Electron-Transport Layers with Air-Stable Dopants for Display Applications

Qiang Huang, Thomas Rosenow, Tobias W. Canzler, Mauro Furno, Carsten Rothe, Sascha Dorok, Ulrich Denker, Omrane Fadhel, Jan Birnstock

Novaled AG, Tatzberg 49, D-01307 Dresden, Germany

Abstract We report on a new class of doped electron transport layers for display applications. New air-stable dopant materials are presented together with matching host material. This new ETL concept allows for optimum balance of long lifetime and high efficiency. Device data for top-emission deep blue OLEDs are shown and options for optimization of OLEDs according to specific application requirements are discussed.

1. Introduction Power consumption and lifetime of OLED displays are very important performance parameters for both mobile and large area display applications. In case of mobile devices, the display consumes a significant fraction of the energy of the whole device and therefore has a big influence on how long the device can be used before the battery needs to be recharged. For large area displays, low power consumption is desired due to environmental concerns. On the other hand, OLED lifetime is of utmost importance for both applications and the requirements for large area stationary applications are even stricter than in case of mobile devices.

Since many years, the performance and lifetime of the blue subpixel is the limiting factor for display applications using OLEDs. Therefore, display development is very much focused on improvement in blue pixel efficiency and lifetime.

2. Background and Objective The use of doped transport layers [1] in OLEDs allows combining very good power efficiency with long lifetime. Novaled has developed a new generation of hosts and dopants for electron transport layers which can be tailored to achieve the best possible combination of efficiency, voltage and lifetime for the desired application.

We have shown before that Novaled doped transport layers can be used to enhance performance and lifetime for blue OLEDs [2]. The OLEDs presented in reference [2] were bottom emitting OLEDs with color coordinate in the CIE 1931 color system of y = 0.23. At that time, we stated a lifetime of 18 khours which referred to the time required to age the OLED to 50% of initial luminance at 1000cd/m².

Nowadays, much deeper CIE-y color coordinates are required in OLEDs for display applications, the target CIE-y coordinate is usually below 0.060 as a deep blue color is required for the pixels in RGB displays with full HDTV color gamut. Furthermore, lifetime targets are mostly set at a luminance drop of 5 % (LT95) or less from initial luminance. For comparison, the LT95 lifetime of the device presented in [2] was 135 h at a driving current of 10 mA/cm².

Although we have shown very good device performance, one weakness of Novaled’s electron transport materials was that n-side dopant materials were not fully air-stable. Currently, OLED

mass production is exclusively by vacuum deposition methods, but OLED materials have to be sufficiently air stable to allow loading from air atmosphere and equipment maintenance. Therefore, we set out to develop a new generation of dopants to fulfill this requirement of air stability. The results of this development are the air-stable dopants NDN-77 and NDN-87. These two dopants fulfill our air-stability test procedure for host materials, and we therefore consider them as completely air stable. The evaporation temperature of the dopant materials is around 200°C, thermal decomposition sets in at more than 60°C higher temperature. This processing window is considered as fully sufficient for use in mass production. Furthermore, we tested the long term stability of the materials and ascertained that the materials remain stable for more than 1 week at processing temperature.

3. Experimental results for electron only devices

Previous generations of Novaled n-side dopants were designed as universal dopants, that is, basically any electron transport material with suitable LUMO level is dopable by these dopants. The new, air stable dopants are a result of a co-development of dopants and host molecules. Therefore, best results are obtained by combining the dopants with their specially matched hosts.

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Figure 1: Current density as a function of drive voltage of electron-only devices comparing a 60 nm NET-142 layer to a 60 nm NET-142: NDN-77 (8: 2) doped electron-transport layer.

In Fig. 1, we show current-voltage characteristics of electron-only devices using the Novaled air-stable host/dopant system NET-142 and NDN-77. We compare a device with only the host material NET-142 to a device with a co-deposited layer of NET-142 and the air-stable dopant NDN-77. In both cases, a 60 nm thick layer

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of the tested materials was sandwiched between contacts which ensured barrier-free electron injection into the organic layers. The current-voltage curve of NET-142 (hollow symbols) shows that NET-142 alone is not a good electron transport material: Current density is very low below 1 V bias. Current is only observed at voltage > 1 V. The current density increase with increasing voltage can be fitted to a power law j ~ Vn with n > 2 (the fit is indicated by the full line). A power law dependence with an exponent n larger than 2 is an indication for trap limited transport in organic materials, in which electron traps inhibit current flow at lower fields and significant currents are only observed at larger bias when trap levels are filled [3].

In contrast to this, the NET-142: NDN-77 layer (full symbols) shows no indication for trap-limited electron transport. In this layer, NET-142 was co-deposited with 20 wt% NDN-77. We observe current flow even at very low bias and the current density curve is almost a straight line in the double logarithmic plot. Again, we fitted the current density versus voltage curve using the formula j ~ Vn with n as fitting parameter. The fit result (shown as dashed line) indicates that n is very close to 2. A power law dependence j ~ V² is commonly interpreted as indication for a trap-free organic material in which transport follows the space-charge limited current (SCLC) model [4]. Therefore, we can conclude that the co-deposited system NET-142:NDN-77 is a very good electron transport material without traps.

4. OLED Results To confirm the air-stability of the dopants on device level, we produced blue OLEDs using NDN-77 after air-exposure of the dopant material for 24 h. Initial performance and lifetime of the OLEDs were measured and compared to OLEDs with the same stack and made using pristine NDN-77. Both initial performance and lifetime were found to be identical.

In the following, we will discuss device data from blue top-emission OLEDs with the new dopants NDN-77 and NDN-87 in more detail. The devices used a generic blue fluorescent top emission stack structure: Silver anode, NHT-51:NDP-9, NHT-51, blue EML, NET-142:NDN-77 or NET-142:NDN-87, and semi transparent magnesium/silver cathode.

Except for the EML, all materials are from Novaled. For the blue emitter layer, we used Sun Fine Chemicals (SFC Co. Ltd., Korea) fluorescent blue emitter materials. SFC materials have been successfully used with PIN-OLEDs before [5]. In this experiments we used the emitter host ABH112 and the emitter dopant NUBD369, co-evaporated with a concentration of 3 wt% emitter material. Key performance results of the OLEDs are summarized in Fig. 2 and Fig. 3.

Figure 2 shows current efficiency as a function of current density in the OLED. For NET-142:NDN-77 as ETL, we find a mostly flat current efficiency curve. At a current density of 10 mA/cm², the OLED driving voltage is 4.3 V and current efficiency is 6.6 cd/A. In case of NET-142:NDN-87 as ETL, the efficiency is increasing for current densities below 5 mA/cm² and then remains mainly constant. The efficiency at 10 mA/cm² is 5.3 cd/A, driving voltage is 4.8 V. Both OLEDs show almost the same spectrum with identical CIE-y coordinate of 0.059.

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Figure 3: Luminance degradation curve of blue top emission OLEDs driven at constant current of 10mA/cm².

In Figure 3, we compare the lifetime of top emission OLEDs driven at 10 mA/cm² constant current. The more efficient OLED with NET-142:NDN-77 shows a lifetime of 170 h to 95 % initial luminance (LT95). A much higher lifetime is observed with NET-142:NDN-87, where we find a LT95 of 380 h. It shows that a modest reduction in efficiency can translate into much higher lifetimes and vice versa. If we compare the lifetime starting from 500 cd/m² initial luminance, the difference is somewhat reduced. At an initial luminance of 500 cd/m², the LT95 for NET-147:NDN-87 is again 380 h. Due to the higher efficiency of NET-142:NDN-77, the lifetime is significantly larger than at 10 mA/cm², we find a LT95 of 240 h at 500 cd/m².

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Figure 4: Performance comparison of NET-142:NDN-87 to NET-142:NDN-77. While NET-142 based ETLs can be either optimized for lifetime or efficiency, NET164 can offer higher performance for both parameters.

We have illustrated the inverse relationship between blue OLED efficiency and lifetime in Fig. 4 where we plot the lifetime against current efficiency for NET142:NDN77 and NET142:NDN87. The lifetime axis has a logarithmic scale, since we observe that rather small differences in initial performance translate into a large increase in device lifetime.

We have tested further ETL host materials (not shown) with the blue EML ABH112:NUBD369 and found that it is relatively easy to find further materials which follow the trend line through the two OLEDs we presented above. In this way, one can either boost lifetime with a more or less modest decrease in performance or increase initial OLED performance at the cost of reduced lifetime.

A more challenging target is to find a material which can improve both lifetime and efficiency and enter the indicated target performance range above the trend line for the NET-142 ETL material system.

Further development has yielded a material system which can indeed break out of the trend line toward higher performance in both efficiency and lifetime. This material is NET-164 which can also be doped with either NDN-77 or NDN-87. We have fabricated top emission OLEDs with the same stack as discussed above and NET164:NDN87 as ETL.

Figure 5 and Figure 6 plot both lifetime and current efficiency for an OLED with NET164:NDN87 ETL. For this OLED driving voltage at 10mA/cm² is 4.5 V, current efficiency is 6.8 cd/A and the CIE-y coordinate is 0.058. In Figure 5, the luminance degradation curve for constant current driving at a current density of 10 mA/cm² is plotted. The lifetime to 95% of initial luminance is about 300h. Figure 6 shows the current efficiency as a function of current density. Using NET164:NDN87, we obtain higher current efficiency than with either NET142:NDN87 or NET142:NDN77.

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Figure 6: Current efficiency as a function of current density for blue top emission OLEDs using NET-164:NDN-87.

Lifetime at constant current is lower for NET164:NDN87 than for NET142:NDN87, but due to the much higher efficiency of NET164:NDN87 this system has the longest lifetime in this comparison, LT95 starting from 500cd/m² is more than 400h.

Current development is now focusing on testing the NET-164 material system in detail. Using a different emitter system, blue lifetimes of up to 1500 h at 10 mA/cm² have been achieved with NET-142 and NET-164.

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5. Conclusion We introduce a new generation of hosts and dopants for electron transporting layers. The new dopants are completely air-stable and we demonstrate device data for blue top-emission OLEDs with high efficiency and lifetime. Using different dopants and hosts we can optimize OLED performance for lifetime or efficiency depending on requirements for the desired application.

6. Acknowledgements The development of NET materials was supported by the German federal government funded project So-Light (FKZ 13N10530)

7. References [1] K. Walzer et al., Chemical Reviews 107, 1233 (2007).

[2] T.W. Canzler, P. Wellmann, M. Hofmann, J. Birnstock, A. Werner, and J. Blochwitz-Nimoth, IDMC 07 Proceedings, S17-03, p. 332 (2007).

[3] M. M. Mandoc, B. de Boer, G. Paasch, and P. W. M. Blom, Phys. Rev. B 75, 193202 (2007).

[4] M. A. Lampert and P. Mark, Current injection in Solids (Academic, New York, 1970).

[5] Ulrich Denker, Carsten Rothe and Jan Birnstock, IMID/IDMC/ASIA Display 2010 Digest, 253 (2010)

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