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Plausible degradation mechanisms in organic light-emitting diodes
Organic Electronics 67 (2019) 222–231
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Plausible degradation mechanisms in organic light-emitting diodes a,∗
a
a
a
T
a,c
Jwo-Huei Jou , You-Ting Lin , Yu-Ting Su , Wei-Chi Song , Shiv Kumar , Deepak Kumar Dubeya, Jing-Jong Shyueb,d, Hsun-Yun Changd, Yun-Wen Youd, Tzu-Wei Liange a
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, 30013, Taiwan Department of Materials Science and Engineering, National Taiwan University, Taipei, 106, Taiwan Hiroshima Synchrotron Radiation Center, Hiroshima University, Kagamiyama 2-313, Higashi-Hiroshima, 739-0046, Japan d Research Center for Applied Sciences, Academia Sinica, Taipei, 115, Taiwan e Global Science Instruments Co., Ltd., Taiwan b c
A R T I C LE I N FO
A B S T R A C T
Keywords: OLEDs Degradation mechanisms ToF-SIMS Lifetime
Organic light emitting diode has become a highly attractive technology for high quality displays and lighting. These applications, however, strongly rely on their lifetime. Probing all the possible failure mechanisms has hence become crucial. We reveal here that the device lifespan depends on the dielectric strength, internal electric field, morphology, thermal stability, and migration of the composing organic and inorganic materials as well as span of recombination zone and device efficiency. Additionally, the lifetime is highly sensitive to the thickness of electron transporting layer. By taking a green emitter doped in 4,4-bis(carbazol-9-yl)biphenyl host for example, the device lifetime can be increased from 51 to 209 h at 1000 cd/m2, an increment of 310%, and its efficacy increased from 21 to 41 lm/W, an increment of 96%, as the thickness is increased from 20 to 40 nm. The results show high device reliability to be achievable provided it composes materials with high dielectric strength, high glass transition property and low migration tendency, and uniform layered structure with low built-in internal electric field, wide recombination zone and high efficiency.
1. Introduction Organic light emitting diodes (OLEDs) have become increasingly attractive in high quality display and lighting applications [1–3]. They have achieved great progress in efficiency over the past years [4–7]. Whilst, lifetime is always a crucial reliability issue before they can be adopted more extensively, especially for lighting purpose [8–10]. Stronger efforts are hence demanded in the pursuit of further understanding regarding why they fail and how to improve, since either energy-saving or sustainability can only be better realized on devices with a longer lifespan [11]. The stability and lifetime in OLEDs are closely related to multiple device degradation factors, which can be further ascribed into intrinsic and extrinsic issues. Intrinsic degradation mechanisms include thermal decomposition [12–17], electrochemical decomposition [18–20], diffusion and drift [21–23], molecular migration [24–26], interfacial degradation [27–31], and narrow recombination zone [32–34], etc. The extrinsic degradations include attack by the ambient moisture and oxygen [35–38], impurity-induced degradation [39,40], and ultraviolet degradation, etc. [41,42]. Many approaches have been proposed to improve the device ∗
lifetime and efficiency. In 1995, Adachi et al. reported high device durability to be achievable by having a small energy barrier at the interface between the hole transporting layer and anode [43]. In 1999, Murata applied a high glass transition material to keep the high quantum efficiency at elevated temperature [44]. In 2011, Jou's group proposed a double mixed-host device architecture to disperse the injected carriers into three different recombination zones to enhance lifetime [45]. It is believed that many more improving approaches can be devised if in-depth investigations are carried out. Hence, we have performed electric simulation as well as device fabrication and lifetime testing for OLED devices with different hosts and with different thicknesses for the employed electron transporting layer (ETL). Atomic force microscopy (AFM) was used to investigate the surface morphologies and the presence of spikes in the aluminum electrode before and after operation, and the effect of spike on dielectric breakdown was examined. Secondary ion mass spectrometry (SIMS) was used to analyze how the migrations of the organic and inorganic materials affect the lifetime.
Corresponding author. E-mail address: [email protected] (J.-H. Jou).
https://doi.org/10.1016/j.orgel.2019.01.035 Received 8 November 2018; Received in revised form 21 January 2019; Accepted 21 January 2019 Available online 24 January 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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2.2. Device measurement 2.2.1. Efficiency All the devices were encapsulated in a nitrogen purge globe chamber and then measured under atmospheric conditions. The luminance, spectrum and CIE chromatic coordinate results were obtained using a PR-655 spectroradiometer, and a Keithley 2400 electrometer was used to measure the current–voltage (IeV) characteristics. 2.2.2. Lifetime The lifetimes were obtained using a programmable Keithley 2400 electrometer. Accelerated lifetime testing was conducted at a constant current with a corresponding initial luminance of 10,000 cd/m2 and 5000 cd/m2 for the CBPe and TCTA-host containing devices, respectively. The lifetime was based on the T50 (the time as the brightness drops to 50% of its initial magnitude) of the pristine device performed at the above accelerated testing condition. 2.2.3. Surface morphology AFM images of the aluminum films before and after applying a 7 V forward bias for 30 min were obtained on a Bruker ICON SPM in tapping mode at room temperature. 2.2.4. SIMS measurements SIMS experiments were performed with a PHI TRIFTV nanoTOF (Chigasaki, Japan) ToF-SIMS system. The pulsed (8284 Hz, 4.5 Na length) primary ion source was operated with C60+ at 20 kV and 1 nADC with a 100 μm × 100 μm rastering area for 10 min. The co-sputtering was done with a C60+ ion beam operated at 20 kV and 2.5 nA with a 800 μm × 800 μm rastering area and with an Ar+ ion beam at 0.5 kV and 620 nA with a 6000 μm × 6000 μm. A dual-beam charge neutralizer, containing Ar+ beam and electron beam, was used to compensate charge on the device surface during data acquisition. 2.2.5. Electrical simulation Thickness-dependent internal electric field and recombination zone in each organic layer of the studied OLEDs were performed using a commercial software package SETFOS [46]. The setting parameters included hole mobility, electron mobility, lowest unoccupied molecular orbital (LUMO), and highest occupied molecular orbital (HOMO), as shown in Table 1. The specific structure for the simulation consisted of ITO(100 nm)/HAT-CN(3 nm)/TCTA (40 nm)/EML(15 nm)/TPBi (20, 40, 60, and 80 nm)/LiF (1 nm)/Al (100 nm) on glass substrate. The work function set was 5.2 eV for the anode and 4.3 eV for the cathode.
Fig. 1. Schematic energy-level diagrams of the green OLED devices with (a) CBP and (b) TCTA hosts. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
2. Experiment
3. Results and discussion
2.1. Device fabrication
Table 2 shows the device performance of the studied CBPe and TCTA-host containing green devices with different ETL thicknesses from 20 to 80 nm. At 1000 cd/m2 for example, the respective power efficiency is 21.1, 41.3, 39.2, and 16.5 lm/W for the CBP-host containing devices, and 3.9, 19.4, 20.5, and 8.3 lm/W for the TCTA counterparts. At 10,000 cd/m2, the resultant devices show a respective
Fig. 1a–b shows the studied green OLED device structures and their corresponding energy level diagrams. The device structure consisted of an indium tin oxide (ITO) as the anode layer, a 3 nm 1,4,5,8,9,11hexaazatriphenylene hexacarbonitrile (HAT-CN) as a hole injection layer, a 40 nm 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) as a hole transporting layer, a 15 nm single emission layer (EML), a 20, 40, 60, or 80 nm 1,3,5-tris(N -phenyl-benzimidazol2-yl) benzene (TPBi) as ETL, a 1 nm lithium fluoride (LiF) as electron injection layer, and a 100 nm aluminum as cathode. The employed light-emitting green dye was tris(2-phenylpyridine) iridium(III) (Ir (ppy)3). The employed host was 4,4-bis(carbazol-9-yl)biphenyl (CBP) for Device I and III, and 4,4′,4’’-tri(N-carbazolyl) triphenylamine (TCTA) for Device II and IV. Additionally, Devices III and IV were doped with a red dye, bis(1-phenylisoquinolinolato-C2,N) iridium (acetylacetonate) (Ir(piq)2(acac)), at the interface between the EML and ETL. Amongst, all the layers were deposited via a thermal evaporation process under high vacuum condition at 5 × 10−6 Torr.
Table 1 Setting parameters for the electric simulation using SETFOS; they are the hole mobility, electron mobility, highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO).
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Material
HOMO (eV)
LUMO (eV)
Hole mobility (10−5 cm2/V)
Electron mobility (10−5 cm2/V)
Ref.
TPBi TCTA CBP TAPC
6.2 5.7 6.0 5.5
2.7 2.4 2.9 2.0
0.033 30 200 1000
3.3 0.3 30 10
[48] [49] [49] [50]
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Table 2 Effects of host material and ETL thickness on the operation voltage (OV), power efficiency (PE), current efficiency (CE), external quantum efficiency (EQE), and half lifetime at 1000 cd/m2 of the green devices. Device
Host
ETL thickness (nm)
Operation Voltage (V)
PE (lm/W)
CE (cd/A)
EQE (%)
Lifetime T50 (hours) @ 1000 cd/m2
@ 100/1000/10,000 cd/m2 1–1 1–2 1–3 1–4
CBP
20 40 60 80
3.1/3.8/5.6 3.0/3.7/5.7 3.2/4.3/6.8 3.5/4.9/8.5
4.2/21.1/15.3 6.5/41.3/27.9 46.3/39.2/20.2 18.1/16.5/8.9
4.6/25.3/27.4 6.8/48.1/50.6 47.8/53.9/43.9 20.1/25.8/23.8
1.3/6.9/7.5 1.9/13.4/14.1 13.7/15.4/12.6 5.8/7.5/7
51 209 116 92
II-1 II-2 II-3 II-4
TCTA
20 40 60 80
3.1/3.7/5.8 3.1/3.7/5.8 3.1/4.1/6.8 4.1/5.3/8.2
0.6/3.9/4.3 3/19.4/9.6 17.8/20.5/8.1 2.8/8.3/4.2
0.7/4.7/7.9 3.3/22.7/17.6 17.9/26.6/17.6 3.6/13.9/11
0.2/1.3/2.3 0.9/6.5/5.1 5.2/7.7/5.2 1.1/4.1/3.2
2.9 3.6 4.4 4.3
power efficiency of 15.3, 27.9, 20.2, and 8.9 lm/W for the former devices, and 4.3, 9.6, 8.1, and 4.2 lm/W for the latter. As seen, the CBPhost containing devices show a much better power efficiency performance as comparing with the TCTA-host counterparts based on the same ETL thickness. From lifetime perspective, the resultant lifetime of the CBP-host containing device is increased from 51 to 209 h, an increment of 310%, as the ETL increases from 20 to 40 nm. At 60 nm, the lifetime drops to 116 h. By further increasing to 80 nm, the lifetime is decreased to 92 h. For the TCTA-host containing devices, the resultant lifetime is increased from 2.9 to 3.6 h, an increment of 24%, as ETL thickness increases from 20 to 40 nm. By continuously increasing to 60 nm, the device lifetime is further increased from 3.6 to 4.4 h. At 80 nm, the lifetime slightly drops to 4.3 h. Fig. 2 shows the correlation between the power efficiency and lifetime. Taking the CBP-host containing device for example, its lifetime is drastically increased from 51 to 209 h as its efficiency increases from 21.1 to 41.3 lm/W, indicating the devices to show better lifetime at higher power efficiency. The same trend appears for the TCTA counterparts. As the resultant efficiency increases from 3.9 to 20.5 lm/W, the corresponding lifetime is increased from 2.9 to 4.4 h. Overall speaking, the CBP-host containing devices exhibit much better lifetime than the TCTA counterparts. Fig. 3 shows the effect of power efficiency on device temperature for the CBPe and TCTA-host containing devices. The device temperature did not change much for both types of devices operated at 1000 cd/m2, regardless of their power efficiency. However, at higher luminance, such as 10,000 cd/m2, the device temperature markedly increased as the power efficiency decreased. Specifically, it increased from 29 to 31 °C as the power efficiency decreased from 27.9 to 8.9 lm/W for the
Fig. 3. Effect of power efficiency on the device temperature for the CBPe and TCTA-host containing devices at 1000 and 10,000 cd/m2.
CBP-host containing device. It increased from 33 to 42 °C as the power efficiency decreased from 9.6 to 4.2 lm/W for the TCTA-host containing device. The significant rise in device temperature can be attributed to low luminous efficacy, in which a significant part of the applied electric energy was converted into Joule heat instead of light. The generation of Joule heat can be used to explain at least partly why the device lifetime decreased as the power efficiency dropped, and why the comparatively low efficiency TCTA-host containing device showed a much poorer lifespan. Moreover, the mobilities of charge carriers, device turn-on voltage (voltage at 1 cd/m2), activation energy and lifetime are also directly influenced by the temperature due to a shift of the quasi-Fermi levels. Owing to the generation of excessive heat resulting from the electrical stress and/or the thermionic emission, large numbers of holes and/or electrons generated inside the recombination zone, which may unbalance the charge carrier distribution, hence reduced device efficiency. We had further investigated the effect of recombination zone on device lifetime. The recombination zone(s) were first identified by electric simulation using the software package SETFOS, details of which are revealed in the simulation section. As seen in Fig. 4, the recombination of the entering holes and electrons would mainly occur within the emission layer, but be dispersed near the two interfaces for the CBP-host composing device. Whilst, the recombination would still mainly occur within the emission layer, but concentrate near the interface between EML and ETL for the TCTA-host composing counterpart. These may explain why the latter to have a poorer device efficiency due to its more narrow recombination zone. To verify the simulation findings experimentally, we fabricated two additional devices with a thin (2 nm) red emission layer deposited
Fig. 2. Effect of power efficiency on the device lifetime of the CBPe and TCTAhost containing devices. 224
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Fig. 4. Recombination distributions in the CBPe and TCTA-host containing devices.
Fig. 7. Internal electric field distributions in the studied OLEDs with an ETL thickness of 40 nm under a forward bias of 3.7 V, at which both devices exhibit a 1000 cd/m2 brightness.
Fig. 5. Electroluminescent spectra of the CBPe and TCTA-host containing green devices with an additional red emission layer in between the HTL (TAPC) and EML. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 8. Effect of a 15 nm aluminum spike on the internal electric field in the (a) CBPe and (b) TCTA-containing devices with a 40 nm ETL at applied voltages as the devices (i) emit 1000 cd/m2, (ii) are under lifetime measurement and (iii) reach their maximum brightness. Black and red lines represent the internal electric fields of the OLED devices with the presence and absence of an aluminum (Al) spike at specified voltage in the figure, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Electroluminescent spectra of the CBPe and TCTA-host containing green devices with an additional red emission layer in between the EML and ETL (TPBi). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 9. Atomic force microscopic images of the aluminum electrode surface (a) pristine and (b) after electric poling with a forward bias of 7 V.
confirming which to have a much dispersed recombination zone. Again, these experimental evidences verify what had been revealed from the electric simulation. As a result, the much narrow recombination zone may be used to explain why the TCTA-host containing device was much poorer in both efficiency and lifetime performance. Although the applied voltage required to trigger the emission of OLED devices is not high as usual, the resulting internal electric field in each of organic layer may approach or even exceed its dielectric strength due to the extremely thin layer structure, by noting the field to be in proportion to the inverse of layer thickness. We had hence measured the dielectric strength of the comprising organic materials. Specifically, the dielectric strength of TAPC (HTL) ranges from 2.3 to 2.9 MV/cm for thickness varying from 40 to 80 nm. Whilst, the dielectric strength of the hosts, CBP and TCTA, ranges from 2.4 to 3.1 MV/cm, and 2.4–3.6 MV/cm, respectively. It is from 2.8 to 3.5 MV/ cm for TPBi (ETL). Any resulting internal electric field that is beyond the dielectric strength of the corresponding material might cause break down electrically, which would happen when the device is driven at high voltage or the electrodes have spikes penetrating into the layer. It was learned that the nominal dielectric strength was sensitive to
between the original HTL and EML, as shown in Fig. S1. The resultant electroluminescent spectra are shown in Fig. 5. As observed, the emissive layers of both devices show the characteristic green and red emissions peaking at 516 and 624 nm, respectively. However, the red peak is much weaker in the TCTA-host composing device, especially as comparing with the CBP-host composing counterpart. The comparatively much weaker red emission implies the recombination to have occurred somewhere away from the red emission layer for the TCTAhost containing device. In contrast, the recombination in the CBP-host containing device had occurred at least partly near the red emission layer as well as in the main green EML so that both the characteristic red and green emissions appeared markedly. These experimental results are in good accordance with what have been predicted from the electric simulation shown above. Actually, in another fabricated devices that with the thin red emission layer deposited in between the EML and ETL (Fig. 6), the red emission was very marked as comparing with the green one for the TCTA-host containing device. This confirms the recombination to have concentrated at the EML/ETL interface. Whilst, the green emission is still comparatively strong for the CBP-host containing counterpart, 226
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Fig. 11. SIMS depth-profiles of the OLED device containing organic molecules, viz. ETL (TBPi), host (TCTA), HTL (TAPC), and HIL (HATCN), and the inorganic materials, viz. the top Al electrode, Li of EIL (LiF) and In of bottom electrode (ITO), for the TCTA-host containing device (a) before and (b) after applying a forward bias of 9.7–10.5 V for 15 min.
Fig. 10. SIMS depth-profiles of the OLED device containing organic molecules, viz. ETL (TBPi), host (CBP), HTL (TAPC), and HIL (HATCN), and the inorganic materials, viz. the top Al electrode, Li of EIL (LiF) and In of bottom electrode (ITO), for the CBP-host containing device (a) before and (b) after applying a forward bias of 8.4–9.0 V for 15 min.
materials, making dielectric breakdown unlikely to occur. Figs. S2a–b show the thickness-dependent internal electric fields of the CBPe and TCTA-host containing devices. As seen, the electric field distribution is highly sensitive to the thickness of ETL. Overall speaking, the electric field in either EML or ETL markedly decreases as the ETL thickness increases, indicating the tuning of ETL thickness to be a highly effective approach to prevent electric breakdown caused device failure, if any. The resulting electric fields at low applied voltage (3.7 V) are all well below the dielectric strengths of the OLED materials. However, the electric fields would become much higher as the applied voltage increase. Figs. S3a–b show the internal electric fields of the devices at their maximum brightness. Their respective voltages were 10, 10.5, 11.5, and 12.5 V for the CBP-host containing devices, and 8, 9, 9.5, and 10.5 V for the TCTA-host containing counterparts, as the ETL thickness was 20, 40, 60, and 80 nm. It would not be too surprising to see the devices start to breakdown as the ETL was reduced to 20 nm, since which would result in an extremely high electric field and hence cause device failure. In the CBP-host containing case, the resulting electric fields had approached the lower limits of dielectric strength of CBP and TPBi, while the field had exceeded the lower limit of TPBi in the TCTA case. Electric breakdown can be used to explain why these 20 nm ETL composing devices showed a comparatively poorest lifetime. The breakdown may have occurred within the CBP-host and TPBi-ETL layers for the CBP-host containing device, and solely within the TPBiETL layer for the TCTA-host containing counterpart. Nevertheless, what might be puzzling and remains to be solved is why those devices that
the surface roughness of the employed electrodes [47]. Taking aluminum for example, spikes inevitably appear throughout the entire specimen, which might cause the aforementioned dielectric breakdown in the organic layer(s) at considerably low voltage(s). The spiking surface would also make the determination of dielectric strength very difficult, especially when the layer is relatively thin. This non-ideal, rough surface is actually an unavoidable fact in numerous real devices and products. Nevertheless, it would be worthy to further establish a convenient, standard method for measuring more precisely the dielectric strength of the increasing OLED materials to ensure better device reliability. Fig. 7 shows the internal electric field distribution in each organic layer of the studied OLEDs. As seen, the electric fields are markedly different in different layers. Moreover, the distribution changes drastically with the change of employed host. In the TCTA-host containing device, a relatively highest electric field is built within the ETL layer with TPBi; i.e. it is about 0.59 MV/cm, while only 0.15 MV/cm within the EML layer with TCTA. By changing the host from TCTA to CBP, the electric field in the TPBi ETL decreases from 0.59 to 0.47 MV/cm, but increases from 0.15 to 0.46 MV/cm in the CBP EML. It is important to note that the applied voltage input in this simulation is only 3.7 V, at which both devices exhibit a 1000 cd/m2 brightness. The magnitude of the electric fields will become two times larger if the applied voltage is increased to 11.1 V. The resulting electric fields are, however, still much lower than the average dielectric strength of the studied 227
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Fig. 13. SIMS depth-profiles of (a) TCTA and (b) TPBi molecules before and after being driven at from 9.7 to 10.5 V for 15 min. Shadow shows the signal from C60 having the same mass as that of TPBi.
Fig. 12. SIMS depth-profiles of (a) CBP and (b) TPBi molecules before and after being driven at from 8.4 to 9.0 V for 15 min. Shadow shows the signal from C60 having the same mass as that of TPBi.
For the TCTA-host containing device with a perfectly smooth surface, the respective electric fields in ETL are 0.59, 0.86, and 1.88 MV/ cm, at 3.7 V for 1000 cd/m2, 4.8 V for lifetime measurement with an initial brightness of 5300 cd/m2, and 9 V at the maximum brightness (Fig. 8b). By taking the additional 15 nm Al spike into account, the electric fields are increased to 1.34 MV/cm at 4.8 V and 2.95 MV/cm at 9 V. Although the average electric field throughout the entire ETL may not be too high, the ETL at anywhere with the protruding Al spike would exhibit a relatively strong electric field that exceeds the dielectric strength of ETL (TPBi) and hence causes dielectric breakdown, at least locally. This might explain why this device showed a much shorter lifetime. As the ETL thickness decreases to 20 nm, the respective electric fields of the devices with a smooth surface are 0.72 and 0.73 MV/cm in the EML and ETL of the CBP-host containing device, while 0.15 and 1.15 MV/cm for the TCTA-host containing counterpart, as shown in Figs. S4a–b. Similarly, by taking the 15 nm spike into consideration, the respective electric fields in EML and ETL would be increased to 4.04 and 4.45 MV/cm at 10 V as the CBP-host composing device reached its maximum brightness. These relatively high internal electric fields might be used to explain why the device's lifetime dropped drastically as its ETL was reduced to 20 nm, and the breakdown might occur in both CBP-host and TPBi-ETL. The field might, theoretically, increase to 5.79 MV/cm under lifetime testing at 4.9 V, or 8.36 MV/cm at maximum luminance at 8 V for
compose 40, 60, and 80 nm ETL exhibit no electric field greater than the dielectric strength had failed in a similar pattern, but just at a slightly higher voltage. Fig. 8a shows the internal electric fields of the CBP-host containing device with the presence and absence of an aluminum (Al) spike under a forward bias of 3.7, 5.9, and 10.5 V, where the ETL thickness is fixed at 40 nm. The first voltage, 3.7 V, was the applied voltage when the device emitted a 1000 cd/m2 luminance; the second, 5.9 V, was the voltage applied for lifetime measurement with an initial brightness of 13,340 cd/m2; and the third, 10.5 V, was the corresponding voltage when the device reached its maximum brightness and started to degrade. As seen, the corresponding electric fields are 0.46, 0.85, and 1.64 MV/cm in the EML. The above results were obtained by assuming the electrodes to have a perfectly smooth surface. However, such an ideal situation did not appear in the real devices as evidenced by the atomic force microscope images shown in Fig. 9. In contrary, a 10 nm spike presented in the Al electrode in the pristine stage, which grew to 15 nm after applying a forward bias of 7 V. By taking such an additional 15 nm spike into account, the resultant electric field in the spike facing ETL would be increased from 0.46 to 0.63 MV/cm at 3.7 V, or to 2.24 MV/cm at 10.5 V. Although the latter approached the lower limit of dielectric strength of the CBP-host, this device was less likely to degrade from dielectric breakdown, especially as comparing with the TCTA-host composing counterpart. 228
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Fig. 14. SIMS depth-profiles showing the atoms of (a) Al and (b) In of ITO in the CBP-host containing device to have migrated toward the opposite electrode after being driven at from 8.4 to 9.0 V for 15 min.
Fig. 15. SIMS depth-profiles showing the atoms of (a) Al and (b) In of ITO in the TCTA-host containing device to have also migrated toward the opposite electrode after being driven at from 9.7 to 10.5 V for 15 min.
the TCTA-host composing counterpart with a 20 nm ETL. The resultant field has far beyond the dielectric strength of the TPBi-ETL, and, undoubtedly, would cause device failure due to dielectric breakdown. These findings may be used to explain why the TCTA-host containing device showed a much poorer lifespan than the CBP-host containing counterpart. The failure of these devices may partly be attributed to the migration of the composing organic and inorganic materials, besides the aforementioned degradation mechanisms, i.e. joule heat, narrow recombination zone, and dielectric breakdown. We had actually observed significant molecular migration in several of the OLED materials as well as atomic migration of the Al, Li and In by using SIMS. The varying composition was probed along the thickness direction before and after these undoped devices were applied a forward bias to produce an initial brightness of 500 cd/m2 for 15 min. Fig. 10a–b shows the SIMS depth-profiles of the organic molecules, including the ETL (TBPi), host (CBP), HTL (TAPC), and HIL (HATCN), and the inorganic materials, including the top Al electrode, Li of EIL (LiF) and In of bottom electrode (ITO), of the CBP-host containing device before and after being driven at from 8.4 to 9.0 V for 15 min. All the organic molecules, except CBP, had obviously migrated toward the ITO side. The inorganic components, Al and Li of LiF, had also migrated toward the same side, while the In of ITO migrated toward the opposite (Al) side. The same phenomena happened to all the organic and inorganic materials in the TCTA-host containing device, as seen in
Fig. 11a–b. An unprecedented 'back-migration' was observed in the host molecule, CBP; i.e. that it had migrated toward the Al instead of ITO electrode, unlike all the other organic molecules. Such an extraordinary phenomenon may be attributed to the intrinsically low glass transition temperature of CBP (78 °C), and the comparatively fast migration rate of TPBi. This is since these devices were undoped and their luminous efficiencies were hence relatively low. Most of the applied current was therefore turned into heat, as mentioned earlier. The high heat would then cause the CBP layer to melt or become very soft, which made easier and faster the penetration of the migrating TPBi molecules. Actually, the faster migration of the TPBi molecule in the CBP-host containing device can be evidenced by comparing its SIMS result (Fig. 12b), against that in the TCTA-host containing counterpart (Fig. 13b). The migration of CBP was seemly much slower for some unknown reasons. As a result, the molecules of CBP were forced to move backward by the penetrating TPBi, leading to the unique backmigration as observed. In contrary, the migration depth of TPBi molecule was much shorter in the TCTA than in the CBP device. The comparatively higher glass transition temperature (151 °C) of TCTA host had somewhat been capable to block the migrating TPBi. Nevertheless, the comparatively faster migration of TPBi and CBP-melt-down caused back-migration might be used to explain why the undoped CBP device showed a much shorter lifespan, which was 2.9 h, as comparing with the 11.9 h measured for the undoped TCTA device.
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Upon doping, the luminous efficiency of the CBP-host containing device was drastically increased; it was increased from 0.3 to 6.5 at 100 cd/m2. Its maximum brightness also greatly increased from 2863 to 99,550 cd/m2. These verify the applied current to have been effectively converted into visible light rather than that of Joule heat, which also significantly decreases the temperature inside the device at higher brightness. The resultant much lower device temperature should have hence kept the CBP layer intact to some degree, which had in turn enabled a device lifetime much longer the TCTA-host containing counterpart with dopant. Moreover, the Arrhenius equation also suggested that the lifetime of the device adversely affected by the temperature. Fig. 14a and b shows the atoms of (a) Al and (b) In of ITO in the CBP-host containing device to have migrated toward the opposite electrode after being driven at from 8.4 to 9.0 V for 15 min. Similarly, the same phenomena happened to the TCTA-host containing device, as shown in Fig. 15a and b. These SIMS results reveal two additional facts. The first is an observation of atomic migration in all the inorganic layers that had simultaneously taken place with the molecular migration. Further in-depth investigation is required to distinguish whether the organic or inorganic migration is the key fatal cause. The second is that electron driven migration of the two electrode materials might be the origin of the spike growth, as indicated by the prior discussed AFM images of the rough Al surface. Consequently, in addition to the growth of the Al spikes, the growth of ITO spikes might add up a catastrophic dielectric burden and speed up the deterioration of the given devices.
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4. Conclusion In this study, we have demonstrated that the degradation in OLED devices might be resulted from manifold, including but not limited to low efficiency, high Joule heating, concentrated recombination zone, high internal electric field, limited dielectric strength, low glass transition temperature, spiking electrodes, and migrations of the composing organic and inorganic materials. It is the first time that the migrations of all of the composing organic and inorganic materials are investigated, simultaneously. The lifetime of devices is found highly sensitive to the thickness of ETL. The growth of Al spikes, for example, might lead to a catastrophically high electric field, locally, and cause dielectric breakdown. The findings might help domain experts to carry out extensive studies to advance the understanding of lifetime issues of organic or even inorganic devices and to devise new approaches to further enhance the device performance. Acknowledgments This work was financially supported in part by Ministry of Economic Affairs (Grant No.104-EC-17-A-07-S3-012) and Ministry of Science and Technology (Grant Nos. 104-2119-M-007-012 and 103-2923-E-007003-MY3), Taiwan. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.01.035. References [1] F. So, J. Kido, P. Burrows, Organic light-emitting devices for solid-state lighting, MRS Bull. 33 (07) (2008) 663–669. [2] S.R. Forrest, The path to ubiquitous and low-cost organic electronic appliances on plastic, Nature 428 (6986) (2004) 911–918. [3] S. Reineke, M. Thomschke, B. Lüssem, K. Leo, White organic light-emitting diodes: status and perspective, Rev. Mod. Phys. 85 (3) (2013) 1245. [4] L. Duan, et al., Solution processable small molecules for organic light-emitting diodes, Mater. Chem. 20 (31) (2010) 6392–6407. [5] G.M. Farinola, R. Ragni, Electroluminescent materials for white organic light emitting diodes, Chem. Soc. Rev. 40 (7) (2011) 3467–3482 2011.
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Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Plausible degradation mechanisms in organic light-emitting diodes a,∗
a
a
a
T
a,c
Jwo-Huei Jou , You-Ting Lin , Yu-Ting Su , Wei-Chi Song , Shiv Kumar , Deepak Kumar Dubeya, Jing-Jong Shyueb,d, Hsun-Yun Changd, Yun-Wen Youd, Tzu-Wei Liange a
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, 30013, Taiwan Department of Materials Science and Engineering, National Taiwan University, Taipei, 106, Taiwan Hiroshima Synchrotron Radiation Center, Hiroshima University, Kagamiyama 2-313, Higashi-Hiroshima, 739-0046, Japan d Research Center for Applied Sciences, Academia Sinica, Taipei, 115, Taiwan e Global Science Instruments Co., Ltd., Taiwan b c
A R T I C LE I N FO
A B S T R A C T
Keywords: OLEDs Degradation mechanisms ToF-SIMS Lifetime
Organic light emitting diode has become a highly attractive technology for high quality displays and lighting. These applications, however, strongly rely on their lifetime. Probing all the possible failure mechanisms has hence become crucial. We reveal here that the device lifespan depends on the dielectric strength, internal electric field, morphology, thermal stability, and migration of the composing organic and inorganic materials as well as span of recombination zone and device efficiency. Additionally, the lifetime is highly sensitive to the thickness of electron transporting layer. By taking a green emitter doped in 4,4-bis(carbazol-9-yl)biphenyl host for example, the device lifetime can be increased from 51 to 209 h at 1000 cd/m2, an increment of 310%, and its efficacy increased from 21 to 41 lm/W, an increment of 96%, as the thickness is increased from 20 to 40 nm. The results show high device reliability to be achievable provided it composes materials with high dielectric strength, high glass transition property and low migration tendency, and uniform layered structure with low built-in internal electric field, wide recombination zone and high efficiency.
1. Introduction Organic light emitting diodes (OLEDs) have become increasingly attractive in high quality display and lighting applications [1–3]. They have achieved great progress in efficiency over the past years [4–7]. Whilst, lifetime is always a crucial reliability issue before they can be adopted more extensively, especially for lighting purpose [8–10]. Stronger efforts are hence demanded in the pursuit of further understanding regarding why they fail and how to improve, since either energy-saving or sustainability can only be better realized on devices with a longer lifespan [11]. The stability and lifetime in OLEDs are closely related to multiple device degradation factors, which can be further ascribed into intrinsic and extrinsic issues. Intrinsic degradation mechanisms include thermal decomposition [12–17], electrochemical decomposition [18–20], diffusion and drift [21–23], molecular migration [24–26], interfacial degradation [27–31], and narrow recombination zone [32–34], etc. The extrinsic degradations include attack by the ambient moisture and oxygen [35–38], impurity-induced degradation [39,40], and ultraviolet degradation, etc. [41,42]. Many approaches have been proposed to improve the device ∗
lifetime and efficiency. In 1995, Adachi et al. reported high device durability to be achievable by having a small energy barrier at the interface between the hole transporting layer and anode [43]. In 1999, Murata applied a high glass transition material to keep the high quantum efficiency at elevated temperature [44]. In 2011, Jou's group proposed a double mixed-host device architecture to disperse the injected carriers into three different recombination zones to enhance lifetime [45]. It is believed that many more improving approaches can be devised if in-depth investigations are carried out. Hence, we have performed electric simulation as well as device fabrication and lifetime testing for OLED devices with different hosts and with different thicknesses for the employed electron transporting layer (ETL). Atomic force microscopy (AFM) was used to investigate the surface morphologies and the presence of spikes in the aluminum electrode before and after operation, and the effect of spike on dielectric breakdown was examined. Secondary ion mass spectrometry (SIMS) was used to analyze how the migrations of the organic and inorganic materials affect the lifetime.
Corresponding author. E-mail address: [email protected] (J.-H. Jou).
https://doi.org/10.1016/j.orgel.2019.01.035 Received 8 November 2018; Received in revised form 21 January 2019; Accepted 21 January 2019 Available online 24 January 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.
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2.2. Device measurement 2.2.1. Efficiency All the devices were encapsulated in a nitrogen purge globe chamber and then measured under atmospheric conditions. The luminance, spectrum and CIE chromatic coordinate results were obtained using a PR-655 spectroradiometer, and a Keithley 2400 electrometer was used to measure the current–voltage (IeV) characteristics. 2.2.2. Lifetime The lifetimes were obtained using a programmable Keithley 2400 electrometer. Accelerated lifetime testing was conducted at a constant current with a corresponding initial luminance of 10,000 cd/m2 and 5000 cd/m2 for the CBPe and TCTA-host containing devices, respectively. The lifetime was based on the T50 (the time as the brightness drops to 50% of its initial magnitude) of the pristine device performed at the above accelerated testing condition. 2.2.3. Surface morphology AFM images of the aluminum films before and after applying a 7 V forward bias for 30 min were obtained on a Bruker ICON SPM in tapping mode at room temperature. 2.2.4. SIMS measurements SIMS experiments were performed with a PHI TRIFTV nanoTOF (Chigasaki, Japan) ToF-SIMS system. The pulsed (8284 Hz, 4.5 Na length) primary ion source was operated with C60+ at 20 kV and 1 nADC with a 100 μm × 100 μm rastering area for 10 min. The co-sputtering was done with a C60+ ion beam operated at 20 kV and 2.5 nA with a 800 μm × 800 μm rastering area and with an Ar+ ion beam at 0.5 kV and 620 nA with a 6000 μm × 6000 μm. A dual-beam charge neutralizer, containing Ar+ beam and electron beam, was used to compensate charge on the device surface during data acquisition. 2.2.5. Electrical simulation Thickness-dependent internal electric field and recombination zone in each organic layer of the studied OLEDs were performed using a commercial software package SETFOS [46]. The setting parameters included hole mobility, electron mobility, lowest unoccupied molecular orbital (LUMO), and highest occupied molecular orbital (HOMO), as shown in Table 1. The specific structure for the simulation consisted of ITO(100 nm)/HAT-CN(3 nm)/TCTA (40 nm)/EML(15 nm)/TPBi (20, 40, 60, and 80 nm)/LiF (1 nm)/Al (100 nm) on glass substrate. The work function set was 5.2 eV for the anode and 4.3 eV for the cathode.
Fig. 1. Schematic energy-level diagrams of the green OLED devices with (a) CBP and (b) TCTA hosts. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
2. Experiment
3. Results and discussion
2.1. Device fabrication
Table 2 shows the device performance of the studied CBPe and TCTA-host containing green devices with different ETL thicknesses from 20 to 80 nm. At 1000 cd/m2 for example, the respective power efficiency is 21.1, 41.3, 39.2, and 16.5 lm/W for the CBP-host containing devices, and 3.9, 19.4, 20.5, and 8.3 lm/W for the TCTA counterparts. At 10,000 cd/m2, the resultant devices show a respective
Fig. 1a–b shows the studied green OLED device structures and their corresponding energy level diagrams. The device structure consisted of an indium tin oxide (ITO) as the anode layer, a 3 nm 1,4,5,8,9,11hexaazatriphenylene hexacarbonitrile (HAT-CN) as a hole injection layer, a 40 nm 4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC) as a hole transporting layer, a 15 nm single emission layer (EML), a 20, 40, 60, or 80 nm 1,3,5-tris(N -phenyl-benzimidazol2-yl) benzene (TPBi) as ETL, a 1 nm lithium fluoride (LiF) as electron injection layer, and a 100 nm aluminum as cathode. The employed light-emitting green dye was tris(2-phenylpyridine) iridium(III) (Ir (ppy)3). The employed host was 4,4-bis(carbazol-9-yl)biphenyl (CBP) for Device I and III, and 4,4′,4’’-tri(N-carbazolyl) triphenylamine (TCTA) for Device II and IV. Additionally, Devices III and IV were doped with a red dye, bis(1-phenylisoquinolinolato-C2,N) iridium (acetylacetonate) (Ir(piq)2(acac)), at the interface between the EML and ETL. Amongst, all the layers were deposited via a thermal evaporation process under high vacuum condition at 5 × 10−6 Torr.
Table 1 Setting parameters for the electric simulation using SETFOS; they are the hole mobility, electron mobility, highest occupied molecular orbital (HOMO), and lowest unoccupied molecular orbital (LUMO).
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Material
HOMO (eV)
LUMO (eV)
Hole mobility (10−5 cm2/V)
Electron mobility (10−5 cm2/V)
Ref.
TPBi TCTA CBP TAPC
6.2 5.7 6.0 5.5
2.7 2.4 2.9 2.0
0.033 30 200 1000
3.3 0.3 30 10
[48] [49] [49] [50]
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Table 2 Effects of host material and ETL thickness on the operation voltage (OV), power efficiency (PE), current efficiency (CE), external quantum efficiency (EQE), and half lifetime at 1000 cd/m2 of the green devices. Device
Host
ETL thickness (nm)
Operation Voltage (V)
PE (lm/W)
CE (cd/A)
EQE (%)
Lifetime T50 (hours) @ 1000 cd/m2
@ 100/1000/10,000 cd/m2 1–1 1–2 1–3 1–4
CBP
20 40 60 80
3.1/3.8/5.6 3.0/3.7/5.7 3.2/4.3/6.8 3.5/4.9/8.5
4.2/21.1/15.3 6.5/41.3/27.9 46.3/39.2/20.2 18.1/16.5/8.9
4.6/25.3/27.4 6.8/48.1/50.6 47.8/53.9/43.9 20.1/25.8/23.8
1.3/6.9/7.5 1.9/13.4/14.1 13.7/15.4/12.6 5.8/7.5/7
51 209 116 92
II-1 II-2 II-3 II-4
TCTA
20 40 60 80
3.1/3.7/5.8 3.1/3.7/5.8 3.1/4.1/6.8 4.1/5.3/8.2
0.6/3.9/4.3 3/19.4/9.6 17.8/20.5/8.1 2.8/8.3/4.2
0.7/4.7/7.9 3.3/22.7/17.6 17.9/26.6/17.6 3.6/13.9/11
0.2/1.3/2.3 0.9/6.5/5.1 5.2/7.7/5.2 1.1/4.1/3.2
2.9 3.6 4.4 4.3
power efficiency of 15.3, 27.9, 20.2, and 8.9 lm/W for the former devices, and 4.3, 9.6, 8.1, and 4.2 lm/W for the latter. As seen, the CBPhost containing devices show a much better power efficiency performance as comparing with the TCTA-host counterparts based on the same ETL thickness. From lifetime perspective, the resultant lifetime of the CBP-host containing device is increased from 51 to 209 h, an increment of 310%, as the ETL increases from 20 to 40 nm. At 60 nm, the lifetime drops to 116 h. By further increasing to 80 nm, the lifetime is decreased to 92 h. For the TCTA-host containing devices, the resultant lifetime is increased from 2.9 to 3.6 h, an increment of 24%, as ETL thickness increases from 20 to 40 nm. By continuously increasing to 60 nm, the device lifetime is further increased from 3.6 to 4.4 h. At 80 nm, the lifetime slightly drops to 4.3 h. Fig. 2 shows the correlation between the power efficiency and lifetime. Taking the CBP-host containing device for example, its lifetime is drastically increased from 51 to 209 h as its efficiency increases from 21.1 to 41.3 lm/W, indicating the devices to show better lifetime at higher power efficiency. The same trend appears for the TCTA counterparts. As the resultant efficiency increases from 3.9 to 20.5 lm/W, the corresponding lifetime is increased from 2.9 to 4.4 h. Overall speaking, the CBP-host containing devices exhibit much better lifetime than the TCTA counterparts. Fig. 3 shows the effect of power efficiency on device temperature for the CBPe and TCTA-host containing devices. The device temperature did not change much for both types of devices operated at 1000 cd/m2, regardless of their power efficiency. However, at higher luminance, such as 10,000 cd/m2, the device temperature markedly increased as the power efficiency decreased. Specifically, it increased from 29 to 31 °C as the power efficiency decreased from 27.9 to 8.9 lm/W for the
Fig. 3. Effect of power efficiency on the device temperature for the CBPe and TCTA-host containing devices at 1000 and 10,000 cd/m2.
CBP-host containing device. It increased from 33 to 42 °C as the power efficiency decreased from 9.6 to 4.2 lm/W for the TCTA-host containing device. The significant rise in device temperature can be attributed to low luminous efficacy, in which a significant part of the applied electric energy was converted into Joule heat instead of light. The generation of Joule heat can be used to explain at least partly why the device lifetime decreased as the power efficiency dropped, and why the comparatively low efficiency TCTA-host containing device showed a much poorer lifespan. Moreover, the mobilities of charge carriers, device turn-on voltage (voltage at 1 cd/m2), activation energy and lifetime are also directly influenced by the temperature due to a shift of the quasi-Fermi levels. Owing to the generation of excessive heat resulting from the electrical stress and/or the thermionic emission, large numbers of holes and/or electrons generated inside the recombination zone, which may unbalance the charge carrier distribution, hence reduced device efficiency. We had further investigated the effect of recombination zone on device lifetime. The recombination zone(s) were first identified by electric simulation using the software package SETFOS, details of which are revealed in the simulation section. As seen in Fig. 4, the recombination of the entering holes and electrons would mainly occur within the emission layer, but be dispersed near the two interfaces for the CBP-host composing device. Whilst, the recombination would still mainly occur within the emission layer, but concentrate near the interface between EML and ETL for the TCTA-host composing counterpart. These may explain why the latter to have a poorer device efficiency due to its more narrow recombination zone. To verify the simulation findings experimentally, we fabricated two additional devices with a thin (2 nm) red emission layer deposited
Fig. 2. Effect of power efficiency on the device lifetime of the CBPe and TCTAhost containing devices. 224
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Fig. 4. Recombination distributions in the CBPe and TCTA-host containing devices.
Fig. 7. Internal electric field distributions in the studied OLEDs with an ETL thickness of 40 nm under a forward bias of 3.7 V, at which both devices exhibit a 1000 cd/m2 brightness.
Fig. 5. Electroluminescent spectra of the CBPe and TCTA-host containing green devices with an additional red emission layer in between the HTL (TAPC) and EML. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 8. Effect of a 15 nm aluminum spike on the internal electric field in the (a) CBPe and (b) TCTA-containing devices with a 40 nm ETL at applied voltages as the devices (i) emit 1000 cd/m2, (ii) are under lifetime measurement and (iii) reach their maximum brightness. Black and red lines represent the internal electric fields of the OLED devices with the presence and absence of an aluminum (Al) spike at specified voltage in the figure, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Electroluminescent spectra of the CBPe and TCTA-host containing green devices with an additional red emission layer in between the EML and ETL (TPBi). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 9. Atomic force microscopic images of the aluminum electrode surface (a) pristine and (b) after electric poling with a forward bias of 7 V.
confirming which to have a much dispersed recombination zone. Again, these experimental evidences verify what had been revealed from the electric simulation. As a result, the much narrow recombination zone may be used to explain why the TCTA-host containing device was much poorer in both efficiency and lifetime performance. Although the applied voltage required to trigger the emission of OLED devices is not high as usual, the resulting internal electric field in each of organic layer may approach or even exceed its dielectric strength due to the extremely thin layer structure, by noting the field to be in proportion to the inverse of layer thickness. We had hence measured the dielectric strength of the comprising organic materials. Specifically, the dielectric strength of TAPC (HTL) ranges from 2.3 to 2.9 MV/cm for thickness varying from 40 to 80 nm. Whilst, the dielectric strength of the hosts, CBP and TCTA, ranges from 2.4 to 3.1 MV/cm, and 2.4–3.6 MV/cm, respectively. It is from 2.8 to 3.5 MV/ cm for TPBi (ETL). Any resulting internal electric field that is beyond the dielectric strength of the corresponding material might cause break down electrically, which would happen when the device is driven at high voltage or the electrodes have spikes penetrating into the layer. It was learned that the nominal dielectric strength was sensitive to
between the original HTL and EML, as shown in Fig. S1. The resultant electroluminescent spectra are shown in Fig. 5. As observed, the emissive layers of both devices show the characteristic green and red emissions peaking at 516 and 624 nm, respectively. However, the red peak is much weaker in the TCTA-host composing device, especially as comparing with the CBP-host composing counterpart. The comparatively much weaker red emission implies the recombination to have occurred somewhere away from the red emission layer for the TCTAhost containing device. In contrast, the recombination in the CBP-host containing device had occurred at least partly near the red emission layer as well as in the main green EML so that both the characteristic red and green emissions appeared markedly. These experimental results are in good accordance with what have been predicted from the electric simulation shown above. Actually, in another fabricated devices that with the thin red emission layer deposited in between the EML and ETL (Fig. 6), the red emission was very marked as comparing with the green one for the TCTA-host containing device. This confirms the recombination to have concentrated at the EML/ETL interface. Whilst, the green emission is still comparatively strong for the CBP-host containing counterpart, 226
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Fig. 11. SIMS depth-profiles of the OLED device containing organic molecules, viz. ETL (TBPi), host (TCTA), HTL (TAPC), and HIL (HATCN), and the inorganic materials, viz. the top Al electrode, Li of EIL (LiF) and In of bottom electrode (ITO), for the TCTA-host containing device (a) before and (b) after applying a forward bias of 9.7–10.5 V for 15 min.
Fig. 10. SIMS depth-profiles of the OLED device containing organic molecules, viz. ETL (TBPi), host (CBP), HTL (TAPC), and HIL (HATCN), and the inorganic materials, viz. the top Al electrode, Li of EIL (LiF) and In of bottom electrode (ITO), for the CBP-host containing device (a) before and (b) after applying a forward bias of 8.4–9.0 V for 15 min.
materials, making dielectric breakdown unlikely to occur. Figs. S2a–b show the thickness-dependent internal electric fields of the CBPe and TCTA-host containing devices. As seen, the electric field distribution is highly sensitive to the thickness of ETL. Overall speaking, the electric field in either EML or ETL markedly decreases as the ETL thickness increases, indicating the tuning of ETL thickness to be a highly effective approach to prevent electric breakdown caused device failure, if any. The resulting electric fields at low applied voltage (3.7 V) are all well below the dielectric strengths of the OLED materials. However, the electric fields would become much higher as the applied voltage increase. Figs. S3a–b show the internal electric fields of the devices at their maximum brightness. Their respective voltages were 10, 10.5, 11.5, and 12.5 V for the CBP-host containing devices, and 8, 9, 9.5, and 10.5 V for the TCTA-host containing counterparts, as the ETL thickness was 20, 40, 60, and 80 nm. It would not be too surprising to see the devices start to breakdown as the ETL was reduced to 20 nm, since which would result in an extremely high electric field and hence cause device failure. In the CBP-host containing case, the resulting electric fields had approached the lower limits of dielectric strength of CBP and TPBi, while the field had exceeded the lower limit of TPBi in the TCTA case. Electric breakdown can be used to explain why these 20 nm ETL composing devices showed a comparatively poorest lifetime. The breakdown may have occurred within the CBP-host and TPBi-ETL layers for the CBP-host containing device, and solely within the TPBiETL layer for the TCTA-host containing counterpart. Nevertheless, what might be puzzling and remains to be solved is why those devices that
the surface roughness of the employed electrodes [47]. Taking aluminum for example, spikes inevitably appear throughout the entire specimen, which might cause the aforementioned dielectric breakdown in the organic layer(s) at considerably low voltage(s). The spiking surface would also make the determination of dielectric strength very difficult, especially when the layer is relatively thin. This non-ideal, rough surface is actually an unavoidable fact in numerous real devices and products. Nevertheless, it would be worthy to further establish a convenient, standard method for measuring more precisely the dielectric strength of the increasing OLED materials to ensure better device reliability. Fig. 7 shows the internal electric field distribution in each organic layer of the studied OLEDs. As seen, the electric fields are markedly different in different layers. Moreover, the distribution changes drastically with the change of employed host. In the TCTA-host containing device, a relatively highest electric field is built within the ETL layer with TPBi; i.e. it is about 0.59 MV/cm, while only 0.15 MV/cm within the EML layer with TCTA. By changing the host from TCTA to CBP, the electric field in the TPBi ETL decreases from 0.59 to 0.47 MV/cm, but increases from 0.15 to 0.46 MV/cm in the CBP EML. It is important to note that the applied voltage input in this simulation is only 3.7 V, at which both devices exhibit a 1000 cd/m2 brightness. The magnitude of the electric fields will become two times larger if the applied voltage is increased to 11.1 V. The resulting electric fields are, however, still much lower than the average dielectric strength of the studied 227
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Fig. 13. SIMS depth-profiles of (a) TCTA and (b) TPBi molecules before and after being driven at from 9.7 to 10.5 V for 15 min. Shadow shows the signal from C60 having the same mass as that of TPBi.
Fig. 12. SIMS depth-profiles of (a) CBP and (b) TPBi molecules before and after being driven at from 8.4 to 9.0 V for 15 min. Shadow shows the signal from C60 having the same mass as that of TPBi.
For the TCTA-host containing device with a perfectly smooth surface, the respective electric fields in ETL are 0.59, 0.86, and 1.88 MV/ cm, at 3.7 V for 1000 cd/m2, 4.8 V for lifetime measurement with an initial brightness of 5300 cd/m2, and 9 V at the maximum brightness (Fig. 8b). By taking the additional 15 nm Al spike into account, the electric fields are increased to 1.34 MV/cm at 4.8 V and 2.95 MV/cm at 9 V. Although the average electric field throughout the entire ETL may not be too high, the ETL at anywhere with the protruding Al spike would exhibit a relatively strong electric field that exceeds the dielectric strength of ETL (TPBi) and hence causes dielectric breakdown, at least locally. This might explain why this device showed a much shorter lifetime. As the ETL thickness decreases to 20 nm, the respective electric fields of the devices with a smooth surface are 0.72 and 0.73 MV/cm in the EML and ETL of the CBP-host containing device, while 0.15 and 1.15 MV/cm for the TCTA-host containing counterpart, as shown in Figs. S4a–b. Similarly, by taking the 15 nm spike into consideration, the respective electric fields in EML and ETL would be increased to 4.04 and 4.45 MV/cm at 10 V as the CBP-host composing device reached its maximum brightness. These relatively high internal electric fields might be used to explain why the device's lifetime dropped drastically as its ETL was reduced to 20 nm, and the breakdown might occur in both CBP-host and TPBi-ETL. The field might, theoretically, increase to 5.79 MV/cm under lifetime testing at 4.9 V, or 8.36 MV/cm at maximum luminance at 8 V for
compose 40, 60, and 80 nm ETL exhibit no electric field greater than the dielectric strength had failed in a similar pattern, but just at a slightly higher voltage. Fig. 8a shows the internal electric fields of the CBP-host containing device with the presence and absence of an aluminum (Al) spike under a forward bias of 3.7, 5.9, and 10.5 V, where the ETL thickness is fixed at 40 nm. The first voltage, 3.7 V, was the applied voltage when the device emitted a 1000 cd/m2 luminance; the second, 5.9 V, was the voltage applied for lifetime measurement with an initial brightness of 13,340 cd/m2; and the third, 10.5 V, was the corresponding voltage when the device reached its maximum brightness and started to degrade. As seen, the corresponding electric fields are 0.46, 0.85, and 1.64 MV/cm in the EML. The above results were obtained by assuming the electrodes to have a perfectly smooth surface. However, such an ideal situation did not appear in the real devices as evidenced by the atomic force microscope images shown in Fig. 9. In contrary, a 10 nm spike presented in the Al electrode in the pristine stage, which grew to 15 nm after applying a forward bias of 7 V. By taking such an additional 15 nm spike into account, the resultant electric field in the spike facing ETL would be increased from 0.46 to 0.63 MV/cm at 3.7 V, or to 2.24 MV/cm at 10.5 V. Although the latter approached the lower limit of dielectric strength of the CBP-host, this device was less likely to degrade from dielectric breakdown, especially as comparing with the TCTA-host composing counterpart. 228
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Fig. 14. SIMS depth-profiles showing the atoms of (a) Al and (b) In of ITO in the CBP-host containing device to have migrated toward the opposite electrode after being driven at from 8.4 to 9.0 V for 15 min.
Fig. 15. SIMS depth-profiles showing the atoms of (a) Al and (b) In of ITO in the TCTA-host containing device to have also migrated toward the opposite electrode after being driven at from 9.7 to 10.5 V for 15 min.
the TCTA-host composing counterpart with a 20 nm ETL. The resultant field has far beyond the dielectric strength of the TPBi-ETL, and, undoubtedly, would cause device failure due to dielectric breakdown. These findings may be used to explain why the TCTA-host containing device showed a much poorer lifespan than the CBP-host containing counterpart. The failure of these devices may partly be attributed to the migration of the composing organic and inorganic materials, besides the aforementioned degradation mechanisms, i.e. joule heat, narrow recombination zone, and dielectric breakdown. We had actually observed significant molecular migration in several of the OLED materials as well as atomic migration of the Al, Li and In by using SIMS. The varying composition was probed along the thickness direction before and after these undoped devices were applied a forward bias to produce an initial brightness of 500 cd/m2 for 15 min. Fig. 10a–b shows the SIMS depth-profiles of the organic molecules, including the ETL (TBPi), host (CBP), HTL (TAPC), and HIL (HATCN), and the inorganic materials, including the top Al electrode, Li of EIL (LiF) and In of bottom electrode (ITO), of the CBP-host containing device before and after being driven at from 8.4 to 9.0 V for 15 min. All the organic molecules, except CBP, had obviously migrated toward the ITO side. The inorganic components, Al and Li of LiF, had also migrated toward the same side, while the In of ITO migrated toward the opposite (Al) side. The same phenomena happened to all the organic and inorganic materials in the TCTA-host containing device, as seen in
Fig. 11a–b. An unprecedented 'back-migration' was observed in the host molecule, CBP; i.e. that it had migrated toward the Al instead of ITO electrode, unlike all the other organic molecules. Such an extraordinary phenomenon may be attributed to the intrinsically low glass transition temperature of CBP (78 °C), and the comparatively fast migration rate of TPBi. This is since these devices were undoped and their luminous efficiencies were hence relatively low. Most of the applied current was therefore turned into heat, as mentioned earlier. The high heat would then cause the CBP layer to melt or become very soft, which made easier and faster the penetration of the migrating TPBi molecules. Actually, the faster migration of the TPBi molecule in the CBP-host containing device can be evidenced by comparing its SIMS result (Fig. 12b), against that in the TCTA-host containing counterpart (Fig. 13b). The migration of CBP was seemly much slower for some unknown reasons. As a result, the molecules of CBP were forced to move backward by the penetrating TPBi, leading to the unique backmigration as observed. In contrary, the migration depth of TPBi molecule was much shorter in the TCTA than in the CBP device. The comparatively higher glass transition temperature (151 °C) of TCTA host had somewhat been capable to block the migrating TPBi. Nevertheless, the comparatively faster migration of TPBi and CBP-melt-down caused back-migration might be used to explain why the undoped CBP device showed a much shorter lifespan, which was 2.9 h, as comparing with the 11.9 h measured for the undoped TCTA device.
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Upon doping, the luminous efficiency of the CBP-host containing device was drastically increased; it was increased from 0.3 to 6.5 at 100 cd/m2. Its maximum brightness also greatly increased from 2863 to 99,550 cd/m2. These verify the applied current to have been effectively converted into visible light rather than that of Joule heat, which also significantly decreases the temperature inside the device at higher brightness. The resultant much lower device temperature should have hence kept the CBP layer intact to some degree, which had in turn enabled a device lifetime much longer the TCTA-host containing counterpart with dopant. Moreover, the Arrhenius equation also suggested that the lifetime of the device adversely affected by the temperature. Fig. 14a and b shows the atoms of (a) Al and (b) In of ITO in the CBP-host containing device to have migrated toward the opposite electrode after being driven at from 8.4 to 9.0 V for 15 min. Similarly, the same phenomena happened to the TCTA-host containing device, as shown in Fig. 15a and b. These SIMS results reveal two additional facts. The first is an observation of atomic migration in all the inorganic layers that had simultaneously taken place with the molecular migration. Further in-depth investigation is required to distinguish whether the organic or inorganic migration is the key fatal cause. The second is that electron driven migration of the two electrode materials might be the origin of the spike growth, as indicated by the prior discussed AFM images of the rough Al surface. Consequently, in addition to the growth of the Al spikes, the growth of ITO spikes might add up a catastrophic dielectric burden and speed up the deterioration of the given devices.
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4. Conclusion In this study, we have demonstrated that the degradation in OLED devices might be resulted from manifold, including but not limited to low efficiency, high Joule heating, concentrated recombination zone, high internal electric field, limited dielectric strength, low glass transition temperature, spiking electrodes, and migrations of the composing organic and inorganic materials. It is the first time that the migrations of all of the composing organic and inorganic materials are investigated, simultaneously. The lifetime of devices is found highly sensitive to the thickness of ETL. The growth of Al spikes, for example, might lead to a catastrophically high electric field, locally, and cause dielectric breakdown. The findings might help domain experts to carry out extensive studies to advance the understanding of lifetime issues of organic or even inorganic devices and to devise new approaches to further enhance the device performance. Acknowledgments This work was financially supported in part by Ministry of Economic Affairs (Grant No.104-EC-17-A-07-S3-012) and Ministry of Science and Technology (Grant Nos. 104-2119-M-007-012 and 103-2923-E-007003-MY3), Taiwan. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.orgel.2019.01.035. References [1] F. So, J. Kido, P. Burrows, Organic light-emitting devices for solid-state lighting, MRS Bull. 33 (07) (2008) 663–669. [2] S.R. Forrest, The path to ubiquitous and low-cost organic electronic appliances on plastic, Nature 428 (6986) (2004) 911–918. [3] S. Reineke, M. Thomschke, B. Lüssem, K. Leo, White organic light-emitting diodes: status and perspective, Rev. Mod. Phys. 85 (3) (2013) 1245. [4] L. Duan, et al., Solution processable small molecules for organic light-emitting diodes, Mater. Chem. 20 (31) (2010) 6392–6407. [5] G.M. Farinola, R. Ragni, Electroluminescent materials for white organic light emitting diodes, Chem. Soc. Rev. 40 (7) (2011) 3467–3482 2011.
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