Defect emission

Photoluminescence of the films varies greatly, both in intensity and in spectral shape, from one report to another. This is not surprising, since this property is very dependent on the state of the surface of the individual crystals. A red (ca. 1.8 eV) defect emission is usually seen, but green, yellow, and infrared peaks have also been reported. The various wavelengths are related to different defects in/on the crystals even the green emission is probably due to a shallow defect emission. 

Photoluminescence of the films (in the absence of water vapor) usually is dominated, as for CdS, by a broad defect emission varying from ca. 1.4 to 1.6 eV. A (close to) band-to-band emission is often also observed, usually (but not always) at a lower intensity than the broad defect emission. In the presence of water vapor, however, the band-to-band emission often dominates. 

Figure 9 (A) Typical TEM image of a CdSe nanoparticle synthesized by the direct reaction of Se and CdO (see main text). (B) Typical emission spectrum of a CdSe nanoparticle. The spectrum exhibits two main features (1) a sharp Gaussian-shaped peak due to band-edge emission and (2) a broad feature at lower energies due to defect emission. (C) The size-dependence of the peak wavelength for CdSe nanoparticles, determined using data provided in Murray et al. (1993).

That even low levels of defects can produce strong emission is exemplified by the case of Ph-LPPP (108). The PL emission from 108 is very similar to that from 106 with maxima at 460 and 490 nm. However, the EL spectrum shows an additional long wavelength band. This is not a broad featureless band as seen for the defect emission from 5 or 106, but one with well-resolved maxima at 600 and 650 nm. Photophysical investigation of this emission showed the feature at 600 nm to be emission from a triplet exciton (phosphorescence) with a vibronic shoulder at 650 nm [158]. Elemental analysis of the polymer showed it contained 80 ppm of palladium (cf. <2 ppm in 106). It was therefore proposed that residues of the palladium catalyst used to make the precursor polymer 103 reacted with the phenyllithium and the polymer to introduce covalently bound palladium centres onto the polymer chain. These then act as sites for phosphorescent emission. 

Enhancement of the Green Defect Emission in the Solid State... 

Green Defect Emission Emerging Under Device Operation. 286... 

In addition to the above mentioned defect emission band at 2.3 eV upon device operation, one also finds an additional individual emission band that is located at 2.45-2.6 eV (energetically located between the regular blue emission bands at 2.9 eV and the broad keto-related emission at 2.3 eV). These emission features appear most strongly for devices with calcium electrodes. It was found that this spectral feature is located close to the cathode of the device and is most probably related to a chemical degradation reaction caused by the low work function metal electrodes [44]. 

We are of the opinion that our single molecule results on PFs copolymerised with lluorenone units represent compelling proof for the monomolec-ular nature of the green defect emission. The development of new synthetic strategies for reaching a blue stable emission in PF-based OLEDs should therefore concentrate on formulating protective groups, rather than on spacers to control intermolecular interactions, which inevitably lower the transport performances. 

We found that OMBE-grown PSP films on KC1 substrates at different substrate temperatures show a drastic difference in the delay emission spectra while their cw-PL spectra are rather similar. The broad structureless defect emission band dominates the delayed PL emission of PSP films consisting of lying molecules on the substrate and no such band has been observed in films composed of upright standing molecules. This clearly indicates a structure-related origin of the observed defects, implying that their concentration could be minimized in most perfect structures of PSP crystallites. 

Almost all of the semiconductor nanoclusters studied so far show red-shifted broad emission bands that can be qualitatively attributed to surface defects. The exact nature of these defects is not well understood. A major barrier for understanding the nature of the defect emission is the... 

Figure 7.5. The radiative recombination of band tail electrons e" and holes h, which yields the main 1.3 eV emission band, the radiative decay of H into neutral dangling bonds db", which yields the defect emission band at 0.9 eV, and the non-radiative decay of e" into db in a-Si H.

Lupton, J.M., M.R. Craig, and E.W. Meijer. 2002. On-chain defect emission in electroluminescent polyfluorenes. Appl Phys Lett 80 (24) 4489-4491. 

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