Operation lifetimes of organic light-emitting devices with different layer structures
Introduction
The organic light-emitting device (OLED) is one of the most promising candidates for the next generation display technologies since it exhibits advantages of low-power consumption, high brightness, high contrast, wide-view angle, and potentially low cost [1], [2]. However, the operation lifetime is still one of the major limitations for OLED applications as compared to other competitive display technologies such as semiconductor light-emitting diodes (LEDs) and liquid crystal displays (LCDs).
During the long-time operation of an OLED, indium ions from the indium–tin–oxide (ITO) anode and metal ions from the cathode (Mg:Ag or LiF/Al) will diffuse through the hole transport layer (HTL) and the electron transport layer (ETL) into the emitting layer (EML) [3], [4]. They form fluorescence quenchers and decrease the luminance under a fixed driving current. Meanwhile, the mobile ions also create a built-in voltage that increases the driving voltage under the same electrical current. On the other hand, the unstable cation in the EML is another degradation mechanism proposed in [5]. Since the HTL/EML hetero-junction in an OLED confines the carriers, they pile up near the interface when applying an electrical field. Injected holes from the HTL to the EML form cations in the EML material which are chemically unstable [6]. It will accelerate the formation of non-radiative trapping centers and result in luminance decay and voltage increase of a device. Kondakov et al. found that the decrease of luminance efficiency is linearly correlated to the accumulation of immobile positive charges at the HTL/EML interface. The electrical aging will generate carrier traps which act as non-radiative centers [7].
In this Letter, we conduct a series of experiments with the same organic materials but with different device structures. Devices with thicker HTLs or ETLs do not necessarily processes longer lifetimes. That is, metal ion diffusion is not the major lifetime mechanism in our experiment. On the contrast, the operation lifetime decreases with the increase of layer thicknesses. We found that devices with higher power efficiency (in terms of lm/W) have longer device lifetimes. For devices with the same power efficiency, those with more holes at EML have shorter operation lifetimes. That implies that the operation lifetimes are effected by the cation formation in the EML. Using the model developed by Kondakov et al., it was shown that the fixed charge formation rate is proportional to that of quencher formation at the HTL/EML interface in our experiment. Although, it is not the direct evidence to prove that the generated trapping carriers become non-radiative centers during electrical aging, it shows, to some degree, those two parameters correlated to each other. We will present experimental results and related discussions in Section 2 and conclude in Section 3.
Section snippets
Results and discussion
In all of our experiments, we used ITO glass substrates with low sheet resistivity (10 Ohm/square) and flat surface roughness (Ra < 1 nm). The size of the active region in our test pixel is 1 cm × 1 cm. In our devices, we used copper–phthalocyanine (CuPc) as the hole injection layer (HIL) material, N,N′-diphenyl-N,N′-bis(1-napthyl)-1,1′-biphenyl-4,4′-diamine (NPB) as the HTL material, 9-benzothiazol-2-yl-1,1,6,6-tetramethyl-2,3,5,6-tetrahydro-1H,4H-11-oxa-3a-aza-benzo anthracene-10-one (C545T) as the
Conclusions
In summary, we have discussed device performance and operation lifetimes with different device structures. Devices with higher power efficiencies exhibit longer lifetimes. A thicker device does not have a longer lifetime. It implies the mobile-ion diffusion from the electrode is not the dominant factor in our experiments. Under charge-unbalanced conditions, a hole-rich device had a shorter lifetime than an electron-rich device under the same power efficiency. Also, the fixed charge increase
Acknowledgements
This work is supported by the AIXTRON Taiwan Corporation.
References (10)
- et al.
Synth. Met.
(2000) - et al.
Synth. Met.
(2004) - et al.
Org. Electron.
(2001) - et al.
Appl. Phys. Lett.
(1987) - et al.
J. Appl. Phys.
(1989)
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