As noted before all dendrimers discussed in this thesis were synthesised in Oxford, all in varying quan- tities. Consequently it was often the case that a popular dendrimer was used to completion. In this case a second version (or batch) of the same dendrimer was required to be made. Before dispatch extensive comparison measurements between the batches were made in Oxford using techniques of TLC, NMR and MALDI mass spectroscopy to ascertain the equivalence of the two batches. However it was still necessary to check whether photophysical and device properties were the same.
All the devices discussed thus far in the thesis have used the same batch of Dendrimer 1(Oxford batch code SVS01-3F) as the electroluminescent layer. In later cases a second batch (ZHL4-35) of this dendrimer was also used. This section briefly establishes the complete equivalence of these two batches through device and PL measurements. Consequently in subsequent chapters of the thesis no determination of the exact batch will be given unless particulary relevant, with all batches just referred to as Dendrimer1.
4.6.1 Photoluminescence of Dendrimer 1
The resulting solution and film absorption and emission spectra for the two batches of Dendrimer1are shown in Figure 4.18. As can be seen both batches had effectively the same characteristics as previously found for Dendrimer1[119]. That is, in both solution and film, the absorption spectra consisted of two components: the absorption of the phenylene dendrons causing the large peak around 272 nm, and the absorption of thefac-tris(2-phenylpyridyl) iridium core between 325 and 475 nm. While in the emission spectra the peak for solution and film was around 520 nm as previously found. The similarity in emission spectra of the two dendrimers was reflected in the CIE coordinates: for the solution emission spectrum of batchA(SVS01-34F) a CIE coordinate of (0.335, 0.582) was calculated, with almost no change in batchB(ZHL4-35) which gave a CIE coordinate of (0.334, 0.582). Moving from solution to film gave a slight red-shift in the emission spectra leading to a CIE coordinate of (0.341, 0.617) for batch A, and (0.347, 0.614) for batchB. In both cases the equivalence of spectra confirmed that both dendrimer batchesAandBwere the same. Further chemistry analysis techniques also confirmed this [134].
The solution and film PLQY were measured for both batches of Dendrimer1and were found to be the same. In film a value of 62 % was found for both batches, well within the error of the 65 % measurement
Figure 4.18: Solution and film absorption and emission plots showing the equivalence of the two batches of Dendrimer1used in the thesis, left hand plot shows solution results, right hand plot shows film results
Dendrimer 1
Batch
Max EQE EQE at 100 cd/m2 CIE coordinate BatchA 11.6 % (4.8 V, 27.7 lm/W, 41.7 cd/A) 9.6 % (3.6 V, 34.1 lm/W, 33.8 cd/A) (0.341, 0.616) BatchB 12.1 % (6.0 V, 22.9 lm/W, 43.9 cd/A) 12.1 % (6.0 V, 22.9 lm/W, 43.9 cd/A) (0.331, 0.623)
Table 4.7: Summary of bilayer device characteristics of Dendrimer1for various batches
reported previously by Dr Jean-Charles Ribierre [124] (note this was for batchA). In solution a value of around 80 % was obtained for each dendrimer, which although was slightly higher than the published value [119], was within the experimental error of±10 %. The similarity in the results of the two batches of Dendrimer1reflected one of the main advantages of dendrimers; they are monodispersive and thus excellent batch-to-batch reproducibility is possible even for small batches.
4.6.2 Dendrimer 1 devices
To complete the characterisation studies of Dendrimer 1 to confirm the equivalence between the two batches a set of bilayer devices was made. For this the device structure was ITO/dendrimer/TPBI/LiF/Al where neat film layers of batch A and B of Dendrimer 1 formed the electroluminescent layer. The resulting emission spectra are shown in Figure 4.19. As in photoluminescence the EL emission spectra are equivalent; both peak at around 520 nm with batchAgiving a CIE coordinate of (0.341, 0.616) close to that found for PL, while for batchB there was a slight shift in the CIE coordinate to give (0.331, 0.623).
Figure 4.19: Bilayer device characteristics showing the equivalence of the two batches of Dendrimer1
used in the thesis
In order to fully compare batches of dendrimers in devices an optimised device structure was re- quired. By using neat films of Dendrimer1from the two different batches this was attempted using the standard bilayer structure with a TPBI layer included within the device structure as an ETL/HBL.
For batchAof Dendrimer1a number of device runs were made (data not shown) where the thickness of the TPBI layer was varied. This involved attempting layer thicknesses in the range 30 to 80 nm, with the optimum thickness for maximum device efficiency found to be 70 nm. The resulting maximum EQE achieved for this device was 11.6 % at 4.8 V. At the standard brightness of 100 cd/m2the EQE was 9.6 % at 3.6 V, an improvement of almost 2% on that reported previously for this dendrimer. A plot of the EQE versus applied voltage for this device is shown in Figure 4.19.
For batch Bof Dendrimer 1, the same device structure with a 70 nm layer of TPBI as found for batch A was attempted, but in this case this was found to be no more successful. For an additional improvement on this device performance it was found necessary to change the solvent used for solution- processing of the film from chloroform to dichloromethane. Dichloromethane has a lower boiling point (40oC) in comparison to chloroform (61oC) which means it has a higher vapour pressure (290 mm Hg at 20oC, against 159 mm Hg at 20oC in chloroform). Consequently dichloromethane is a more volatile solvent than chloroform and so will evaporate more quickly during spin-coating leading to slightly thin- ner films. A film thickness of approximately 100 nm has been found to be standard. The resulting bilayer device using this dendrimer layer in combination with the 70 nm thick TPBI layer, an ITO anode (Merck 800 735 X0), and a cathode of 1.2 nm LiF capped with 100 nm of Al, gave a maximum EQE of 12.1 % at 6.0 V which also corresponded to the efficiency at the standard brightness of 100 cd/m2. The effi-
in the standard bilayer structure there is nothing done to increase theΦESCAP E term - in this device due to the large refractive index contrast with surrounding air a lot of emitted light becomes trapped in various waveguided modes, where it may be reabsorbed or emitted from the edges of the device. Lateral emission is not in the intended and useful forward direction and hence is regarded as a loss. A simplistic analysis gives the fraction of power coupling to leaky air modes as 4n12, or 2n12 if as done here a metal cathode is used to reflect light emitted backwards back out of the device (where n is the refractive index of the active layer) [100, 136, 137]. In general organic semiconductors have refractive indices of around 1.6, henceΦESCAP Ewill be equal around 0.2; up to 80 % of the light generated within a device remains trapped within the device. Consequently in Equation 4.1 the maximum theoreticalΦEXT is limited to only 20 % assuming all other terms are, as can be possible using dendrimers, equal to 1.
For Dendrimer 1 the neat film PLQY was measured to be 65 %, therefore for devices with this dendrimer, from Equation 4.1, the maximum theoretical external quantum efficiency was limited to only 13 %. The device shown in Figure 4.19 gave a maximum EQE of 12.1 % and was thus almost as efficient as the maximum theoretically possible using this dendrimer. It is recalled that Dendrimer1was a single dendron first generation dendrimer where the effect of intermolecular interactions would be greatest. The results implied that with higher generations of dendrimer where the effect of such emission quenching interactions were reduced leading to higher PLQYs, the device performance could be further improved over those values so far reported.