Probing the Emission Zone Length in Organic Light Emitting Diodes

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Probing the Emission Zone Length in Organic Light Emitting Diodes via Photoluminescence and Electroluminescence Degradation Analysis Cheng Peng, Amin Salehi, Ying Chen, Michael Danz, Georgios Liaptsis, and Franky So ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13537 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 2, 2017

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ACS Applied Materials & Interfaces

Probing the Emission Zone Length in Organic Light Emitting Diodes

via

Photoluminescence

and

Electroluminescence

Degradation Analysis Cheng Peng1, Amin Salehi2, Ying Chen3, Michael Danz4, Georgios Liaptsis4, Franky So3* 1

Department of Materials Science and Engineering, University of Florida, Gainesville, Florida,

32611, United States of America 2

Department of Physics, North Carolina State University, Raleigh, North Carolina, 27695,

United States of America 3

Department of Materials Science and Engineering, North Carolina State University, Raleigh,

North Carolina, 27695, United States of America 4

CYNORA, GmbH, Werner-von-Siemens-Straße 2-6, Building 5110, 76646 Bruchsal, Germany

KEYWORDS: OLED, OLED operational stability, photoluminescence, emission zone, mixed host ABSTRACT: The understanding and control of the emission zone in organic light emitting diodes (OLEDs) is crucial to the device operational stability. Using the photoluminescence and electroluminescence degradation data, we have developed a modeling methodology to quantitatively determine the length of the emission zone and correlate that with the

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degradation mechanism. We first validate the modeling results by studying the emitter concentration effect on operational stability of devices using the well-studied thermal activated

delayed

fluorescent

(TADF)

emitter,

(4s,6s)-2,4,5,6-tetra(9H-carbazol-9-

yl)isophthalonitrile (4CzIPN), and our results are consistent with previous published data. We further applied this methodology to study the emitter concentration effect using another TADF emitter, 4-carbazolyl-2-methylisoindole-1,3-dione (Dopant 1). The results show that the emission zone of the Dopant 1 devices is narrower than the 4CzIPN device leading to faster degradation. While a higher emitter concentration does not result in widening of the emission zone, we were able to widen the emission zone and hence extend the device lifetime using a mixed host. Introduction Since the first organic light emitting diode (OLED) was demonstrated in 1987 by Tang and Van Slyke,1 OLED has been attracting a lot of attention because of its applications in displays and lighting. The inventions of phosphorescent emitter and thermally activated delayed fluorescent (TADF) emitter have realized nearly 100% OLED internal quantum efficiency (IQE).2, 3 Green and red phosphorescent OLEDs are already used in flat panel displays. However, neither TADF nor phosphorescent blue OLEDs achieve the operational stability required by display applications so far.4,

5

Therefore, a detailed understanding of the OLED degradation

mechanism is thus still of great significance and interest. It is well accepted that the control of the emission zone is crucial to improve the OLED stability. The emission zone profile determines the spatial distribution of excitons and polarons, which have a large influence on the degradation mechanisms such as triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) in the emitting layer as well as its neighboring layers. 2 ACS Paragon Plus Environment

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The chemical reaction products associated with these degradation mechanisms are believed to be responsible for the intrinsic degradation of the active layer molecules.4, 6, 7 However, an accurate determination of the emission zone is challenging. A common approach to probe the exciton profile is to add a thin sensing layer that generates electro luminescence (EL) at longer-wavelengths at various positions within the emitting layer. By analyzing the emission peak of the sensing layer, the exciton density can be estimated.8-10 Furthermore, researchers were able to determine the spatial distribution of the exciton density. The approach assumes that the sensing layer causes negligible effects on the charge transport properties of the emitting layer. This assumption might be questionable as the sensing layer can act as a trap for charge carriers within the emission layer. In this work, we introduce a methodology to determine the length of the emission zone in an OLED using the photoluminescence (PL) degradation data obtained during electrical stress. Compared with the sensing layer approach, this methodology is based on direct measurements on the actual device without any perturbation to the original device structure. We first validate this methodology by studying the emitter concentration effect on the emission zone and the stability of an OLED with the commonly used (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) TADF emitter. The obtained results are in line with the published work by Song et al. and Nakanotani et al. implicating that our approach is working.8, 11 We further study the OLED degradation mechanism using a TADF emitter 4-carbazolyl-2-methylisoindole-1,3-dione (Dopant 1). We found that the electron transport of the emitting layer and hence the emission zone length limits the device lifetime. Using a mixed host, we show that the emission zone length can be extended as well as the device operating lifetime can be improved. Results and Discussion 3 ACS Paragon Plus Environment


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Characterization of Degradation. In our OLED degradation study, while we stressed the devices under constant-current at J = 25 mA/cm² and recorded the EL intensity as it degraded continuously, at various time intervals we monitored the PL degradation of the emitting layer by momentarily turning off the driving current, and exciting the sample using a monochromatic light source at a wavelength of λex = 405 nm. We verified that the degradation caused by the near-UV exposure was negligible due to the very short exposure time. The EL intensity right after each PL measurement was consistent with the EL intensity before the PL measurement. Figure 1 shows the measurement setup and an example of the detected signal. A detailed description of the measurement setup is presented in the Experimental Section.

Figure 1. The measurement setup of PL measurement during a constant-current degradation test. Inset: A plot showing an example data set measured from a green OLED. To correlate the PL degradation data with the length of the emission zone, we need to understand how the emission zone influences the PL degradation of OLEDs. Considering an OLED stack with an unbalanced charge transport having a hole (or electron) dominant emitting layer with a hole (or electron) blocking layer (HBL or EBL) with a large HOMO (or LUMO) 4 ACS Paragon Plus Environment

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level offset, the emission zone is narrow with the exciton concentration in the emitting layer higher near the EML/HBL (or EML/EBL) interface during operation.8,

11, 12

As the device

degrades under electrical stress, the concentration of the degradation products, such as molecular fragments,

4, 12

is also higher near the interface close to the HBL (or EBL), which further

accelerates the EL degradation. On the other hand, as excitons are generated nearly uniform across the emitting layer under photo-excitation, interface-concentrated degradation does not have a major impact on PL as the degradation products are produced only in the narrow emission zone concentrated near the interface. Therefore, PL suffers less than EL from degradation induced by degradation products (quenching). The latter results in a slower PL degradation compared to EL degradation as shown in the inset of Figure 1, and this difference in PL and EL degradation rates has also been previously reported.13-15 For a device with a good charge balance and a wide emission zone with a more uniform distribution of excitons, as the degradation products are distributed more uniformly across the emitting layer, the difference in PL and EL degradation will be less pronounced. Therefore, comparing the PL and EL degradation of an OLED during operation can yield information on the emission zone under electrical bias. To understand the correlation between the width of emission zone and PL degradation, we fabricated OLEDs with a TADF emitter (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN).11,

12

We have chosen a hole dominant material, 3,3-di(9H-carbazol-9-yl)biphenyl

(mCBP) as the host and 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T) as hole blocking layer. This material combination (mCBP:4CzIPN/T2T) used for OLEDs has been intensively studied by several groups.8, 11 It has been found that for a low emitter concentration (10 vol. %), the emission zone is located close to the EML/HBL interface as the host is hole dominant. Given the LUMO level difference between 4CzIPN and mCBP is larger than 1 eV, the 4CzIPN molecules



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are deep electron traps in the EML.8, 11 For a higher emitter concentration, electron transporting via the 4CzIPN molecules is enhanced, resulting in a higher electron mobility in the EML and a broader emission zone. Therefore, it is expected that the device with a higher emitter concentration results in a slower OLED EL degradation compared to a device with a lower emitter concentration. By studying how the emitter concentration affects the EL and PL degradation in this series of devices, we can determine how the length of the emission zone affects the EL and PL degradation rates under electrical stress. Here, we made devices with three different emitter concentrations: 10 vol.%, 20 vol.% and 30 vol.% volume ratios in the EML. We used 1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile (HAT-CN) as the hole injection layer, and N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB), Tris(4-carbazoyl-9-ylphenyl)amine (TcTa) as the hole transport layers, 2,9Bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (NBphen) as the electron transport layer, 8-Hydroxyquinolinolato-lithium (Liq) as the electron injection layer and aluminum as the top contact. The device structure as well as the energy diagram are shown in Figure 2 (b).11, 16-19


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Measured EL Measured PL Fitted EL Fitted PL

1.1 1.0 0.9 0.8 0.7 0.6

10% 0

2

4

6

8

time (h)

L=4.5±0.4 nm

10


1.1 1.0 0.9 0.8 0.7 0.6

20% 0

2

4

6

8 10 12 14 16

Normalized Intensity

a


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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1.1


1.0 0.9 0.8 0.7 0.6

30% 0

4

8

12

16

20

time (h)

time (h)

L=7±1 nm

L=16±9 nm

24

b

Figure 2. (a) Normalized measured and fitted OLED EL and PL degradation of 4CzIPN devices. (b) Device structure and energy diagram. The thickness of emitting layer is 20 nm. The normalized OLED EL and PL degradations of the three devices driven at 25 mA/cm2 are shown in Figure 2 (a). As shown in the figures, the devices with a higher emitter concentration have a better stability while the device with a lower emitter concentration has a lower stability, and the results are consistent with those reported by other groups.

8, 11

What is important here is

the correlation of the EL and PL degradation data as a function of the emitter concentration. The different PL and EL degradation rates in the 10% and 20% devices indicate an emission zone narrower than the EML. It is clear to see that the difference in the degradation rate between PL and EL decreases with increasing emitter concentrations, indicating a broader emission zone with a higher emitter concentration. This conclusion is consistent with previous researchers’ findings, as mentioned in the last section, that in mCBP:4CzIPN/T2T devices a higher emitter 7 ACS Paragon Plus Environment


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concentration leads to a broader emission. Therefore the PL and EL degradation data can be used to determine the OLED emission zone length. A mathematical model based on EL degradation, transient PL and voltage rise has previously been reported for phosphorescent OLED degradation to provide information such as emission zone length and degradation product generation rate as the result of degradation.6 In this work, we built a simple mathematical model to analyze the PL degradation data instead of the previously thoroughly discussed transient PL data. Below are the assumptions made in our model. First, the distribution of exciton generation inside the emitting layer is assumed to have an exponential profile with a maximum at the EML/HBL interface. This assumption was also used in the previously reported model, 6 and found to be consistent with the exciton profile measured from the mCBP:4CzIPN devices with a sensing layer in another work.8 Second, we assume that the rate of degradation reaction is only proportional to the concentration of excitons. Third, we assume the exciton quenching rates are the same whether the excitons are generated by photoexcitation or electrical excitation. Previous studies suggested the difference in the PL and EL degradation rates to the changes in the charge balance.13-15 In this work, we assume the EL degradation due to a change in charge balance is negligible compared to the degradation due to quenching. Indeed, as shown in Figure 2 (a) for a device with 30% emitter concentration, there is no difference in PL and EL rates indicating that the change in charge balance is negligible during electrical stress. Our conclusion is consistent with the results from Sandanayaka, et al. that the EL degradation is mainly due to the formation of quenching sites rather than changes in charge balance in devices with mCBP:4CzIPN/T2T as the EML/HBL.12 Based on the above assumptions, the rate equations for exciton concentration N(x,t) can be 8 ACS Paragon Plus Environment

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written as,

= ∙ , + ∙ , ∙ , ,

(1)

where G(x) is the generation rate of excitons having an exponential dependence of distance. Hence

= = 0 ∙ exp − ,

(2)

where L is the effective emission zone length, x is the distance from the interface where the generation rate of excitons is maximum, τ is the exciton lifetime, and Q(x,t) is the concentration of quenching sites with a rate constant of KQ. Both the exciton concentration N and the quenching site concentration Q are functions of position x as well as time t. The generation of quenching sites can be described by the rate equation below: ,

= ∙ ,

(3)

The generation rate of quenching sites is proportional to the exciton concentration by a rate constant KG. In a fresh device, Q(x,0) = 0. Then we can write the equations for EL and PL degradation normalized to the initial intensity: (

(

!"# $% = &') # ∙ , * / &') # ∙ , 0* (

!"# ,% = -') # ∙ ./

0 ∙,∙

1 /2

(4) (5)

Here, W is the thickness of the emitting layer, which is 20 nm in all the devices. The distribution of photo-generated exciton concentration is assumed to be uniform across the emitting layer. We also did a more accurate calculation based on the transfer matrix method and found the deviation from uniform distribution to be within 10% (Figure S1 in Supporting Information). Combining the previous equations and using the effective emission zone length L 9 ACS Paragon Plus Environment


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and the product of two rate constants KQ·KG as the fitting parameters, we can fit the measured normalized EL and normalized PL as a function of degradation time t. By using the exciton lifetime of 4CzIPN (τ=3.7 µs), 3 we can fit the measured EL and PL degradation data of the three 4CzIPN devices for three emitter concentrations as shown in Figure 2. The obtained results for the effective emission zone length are L: 4.5±0.4 nm for the 10% emitter device, 7±1 nm for the 20% emitter device and 16±9 nm for the 30% emitter device. Note that the physical meaning of this effective emission zone length is the distance over which the concentration decreases to e-1 ≈ 37%. Comparing the results with the ones obtained by Song et al. who determined the exciton distribution profile by inserting a sensing layer in the mCBP:4CzIPN/TPBi device, the results agree very well. Song et al. found L ~ 8 nm for the 10 vol.% emitter device and larger than 18 nm for the 30 vol.% emitter device.8 Emitter Concentration Effect on Dopant 1 OLEDs Stability. After successfully modeling the degradation data in the mCBP:4CzIPN/T2T device, we studied the emitter concentration effect in OLEDs having a TADF emitter Dopant 1. For the sake of consistency, we used the same device stack to minimize the variation in device parameters. The molecular structure of the emitter as well as the device energy diagram are shown in Figure 3 (a). This emitter has a similar LUMO alignment with the host similar to the 4CzIPN emitter, and therefore we expected a comparable concentration effect, and a broader emission zone with a higher emitter concentration. a

b


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Figure 3. (a): Device structure and energy diagram of Dopant 1 OLEDs. The thickness of the emitting layer is 20 nm, and (b): Molecular structure of Dopant 1. We made three types of devices with three different emitter concentrations: 10%, 20% and 30% and monitored the changes in EL and PL intensities with a constant current density of 25 mA/cm2. The exciton lifetime was measured to be 6.8 µs by transient PL measurements. The fits


1.0 0.9 0.8 0.7 0.6

10%

0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

time (h)

L=3.3±0.2 nm


1.1 1.0 0.9 0.8 0.7 0.6

20%

0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0


1.1


to the measured data are shown in Figure 4.


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time (h)

L=3.0±0.1 nm

1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3


30% 0 2 4 6 8 10 12 14 16 18 20

time (h)

L=3.5±0.1 nm

Figure 4. Measured and fitted EL and PL degradation of Dopant 1 devices with different emitter concentrations. Based on our fitting, we found the effective emission zone length to be 3.3±0.2 nm for the 10% emitter device, 3.0±0.1 nm for the 20% emitter device, and 3.5±0.1 nm for the 30% emitter device. In contrast to the 4CzIPN devices, we did not observe any changes in the effective emission zone length due to the changes in the emitter concentration, indicating that Dopant 1 does not affect the electron transport even with a concentration up to 30%. We also compared the 11 ACS Paragon Plus Environment


driving voltages of all OLEDs with different emitter concentrations using two emitters, and the driving voltages at 1 cd/m2 luminance as well as 1 mA/cm2 current density are shown in Figure 5. Here, the voltage at 1 cd/m2 is chosen because this voltage is considered the device turn on voltage. As it is clearly shown, the drive voltages of the 4CzIPN devices are lower than those of the Dopant 1 devices with the drive voltage decreasing with increasing 4CzIPN concentration while the drive voltage increases with increasing concentration of Dopant 1, further confirming that Dopant 1 is not an effective electron transport material in the hole-dominant mCBP host. The ineffective electron transport due to Dopant 1 leads to a narrow emission zone, limiting the device operational stability.

a

4CzIPN OLEDs 4.5

3.5 3.0 2.5 2.0

4.5

V @ 1nit V @ 1mA/cm2

4.0

Dopant 1 OLEDs

b

Voltage (V)

Voltage (V)

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4.0 3.5 3.0

V @ 1nit V @ 1mA/cm2

2.5 10

20

30

2.0

10

20

30

emitter concentration (%)

Emitter Concentration (%)

Figure 5. Drive voltages for (a) the 4CzIPN device and (b) for the Dopant 1 device. Mixed-host Effect on Dopant 1 OLEDs Stability. As we cannot control the emission zone by changing the emitter concentration, we attempted to do that by changing from a single host EML to a co-host EML.4 Here, we added the electron conductive material T2T into the emitting layer. We fabricated two sets of devices: one set with a low emitter concentration of 6 vol. % and the 12 ACS Paragon Plus Environment

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second set with a high emitter concentration of 30 vol. %. In each set of the devices, devices with 2 different T2T concentrations (10 vol. % and 20 vol. %) were used in the emitting layer. In addition, we fabricated a control device without T2T co-host. All the devices were again operated under the same conditions as before (J = 25 mA/cm2). The results are shown in Figure 6 (a).



a

High emitter concentration

1.00

mCBP single host 10% T2T in EML 20% T2T in EML

0.95 0.90 0.85 0.80 0.75 0.70 0.0

0.4

0.8

1.2

1.6

2.0

Normalized EL Intensity

Normalized EL Intensity

Low emitter concentration

1.00

mCBP single host 10% T2T in EML 20% T2T in EML

0.95 0.90 0.85 0.80 0.75

0.70 0.0 0.5 1.0 1.5 2.0 2.5 3.0

Time (h)

Time (h) Measured EL Measured PL Fitted EL Fitted PL

1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.0

0.5

1.0

1.5

2.0


b Normalized Intensity

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1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.0

0.5

1.0

1.5

time (h)

time (h)

L=2.6±0.1 nm

L=6.5±0.2 nm

2.0

Figure 6. (a) OLED EL degradation data from devices with a mixed host and two different emitter concentrations. (b) Measured and fitted EL and PL degradation of low emitter concentration (6%) Dopant 1 devices with a single host and a mixed host with 20% T2T. The effect of mixed-host on lifetime is more significant on the device having a low emitter concentration compared with the device having a high emitter concentration. This can be explained by the LUMO level alignment between mCBP (-2.34 eV), T2T (-2.8 eV) and Dopant 1 (-3.17 eV). Dopant 1 is a strong electron trap in the emitting layer. Therefore, a higher emitter concentration in the emitting layer implies more electron trapping sites such that a moderate concentration of T2T up to 20% could not facilitate the transport of electrons effectively. With a low emitter concentration, the electron traps are filled at a current density of 25 mA/cm2. 14 ACS Paragon Plus Environment

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Therefore, the addition of T2T in the emitting layer provides extra electron transport pathways which leads to a broadening of the emission zone. We can also quantify the effect of mixed host on the device with a lower emitter concentration as shown in Figure 6 (b) by monitoring the PL degradation during electrical stress as described above. Based on the fitting to our model, the effective emission zone length increased from 2.6±0.1 nm with the single mCBP host to 6.5±0.2 nm with 20% T2T mixed in the emitting layer. A further analysis on the effect of broadened emission zone can be done using our modeling methodology. We did a simulation of EL degradation and PL degradation over time based on the measured data with 10 vol.% emitter Dopant 1 concentration in a single mCBP host device. We have shown the simulation data in Figure 7. If the effective emission zone length is doubled from 3.1 nm to 6.2 nm, the device lifetime at 80% of the initial luminance (LT80) at 25 mA/cm2 can be improved by ~70%, which is in line to the measured device data in the last section where about 85% enhancement in LT80 was achieved by adding 20% T2T as co-host. Furthermore, the results of our simulation also give a hint to the limitation of emission zone broadening. The maximum improvement can only be as much as 100%. To further improve the OLED stability, more effort has to be done in designing new stable materials and advanced device architectures like tandem devices were the luminance can be kept constant while simultaneously reducing the operating current.


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Normalized EL and PL intensity


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measured data L=3.1nm simulated L=6.2nm simulated L=9.3nm simulated L=30nm

1.0 0.9 0.8 0.7 0.6 0.5 0

1

2

3

4

time (h) Figure 7. Simulated data with different effective emission zone lengths based on measured 10% emitter concentration single host Dopant 1 OLED data. Discussion In the description of the model to analyze the OLED PL and EL degradation data, several simplifications are included. It is necessary to discuss each simplification to assess the usefulness as well as the limitation of this methodology. The exponential profile with the maximum at the EML/HBL interface is assumed to simplify the distribution of the exciton concentration. However, there do exist situations where the exciton concentrations have the maximum away from the interface, particularly in the OLED structures with a bipolar EML. Our assumption of exponential profile does not conflict with such situations. In those situations, it is expected to see a long effective emission zone length. The 30% emitter concentration 4CzIPN device is likely to be in such a situation. The long effective emission zone length with a big fitting error implies the deviation from the exponential profile.


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It is assumed that all the device degradation happens in the EML. This is guaranteed in this study.

Provided that the PL only originate from the EML as only the PL emission with

wavelengths longer than 530 nm is considered, the same PL and EL degradation rate in the 30% emitter concentration 4CzIPN device shows that the degradation of other layers is negligible. As the composition of the EML is changed with different concentrations and a different emitter, the EL degradation is faster, further indicating the degradation of the EML is the only contributor to the device degradation. One may argue that the change in NPB absorption could be possibly influencing the PL as the excitation light passes the NPB layer before reaches the EML. In fact, NPB is not expected to degrade under the test conditions in this study with the 25 mA/cm2 constant current density. Kondakov et al. reported that NPB HTL does not show any significant degree of degradation even 50% initial device luminance has been degraded after ~80 h at 40 mA/cm2.20 Our analysis assumes the emission zone length does not change during the device degradation. The transport properties of the EML can change due to the formation of charge traps. In this case, the effective emission zone length extracted in this study would be an averaged value over the measurement time. Our analysis assumes that the charge balance change does not contribute to the different PL and EL degradation rates. Indeed, the same PL and EL degradation rate in the 30% emitter concentration 4CzIPN device indicates the quenching effect is the only contributor to the device degradation and this result is consistent with literature.12 Besides, the HOMO offset ~0.5 eV from the EML to the HBL provides near-complete hole blocking as confirmed in literature.11 We do not expect any hole leak into the HBL during the degradation, implying the charge balance is maintained.



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Though in this study we propose a methodology to probe the emission zone length in the OLED, the emission zone length is not the only factor that influences the device stability. As shown in Figure 7, even if the effective emission zone length of the 10% emitter concentration Dopant 1 device is extended to 30 nm, the LT80 at 25 mA/cm2 can only be enhanced to about 2.2 h. It is still shorter than that of the 10% emitter concentration 4CzIPN device, which is above 3 h, though the effective emission zone length of the 10% emitter concentration 4CzIPN device is only 4.5 nm. The more fundamental properties of the emitter such as the exciton lifetime and the exciton energy play important roles, too. Overall, this methodology is expected to be particularly useful in the development of new emitters, where the performance of the OLED is limited by the emitters, hence the EML, while the rest of the OLED stack is far more stable so that the contribution to the device degradation from other layers is negligible. Summary and Conclusion We introduced a quantitative methodology to study the PL degradation of the emission layer during electrical stress. We show that the different PL and EL degradation rates can be used to probe the emission zone during electrical stress. The emission zone length can be determined by analyzing the PL and EL degradation of a set of devices without influencing the device architecture by e. g. the using of sensing layers. This methodology was first validated with a well-studied OLED structure using 4CzIPN as an emitter and the results are consistent with works published in the literature. Subsequently, the degradation mechanism of OLEDs based on a new blue TADF Dopant 1 was studied and the conclusion is that a narrow emission zone is limiting the device stability due to the relatively poor electron transport property of the emitter. A mixed host strategy was then used to improve the stability. It was found the effect of mixed


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host was significant in the device with a lower emitter concentration, while it only brought a small enhancement in the device with a higher emitter concentration due to the strong electron trapping nature of the emitter molecule in the emitting layer. At last, by simulation, we show that the effect of a broader emission zone is limited, for example, to around 100% in LT80 for the 10% emitter concentration Dopant 1 device. The more fundamental properties of the emitters such as the exciton lifetime and the exciton energy have more significant influences on the device stability. Experimental Section OLED fabrication and characterization. For device fabrication, ITO coated glass substrates with an ITO thickness of 95 nm have been used. The substrates were cleaned subsequently in acetone and 2-propanol for 15 min in an ultra-sonic bath. After cleaning, the substrates were treated by ultra-violet ozone for 15 min. Afterwards PEDOT:PSS CleviosTM Al4083 was spincast from a 25°C, PVDF 45 µm filtered solution. The respective coating parameters were 4000 rpm for 40 s followed by an annealing step at 160 oC for 30 min. The spin-coating process and annealing were done in a fume hood. Then, samples were transferred into the vacuum chamber with the pressure lower than 10-6 torr and were kept in there for 1 hour before starting the deposition of the organic layers. All organic materials were obtained from CYNORA GmbH. Deposition rates were about 1-2 Å/s for organic materials and 5 Å/s for aluminum. After deposition, samples were encapsulated with glass and UV curable epoxy resin in a glovebox. OLED device JV characterization was measured with a Keithley 2400 source meter. The OLED luminance was measured with a pre-calibrated silicon photodiode. The OLED spectra were measured using a calibrated Ocean Optics HR4000 high-resolution spectrometer. The calibration was done by a standard Ocean Optics LS-1 tungsten halogen lamp. The OLED pixel area was



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2×2 mm2 defined by pre-patterned ITO and shadow mask patterned aluminum electrodes. The degradation was done at a constant current density J = 25 mA/cm2. Photoluminescence measurement during electrical degradation. Photoluminescence spectra were measured in a semi in-situ setup shown in Figure 1. Except for the pixel area, the backside of the sample was covered by black tape to ensure the PL emission only from the pixel area. During the electrical degradation, the electroluminescence spectra were captured by an Ocean Optics HR4000 high-resolution spectrometer through an optical fiber. When a PL spectrum measurement was needed, the current through the device was paused and the sample was photo-excited with a continuous λ = 405 nm laser diode. PL spectrum was captured by the same spectrometer as mentioned before. To recognize any fluctuation in the laser density, the laser beam was split and guided to silicon photodiode. The PL intensity was calculated by integrating from λ1 = 530 nm to λ2 = 650 nm to exclude the PL emission from other functional layers. The EL intensity was calculated by integration over the whole visible range (λ1 = 380, λ2 = 780 nm). ASSOCIATED CONTENT Supporting Information Light field distribution under photo-excitation by transfer matrix method; 4CzIPN OLEDs’ characteristics; Dopant 1 OLEDs’ characteristics AUTHOR INFORMATION Corresponding Author *[email protected]


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Author Contributions The manuscript was written through contributions of all authors. Funding Sources CYNORA GmbH, Germany Acknowledgement: The authors acknowledge the support of CYNORA GmbH, Germany for this work. References 1.

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Probing the Emission Zone Length in Organic Light Emitting Diodes

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