Results and Discussion
3.7. High-efficiency phosphorescent PLED
J is the current density, ε is the permittivity of the polymer, μe is the electron mobility, V is driving voltage, Vbi is the built-in voltage, and L is the polymer thickness. Both the current and the electron mobility of HMw-PFO is higher than that of PFO. This might be due to the greater chain length of HMw-PFO for longer intrachain transport and the reduced impurities which act as electron traps as discussed above. Finally we compare the photoluminescence (PL) quantum efficiency. The PL efficiency is 40 % for PFO and 50
% for HMw-PFO. More surprisingly the PL efficiency is enhanced to 75 % by slightly doping TFB into HMw-PFO, which might be attributed to the decrease of the aggregation formation in polyfluorene.[82]
3.7. High-efficiency phosphorescent PLED
It is believed that phosphorescent PLED is a promising way for high efficiency. In general, most the phosphorescent PLEDs use only one triplet emitter in one device. Rare studies report one PLED has two or more triplet emitters. In this part I studied four kinds of PLED, including a green one based on Ir(mppy)3 (device I), a yellow broad-band one based on Ir(mppy)3 : Ir(piq)2 (device II) and two white ones based on Ir(mppy)3 : Ir(piq)2/ blue (phosphorescent or fluorescent) bilayer structure (device IV). The fabrication process is described in page 29. Figure 3.20 shows the performance of device I and
device II.
Ir(mppy)3 single layer spin-rinsed TFB/Ir(mppy)3 TFB/Ir(mppy)3:Ir(piq)2=17.5:1
Voltage (V)
Ir(mppy)3 single layer spin-rinsed TFB/Ir(mppy)3 TFB/Ir(mppy)3:Ir(piq)2=17.5:1
Current density (mA/cm2)
FIG. 3.20: The performances of PLED including single green emitter and the mixing of the green and red iridium materials: single layer Ir(mppy)3 blending system (square), spin-rinsed TFB/ Ir(mppy)3 system (circle), spin-rinsed TFB/ Ir(mppy)3: Ir(piq)2
blending system (triangle). (a) The current density and luminance. Inset is the EL spectrum. Circle is TFB/ Ir(mppy)3, triangle is TFB/ Ir(mppy)3: Ir(piq)2=17.5:1(b) The current efficiency and power efficiency. Inset is the lifetime of green TFB/ Ir(mppy)3
device.
(a)
(b)
For green light devices based on Ir(mppy)3, the current efficiency of the device with spin rinsed TFB layer was 47.9 cd/A with power efficiency of 29.6 lm/W, and the external quantum efficiency (EQE) of 14%, about 20% increase from device without TFB layer. The current density was also increased, indicating that the TFB layer facilitates the hole injection. The luminance was also enhanced from 2445 cd/m2 to 7318 cd/m2 at 10 V.
For yellow light device, we doped Ir(piq)2 together with the Ir(mppy)3 with the ratio of 1:17.5. The current efficiency was 23 cd/A with power efficiency of 9.8 lm/W and the EQE of 10%, only slightly smaller than green Ir(mppy)3 devices (14%). The luminance was 9100 cd/m2 at 14 V. The current density and luminance decrease relative to the green light device suggesting Ir(piq)2 plays a stronger carrier trap compare with Ir(mppy)3. As a result, we have gotten yellow light with strong red component by blending little amount of Ir(piq)2 in Ir(mppy)3. The inset of Fig. 3.20 shows the EL spectra of the green light and yellow light devices and the lifetime of the green TFB/ Ir(mppy)3 device at the constant current mode. The yellow light device are composed of two distinct peaks and the CIE coordinate is (0.40, 0.56). Even though not white, this spectrum is very broad band and covers emission from 500 nm up to 720 nm. This result demonstrates that highly efficient multi-color PLED can be made by properly selecting the combination of Ir complexes.
On the basis green-red emission using Ir complexes, we made white PLED by adding a blue emitter. Two kinds of blue materials were used, one was FIrpic (device III) and the other was PFO (device IV). We first added all three phosphorescent dyes FIrpic, Ir(mppy)3 and Ir(piq)2 into the PVK blend host and it turned out that there was no blue emission at all even as the the FIrpic concentration was 20 times higher than the Ir(mppy)3, in sharp contrast with the above case where a small amount of red dye was enough to cause a large red emission. This implies that the carrier trapping capability of FIrpic is much weaker than the other two dyes. In Fig. 2.7(b) one can see that the EA (2.9
eV) and IP (5.8 eV) values of FIrpic are both much larger than the green and red dyes. In particular, EA is 0.4 eV and 0.5 eV higher than the red and green dye respectively. It is also 0.5 eV higher than the electron transport molecule PBD. We suspect that the much deeper molecular level of FIrpic makes it unlikely for an electron to be trapped since a large amount of energy needs to be released. On the other hand, the holes are also not trapped due to the large IP value. The absence of blue emission in the triple Ir blend therefore indicates that in order to have a balanced emission among the three primary colors, the molecular levels of the dyes must be more or less aligned to have an even carrier trapping capability. In order to achieve white light, the blue emission is realized by adding the second blue layer containing FIrpic or fluorescent polymer. Assisted by the poor solubility of high molecular weight PVK in toluene, the blue layer is formed by directly spinning upon the layer. The difference in thickness between the total film and sum of Ir(mppy)3: Ir(piq)2 film and PFO film is about 10 nm, indicating that the procedure is feasible for multi-layer structure devices. The performances of white light PLED are shown in Figure 3.21.
0 2 4 6 8 10 12 14
250 TFB/Ir(mppy)3:Ir(piq)2=1:4/FIrpic TFB/Ir(mppy)3:Ir(piq)2=1:1/PFO
Voltage(V)
10 TFB/Ir(mppy)3:Ir(piq)2=1:4/FIrpic TFB/Ir(mppy)3:Ir(piq)2=1:1/PFO
Current density(mA/cm2)
FIG. 3.21: The performances of white PLED by adding phosphorescent materials (FIrpic, triangle) and fluorescent (PFO, circle) (a) The current density and luminance. (b) The current efficiency and power efficiency.
For device III, with Firpic blended in PVK as the blue layer, the maximum current efficiency was 8.1 cd/A with the corresponding EQE of 4.28% and power efficiency of 3.62 lm/W. The maximum luminance was 4130 cd/m2. The CIE coordinates were (0.34, 0.43) at 7V and (0.32, 0.42) at 10V. For device IV with pure PFO as the blue layer, the
(a)
(b)
maximum current efficiency was 5.7 cd/A, with the corresponding EQE of 2.1% and power efficiency of 2.2 lm/W. The maximum luminance was 8900 cd/m2. The CIE coordinates were (0.34, 0.45) at 7V and (0.32, 0.42) at 10V. Fig 3.22 shows the spectrum of device III and device IV.
FIG. 3.22: The EL spectra of white PLED using (a) FIrpic and (b) PFO for the second layer at different voltages.
(a)
(b)
The spectrum exhibits three distinct blue, green and red emissions and with the corresponding CRI value of 86 and CIE coordinates of (0.33, 0.44) for device III . Although the CIE coordinate is not pure white (0.33, 0.33), the CRI value is very high for display application using color filter to separate the three primary colors. The device performances are summarized in Table 3.5.
TABLE 3.5: Performance of phosphorescent PLED in this work.
Label Max. Current
One of the remarkable features of the multiple doped phosphorescent PLED is that for a given ratio of Ir(mppy)3 and Ir(piq)2 in the first layer, the relative emission intensity of the green and red depends depends dramatically on the presence and the nature of the second blue layer. For single layer device in order to have the comparable green and red emission as shown in Fig. 3.20(a), the Ir(mppy)3 to Ir(piq)2 dye ratio in the blend is 17.5:1 where the red dye only constitute a small fraction. Surprisingly as the second PFO layer is added, the red ratio needs to be raised to 1:1 in order to have the balanced emission shown in Fig.
3.22. Furthermore, as FIrpic doped in PVK blend is used as the second blue layer, the red
dye have to be further increased to as much as 1:4 and become the predominant component in the first layer in order to obtain the spectrum in Fig.3.22. We believe that this feature indicates that the operation of the device is dominated by the amount of the electron flow into the first layer. Apparently, the relative electron trapping capability of Ir(piq)2 and Ir(mppy)3 depends sensitively on the electron current density. As the electron supply is high, Ir(mppy)3 may become easily saturated and the rest of the electron has to recombine in Ir(piq)2. This is why we only need a small amount of Ir(piq)2 for single layer where the electrons are directly injected from the CsF cathode. As the blue layer is inserted between the cathode and the first layer, electrons are blocked in the blue layer with poor electron mobility and the electron current supplied to the first layer become limited. In that case Ir(mppy)3 can trap electrons as efficiently as Ir(piq)2, so the red dye needs to be increased. Another reason for this is the red dye is the deepest hole trap in the PVK system and trapped holes will attract electrons and facilitate electron injection from the cathode. FIrpic doped PVK appears to have poorer electron transport than PFO since even higher red dye is necessary. Further support comes from the dependence of the green-red spectrum on the thickness of the blue layer and the CsF in cathode which modulates the electron flow. CsF is increased from 1 nm to 2 nm for device III for better electron injection.
Above we have reported PLED with green, green plus red, and green plus red plus blue iridium emitters. It turns out that before the addition of blue emitter Firpic the efficiency is high and comparable to small molecule OLED: external quantum efficiency of 14 % for green, 10 % for green plus red. Unfortunately when blue emitter is added the quantum efficiency drops to 4.3 % which is even lower than the best fluorescent white PLED. Such reduced efficiency directly results from the low efficiency for PLED with blue emitter only. In Fig. 3.23, the purely blue PLED with Firpic emitter shows efficiency up to only 9
cd/A (4 %) no matter how we tuned the ratio other small molecules including electron
FIG. 3.23: The performances of blue Firpic PLED (a) The current density and luminance.
(b) The current efficiency and power efficiency.
Neher et al were the first to report high efficiency PLED with green Ir complex, but their blue PLED efficiency is only 5.7 % which is close to our result[83]. It is probably
-2 0 2 4 6 8 10 12 14
because the triplet energy of PVK is not high enough to confine the triplet excitons of blue Ir complex. Indeed the host used for blue Ir OLED is much higher than PVK. Such superior host for PLED seems yet to be discovered. Recently high efficiency is reported for blue Ir PLED by Mathai et al using the same materials and device structure. The difference in processing conditions between their and our works is not understood so far.
Regardless of the problem of the blue emitter, the major result of our work is that very high efficiency yellow emission can be achieved with both green and red dopants. As for white PLED more works are needed to solve the reproducibility of blue PLED efficiency first. Furthermore the stability of the device is not good compared with fluorescent PLED.
In fact this is general problem for PLED with PVK host doped with Ir complexes.
Chapter 4 Conclusion
In conclusion different ways to achieve high-efficiency PLED have been studied, including charge balance achieved by several multilayer device designs, triplet emitters doped into a polymer host, and other parameters adjustment such as cathode adjustment, fabrication environment, annealing temperature…etc. In order to fabricate all-solution-processed multilayer PLED, liquid buffer layer method and blade coating method have been successfully applied. Several multilayer PLEDs have been made to verify the feasibility of methods. To serve as a buffer layer, the material must be a non-dissolvent liquid with high viscosity in order to protect the underneath layer. On the other hand it must have low boiling-point and small molecular weight for easy removal by baking. 1,2-propylene glycol appears to be the best choice. This method can be applied to not only multilayer PLEDs but also other solution-process multilayer polymer devices like solar cells which also need multilayer structure to increase the efficiency.
In addition to liquid buffer method, the blade coating provides a way to simultaneously reduce the cost of PLED and prevent the dissolution between two polymer layers by blade coating. This is a very simple method to fabricate all-solution-processed multilayer polymer devices in potentially very large area up to meter scales. There is no need to design new functional materials. The film uniformity is about the same as standard spin coated films in both large and small scales. The performance of the single layer PLED by blade coating is the same as spin coated ones. For bilayer PLED made by blade coating the efficiency is more than double compared with single-layer spin coated PLED. This method can be applied to not only PLED but also other solution-process multilayer polymer devices like solar cells.
Besides the multilayer fabrication methods, the factors that decide the device efficiency have been discussed. For high PL efficiency polymers, the electron current is demonstrated to be the most important factor to determine the EL efficiency. The universal features to get
higher EL efficiency are that the carrier mobility needs to be in the same order and, more importantly, the electron currents are larger than hole currents. To get larger electron currents, the purity of polymer is significant and the electron injection barrier should not be too high.
This is a direction not only for chemists to design high-purity polymers but also for physicists to design proper device structure in balancing electron and hole currents.
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