated by the solvent, confonnational defects, typically kinks, may generate interconjugation-segment polaron pairs, which are dynamically similar to interchain pairs [30,49]. Indeed, it should be noted that toluene solutions of P3HT did not yield any ODMR, and in CCI4 solutions the polaron resonance decayed below the noise level after 488 nm photoexcitation at room temperature for a few minutes [23,59.73]. This clearly demonstrates that the polaron resonance is sensitive to the interaction between the polymer and the solvent, and may possibly involve its effect on chain separation and/or formation of kinks in the chains. 

The role of the dopant potential on the stability and magnetic and optical properties of polarons and bipolarons in conducting polymers is shown with the aid of calculations of singlet and triplet states of a bipolaron [167] and by spectroelectrochemical and conductivity measurements [168-170]. The X-band optically detected magnetic resonance of PHT and PDDT shows that the distant intrachain polaron recombination is temperature-independent and identical in films and solutions. However, the triplet polaronic excitation decay is observable in films, but not in solutions [171], Electrochemical in situ conductivity and EPR measurements of PT films were performed in several solutions [172]. The results indicate that polarons merely seem to initiate the electrical conductivity. The electronic delocalization of polarons is restricted to a relatively short chain length at low potentials. As the polaron concentration increases (spin density maximum), bipolarons are generated immediately (probably too fast for the detection of polarons by EPR). Thus the bipolarons prevail in the fully conducting polymer films and as a consequence should be mainly responsible of the intrinsic conductivity [172]. Asymmetrically disub-stituted PBT display well-defined redox processes which are correlated to the consecutive formation of radical cations, dimerized radical cations, and dications [173]. 

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. 

The authors found the neutral form of the polymer to be epr inactive. With increasing potential the number of spins detected increased from zero, reached a maximum and then decreased, as shown in Figures 3.79(a) and (b). The data were interpreted by the authors in terms of the initial formation of polarons followed by their subsequent recombination into bipolarons. 

W Wohlgenannt, XM Jiang, C Yang, OJ Korovyanko, and ZV Vardeny, Spin-dependent polaron pair recombination in TT-conjugated polymers enhanced singlet exciton densities, Synth. Met., 139 921-924, 2003. 

Giebink NC, Lassiter BE, Wiederrecht GP, Wasielewski MR, Forrest SR (2010) Ideal diode equation for organic heterojunctions, n. The role of polaron pair recombination. Phys Rev B... 

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