Single layer devices

In the first simple tests, single layer devices were fabricated using neat films of the carbazole dendrimers as the electroluminescent layer sandwiched between an ITO anode and a cathode of calcium-aluminium. For these devices the old solution-processing protocol was used, where the time left for the dendrimer in solvent to dissolve prior to spin-coating was not minimised, unlike for all other subsequent devices which followed the solution-processing protocol as described in Chapter 4, where the time of the den- drimer in solution was minimised. The resulting device characteristics are shown in Figure 5.14, and are summarised in Table 5.4.

Dendrimer Max EQE EQE at 100 cd/m2 CIE coordinate Dendrimer5, Ir- CarbG1 0.35 % (8.9 V, 0.3 lm/W, 0.8 cd/A) 0.28 % (0.36 lm/W, 0.6 cd/A, 6.9 V) (0.376, 0.575) Dendrimer6, Ir- CarbG2 0.04 % (10.8 V, 0.02 lm/W, 0.07 cd/A) 0.04 % (10.8 V, 0.02 lm/W, 0.07 cd/A) (0.359, 0.568) Dendrimer7, Ir- CarbG3 0.04 % (10.9 V, 0.04 lm/W, 0.13 cd/A) 0.04 % (10.1 V, 0.04 lm/W, 0.13 cd/A) (0.319, 0.595) Dendrimer8, Ir- DDCarbG1 0.75 % (7.6 V, 0.9 lm/W, 2.2 cd/A) 0.51 % (5.4 V, 0.9 lm/W, 1.5 cd/A) (0.470, 0.521)

Table 5.4: Summary of device characteristics of single layer devices for the carbazole family

For all these dendrimers the turn-on voltage was found to be around 3-6 V, well in excess of the organic bandgap and nearer to the work function difference of the two electrodes which is typical to most organic materials. A comparison of the device curves showed that the first to third generation carbazole dendrimers all drew a similar current through the device, with the double dendron device giving much more current and light output. The light output of the second and third generation carbazole devices were similar but the first generation carbazole device was capable of giving out slightly more

Figure 5.14: Device characteristics of single layer devices from the carbazole dendrimer family. Devices were made on following the old solution-processing protocol

light, consequently this device was considerably more efficient. In fact for the device with Ir-CarbG1, the maximum external quantum efficiency (EQE) was 0.35 % at 8.9 V. At the standard display brightness of 100 cd/m2, the EQE was 0.28 % at 6.9 V, this corresponded to a power efficiency of 0.29 lm/W or 0.63 cd/A. Despite the greater photoluminescence efficiency of both Ir-CarbG2 and Ir-CarbG3 in comparison to Ir-CarbG1, for both these higher generation dendrimers, the device performance was in fact worse. The slight reduction in current and large drop in luminance meant that both dendrimers were only capable of attaining a maximum EQE of around 0.04 %.

It is recalled from Section 5.4, that in the carbazole dendrimers the hole mobility increased with dendrimer generation, yet in these device structures the current was found to decrease with generation. The reduction in current with dendrimer generation was the same as has been observed previously in dendrimers with non-charge transporting phenylene dendrons [27, 93]. In this case the behaviour was explained by the enlarged distance between cores on increasing the dendrimer generation. This resulted in a reduction in charge mobility, because with an increased hopping distance between the cores the prob- ability of hopping between the cores decreased resulting in a slowing of the charge carriers. However, in

the case of the carbazole dendrimers, where the hole mobility increased with dendrimer generation this cannot be the case. For these dendrimers, as shown in Figure 5.9, the photocurrent was reduced with dendrimer generation, although as shown in Figure 5.11, the hole mobility increased with dendrimer generation. While the change in dendron resulted in a change in the hole charge transport properties, the similarity of the LUMO distributions in both dendron types [160], suggested that the electron charge transport behaviour was the same. That is, electron transport remains via the cores in the carbazole den- drimers; electrons and holes are thus widely spaced and so the probability of exciton formation decreases in these dendrimers [163].

The best single layer device performance from a carbazole dendrimer was found for the double dendron dendrimer Ir-CarbDDG1. This device gave a maximum efficiency of 0.75 %, and at a brightness of 100 cd/m2 the EQE was 0.51 %. Unfortunately, once again the lack of a sufficient quantity of the Ir-CarbDDG2 dendrimer prevented devices from being attempted with this dendrimer to test whether further efficiency improvements were possible.

The high single layer device efficiency with the double dendron Ir-CarbDDG1 carbazole dendrimer was found despite this dendrimer showing the highest hole mobility, and thus by intuition would be pre- dicted to have the most unbalanced charge transport. Despite the possible charge transport imbalance problems within Ir-CarbDDG1 the light output was greater in the Ir-CarbDDG1 device than in any of the devices from the other carbazole dendrimers. Also in Section 5.4.3, it was discovered that Ir-CarbDDG1 gave hole transport behaviour that indicated a more ordered system than found for the other carbazole dendrimers. The fact that this was still able to give an efficient device may have arose from increased electron transport within this ordered dendrimer system due to the smaller core-to-core separation com- pared to the higher generations of carbazole dendrimer.

A further reason for the high device efficiency may be the higher neat film photoluminescence quan- tum yield of Ir-CarbDDG1, but a 15 % improvement in film PLQY over that of Ir-CarbG2 and Ir-CarbG3 can not fully account for the nearly 20 times improvement in the device efficiency. Of course, as a fi- nal explanation, as has been seen previously with double dendron dendrimers, Ir-CarbDDG1 may have suffered less photodegradation effects in comparison to the other dendrimers considered here where all were prepared under the old protocol solution-processing technique.

The corresponding emission spectra for each of the three generations of dendrimer are also shown in Figure 5.14, from which it can be seen they were very close to their PL counterparts with CIE co-