Root Causes of the Limited Electroluminescence Stability of Organic

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Organic Electronic Devices

The Root Causes of the Limited Electroluminescence Stability of Organic Light-emitting Devices Made by Solution-coating Yong Joo Cho, and Hany Aziz ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00926 • Publication Date (Web): 07 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Applied Materials & Interfaces

The Root Causes of the Limited Electroluminescence Stability of Organic Lightemitting Devices Made by Solution-coating Yong Joo Cho* and Hany Aziz Department of Electrical & Computer Engineering, and Waterloo Institute for Nanotechnology (WIN), University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2G 3G1, Canada E-mail: [email protected] Keywords: solution-coating, stability, degradation mechanisms, exciton-induced degradation, UV irradiation

Abstract

Although organic electroluminescent materials have long promised the prospect of making Organic Light Emitting Devices (OLEDs) via low-cost solution-coating techniques, the electroluminescence stability of devices made by such techniques continues to be rather limited making them unsuitable for commercialization. The root causes of the lower stability of OLEDs made by solution-coating versus the more conventional vacuum-deposition remain unknown. In this work, we investigate and compare between solution-coated and vacuum-deposited materials under prolonged excitation, using the archetypical host material 4,4 ′ -bis(N-carbazolyl)-1,1 ′ biphenyl (CBP) as a model OLED material. Results show that solution-coated films are more susceptible to degradation by excitons in comparison to their vacuum-deposited counterparts, resulting in a faster decrease in their luminescent quantum yield. The degradation rate also 1 ACS Paragon Plus Environment

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depends on the choice of solvent that was used in the solution-coating process. Results also show that the decrease in quantum yield is caused by exciton-induced chemical decomposition in the material as well as some possible molecular reorganization or aggregation, both of which are induced by excitons and proceed more quickly in case of solution-coated films. The faster degradation in the solution-coated films appears to originate primarily from their different morphological makeup and not due to chemical impurities. The findings uncover what appears to be one of the fundamental root causes of the lower stability of solution-coated OLEDs in general.

Introduction Thirty years after their first demonstration in the lab,1 Organic Light Emitting Devices (OLEDs) are currently establishing their place in the mobile displays market and expanding their presence to other areas of applications such as TVs, instrument panels, wearable electronics, automotive applications and solid state lighting.2–5 Currently, OLEDs in commercial products are almost exclusively fabricated by vacuum deposition techniques. However, the prospect of fabricating OLEDs via solution-coating techniques has always attracted significant interest because of its potential cost saving advantage over vacuum-deposition techniques especially for large-area products.6–9 Towards this end, the last few years have witnessed a significant progress in developing high quantum efficiency solution-processed devices utilizing small molecule materials, and impressive efficiencies have been reported.10–15 Despite this progress, the electroluminescence stability of solution-processed OLEDs continues to be significantly lower relative to that of their vacuum-deposited counterparts.16–20 The lower stability continues to be an obstacle that hampers the commercialization of solution-based OLED technology. The root causes of their lower stability remain unclear.16,17,19 2 ACS Paragon Plus Environment

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In general, the deterioration in the electroluminescence of OLEDs under prolonged electrical bias can attributed to a number of degradation mechanisms. For example, considering that the exciton energy in OLED materials in on the same order of the bond dissociation energy of organic molecules, bond cleavage can occur under exciton stress, especially by singlet excitons. This could lead to chemical decomposition of the materials and thus the decrease in their quantum yield over time.13,21–24 Excitons can also be quenched by collisions with polarons that accumulate in the various OLED layers over time, adversely affecting their quantum efficiency.25–28 Recently, we found that interactions between excitons and polarons can induce aggregation in a wide range of OLED host materials; a process referred to as exciton-polaron induced aggregation (EPIA).

29–32

Interestingly, such process was also found to occur more

readily in solution-coated OLEDs suggesting it may play a role in their lower stability.16,17 The reasons behind the stronger susceptibility of solution-coated materials to this behavior remain unclear.

In this work, we investigate and compare between solution-coated and vacuum-deposited OLED materials under prolonged excitation using UV irradiation. The UV irradiation is used as a tool to create singlet excitons like those normally created in OLEDs under electrical bias. This provides an opportunity to examine the effect of exciton stress, without any confounding effects from current flow. The study is conducted on the archetypical host material 4,4 ′ -bis(N-

carbazolyl)-1,1′-biphenyl (CBP). CBP is selected not only because it is one of the most widely used OLED host materials but also because it can serve as a model compound for carbazoles in general; the class of materials commonly used for host materials in the emitter layers of 3 ACS Paragon Plus Environment


phosphorescent OLEDs. Optical absorption measurements, photoluminescence (PL) spectral measurements, Time-resolved PL measurements (TRPL) and High Performance Liquid Chromatography (HPLC) measurements are used to study and compare between the two material systems and the effect of UV irradiation (i.e. exciton stress) on them. Results show that irradiation brings about larger PL spectral shifts and a faster deterioration in the PL quantum yield (PLQY) of solution-coated films revealing that they are more susceptible to degradation by excitons in comparison to vacuum-deposited films. Results also show that the PL characteristics of solution-coated films depended on the solvent used, showing that film morphology not only influences device efficiency33 but also the photophysical properties of these materials and their susceptibility to degradation by excitons. The results provide new and important insights on the root causes of the lower EL stability of solution-coated OLEDs.

Experimental Section Materials: CBP and TPBi were purchased from Electronic Materials Index Co. All materials have purity above 99% and were thus used without further purification. Device Fabrication: All devices were fabricated on pre-patterned ITO substrates obtained from Kintec. Solution-processed layers: The organic materials were first dissolved in MC, chloroform or toluene to from 5.0, 5.0 or 1.0 mg mL−1 solution, respectively, and then were coated using a WS-400/500 series spinner from Laurell Technologies Corp. at a spin rate of 500/2000 rpm during 5/25 s.17 The coated films were then baked at 60 °C on a hot plate in a glove box. Vacuum-deposited films: All layers were deposited using an Angstrom Engineering EvoVac system. All materials were evaporated at a rate of 0.1–1.0 Å s−1 at a base pressure of below 5 × 10−6 Torr. 4 ACS Paragon Plus Environment

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Device Measurement: Photoluminescent spectra were collected under monochromatic illumination from a 200 W Hg lamp equipped with an Oriel-77720 monochromator. All devices were kept and measured in a nitrogen atmosphere at all times. Fluorescence exciton lifetime data were collected by an Edinburgh Instruments FL920 spectrometer. High performance liquid chromatography (HPLC) was performed using a C-18 reverse phase column (Particle size: 5 μm, inner diameter: 4.6 mm and length: 250 mm) of Shiseido CAPCELL PAK. The HPLC was a multiple set up system.17 The detector was a Waters 486 tunable absorbance detector. The controller was a Waters 600 controller.17

Results and discussion First, changes in the PL characteristics of solution-coated and vacuum-deposited CBP films under UV irradiation were investigated and compared. In order to study the films in environments that closely simulate the conditions in OLEDs, the films were made as part of bilayer device structures of the following structure: ITO/MoO3 (5 nm)/ CBP made by solution-coating (Sol) or vacuum-deposition (Vac), (30 nm)/TPBi (45 nm)/LiF (1 nm)/Al (80 nm). In one set of devices (i.e. Vac), all layers were made by standard thermal deposition in vacuum. In the other set (i.e. Sol), the CBP layer was made by spin coating in a glove box with a dry nitrogen atmosphere utilizing CBP solutions with solvents of different boiling points; dichloromethane (MC, 40 °C), chloroform (61 °C) and toluene (111 °C). Because differences in solvent boiling points have different vapor pressure that lead to differences in film thickness34, the CBP concentration in the solutions was adjusted in the various solutions so as to obtain a final film thickness of ~30 nm in each case. Aside from the CBP layer, the other layers were fabricated via vacuum deposition. After fabrication, the test devices were irradiated by UV light (365 nm, and an irradiation power 5 ACS Paragon Plus Environment


density of 2 mW/cm2) for 72 hours while being kept in a dry nitrogen atmosphere. This wavelength was selected because it can excite CBP without significantly exciting TPBi (due to the limited absorption of TPBi at this wavelength.35 An identical set of devices, fabricated on the same substrates as the UV irradiated ones, was shielded from the irradiation via black masking tape and used as an “un-irradiated” reference. Figure 1 shows the PL spectra collected from the test devices before and after the irradiation. Spectra collected from the non-irradiated control test devices after the same period of time (i.e. 72 hours) are also shown in each case. As can be seen, the PL spectra corresponded to the familiar CBP spectrum with peak at ~400 nm which can be ascribed to the S1 S0 singlet transition in CBP, indicating that the PL originated almost exclusively from the CBP layers and verifying that PL from TPBi was insignificant. Notably, aside from the 400 nm band which was present in all spectra, herein referred to as the unimolecular band (or CBP monomer band), the spectra of the irradiated samples showed significant broadening at longer wavelengths. The spectral broadening was generally more significant in case of the solution-coated samples and its magnitude varied among the samples depending on the solution that was used for fabricating the layer. In this regard, the films made from the chloroform solution exhibited the largest shift. The net spectral change due to the irradiation, obtained by mathematically subtracting the initial (i.e. before UV irradiation) spectrum from the final (i.e. after UV irradiation spectrum), corresponded to the emergence of a new broad emission band with peak in the 475-500 nm range. Those traces are shown in the figure insets. The 475-500 nm bands closely resembled those produced by exciton-polaron interactions in OLEDs under electrical stress and ascribed to CBP aggregation.16,29,36,37 Such bands were found to be suppressed when a second material is mixed with CBP verifying that they arise from CBP-CBP interactions.16 The fact that this band did not appear in the spectra of the non-irradiated controls indicated that it was caused exclusively by the irradiation and suggested 6 ACS Paragon Plus Environment

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that excitons could induce aggregation in organic materials even when polarons are absent. The fact that the intensity of this band varied among the devices suggested that the rate of such exciton-induced changes varied among them, and was fastest in case of the solution-coated film made using the chloroform solution. The results reveal that the choice of solvent has a significant influence on the stability of the solution-coated materials under exciton stress. We also collected spectra at 5 minute intervals during the first 24 hours of the UV irradiation to monitor changes in the PL intensity over time. Figure 2(a) shows the PL intensity at 400 nm (the CBP monomer band peak) over that period of time normalized to the initial value. Clearly, the PL intensity did not change appreciably after 24 hours of irradiation in case of the vacuum-deposited film. In contrast, there was a marked decrease in PL of all the solution-coated films over the same period of time. The chloroform film once again exhibited the fastest decrease. Considering that the excitation power was kept constant, such decrease in PL intensity reflected a decrease in the PLQY of the materials. We also similarly compared changes in PL intensity under continuous UV irradiation in case of tris[2-phenylpyridinato-C2,N]iridium(III) (Ir(ppy)3)–doped CBP films (5% doping) made by solution-coating versus vacuum-deposited. The solution-coated film was spin cast from a toluene solution. Figure 2(b) shows the PL intensity at 512 nm (the Ir(ppy)3 band peak) versus irradiation time normalized to the initial value. As with the non-doped CBP films, the PL intensity of the solution-coated doped film is found to decrease much faster relative to its vacuum-deposited counterpart. The results therefore show that prolonged excitation (i.e. exciton stress) causes the quantum yield of a material to degrade faster when the material is solutioncoated than when it is vacuum-deposited. The results are consistent with recent observations from other materials and attributed the phenomenon to differences in molecular packing and lower film density.38,39 Interestingly, our results show that the exciton-induced degradation rate also depends on the solvent that was used in fabricating the solution-coated film. Although on first sight one 7 ACS Paragon Plus Environment


might attribute the effect to possible presence of residual solvent, the fact that the film made from the chloroform solution exhibited the fastest degradation among the three solution-coated samples despite the fact that chloroform does not have the highest boiling point among the three precludes this possibility. We should also note that all the solution-coated films were cured at 60oC after the spin coating step and were also subsequently placed in a high vacuum environment for several hours in the process of fabricating the other layers. This makes the possibility that any residual solvent might be present highly unlikely. In the absence of residual solvents, the root cause behind the different degradation rates among the solution-coated films might be due to differences in molecular packing and film morphologies among the films due to the different solvents.

We also tested the optical absorption and TRPL characteristics of CBP films made by solutioncoating and vacuum-deposition. For this test we used CBP films coated on quartz substrates and subjected them to UV irradiation under the same conditions as above. The use of single films instead of device stacks as was done above was pursued for the TRPL measurements in order to avoid any modifications to the PL decay rates due to micro cavity effects.40 Figure 3(a) shows the UV-Vis absorption spectra collected from ~30 nm thick vacuum-deposited and solutioncoated (with toluene as the solvent) CBP films before and after UV irradiation for 72 hours. As can be seen, all spectra had essentially the same characteristics. The slightly lower absorption in the solution-coated films could be attributed to a slightly lower density relative to the vacuum deposited films and/or small deviations in thickness. Clearly, the optical density did not change to any appreciable extent as a result of the UV irradiation in both material systems indicating that photo-bleaching was insignificant. This observation confirms that the decrease in the PL intensity after the irradiation observed in Figure 2 was indeed the result of a decrease in the PLQY of the 8 ACS Paragon Plus Environment

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materials and not due to a decrease in their optical density. Figure 3(b) shows the TRPL characteristics at 400 nm of the same films collected under pulsed excitation at 380 nm. The PL decay rate was faster in case of the pristine (before irradiation) solution-coated film relative to its vacuum-deposited counterpart, indicating that the exciton lifetime was shorter. The shorter exciton lifetime in case of solution-coated films was consistent with our previous observations, and can be attributed to morphological differences between the two systems.16 The PL decay rate became faster after the UV irradiation in both material systems, pointing to a decrease in exciton lifetime in both cases. The decrease was however much more significant in case of the solutioncoated film and was consistent with the larger deterioration in quantum yield due to the irradiation that was observed in figure 2.41 The more significant decrease in exciton lifetime was in line with the faster CBP aggregation as a result of the UV irradiation observed in figure 1. Increased aggregation

would lead to increased exciton quenching due the increased

intermolecular interactions, and hence the decrease in the quantum yield.42,43 The decrease in exciton lifetime could also be a result of the formation of exciton quenchers as a result of possible photochemical decomposition under the effect of the irradiation. This last point will be addressed later in the manuscript.

The results in Figure 1 suggested that the solution-coated films were more susceptible to exciton-induced aggregation (relative to the vacuum-deposited one) and that the aggregation rate also depended on the solvent that was used in the spin-coating solutions. One can therefore expect the solution-coated films to also have a lower temporal morphological stability. We therefore fabricated another set of films on glass substrates. The films were then stored at room temperature in a glove box, and were examined periodically for signs of aggregation or crystallization using an optical microscope. The results are shown in Figure 4. The vacuum9 ACS Paragon Plus Environment


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deposited film showed no detectable changes in morphology after two days of fabrication, whereas the solution-coated films started to exhibit clear morphological changes, and in some cases even crystallites, after only one day. Such morphological changes must obviously occur via a mass transfer process that is driven by a thermodynamic driving force to attain (the more energetically favourable) increased molecular ordering and aggregation.

That such

morphological changes were more significant in case of the solution-coated films verifies that solution-coating produces films with morphologies that facilitate molecular aggregation and crystallization; i.e. morphologies that contain more nucleation and/or allow faster growth of crystalline domains (the latter perhaps due to the presence of more intermolecular voids in case of solution-coated films.38,44 Also, quite notably, the morphological changes took on different patterns or features in the various solution-coated films depending on the solvent that was used in the spin-coating solution. This was verified by Atomic Force Microscopy (AFM) measurements that showed that the morphology of the solution-coated films varied depending on the solvent type that was used. The results are shown in Figure 5. The vacuum-deposited film and the solution-coated film using toluene showed similar surface morphology, whereas the solutionprocessed films using MC and chloroform showed pin-holes over the entire film surface. These results convincingly show that the film fabrication process leaves a morphological “disposition” in the films that influences their susceptibility to morphological changes and aggregation later. The results therefore fully support the conclusion that not only are solution-coated films more susceptible to exciton-induced aggregation but also that this phenomenon depends on the solvent that was used in their fabrication.

As was noted earlier, CBP exciton lifetime was found to be shorter in case of solution-coated films than in vacuum-deposited films. Seeing that the type of solvent also influenced the 10 ACS Paragon Plus Environment

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morphological stability and exciton-induced degradation rate in the solution-coated films, we tested the PL decay rates in the different material systems using TRPL measurements. The results are shown in Figure 6(a). As can be seen in the figure, exciton lifetimes of all the solutionprocessed films were shorter than that of the vacuum-deposited film, and varied depending on the solvent type. Although one cannot rule out the possibility that the variations in PL decay rates might be due to the presence of trace amounts of quenching contaminants from the solvents, such variations may simply be due to the morphological differences among the films as a result of the different film forming and drying kinetics. Variations in the degree of short term molecular ordering or crystallinity among the films would expectedly lead to variations in the degree of intermolecular interactions and hence also in the efficiency of the pathways by which excitons can be dissipated, and therefore cause the PL decay rates to vary. In this regard, it is notable that the film made using chloroform, which showed the highest susceptibility to both crystallization and exciton-induced aggregation, also exhibited the fastest PL decay rate, the latter suggesting a film morphology that may contain more domains that have a higher degree of short term molecular ordering or aggregation. Such domains could then serve as nucleation or “seeding” sites for further aggregation and faster crystal growth to occur over time or under exciton stress. In contrast, and following the same principle, the MC film, which exhibited the slowest exciton induced aggregation among the solution-coated films, also had the slowest PL decay rate suggesting that it had a relatively more amorphous morphology. With a more disordered molecular packing, it is not surprising that crystallization and exciton-induced aggregation was slower in this case. It would follow, based on the same argument and the fact that PL decay rates in all solution-coated films tend to be shorter than in vacuum-deposited films, that solutioncoating in general produces morphologies that allow for stronger intermolecular interactions (i.e. crystalline domains or aggregates).

This may perhaps be due to the fact that in vacuum11

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deposition, due to the rapid condensation kinetics and high sticking coefficients, molecules tend to “freeze” on the substrates almost instantaneously upon arrival and therefore have a relatively limited opportunity for surface migration. In contrast, because in solution-coating the solvent drying does not happen instantaneously, the molecules are afforded more mobility to move and form aggregates before the freezing is completed. This argument is in agreement with findings by Shibata et al. that the morphological state produced by vacuum deposition is only a metastable one from a thermodynamic standpoint, and that thermal annealing would transform it into (the more thermodynamically stable) “solution-coated-like” morphology.44

In order to validate this hypothesis, we studied the effect of exposing vacuum-deposited films to solvent vapours – a technique often referred to as “solvent annealing” – on the PL decay rates. Exposure to solvent vapour would provide the molecules in the films increased mobility and thus an ability to aggregate.45 One would therefore expect to see an increase in PL decay rate (i.e. the CBP exciton lifetime to become shorter) after the solvent annealing. For this test, toluene was poured into a container, and then the CBP film substrate was attached to the lid so as to be exposed to the toluene vapours, and the container was sealed for 2 min. The substrate was then removed and baked at 60 °C for 5 min to remove any residual solvents from the film. Results from TRPL measurements showing the PL decay rates before and after the solvent annealing, denoted by (Vac, as fabricated) and (Vac, after solvent annealing), respectively, are shown in figure 6(b). As expected, the PL decay rate became faster after the solvent annealing, suggesting that some aggregation occurred in the films. We note that optical microscope examination revealed no noticeable morphological changes at this point. We therefore also investigated the effect of subjecting the films to an additional thermal annealing step (at 120 °C for 5 min) until the films showed visible crystallinity. The PL decay characteristics, denoted to on the figure by 12 ACS Paragon Plus Environment

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(Vac, after thermal annealing) showed an additional (but smaller) increase in PL decay rate after the thermal annealing, consistent with the further increase in aggregation and crystallinity. The figure also shows TRPL characteristics from solution-coated CBP, with toluene as solvent, before and after thermal annealing at 120 °C for 5 min, denoted to on the figure by (Sol, as fabricated) and (Sol, after thermal annealing) respectively. (note that in this case the “as-fabricated” film was still subjected to a baking step as detailed in the Experimental section). Quite remarkably the PL decay rate in the vacuum deposited film after the solvent annealing step was very similar to that of the as fabricated solution-coated film. Also, quite remarkably, the PL decay rates in both films were very similar after the thermal annealing. These results convincingly show not only that the PL decay rates mirror morphological changes in the films and become faster as molecular aggregation increases in the films, but also that solution-coated films, in the as-fabricated condition, have more aggregates (are less amorphous) than their vacuum–deposited counterparts.

It has been shown that films produced by vacuum-deposition generally have higher density in comparison to those produced by solution-coating.38,44 Shibata et al. also showed that vacuumdeposition gives more ordered morphologies.44 We should point out that our findings here that solution-coated films exhibit stronger intermolecular interactions and signs of increased aggregation relative to vacuum-deposited ones does not contradict with those previous findings. It is entirely possible, and indeed expected, that solution-coated films will have less homogenous morphologies, that include, within the same film, areas or domains that have higher molecular packing and crystallinity (molecular aggregates) and other areas or domains that have lower molecular packing and larger intermolecular distances and/or voids. Compared to the more homogenous morphology of vacuum deposited films, the inhomogeneity and larger intermolecular voids in the solution-coated films would make their overall density lower despite 13 ACS Paragon Plus Environment


the presence of denser domains of molecular aggregates. The cartoons in Figure 7 show graphical illustrations of such morphologies.

As noted earlier, the PLQY of CBP decreased significantly during the UV irradiation in case of the solution-coated films (Figure 2). It has been reported that UV irradiation of CBP can lead to cleavage of the C-N bond between the centered phenyl and carbazole groups, producing biphenyl carbazole (BPC).23,24 In order to determine if the decrease in PLQY was primarily due to the aggregation in the films or if chemical degradation also contributed to this effect, we used HPLC measurements to test for chemical degradation by products in the irradiated test samples. An identical set of test samples, that was kept in a UV-free glove box, was also tested, to be used as reference. The metal cathodes of the test samples were removed via scotch tape. The organic films were then dissolved in acetonitrile and analyzed by HPLC. Figure 8(a) displays the obtained HPLC chromatograms. As can be seen from the insets, a new band was observed at 8.6 min for the UV-irradiated samples regardless of the film fabrication process. Since these test samples had a layer of TPBi in addition to the CBP layer, and to verify if the new bands originated exclusively from the CBP layers, we also repeated the experiment using samples that did not have the TPBi layers but otherwise the same structure. The results are shown in Figure 8(b). Clearly, new peaks, again at 8.6 min, appeared in the UV-irradiated samples, proving that they indeed originated from the CBP and were caused by the UV irradiation. These peaks resemble those ascribed to BPC in previous reports.23,24 Interestingly, the fraction of the area under this new band in case of the vacuum-deposited film was 0.47% (of the total area under the chromatogram) whereas that in case of the solution-coated film was almost 1.8x larger (0.86%). This result means that the solution-coated film had more UV-induced chemical by-products formed under the same irradiation conditions, indicating that chemical decomposition was faster. 14 ACS Paragon Plus Environment

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It should also be pointed out that the chromatograms from the solution-coated films showed no additional bands. This proves that the lower stability of these films was not due to any new (additional) chemical reactions or decomposition routes that occur in solution-coated films. This suggests that the faster chemical decomposition of CBP by the UV irradiation in the solutioncoated films may have its origins in the different morphological make-up of these systems which makes the molecules less chemically stable relative to those in their vacuum-deposited counterparts (at least when in an excited state).

The notion that the chemical stability of

molecules may be influenced by film morphology is not surprising. For example, C-N bond dissociation energy has been found to depend on the carbazole to centered phenyl distortion angle and therefore on CBP geometry.23,46 Differences in molecular geometries as a result of differences in molecular packing can therefore be expected to affect their chemical stability. A recent study on conjugated molecules has indeed shown that bond twisting in a conjugated polymer can reduce its photostability.38

In order to verify if the faster chemical decomposition of CBP by the UV irradiation in case of the solution-coated film is primarily due to morphological factors, we test the effect of solvent annealing by various solvents on the UV-induced chemical decomposition behaviour of vacuumdeposited films. Vacuum-deposited films were exposed to vapours of MC, chloroform, methanol or toluene. The exposed films were then baked at 60 °C for 5 min to remove any residual solvents. A vacuum-deposited film that was not exposed to solvent was also included in the study to be used as reference. To ensure that any differences between the samples were exclusively the result of the solvent-exposure and not due to the baking step, the non-exposed reference film was similarly baked. The films were then subjected to UV irradiation (again for 72 hours) in an inert atmosphere. Figure 9 displays the HPLC chromatograms collected from these samples after the 15 ACS Paragon Plus Environment


UV irradiation. Clearly, the height of the BPC band varied among the samples depending on the type of solvent used in the solvent annealing step, and was lowest in case of the sample without the solvent annealing. The BPC band integral area was calculated and found to be 0.31% for the chloroform annealed film, 0.22% for the methanol annealed film, 0.18% for the toluene-annealed film, 0.13% for the MC annealed film and 0.09% for the untreated film, showing that the extent of chemical decomposition due to the UV irradiation indeed varied among the films, despite the fact that they were all made by vacuum-deposition, only because they were exposed to different solvents which caused their morphologies to be altered variably. The results clearly show that variations in morphology can indeed lead to large differences in the chemical stability of CBP, proving that the faster chemical decomposition by UV irradiation in case of solution-coated films versus vacuum deposited films has its origins in the differences in the morphological makeup of the films fabricated via these techniques. The changes in the stability with the type of solvent may also be due to the formation of different polymorphs. It has been reported recently that CBP can have three different polymorphs, with varying distortion angle between centered phenyl and carbazole, depending on the solvent type.47 Given the differences in distortion angle, one can expect the polymorphs to have different chemical stability.

Conclusion In conclusion, studies on CBP show that solution-coated films are more susceptible to degradation by excitons in comparison to their vacuum-deposited counterparts, resulting in a faster decrease in their luminescent quantum yield. Results also show that the decrease in quantum yield is caused by exciton-induced chemical decomposition in the material as well as some possible molecular reorganization or aggregation, both of which are induced by excitons and proceed more quickly in case of solution-coated films. The degradation rate is also found to 16 ACS Paragon Plus Environment

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depend on the choice of solvent that was used in the solution-coating process. The faster degradation in the solution-coated films appears to originate primarily from their different morphological makeup and not due to chemical impurities. The findings uncover what appears to be one of the fundamental root causes of the lower stability of solution-coated OLEDs in general, and reveal the critical influence of both film morphology and the fabrication process on the stability of organic electroluminescent materials.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y. C.). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by Natural Science and Engineering Research Council of Canada (NSERC) Discovery Grant (RGPIN-2014-04940)

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Figure 1. Normalized PL spectra collected from the solution-coated with (a) MC, (b) Chloroform, (c) Toluene and (d) vacuum-deposited test samples before and after the UV irradiation. Insets: The net change in the spectra, obtained by subtracting the “before UV irradiation” spectrum from the “after UV irradiation” spectrum in each case.



Figure 2. Normalized PL intensity versus the UV irradiation of (a) un-doped CBP films and (b) Ir(ppy)3-doped CBP films made by vacuum-deposition or solution-coating using various solvents. The solvent that was used in fabricating the solution-coated films is shown between the parentheses.


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Figure 3. (a) UV absorption spectra, and (b) TRPL characteristics collected from vacuumdeposited or solution-coated CBP films on quartz substrates (the later using toluene solution) before and after UV irradiation.



Figure 4. Optical microscope images of CBP films taken at different periods of time after fabrication. The films are fabricated by vacuum-deposition or solution-coating using the shown solvent.


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Figure 5. AFM images of CBP films fabricated by vacuum-deposition or solution-coating using the shown solvents.



Figure 6. TRPL characteristics of (a) vacuum-deposited and solution-coated CBP films, the latter coated using MC, chloroform or toluene solvents, and (b) vacuum-deposited and solution-coated CBP films, the latter coated using a toluene solvent, collected from the as-fabricated films and then again after the solvent annealing and/or thermal annealing as explained the text.


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Figure 7. The anticipation diagram of CBP molecules in the vacuum-processed versus the solution-processed films.



Figure 8. HPLC chromatograms obtained from material collected from UV-irradiated and nonirradiated samples of (a) CPB/TPBi bilayer stacks, and (b) CBP single layer films. The CBP films are made by vacuum-deposition or solution-coating, the latter using a toluene solution. The chromatograms are normalized to the same height at the maximum intensity. Inset: A magnified view of the 8-9.5 min range of the chromatograms.


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Figure 9. HPLC chromatograms obtained from material collected from UV-irradiated CBP films. The films were made by vacuum-deposition and then exposed to vapours of various solvents prior to the UV irradiation. Data from a film that was not exposed to solvents, denoted to by “untreated” is also included. The chromatograms are normalized to the same height. Inset: A magnified view of the 8-9.5 min range of the chromatograms.



Toc graphic


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Root Causes of the Limited Electroluminescence Stability of Organic

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