采用n型掺杂的高性能和高稳定性PIN OLED--Essay代写范文

Essay代写范文:“采用n型掺杂的高性能和高稳定性PIN OLED”，这篇论文主要描述的是经过创新研发的N型掺杂技术被应用到了PIN OLED的试验当中，以研究出性能更高、运行更为稳定、周期寿命更长的产品，采用这种N型掺杂技术能够有效的提高有效寿命的问题，是一种比较有前途的架构方式，如果研究成功将获得更长的寿命，取得突破性的进展，使得PIN能够在热环境下有着热稳定的优势。

Innovative PIN OLED that use a new n-doped architecture have been investigated.Good initial performances as well as good lifetimes have been demonstrated. Thermal stability of these new diodes has been greatly improved by using a new system matrix+n-dopant. As a result, RGB diodes can sustain at least a 500 h storage period-of-time at 90°C with minor degradation. Preliminary studies at 110°C for at least 24 h show also very promising features for such devices.

1. Introduction – Technical summary

Organic light-emitting devices based upon the PIN technology, where the p-doped (n-doped) hole-injecting layer (electroninjectinglayer) is made of a host material where a p-dopant (ndopant) is blended in, together with the active layers sandwiched in between, are promising architectures that may enter the OLED market soon. Such devices provide very high luminance values, along with high power efficiencies. At the same time, extrapolated lifetimes of several thousand hours have already been recorded for red and green emitting devices but today little is known regarding the thermal stability of such diodes. In particular, it is well-known

These high temperature studies have been chosen because they represent critical values for displays that are being used for instance in the automotive industry. In particular, required storage specifications are typically 500 h (20 days) at 85°C and 24 h at 105°C with minor deviations of main parameters that defined the device.

2. Materials and method

The PIN-OLEDdescribed in this paper comprise five organic layers: a p-doped hole transporting layer next to the anode, a first interlayer, the emitting layer, a second interlayer, and a n-doped electron transporting layer next to the cathode (see figure 1). On the p-side, a strong acceptor was used as p-dopant. On the n-side,TND1 was doped into an electron-transporting host. For the emission zone, a phosphorescent system was used for both red and green while a fluorescent system was used for blue. Elementary devices have been made consisting in diodes of 0.1cm2 surface area using ITO as anode and a thick Al layer as a cathode. In all cases, devices have been encapsulated in a dry and

inert atmosphere, using a glass lid glued onto the substrate using an UV-curable resin. Specific getters have been added into the cavity so that the study is not limited by the formation of dark spots due to residual humidity and O2 vapors. Encapsulation resins have also been chosen regarding their thermal stability and permeability properties when heated up to 110°C. Red and green devices stored at 90°C have been characterized once a week during 2 months (25 days for blue ones). Red and green devicesstored at 110°C have been characterized after a 48 h period-oftime (24 h for blue ones). For that purpose, they were extracted from the hot oven and allowed to cool down for at least 10 min before measurement.

Figure 1: Schematic view of PIN OLED that use TND1 as ndopant into ETL

3. Red-emitting devices

3.1 Storage at 90°C

Figure 2 shows the I-V-L (Current-Voltage-Luminance) characteristics of a bottom PIN red-emitting diode before and after a 61 days storage time at 90°C. In this architecture, TND1 is the n-dopant blended in an electron-transporting matrix. First evaluation of device thermal stability is very good, and very little variation can be seen in the I-V as well as in the L-V curves. The main characteristics of red devices before storage are 2.45 V @ 100 cd/m2, 5.8 cd/A @ 100 cd/m2 and a maximum luminance efficiency of 6.2 cd/A. After the storage, these values remain almost unchanged: 2.43 V @ 100 cd/m2, 5.8 cd/A @ 100 cd/m2 and a maximum current efficiency of 6.3 cd/A. Visual inspection shows that the active surface of the diode stays very homogeneous, without any sign of crystallization. Few localized very small dark spots could however be observed under the microscope probably due to the limitation of the encapsulation resin, for which permeability may vary for such a long time at 90°C. Such devices show also good lifetime, with typical value of～ 3,000 h at an initial luminance of 1,600 cd/m2.

Table 1 shows the variations of the color coordinates x and y of the red device before and after the storage at 90°C for a 61 days period-of-time. It is clearly shown that x and y remain constant with values of 0.682 and 0.316, respectively. The shape of the EL spectrum does not vary as well (not shown here).

Figure 2: I-V-L characteristics of a PIN red-emitting device using TND1 as n-dopant – before storage (filled symbols) and after 61 days at 90°C (open symbols) Red Green Blue CIE coordinates before storage @ 90°C

x @ 100 cd/m2 0.682 0.402 0.148

y @ 100 cd/m2 0.316 0.563 0.216

after storage @ 90°C

x @ 100 cd/m2 0.682 0.401 0.147

y @ 100 cd/m2 0.316 0.565 0.213

Table 1:Evolution of the CIE coordinates of red-, green- and blue-emitting diodes before and after the storage at 90°C

3.2 Storage at 110°C

Figure 3 shows the I-V-L characteristics of a bottom PIN redemitting diode using TND1 as n-dopant before and after a 48 h storage time at 110°C. The thermal stability is once again very good, and very little variation can be seen in the I-V as well as in the L-V curves. After the 48 h storage, the main parameter values remain almost unchanged: 2.41 V @ 100 cd/m2, 5.8 cd/A @ 100cd/m2 and a maximum current efficiency of 6.4 cd/A. Visual inspection shows that the active surface of the diode stays very homogeneous, without any sign of crystallization nor appearance of dark spots. In that case again, the CIE coordinates do not vary nor the shape of the EL spectrum.

Figure 3: I-V-L characteristics of a PIN red-emitting device using TND1 as n-dopant – before storage (filled symbols) and after 48 h at 110°C (open symbols)

4. Green-emitting devices

4.1 Storage at 90°C

Figure 4 shows the I-V-L characteristics of a bottom PIN greenemitting using TND1 as n-dopant, before and after a 62 days storage time at 90°C. As for the red-emitting devices, the thermal stability is very good, with little variations observed in the I-V as well in the L-V curves. The main characteristics of green devices before storage are

、2.34 V @ 100 cd/m2, 29 cd/A @ 100 cd/m2 and a maximum current efficiency of 30 cd/A. After the storage period, these values remain unchanged: 2.33 V @ 100 cd/m2, 28 cd/A @ 100 cd/m2 and a maximum current efficiency of 29 cd/A. Again, few localized very small dark spots have been observed under the microscope, but no crystallization has been noticed. Such devices show good lifetime,with typical value of ~ 1,500 h at an initial luminance of 1,600 cd/m2. It can be seen in table 1 that the color coordinates are not affected by the storage, their variations laying below 1%.

Figure 4: I-V-L characteristics of a PIN green-emitting device using TND1 as n-dopant – before storage (filled symbols) and after 62 days at 90°C (open symbols)

4.2 Storage at 110°C

Figure 5 shows the I-V-L characteristics of a bottom PIN greenemitting diode using TND1 as n-dopant before and after a 48 h storage time at 110°C. The main parameter values remain almost unchanged: 2.32 V @ 100 cd/m2, 27 cd/A @ 100 cd/m2 and a maximum current efficiency of 28 cd/A. Visual inspection shows that the active surface of the diode stays very homogeneous, without any sign of crystallization. In that case, the CIE coordinates do not vary nor the shape of the EL spectrum.

Figure 5: I-V-L characteristics of a PIN green-emitting device using TND1 as n-dopant – before storage (filled symbols) and after 48 h at 110°C (open symbols)

4.3 Storage at 90°C for Cs-based devices

Figure 6 shows the I-V-L characteristics of a bottom PIN greenemitting diode before and after a 16 days storage time at 90°C. In this architecture, Cs is the n-dopant blended in the same electrontransporting host used for the TND1. In this case, the thermal stability is very poor. The main characteristics of these green devices before storage are 2.4 V @ 100 cd/m2, 28 cd/A @ 100cd/m2 and a maximum current efficiency of 28 cd/A. After one week at 90°C (not shown on figure 6), these values degrades to: 2.5 V @ 100 cd/m2, 15 cd/A @ 100 cd/m2 and a maximum current efficiency of 16 cd/A. After 16 days, the device is totally degraded. When looking at the lifetime of the Cs doped structures, we obtained typically values of ~ 1,200 h at an initial luminance of 1,600 cd/m2. This means that, before looking at the storage at high temperature, it would have been difficult to spot the inherent instability related to the use of this metallic dopant.

Figure 6: I-V-L characteristics of a PIN green-emitting device using Cs atom as n-dopant – before storage (filled symbols) and after 16 days at 90°C (open symbols) It should be noted here that the same behavior has been observed for red-emitting diodes as well as blue-emitting ones using Cs as n-dopant (not shown here).

5. Blue-emitting devices

5.1 Storage at 90°C

Figure 7 shows the I-V-L characteristics of a bottom PIN blueemitting using TND1 as n-dopant, before and after a 22 days storage time at 90°C. As for previous devices based upon TND1,the device thermal stability is very good, with little variations observed in the I-V as well in the L-V curves. The main characteristics of blue devices before storage are 4.63 V @ 100cd/m2, 3.9 cd/A @ 100 cd/m2 and a maximum current efficiency of 4 cd/A. After the storage period, these values have been a bit improved: 4.48 V @ 100 cd/m2, 4.1 cd/A @ 100 cd/m2 and a maximum current efficiency of 4.2 cd/A. Such devices show a lifetime with typical value of ~ 2,500 h at an initial luminance of 400 cd/m2. Interestingly, the color coordinates of the blueemitting structure do not vary a lot, within 2% for x and y (see table 1).

Figure 7: I-V-L characteristics of a PIN blue-emitting device using TND1 as n-dopant – before storage (filled symbols) and after 22 days at 90°C (open symbols)

5.2 Storage at 110°C

Blue-emitting devices stored at 110°C for 24 h are less stable compared to red and green ones. It can be seen on figure 8 that the I-V curve is mainly affected by this thermal treatment: for a given forward voltage, the current passing through the device is increased slightly leading to a lower efficiency. As a result, the main characteristics become 4.4 V @ 100 cd/m2, 1.8 cd/A @ 100cd/m2 and 1.8 cd/A after the annealing for the maximum current efficiency. At the same time, the color coordinates at 100 cd/m2 change to 0.160 and 0.214 for x and y respectively. This corresponds to a +7.5 % increase in the x coordinate compared to the original one.

This color shift is illustrated on figure 9, where electroluminescent (EL) spectra of blue-emitting devices have been depicted. It can be seen that the EL spectrum of diodes after the 110°C storage shows a different shape when compared to those of blue-emitting ones before any thermal treatment as well as after the 22 days storage at 90°C. Thus, in the former case, the vibronic peak at 450nm is over expressed while a wide feature appears towards the low energy region of the spectrum.

Figure 8: I-V-L characteristics of a PIN blue-emitting device using TND1 as n-dopant – before storage (filled symbols) and after 24 h at 110°C (open symbols)

Figure 9: EL spectra of PIN blue-emitting devices before thermal storage (red line), after the 90°C storage (blue line) and after the 110°C storage (green dashed lines)

The reason to this behaviour has not been elucidated yet.However, the shift of the color coordinates might indicate that an interface with the blue-emitting material has been modified and that a critical interdiffusion might have occurred upon heating.

This is illustrated on figure 8: at low voltage, the current of the annealed sample has increased drastically (about 1 decade at a given voltage between 1 V and ～ 2.5 V), indicating that new preferential pathways should have been created for charge carriers. The same behavior has been noticed for the reverse current density (not shown here).

6 Discussions

The hole-transporting layer in blue devices is made of NPB, and we believe that the stability limitation at 110°C for the blue diodes may come from the Tg of the molecule, which is about 95°C.Chen et al. have already assumed the formation of an interdiffused layer when using the hole-blocking BCP molecule as a “sacrificial” layer in undoped devices [1]. In their work however, they noticed a large improvement of green-emitting device characteristics due to an annealing above the Tg (80°C) of the BCP molecule. Nevertheless, when blue emitters are of concerned, it is reasonable to think that the presence of interfacial mixtures may lead to red shifting for instance by mean of exciplex

formation.

The OLED structures described in this work address the issue of dopant diffusion into the organic layer adjacent to it. It has been shown that the replacement of the Cs atom by another element (TND1) in the n-doped region of the structure could improve noticeably the stability and reliability of devices under severe storage conditions. Cs has been already known to diffuse in OLED, and as a consequence to limit the storage capability of the device [2]. Some recent work has demonstrated that the nature of the organic themselves, especially the phosphorescent emitter, is of great importance for the device in order for it to be stable at high temperature [3]. Thus thermally stable structures are of great importance today and this is why it is crucial to go on developing such high temperature investigations for PIN OLED. At Thomson, further experiments are on the way to implement thermal studies of red, green and blue architectures under DC operation.

7. Conclusions

It was found that a new n-dopant, named TND1, can be used into PIN organic device. This dopant does not change the initial characteristics of devices nor their lifetime when compared to devices that use Cs atom as n-dopant. At the same time, and this is the main advantage of using this new material, the thermal stability of PIN structures, when stored at elevated temperatures,is strongly increased for red-, green- and blue-emitting devices.

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