Extrinsic excitons

The excitons can react with the surface and, owing to the presence of absorbed oxygen, impurities and structural defects or due to electrode interactions may dissociate to produce separated electrons and holes. The probability of the generation of charge carriers will be higher for excitons created near the surface. The extrinsic exciton-surface mechanism may be distinguished from the direct process as follows ... 

Our results also shed light on the long-lived PA3 band detected in transient PM measurements of P3BT (see Fig. 7-19) and can explain changes in the PA spectra observed in several ps transient measurements of films of PPV derivatives at energies around 1.8 eV [9, 25, 60J. In good PPV films the transient PA spectrum shows a PA band of excitons at 1.5 eV whose dynamics match those of the PL and stimulated emission (SE) [9J. However, in measurements of oxidized [25] or C60-doped films 60, there appears a new PA band at about 1.8 eV whose dynamics are not correlated with those of the PL and SE. Based on our A-PADMR results here, we attribute the new PA band at 1.8 eV to polaron pair excitations. These may be created via exciton dissociation at extrinsic defects such as carbo-... 

Poly(phenylmethylsilane) (PMPS) was investigated by Kepler et al. (1983, 1984, 1987). Figure 32 shows the wavelength dependence of the absorption and the normalized photocurrent. The spectral dependence of the photocurrent indicates that carrier generation occurs by an extrinsic process which results primarily from an interaction of charge-transfer excitons with the surface. The argument is similar to that proposed earlier by Kepler (1976) to describe hole... 

The STH resonance is enhanced in ODMR spectra by the addition of parts per million of known electron trapping dopants such as Ir3+ [176], Ni2 + [177], and Rh3+ [178]. This is probably because these dopants enhance distant pair formation at the expense of exciton formation [179,180]. Extrinsic self-trapped hole species have also been observed in the ODMR spectra of AgClj Br samples (see below) [111, 181,182],... 

Low temperature absorption measurements performed in p-type 3C-, 4H-, 6H- and 15R-SiC doped with boron and 6H-SiC doped with Al show extrinsic absorption features which have been attributed to excitons bound to neutral acceptors [69-72]. Electroabsorption measurements performed in boron-doped 6H polytype show that the observed effective microfield is considerably stronger than the expected values generated by the substitutional boron acceptor, suggesting that the excitons may be bound to dipole complexes, which may include the N-donor [73]. 

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