Polaron resonance

The maximum d is reached in the region of the polaron resonance absorption with radiation frequency ft 4/0, // , where the width of the absorp-... 

Photoexcitation at /.e = 488 and 457.9 tim was accomplished by an Ar" " laser. An Oriel high-intensity high-pressure broadband Hg emission lamp provided the UV source. Nearly monochromatic 25 mW excitation at 353, 308, and 248 ntn was provided by 6 nm-bandwidth mirrors blazing at these wavelengths 5 mW excitation at 430, 406, and 380 nm was obtained by appropriate bandpass filters. As the ODMR signal-to-noise ratio was much poorer at these latter the microwave power was increased to up to 1400 mW. The resulting microwave field, however, was then sufficiently intense to broaden the narrow polaron resonance. Detailed descriptions of the visible- and UV-ODMR systems are given elsewhere [59,60]. 

Figure 7.9. The PL-enhancing polaron resonance in poly(3-hexyl thiophene) (P3HT) and poly(3-dodecyl thiophene) (P3DT), showing the decomposition into two Gaussians. While its intensity strongly decreases with increasing temperature, the lineshape is temperature independent [23].

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. 

Figure 7.11 displays the temperature dependence of the amplitude AI/I of the PL-enhancing polaron resonance in P3HT and P3DT films. The amplitude of the resonance strongly decreases with increasing T, but is still observable at room temperature, as it is in solutions. The temperature dependence appears to be quantitatively given by [23,59],... 

Figure 7.10. The PL spectrum and the spectral dependence of the PL-enhancing polaron resonance A/ in P3HT and P3DT films at 20 K and solutions at 295 K.. Note that AI/I is thus independent of emission wavelength [23].

Figure 7,11. Temperature dependence of the PL-enhailcing polaron resonance in P3HT and P3DT [23],...

The lifetime of the trapped interchain polaron pairs responsible for the indirect non-radiative ODMR can be determined from the microwave chopping-frequency dependence of the in-phase and quadrature lock-in detected resonance signals [47]. Such measurements of the PL-enhancing polaron resonance of P3HT yielded a polaron pair lifetime t 5 /js, which is 10 times shorter than in PPVs [79]. The nature of this relatively short lifetime is not clear and obviously warrants additional investigation. Some speculations on this observation are offered in Section 2.1.3 below. 

Figure 7.14. The PL-enhancing polaron resonance in PPV and DHO-PPV prepared by Wudl and co-workers [24].

The PL spectra of the DHO-PPV and the highly-ordered pristine PPV used to obtain the polaron resonance shown in Figure 7.14, and the spectral dependence A/ of the resonance in DHO-PPV are displayed in Figure 7.15. As observed in the P3ATs (Figure 7.10), the spectral dependence of the resonance... 

Figure 7.16 presents the narrow PL-enhancing polaron resonance in CN-PPV excited at 488 nm using 40 mW microwave power. Note the exceptionally narrow linewidth A//i/2 6.7 G. As mentioned above, this narrow width is consistent with a significant contribution of hyperfine coupling of the polaron wavefunction with the protons of the HC=CH vinylene unit to the linewidth, and trapping of the polaron pairs at the CN-substituted site, which is... 

Figure 7.15. PL spectrum of DHO-PPV and the spectral dependence of the polaron resonance A/. Implying that A/// is emission wavelength-independent.

Figure 7.16. The narrow PL-enhanciug polaron resonance in CN-PPV excited at 488 nm. Note the exceptionally narrow linewidth AWi/2 6.7 G...

The PL-enhancing polaron resonance in the trans— rrans-l,4-distirylbenzene-0-alkane block copolymer, displayed in Figure 7.18, is also identical to that of the typical PPVs. This result may not be surprising,... 

Figure 7.17. Polaron resonance in dilute blends of MEH-PPV in ultra-high molecular weight polyethylene.

Figure 7.18. Polaron resonance in franr-rraw-1.4-distirylbenzene-O-alkane block co-polymer. Note that lrans-lrans-, 4-distirylbenzene is an oligomer of PPV [61],...

The PL-enhancing polaron resonance of PDEPDPV, excited at 2cx = 488 and 353 nm, is shown in Figure 7.22. As in the PPVs and PPEs, g 2.0023, identical to that of the free-electron value within experimental error. Note the exceptionally narrow resonance at 2ex = 488 nm, of width G, and the much... 

The PL-enhancing polaron resonance in PDPEA excited at 2ex = 353, 308, and 250 nm is shown in Figure 7.24. The lineshape is clearly independent of 2ex, and the broad line of width AH /2 30 G suggests... 

Figure 7.22. The PL-enhancing polaron resonance in PDEPDPV films excited at /. = 488and 353 nm. Note the exceptionally narrow resonance at A = 488 nm, of width Ai/,/2a 3 G, and the much broader line at Ac = 353 nm, of width AH,/2 24 G [60].

Figure 7.26. The PL-detected polaron resonance in P3AT at various 2,...

Figure 7.28 displays the PL-detected polaron resonance in DHO-PPV and 2,5-dioctoxy-PPV (DOO-PPV) at various X . As is clearly seen, in both DHO-PPV and... 

The behaviour of the polaron resonance in CN-PPV prepared by Holmes and co-workers is qualitatively... 

In the highly ordered pristine PPV synthesized by Holmes and co-workers [80] the polaron resonance was PL-enhancing at 2 = 353 nin, and no PL-quenching was detectable. This is consistent with the rapid decay of free polarons to p, -pi pairs trapped at close... 

Figure 7.29. The PL-detected polaron resonance in CN-PPV at / = 4fi8, 360, and 353 nm. Note that the enhancing and quenching lineshapes are very similar to each other.

Although not shown, the temperature dependence of the amplitude of the EL- and o-detected resonances is also very different from that of the PL-detected resonances Whereas the latter decreases 50-fold from 20 K to room temperature, the EL-detected resonance decreases by only 50% and the cr-detected resonance is essentially temperature-independent. This difference is very significant as it illuminates the nature of the temperature dependence of these resonances. As discussed in Section 2.1.1 above, it was initially suggested that the temperature dependence of the PL-enhancing polaron resonance reflects the temperature dependence of the polaron spin-lattice relaxation rate... 

Figure 7.32. The EL-, a-, and Oph-detected polaron resonances of an early ITO/PPV/Ca/Al LED at an injected current density J = 50 /rA/mm, bias V =r 30.7 V, and T = 20 K. Reprinted from ref. 26 with permission from Elsevier Science Inc.

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