The previous section considered dendrimers containing triazine groups in an attempt to impart the char- acteristics of electron transport to the dendrimer in order to improve the charge balance and thereby improve the device efficiency. However the results showed that the use of a triazine ring and carbazole containing dendrons were not able to achieve this. In this section an alternative method was considered where the carbazole groups were replaced with diphenylamine groups as the dendrons. The resulting structure was that of Dendrimer14(FI05-63C) shown in Figure 6.19.
Figure 6.14: Structure of the soluble triazine dendrimer, Dendrimer14
Diphenylamine is known to have a smaller ionisation and oxidation potential than carbazole and pos- sess more delocalised energy levels. It therefore has improved hole charge transport through decreased hopping distances in comparison to the carbazole dendrons. As a result the hole mobility in dipheny- lamine has been measured to be greater than that of carbazole [173]. Consequently diphenylamine is a stronger donor than carbazole, which meant Dendrimer14should have a lower LUMO level and larger singlet energy gap than Dendrimer11[173]. In order to establish whether this was true the energy levels of the dendrimers considered were measured through electrochemistry studies by cyclic voltammetry measurements in Oxford by Fumiaki Ito, the full details of which are outside the scope of this thesis.
The oxidation potentials were measured allowing values for the HOMO levels of most of the den- drimers considered in this chapter to be determined, these values were given in Table 6.1. For Den- drimer11a value of 5.75 eV was obtained, for Dendrimer12where the only change from Dendrimer11
was the increased alkyl chain length, the HOMO was estimated to be lower at 5.72 eV. For Dendrimer13
the HOMO energy was 5.69 eV, this decreased further in Dendrimer14to give a value of 5.52 eV. Unfortunately only the oxidation potentials of the dendrimers were measured and thus the reversibil- ity or not of the reaction remains questionable. As a result, the technique of using the first absorption peak of the UV-visible absorption spectra with the addition/subtraction of the oxidation potential to es- tablish the unknown reduction potential, and hence from this estimate the LUMO energy could not be reliably assumed to yield the correct number. Albeit, as the HOMO levels obtained for these dendrimers were all close to the HOMO level of an iridium(III) dendrimer with phenylene dendrons at 5.6 eV and emit in the green, a similar value for the LUMO of around the 2.6 eV reported for this dendrimer [26]
Figure 6.15: Solution and film absorption and emission spectra of Dendrimer14
could be predicted.
On making this assumption the ability of the dendrimers with triazine rings included in the structure to impart electron transport ability to the dendrimer becomes questionable. In light of the LUMO levels the dendrimers do not seem to have any particular electron transport (i.e. injection) ability, a fact that has clearly been indicated in the device results so far obtained. Of course, the presence of the triazine ring may have enhanced the electron mobility of the dendrimer, but the device results in no way confirm this. It was also not possible to obtain alternative evidence to support such a claim by for example a measure- ment of the electron mobility. This was due primarily to the small amounts of dendrimer available, but also due to the difficultly in obtaining a suitable charge generation layer to use in combination with a spin-coated layer of this dendrimer.
Due to the difficulty in synthesis of a dendrimer with diphenylamine dendrons, Dendrimer14 was made with no surface groups were included within the dendrimer structure. Consequently this dendrimer was not particularly soluble. Nonetheless films were still able to made at a sufficient concentration to allow photoluminescence measurements to be made. The resulting absorption and emission spectra of both solutions and films of Dendrimer14are shown in Figure 6.15.
As Figure 6.15 shows, for Dendrimer 14 the solution absorption spectra were very similar, both showed peaks around 208 nm and 278 nm. The second of these peaks was due to the presence of the triazine ring. The dendrimer showed minimal absorption at higher wavelengths.
Figure 6.16: Comparison of solution and film absorption and emission spectra of Dendrimer 14 and Dendrimer11
moving from solution to film there was a large 36 nm red-shift in the spectra, shifting the peak to occur at 566 nm in the film. The CIE coordinates changed from (0.382, 0.590) in solution, to (0.452, 0.533) in film. The size of this large red-shift may be related to the poor quality of the films that were able to be made with this dendrimer. This was also reflected in the measurement of the photoluminescence quantum yield: in solution a value of 69 % was found closely replicating, as shown in Table 6.1, the values obtained for most other dendrimers considered in this chapter. In contrast, for a neat film of Dendrimer14, a PLQY value of 20 % was measured. This value was three times less than that of the triazine Dendrimer13 that included solubilising surface groups which enabled good films to be made with the dendrimer. Evidently the choice as whether to use surface groups or not has great consequence on the resultant film quality and hence the quantum efficiency of the dendrimer.
The comparison between Dendrimer 11 (or 12) and Dendrimer 14 was also relevant as the only difference between these dendrimer structures was that of the dendrons: carbazole dendrons were used in Dendrimer11(or12) and diphenylamine dendrons in Dendrimer14. Figure 6.16 repeats the spectra shown in Figure 6.15 but with the addition of the solution spectra obtained for Dendrimer11. As the figure details once again it was found that by simply changing the dendron the resultant absorption and emission of the dendrimer could be modified as desired.
6.2.6 Single layer devices from triazine dendrimers with diphenylamine dendrons