The relation between molecular weight and device efficiency

In addition to the fabrication environment, the relation between molecular weight and device efficiency has been studied. In this part, three types of device were fabricated, including the doped host-guest emission layer (EML) in single layer structure (type I), HTL/EML bilayer device (type II), and HTL/host-guest EML bilayer device (type III).

The fabrication process is described in page 27. Figure 3.17 shows the results of PFO-based devices.

FIG. 3.17: The performances of PFO devices: standard PFO device (solid square), type I (PFO: 1 wt% TFB) device (open square), type II (TFB/PFO) device (solid circle) and type III (TFB/PFO: 1 wt% TFB) device (open circle).(a) The current efficiency. Inset are the EL spectra. (b) The luminance. Inset is the current density.

The maximum efficiency of type I (PFO: 1 wt% TFB) device were 2.3 cd/A with the corresponding EQE of 1.99 %, which was 3 times higher than that of the standard PFO device (EQE=0.63 %) without TFB doping. This indicates that TFB plays the role of hole trap in the host-guest EML and the effective hole mobility is reduced. The efficiency is enhanced due to the better carrier balance. The maximum efficiency of type II device (TFB/PFO) was 1.64 cd/A with the corresponding EQE of 1.29 %, which was also 2 times higher compared to the standard one. As can be seen in Fig. 3.17(b), at the interface between TFB and PFO there is a hole barrier from the TFB side and electron barrier from the PFO side. The recombination zone is hence expected to concentrate hear the PFO region near the interface, unlike the case of single layer PFO where the recombination zone is expected to be near the cathode due to the low electron mobility. The cathode quenching effect in the bilayer structure is therefore less severe than the single layer case, thus explaining the improved efficiency in type II even without any doping. The type III device (TFB/PFO: 1 wt% TFB) combines the functions of type I and type II devices. The maximum efficiency of type III was 1.5 cd/A with the corresponding EQE of 1.18 %. The efficiency of type III did not get better than type I or type II device. Compared with device I, the HTL in device III may cause too much hole injection and destroy the balance already established by TFB doping. Compared with device II, the holes in device III may be less confined at the interface due to the easy injection from HTL into the TFB dopants to cause more cathode quenching.

Now we turn to the case of high molecular weight PFO with Mw equal to 356,000. The basic physical picture and the relative efficiencies of the device types are the same as the low molecular weight PFO. However the absolute values of the performance parameters are greatly enhanced by the increasing molecular weight. The results of HMw-PFO devices are shown in Figure 3.18.

FIG. 3.18: The performances of HMw-PFO devices: standard HMw-PFO device (solid triangle), type I (HMw-PFO: 1 wt% TFB) device of thickness 100 nm (open triangle), type I (HMw-PFO: 1 wt% TFB) device of thickness 60 nm (solid star), type II (TFB/PFO) device (solid rhombus) and type III (TFB/PFO: 1 wt% TFB) device (open rhombus).(a) The current efficiency. Inset are the EL spectra. (b) The luminance. Inset is the current density.

The maximum efficiency of type I device (HMw-PFO: 1 wt% TFB) was 2.8 cd/A for thickness of 100 nm with the corresponding EQE of 2.04 %. When the thickness was reduced to 60 nm, the maximum efficiency was enhanced to the remarkable value of 3.8

cd/A with the corresponding EQE of 3.7 % and power efficiency of 2.6 lm/W. The emission color was deep blue with Commission Internationale de L’Eclairage (CIE) coordinate (0.15,0.09). It is well-known that the main limiting factor for the blue polymer LED is the electron transport. Because of the small electron affinity electrons in PFO are highly susceptible to the impurities, with either chemical or structural nature, which commonly causes electron trap states inside the energy gap. High molecular weight polymers usually imply low impurity level after the synthesis. In addition, in the high molecular weight polymer there are much less chain ends which themselves are considered as some kind of traps.[27] Moreover, the chain entanglement and thermal as well mechanical stability are improved by the larger molecular weight and may result in less structural defects like aggregation formation. It is quite intriguing that the reduction of the thickness has such a dramatic effect on the efficiency. Apparently the high electric field helps the electron injection and mobility, especially for the high molecular PFO with less traps, therefore in the thin samples the charge balance and enhance the efficiency are improved despite of the concurrent negative effect of more cathode quenching. The maximum efficiency of type II (TFB/HMw-PFO) device was 2.1 cd/A with corresponding EQE of 1.8 % which were slightly higher than the low molecular weight counterpart. The maximum efficiency of type III (TFB/HMw-PFO: 1 wt% TFB) was 1.4 cd/A with corresponding EQE of 1.1 %, which was roughly the same as the low molecular weight counterpart. The luminance at a given voltage for HMw-PFO was however always much higher than low molecular weight PFO, supporting the assumption of higher electron current.

In addition to efficiency, another equally critical issue for blue PLED is the color stability at higher voltage. As discussed above polyfluorene is known to have the tendency to show pronounced green emission at the shoulder of the spectrum due to either

aggregate or ketone defects. As a result it is commonly observed that the emission spectrum suddenly changes from blue to green beyond some critical voltage. The maximal luminance below which there is no spectral shift can be taken as a measure for the color stability. As shown in Table 3.4, in general HMw-PFO shows a much higher maximal luminance compared with PFO.

TABLE 3.4: Performance of PLEDs in this work. The Max. Luminance is defined as the highest luminance before the growth of green peak in emission spectrum.

Label Max. Current

In particular, type I device with 100 nm thickness sustains stable deep blue emission up to alomst 7000 cd/m². Such brightness is more than enough for most of the display and lighting applications. Note also that the color stability of the three types is enhanced relative to the standard single layer sample. I suspect that with either TFB blending or bilayer structure the recombination zone of the device is moved away from the cathode where a higher concentration of oxygen or other defects are present and the polymers are

less stable under electroluminescence.

The mechanisms for the efficiency improvement are clarified by further experiments. Fig.

3.19(a) gives the direct evidence that TFB plays the role of hole trap by measuring the hole currents of hole-only devices with and without blending TFB.

FIG. 3.19: (a) Hole-only currents of PFO (solid circle) and PFO: 1 wt% TFB (open circle) devices. (b)Electron-only currents of HMw-PFO (solid square) and PFO (open square) devices. Inset is calculated electron mobility.

The hole current is reduced about one order of magnitude with a light TFB doping of 1 wt%. In addition, electron-only devices for PFO and HMw-PFO are compared. The results are shown in Fig. 3.19(b). The effective work function of CsF/Al is 2.6 eV.[81] It there forms an Ohmic contact with PFO whose electron affinity is 2.8 eV, and the current is bulk-limited instead of injection limited. In the inset we calculate the electron mobility by using space-charge-limited current (SCLC) voltage-current relation