FORMALIZACIÓN DEL COMERCIO

In the final set of bilayer devices with the carbazole dendrimers a further set of blend film devices were investigated but this time using a TCTA host. The structure of TCTA was shown in Figure 3.1. The triphenylamine center of TCTA is considered to impart greater hole-transport character than the biphenyl units in CBP. Also the dendritic nature of TCTA has been found previously to lead to improved film quality on blending with phosphorescent dendrimers and optimum device efficiency [26].

Films of a 20:80 wt % dendrimer-TCTA blend for each of the carbazoles were first measured for PLQY; the numbers obtained, as shown in Table 5.2, were similar to those of both the solution mea- surement and the solid solution film measurement on the dendrimers. While blending with TCTA was not found to be any more beneficial in reducing the concentration quenching effects in the dendrimer films than a CBP host, it does have the additional advantage of better aligned energy levels to the den- drimer than CBP. The HOMO and LUMO energy levels of CBP are, as plotted in Figure 5.16, located at 5.9 eV and 2.3 eV respectively, and thus both holes and electrons would be trapped on the dendrimer in a dendrimer-CBP blend. In contrast TCTA has HOMO and LUMO energies of 5.7 eV and 2.0 eV respectively [26]. The HOMO energy of TCTA is therefore equal to that of all the carbazole dendrimers. As a result the hole density should be more evenly distributed across the blended layer, charge trapping on the dendrimer will be minimised, and charge transport between these materials is optimised, and thus the device efficiency is improved [26].

Using the TCTA host with the dendrimer doped at a 20:80 wt % dendrimer-TCTA blend for each of the carbazole dendrimers, devices were fabricated using a structure of ITO/dendrimer/TPBI/LiF-Al, where TPBI was the electron transport/hole blocking layer. The resulting device characteristics are given

Figure 5.21: Device characteristics of 20:80 wt % dendrimer-TCTA host blended bilayer devices for the carbazole dendrimer family

in Figure 5.21 and summarised in Table 5.9.

As the results show for the Ir-CarbG1-TCTA blend device the maximum EQE was 14.4 % at 7.8 V, and a brightness of 3140 cd/m2, with an emission spectrum corresponding to a CIE coordinate of (0.310, 0.637). At a brightness of 100 cd/m2, the EQE was 11.9 % at 4.4 V. Very efficient devices were thus made with Ir-CarbG1 dendrimer, which strongly realised the TCTA blend film PLQY of 78 % detailed in Table 5.2 for this dendrimer.

Using a TCTA host was also found to improve the performance of the Ir-CarbG2 dendrimer over that obtained with a CBP host, although the resulting device efficiency was still some way short of the maximum theoretically possible given that a film PLQY of 82 % has been measured for the Ir-CarbG2- TCTA blend film. For the Ir-CarbG2 device, the maximum EQE was 9.3 % (at 7.0 V), and at a brightness of 100 cd/m2the EQE was 8.2 % (4.8 V). This device gave an emission spectrum corresponding to a CIE coordinate of (0.273, 0.632).

For the Ir-CarbDDG1 device, at a brightness of 100 cd/m2the EQE was 3.6 % (4.6 V), with the emis- sion spectrum corresponding to a CIE coordinate of (0.406, 0.579). A comparison of the current-voltage

DDCarbG1 15.3 cd/A) 13.1 cd/A)

Table 5.9: Summary table of device characteristics of 20:80 wt % dendrimer-TCTA host blended bilayer devices for the carbazole dendrimer family

and EQE-voltage characteristics for all the devices that used an emissive layer containing Ir-CarbDDG1 is shown in Figure 5.22. As the figure shows the current in the neat film device was greatest but the the efficiency of this device was least. For the low currents in the CBP blend devices the corresponding efficiency was the greatest, with the TCTA blended device between these two situations. It has been shown before with a phenylene dendronised dendrimer that a blend with a TCTA host gave higher hole mobility than a blend with a CBP host [102]. In this way it was concluded that with a CBP host holes preferentially hopped between dendrimer cores, whereas with a TCTA host both the host and the den- drimer played a role in the hole charge transport [102]. Ir-CarbDDG1 has been found to have a high hole mobility, and hence the device has a large current. On blending with CBP the lower device current suggested the mobility was reduced, and with CBP not participating in the charge transport, the device efficiency increased. In contrast, on blending with a TCTA host, the higher current indicated both the dendrimer and host transported the hole charges; the charge balance was modified and the efficiency of the device was reduced. A reason for the differences was proposed to be related the formation of excitons. Excitons, required for efficient radiative light emission, form when electrons and holes are in close proximity - in the double dendron dendrimer, the hole charge transporting dendrons, particularly in a blended film are widely spaced apart, and can be far from the electrons residing on the core; the probability of exciton formation was low and hence the dendrimer was inefficient [163].

Finally, for the third generation carbazole dendrimer, as found for both the first and second generation dendrimer, the use of a TCTA host gave the most efficient devices that have been able to be produced for the dendrimer. In this case for Ir-CarbG3 at the standard 100 cd/m2 brightness the device gave an EQE of 15.0 % (6.0 V), and the maximum EQE of the device was 15.1 % (5.8 V), a high efficiency that finally was able to realise the high photoluminescent quantum yield of this dendrimer. The CIE

(a) (b)

Figure 5.22: A comparison of the bilayer device characteristics for neat and host blended film devices of Ir-CarbDDG1, (a) I-V characteristics, and (b) EQE against voltage characteristics

coordinates of the emission spectrum of this device were (0.280, 0.623). The performance of this device placed it amongst the highest ever recorded for a solution-processed device. Furthermore, as it used a non-optimised device structure, further device efficiency improvements could be possible, through, for example, modification of the blend ratio for maximum charge balance. The results thus clearly show the significant advantage of the carbazole dendrimer structure.