Dependence of Organic Interlayer Diffusion on Glass-Transition

Research Article www.acsami.org

Dependence of Organic Interlayer Diffusion on Glass-Transition Temperature in OLEDs Jake A. McEwan,† Andrew J. Clulow,† Andrew Nelson,‡ Nageshwar Rao Yepuri,§ Paul L. Burn,*,† and Ian R. Gentle*,† †

Centre for Organic Photonics & Electronics, School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia ‡ Australian Centre for Neutron Scattering and §National Deuteration Facility, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia S Supporting Information *

ABSTRACT: Organic light-emitting diodes (OLEDs) are subject to thermal stress from Joule heating and the external environment. In this work, neutron reflectometry (NR) was used to probe the effect of heat on the morphology of thin three-layer organic films comprising materials typically found in OLEDs. It was found that layers within the films began to mix when heated to approximately 20 °C above the glass-transition temperature (Tg) of the material with the lowest Tg. Diffusion occurred when the material with the lowest Tg formed a supercooled liquid, with the rates of interdiffusion of the materials depending on the relative Tg’s. If the supercooled liquid formed at a temperature significantly lower than the Tg of the higher-Tg material in the adjacent layer, then pseudo-Fickian diffusion occurred. If the two Tg’s were similar, then the two materials can interdiffuse at similar rates. The type and extent of diffusion observed can provide insight into and a partial explanation for the “burn in” often observed for OLEDs. Photoluminescence measurements performed simultaneously with the NR measurements showed that interdiffusion of the materials from the different layers had a strong effect on the emission of the film, with quenching generally observed. These results emphasize the importance of using thermally stable materials in OLED devices to avoid film morphology changes. KEYWORDS: organic light-emitting diodes, diffusion, intermixing, photoluminescence, neutron reflectometry

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INTRODUCTION Organic light-emitting diodes (OLEDs) are already used in commercial displays and are considered important candidates for next-generation lighting technology.1 OLEDs are lightweight, have a high power efficiency, vivid color output, and can be flexible.2−4 The most efficient OLEDs generally have complex structures, comprising multiple thin layers of organic materials deposited between the cathode and anode. The organic component of the device is normally composed of a central emissive layer, which itself comprises a minimum of two materials (a light-emitting guest in a host), and hole- and electron-transporting and injecting layers on either side of the emissive layer. The aim of the different layers is to balance charge injection and transport such that the charges are funneled into the emissive layer to maximize recombination and generate light through radiative exciton decay. Through careful design of the materials and device architectures, OLEDs that exhibit external quantum efficiencies >30% have been fabricated.5−10 However, with any complex device architecture there are clear questions that need to be addressed, such as whether the manufacturing process truly gives the expected architecture and/or what happens to the components within the layers over time. For example, the deposition process could © 2017 American Chemical Society

lead to diffuse layer boundaries and not sharp interfaces as generally depicted. Furthermore, thermal cycling of the device over time could lead to the intermixing of the different components and layers. Either of these factors could lead to significant changes in the desired OLED performance. The effect of thermal stress on device performance has not been widely investigated or well understood,11,12 with some reports indicating that exposure to temperatures above ambient can lead to a significant drop in OLED performance,11 whereas in other cases, an improvement in device characteristics is observed.12 Thermal stress can arise from Joule heating during device operation13 or the external environment. Joule heating is also a significant problem during lifetime testing, which is performed on devices under accelerated conditions. In these tests, the devices are run at a high brightness, which would maximize Joule heating and thermal stress.14 As such, the accelerated testing process could lead to the OLEDs appearing to have shorter lifetimes than they might actually possess during normal operation. It is logical to consider that Received: January 30, 2017 Accepted: April 3, 2017 Published: April 13, 2017 14153

DOI: 10.1021/acsami.7b01450 ACS Appl. Mater. Interfaces 2017, 9, 14153−14161

Research Article

ACS Applied Materials & Interfaces Table 1. Film Combinations Studied, with Glass-Transition Temperaturesa sample number

bottom layer

middle layer

upper layer

1 2 3

d-Alq3 (Tg = 175 °C)32b d-TPBi (Tg = 125 °C)31 d-Alq3

TCTA:Ir(ppy)3 (Tg = 152 °C for TCTA)31 TCTA:Ir(ppy)3 TCTA:Ir(ppy)3

d-NPB (Tg = 102 °C)31 d-NAD (Tg = 206 °C)19b d-NAD

a Note: The bottom layer is the layer closest to the Si/SiO2 interface and the upper layer is that nearest to the interface with air. bPrevious measurements on deuterated materials have shown the Tg’s to be of a similar magnitude as those of the protonated materials and hence the reported values for the protonated materials are used in the analysis of the data.

Figure 1. Chemical structures of the materials used.

the layers the higher-Tg material diffused or dissolved into the lower-Tg layer faster than the reverse process.29−31 Given the developing evidence of a correlation between Tg and thermally induced changes in film morphology, in this study, we report the effect of thermal annealing on the structure and photophysical properties of three-layer stacks comprising mostly high-Tg materials. The electron-transport materials studied are deuterated versions of 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (d-TPBi) and tris(8-hydroxyquinolinato) aluminum (d-Alq3), with the emissive layer consisting of 6 wt % fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)3] doped into the high-Tg host, tris(4-carbazol-9-ylphenyl)amine (TCTA), both with natural isotopic abundance. Finally, the hole-transport layers were either deuterated N,N′-bis(naphthalen-1-yl)-N,N′diphenylbenzidine (d-NPB) or N,N′-bis(anthracen-9-yl)-N,N′bis(napthalen-1-yl)benzidine (d-NAD). The use of deuterated electron- and hole-transport layers sandwiching the protonated emissive layer provides the necessary contrast for the NR experiments to enable the structural changes in each of the layers to be elucidated.

degradation of OLED performance under thermal stress could be related to the glass-transition temperatures (Tg’s) of the materials in the active layers,15,16 and consequently, there have been substantial efforts to develop high-Tg compounds.17−19 There are now a number of reports using different techniques that indicate that the drop in device performance is morphologically driven; that is, that the layered structure is altered during heating.20−25 Changes in the layered structure of the device can lead to an imbalance in the injection and transport of charges, resulting in less exciton formation and radiative decay. We have previously made use of neutron reflectometry (NR) with in situ photoluminescence (PL) measurements to probe the organic−organic interfaces and layers in OLED stacks. NR is an ideal technique for probing changes in film structure as it is nondestructive.26,27 In addition, using selective deuteration, it is possible to obtain excellent contrast between different materials and organic layers, which enables better resolution of the interfaces within the samples and of the changes that occur.28 The early NR studies indicated that there was an apparent correlation between the diffusion onset temperature and the Tg of the materials present in the sample, with intermixing occurring after the temperature rose above that of the material with the lowest Tg in the film. Furthermore, it appeared that once the temperature was above the Tg of one of

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RESULTS AND DISCUSSION The film combinations studied and Tg’s of the materials are summarized in Table 1, with the chemical structures of the materials shown in Figure 1 (the syntheses of the new 14154


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ACS Applied Materials & Interfaces Table 2. Initial Film Structures at 40 °C for Samples 1−3 sample number

layer position

layer material

1

bottom middle top bottom middle top bottom middle top

d-Alq3 TCTA:Ir(ppy)3 d-NPB d-TPBi TCTA:Ir(ppy)3 d-NAD d-Alq3 TCTA:Ir(ppy)3 d-NAD

2

3

thickness (Å) 266 351 242 284 335 258 275 353 221

± ± ± ± ± ± ± ± ±

1 1 1 1 1 1 1 1 1

SLD (10−6 Å−2) 5.95 2.56 5.01 5.32 2.55 5.29 5.86 2.55 5.46

± ± ± ± ± ± ± ± ±

0.01 0.01 0.02 0.02 0.01 0.02 0.02 0.01 0.03

roughness (Å) 5 11 14 5 11 13 4 13 14

± ± ± ± ± ± ± ± ±

4 2 1 4 3 1 4 1 1

Figure 2. (a) Reflectivity profiles and (b) SLD vs thickness plots for sample 1 on thermal annealing. Arrows indicate the direction of the fasterdiffusing material at different temperatures.

Figure 3. (a) Reflectivity profiles and (b) SLD vs thickness plots for sample 2 under thermal stress. Arrows indicate the direction of material diffusion. In this case, the position of the interface does not move significantly, although it broadens at higher temperatures, and d-TPBi and TCTA:Ir(ppy)3 diffuse in opposite directions at similar rates.

should first diffuse into the hole-transport layer, whereas for sample 2, it should dissolve into the electron-transport layer. Finally, in the case of sample 3, wherein the light-emitting layer has the lowest Tg, both the electron- and hole-transporting layers should diffuse into the middle emissive layer. The layers in the stack were deposited by thermal evaporation under vacuum, with the individual layer thicknesses, scattering length densities (SLDs), and roughnesses

deuterated materials are described in the Supporting Information S1 and S2 and Figures S1−S24). Three film combinations were chosen such that the lowest Tg material in the stack was placed at the top (sample 1), at the bottom (sample 2), or in the middle (sample 3) of the film. If the earlier reports of high-Tg materials diffusing into layers containing materials of lower-Tg on heating are generally applicable, then in the case of sample 1 the light-emitting layer 14155


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ACS Applied Materials & Interfaces

Figure 4. (a) Reflectivity profiles of and (b) SLD vs thickness plots for sample 3 under thermal stress. Arrows indicate the direction in which materials in each of the layers diffuse.

iformity of the film can be seen in the final reflectivity profile (150−154 °C), which could not be fitted because of significantly weakened fringes with increased background intensity. It is postulated that the roughening of the film is a result of crystallization of d-TPBi:TCTA:Ir(ppy)3 underneath the d-NAD layer. Finally, sample 3, which had the lowest Tg material (152 °C) as the middle emissive layer, was stable up to 176 °C, maintaining its three-layered structure (see Figure 4). Above 176 °C, the morphology of the film was found to rapidly change. In the case of the d-Alq3 and central TCTA:Ir(ppy)3 layers, the interface first became more diffuse and then moved into the d-Alq3 layer (Figure 4b) reminiscent of the TCTA:Ir(ppy)3 diffusing into the d-NPB. However, whereas the model shows the expected increase in the SLD of the TCTA:Ir(ppy)3 layer due to the presence of d-Alq3 (note at this stage d-NAD has also diffused into the emissive layer, increasing the SLD of the latter), the SLD of d-Alq3 is also seen to decrease in advance of the front, indicating that TCTA:Ir(ppy)3 also diffuses in the opposite direction in a manner akin to that in sample 2. In contrast, the interface between the dNAD layer and TCTA:Ir(ppy)3 layers moves into the d-NAD layer, with the SLD of the d-NAD layer not changing significantly. The movement of the interface in this case is consistent with the quicker diffusion of the d-NAD layer into the TCTA:Ir(ppy)3 layer than vice versa. The fact that the changes occurred more rapidly at the d-Alq3/TCTA:Ir(ppy)3 interface is consistent with the d-Alq3 layer having a lower Tg than that of the d-NAD layer (175 vs 206 °C). In summary, in all cases, diffusion was observed to begin after the samples were heated to approximately 20 °C above the lowest Tg. The question that then arises is how does diffusion occur in these organic semiconductor thin-film stacks? Ediger et al. have previously investigated thermal annealing of films composed of stable organic (nonsemiconductor) glasses.34−36 They found that when the films were heated above their glasstransition temperatures, the stable glasses did not transition to a supercooled liquid state evenly throughout the film but rather they observed a supercooled liquid front propagating through the film. In the case of a neat film, the supercooled liquid state front was found to start from the “air” interface and propagate into the film. The mechanism proposed to explain this behavior

determined from fitting the reflectometry profiles for each film structure at 40 °C (summarized in Table 2). The neutron reflectivity profile of each film combination was then recorded as the film was exposed to a 1 °C min−1 temperature ramp starting at 40 °C. Sample 1, in which the lowest Tg material (dNPB, 102 °C) was at the top of the film, was stable up to 119 °C. Above 119 °C, the TCTA:Ir(ppy)3 and d-NPB layers began to intermix, with the interface between the layers moving into the TCTA:Ir(ppy)3 layer. Representative reflectivity profiles and SLD versus thickness plots can be seen in Figure 2a,b, respectively. This is indicative of diffusion of the TCTA:Ir(ppy)3 layer into the adjacent d-NPB layer at a faster rate than that of diffusion of d-NPB into TCTA:Ir(ppy)3. The relative rates of diffusion will be discussed in more detail later but at this point it is important to understand that d-NPB is acting as a diffusive host above its Tg. This is reminiscent of the Kirkendall shift observed for metal/metal diffusion when the position of the interface between two materials shifts in the direction opposite to the direction of diffusion.33 The TCTA:Ir(ppy)3 layer continued to diffuse into the d-NPB layer as the sample temperature was raised until the two layers became completely intermixed. Increasing the temperature above 150 °C resulted in a diffusion front extending into the dAlq3 layer, which indicated diffusion of the higher-Tg d-Alq3 layer into the formed adjacent lower-Tg TCTA:Ir(ppy)3:d-NPB mixed layer. The reflectivity profiles and SLD versus thickness plots are shown in Figure 3 for sample 2, in which the lowest Tg material (d-TPBi, 125 °C) was at the bottom of the stack. The sample was stable up to 144 °C, after which, as expected, the TCTA:Ir(ppy)3 and d-TPBi layers began to intermix. Unlike that in sample 1, the interface between the intermixing layers did not move in as dramatic a manner. Here, interdiffusion was marked by a drop in the SLD of the d-TPBi layer, with a corresponding increase in the SLD of the TCTA:Ir(ppy)3 layer. This behavior clearly indicates that the diffusion rates across the interface must be broadly symmetric, as opposed to the significantly asymmetric diffusion rates seen in sample 1. At higher temperatures, the interface between d-TPBi and TCTA:Ir(ppy)3 became more diffuse. After complete intermixing of the d-TPBi and TCTA:Ir(ppy)3 layers at 150−152 °C had occurred, the sample roughened markedly. The nonun14156


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Figure 5. SLD vs thickness plots and PL spectra of sample 1 films at (a, b) 119 °C, (c, d) 129 °C, and (e, f) 139 °C. The films were held at a fixed temperature for 3 h, with the reflectivity continuously measured. The corresponding reflectivity profiles can be found in Figure S25.

is termed “kinetic facilitation”, whereby molecules with a higher freedom of motion (in the case of neat films, the molecules at

the air interface) induce an increase in mobility in the immediately adjacent molecules in the glass, with a trans14157


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ACS Applied Materials & Interfaces formation front propagating from the interface through the film until it entirely becomes a supercooled liquid. In addition, where glasses of different stabilities (different Tg’s) were used, fronts were found to start growing from the interfaces between the different organic layers. The results for the organic semiconductor layers in the films of this work are consistent with the explanation of Ediger et al. Using sample 1 as an example, when the film is heated to above the Tg of d-NPB, the d-NPB layer is able to transition to a supercooled liquid state. It is interesting that the transition occurs at the Ir(ppy)3:TCTA interface, which, according to the work of Ediger et al., would imply that the d-NPB film is more disordered at the organic−organic interface. That is, the disorder at the organic−organic interface leads to the supercooled state forming there first. The enhanced mobility of these molecules can then induce an increase in mobility of the adjacent TCTA and Ir(ppy)3 molecules, with one or both being able to initially intermix with the d-NPB molecules. The induction of mobility continues to propagate throughout the TCTA:Ir(ppy)3 layer, resulting in the diffusion front extending from the d-NPB layer into the TCTA:Ir(ppy)3 layer, indicating faster diffusion of the emissive layer into d-NPB than that of dNPB into the emissive layer. The next stage in the study was to elucidate the kinetics of the first diffusion process associated with the material with the lowest Tg using time-resolved NR measurements. Sample 1 films were exposed to 10 °C min−1 temperature ramps before being held at the observed transition point (119 °C) or at 10/ 20 °C above the transition point (129/139 °C), and their reflectivity profiles (see Figure S25) and PL spectra were measured over extended periods of time. As can be seen from the SLD versus thickness plots in Figure 5, the initial diffusion of the emissive layer into d-NPB was fast, with the interface quickly migrating into the emissive layer. However, the longer the films were held at a fixed temperature, the slower the diffusion became, until there was no observable progression of the diffusion front into the emissive layer. This was the case for films held at both 119 and 129 °C, with the front extending further into the emissive layer for the higher temperature before diffusion was not observable on the time scale of the NR measurement. Heating to 139 °C was found to provide enough thermal energy to cause the d-NPB and TCTA:Ir(ppy)3 layers to fully intermix on the time scale of the experiment (3 h). The fact that at temperatures above the Tg the front moves and then stops for a particular temperature is interesting, as it provides a plausible explanation for why some OLEDs are observed to have a so-called “burn-in period” in terms of their performance and stability. Initial Joule heating could lead to partial intermixing of the layers (the burn-in period), which would change the charge-transport and emissive properties, and hence device efficiency. After the burn-in period the device reaches a quasistable morphology and is observed to have a relatively stable performance. Fick’s law relates the distance traveled by the diffusing interface (x) to time elapsed (t) according to the relationship x ∝ tn (t = 0 is defined as the time of diffusion onset at 119 °C). When n = 0.5, the diffusion is defined as Fickian, whereas if n = 1, the process is described as case II diffusion.37−39 Plotting the log of the absolute change in interface position against the log of time enables n to be determined. At all three temperatures, sample 1 had an exponent of n < 0.5 for the linear region, which is described as pseudo-Fickian diffusion (see Figure 6), where one material preferentially diffuses into the other with

Figure 6. Plots relating interface position to time for the film at 119, 129, and 139 °C for sample 1, with lines representing Fickian (n = 0.5) and case II (n = 1) diffusions as a point of comparison. Note: in the 139 °C data set, the first recorded point was at 129 °C during the ramp to 139 °C and was thus not included in the fit as the film temperature had not yet equilibrated.

significantly different diffusion coefficients. In the case of sample 1, the SLD of TCTA:Ir(ppy)3 in front of the advancing interface remained constant (Figure 5), indicating that the diffusion coefficient of d-NPB into TCTA:Ir(ppy)3 is essentially zero. For sample 2, the shift in the interfacial position was not as dramatic as that in sample 1. Whereas the drop in the SLD of the d-TPBi layer and the corresponding increase in the SLD of the TCTA:Ir(ppy)3 layer indicates that the rates of diffusion are broadly symmetric (in terms of volume), close inspection of the thicknesses of the TCTA:Ir(ppy)3 and d-TPBi layers (Figures 7a and S26) reveals that the TCTA:Ir(ppy)3 layer grows in thickness by ∼6% over 15 min (338−357 Å), whereas the dTPBi layer shrinks by a similar amount. This is consistent with d-TPBi having a slightly larger diffusion rate than TCTA:Ir(ppy)3, although the difference is much smaller than that observed for sample 1. During this time period, the interface roughens significantly. After the first 14 min of the experiment, interdiffusion of the d-NAD layer into the emissive layer starts to occur. Diffusion of d-NAD into the emissive layer below the Tg of TCTA is likely due to depression of the Tg of the TCTA:Ir(ppy)3 layer due to the intermixing with d-TPBi, which has a lower Tg. An explanation for why the diffusion processes for the diffusion of d-TPBi into TCTA:Ir(ppy)3 and TCTA:Ir(ppy)3 into d-NPB are significantly different is related to the differences in the Tg’s. In the case of diffusion of TCTA:Ir(ppy)3 into d-NPB, the temperature at which d-NPB becomes a supercooled liquid is ≈30 °C below the Tg of TCTA and hence d-NPB acts as a normal diffusive host, with negligible diffusion into the TCTA:Ir(ppy)3 layer. It is important to note that having 6 wt % of Ir(ppy)3 does not depress the Tg of TCTA. In contrast, for the diffusion of d-TPBi into TCTA:Ir(ppy)3, the temperature at which d-TPBi becomes a supercooled liquid (≈145 °C) is only 7 °C lower than the bulk Tg reported for TCTA (152 °C). Thus, it is likely the TCTA molecules will also have a greater degree of mobility at 145 °C, leading to the 14158


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Figure 7. (a) SLD vs thickness plots and (b) PL spectra for sample 2 held at 145 °C. The corresponding reflectivity profile can be found in Figure S26. The arrows show the direction of SLD change in the d-TPBi and TCTA:Ir(ppy)3 layers.

the d-NPB layer was found to be permanently quenched. That is, on cooling after thermal annealing, the fluorescence did not recover (Figure 5). The change in the fluorescence at 430 nm of the film at temperatures above the onset temperature of interdiffusion between the TCTA:Ir(ppy)3 and d-NPB layers is consistent with results of previously observed films in that the fluorescence of d-NPB is quenched due to energy transfer from d-NPB to Ir(ppy)3 that has intermixed with it.31 However, in the previous report, it was also shown that the phosphorescence of Ir(ppy)3 was quenched by energy transfer to the nonemissive d-NPB triplet state. That is, full mixing of the two layers led to mutual luminescence quenching. The fact that some green emission remains for the Si/d-Alq3/TCTA:Ir(ppy)3/d-NPB film after thermal annealing is consistent with fluorescence from the d-Alq3 layer (that has not intermixed) and residual phosphorescence from the TCTA:Ir(ppy)3 layer that has not blended with the d-NPB layer. The behavior of the PL spectra for samples 2 (Figure 7b) and 3 (Figure S27) follow similar trendsduring the annealing process. Initially, there is only emission in the green region, which for sample 2 corresponds to a combination of Ir(ppy)3 phosphorescence and d-NAD fluorescence (NAD emits with a λmax of 525 nm19), whereas for sample 3, all three layers have green emission. The PL intensity of the samples decreased with increasing temperature, and after cooling, the emission remainedt a lower intensity, with a somewhat broader wavelength range. In the case of sample 2, dTPBi mixes with the emissive TCTA:Ir(ppy)3 layer (and vice versa), and it is known that TPBi and TCTA can form a poorly emitting exciplex, which, in principle, could also quench the Ir(ppy)3 emission.40,41 Hence, emission from sample 2 post annealing is likely to arise from residual Ir(ppy)3, exciplexes, and d-NAD emission. For sample 3, at 180 °C, both d-Alq3 and d-NAD diffuse into the emissive layer as well as the emissive layer into d-Alq3, although not fully during the course of the experiment (see Figure 4b). The triplet energies of d-Alq342 and d-NAD (based on its fluorescence energy) are both below those of Ir(ppy)3. Thus, the loss of PL intensity could be due to the Ir(ppy)3 emission being at least partially quenched, with the residual emission mostly being a combination of emission from d-Alq3 and d-NAD.

observed similar diffusivities for the diffusion of d-TPBi into TCTA:Ir(ppy)3 and vice versa. For sample 3, it was not possible to determine the relative rates of diffusion, as intermixing of the layers occurred on a very fast time scale for the NR measurementthere were insufficient time points for the kinetic analysis. In fact, the transition occurred in less than 10 min at a temperature of around 180 °C. Despite this, Figure 4 shows that at around 180 °C the d-Alq3 and central TCTA:Ir(ppy)3 layers interdiffuse in a similar manner to that observed for the d-TPBi/TCTA:Ir(ppy)3 combination. The Tg of Alq3 is reported to be 175 °C; hence, not only will TCTA:Ir(ppy)3 be in a supercooled liquid state but the Alq3 will also be more mobile at 180 °C, leading to interdiffusion of the d-Alq3 and central TCTA:Ir(ppy)3 layers. However, the Tg of d-NAD iis over 200 °C, more than 20 °C above the experimental temperature and so in this case it simply diffuses into the TCTA:Ir(ppy)3 (containing some dAlq3) supercooled liquid. Thus, the temperature at which the lower-Tg material becomes a supercooled liquid and the Tg of the more thermally stable material govern whether the relative rates of interdiffusion are similar or different. If the two temperatures are significantly far apart, then the rates will be greatly different, and if close, they will be more similar. Finally, we relate the changes in film structure to the PL properties during and after the initial layer interdiffusion. The PL spectra were obtained in situ during the NR experiments. The samples were all excited at 365 nm through the holetransport material. It is important to note that the overall thickness of the film meant the full depth of the film was photoexcited and hence the measured PL spectra were a combination of the emission from each of the luminescent materials in the different layers. For sample 1 (Si/d-Alq3/ TCTA:Ir(ppy)3/d-NPB), emission from different species was observed (Figure 5b,d,f) for the as-deposited film held at 40 °C. The shorter-wavelength emission at 430 nm is assigned to dNPB fluorescence,31 with the longer-wavelength green emission assigned to a combination of Ir(ppy)3 phosphorescence31 and the green fluorescence of d-Alq3. The fact that fluorescence is seen from d-NPB is evidence that exciton diffusion from that layer into the emissive layer is limited. On heating to temperatures above the Tg of d-NPB, the fluorescence from 14159



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CONCLUSIONS

A combination of NR and PL measurements was used to determine the relationship between the glass-transition temperature, interlayer diffusion, and emissive properties of OLED films. In all cases, diffusion was initiated when the lowest Tg material present in the sample was heated to above the glasstransition temperature, changing from an amorphous glass state to a supercooled liquid state. The supercooled liquid state enhances the mobility of the molecules at the layer interfaces. When there is a large difference in the Tg’s, the high Tg molecules preferentially diffuse into the adjacent supercooled liquid material, with kinetic measurements confirming that the high Tg molecules diffuse more quickly into the supercooled liquid than the supercooled liquid diffuses into the more stable glass. In all cases, the diffusion was accompanied by a decrease in the luminescence of the films, a result that affirms the importance of utilizing materials with a high thermal stability for each layer in the device to minimize any morphologically derived thermal degradation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (P.L.B.). *E-mail: [email protected] (I.R.G.). ORCID

Paul L. Burn: 0000-0003-3405-3517 Ian R. Gentle: 0000-0001-5573-7868 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to acknowledge the Australian Centre for Neutron Scattering (formerly the Bragg Institute at the time of the experiments), the Australian Nuclear Science and Technology Organisation (ANSTO), and the Australian Institute for Nuclear Science and Engineering (AINSE) for providing the neutron research facilities for the reflectometry experiments and the Australian National Deuteration Facility (NDF) for providing the deuterated compounds used. The NDF is partly supported by the National Collaborative Research Infrastructure Strategy - an initiative of the Australian Government. P.L.B. is an ARC Laureate Fellow (FL160100067). The research was supported by an Australian Research Council Discovery Program (DP120101372).

EXPERIMENTAL SECTION

Film Preparation. The films were prepared by thermal evaporation under high vacuum (≤4 × 10−6 mbar) on silicon wafers. Silicon wafers of 50 mm diameter were cleaned using piranha solution (a 2:1 mixture of 98% aq sulfuric acid and 30% aq hydrogen peroxide) for 5 min and rinsed with distilled water. The native silicon oxide layer was not removed. Before deposition, the wafers were ultrasonicated in acetone for 5 min, rinsed with 2-propanol, and dried under a stream of nitrogen. Neutron Reflectometry. Neutron reflectometry experiments were performed using the Platypus time-of-flight neutron reflectometer and a cold neutron spectrum (2.8 Å < λ < 18 Å) at the OPAL 20 MW research reactor [Australian Nuclear Science and Technology Organisation (ANSTO), Sydney, Australia].43,44 Neutron pulses (24 Hz) were generated using a disk chopper system (EADS Astrium GmbH) in the medium resolution mode (Δλ/λ = 4%) and recorded on a two-dimensional helium-3 neutron detector (Denex GmbH). Reflected beam spectra were acquired at an incident angle of 1.0° in the event mode (time of flight, x and y positions, and frame number time are recorded for each neutron) and re-binned to 2 min time windows for analysis. A custom-built experimental cell with in situ annealing capabilities was used for simultaneous NR and PL measurements. The samples were placed on top of an aluminum block (silicon side down), with the neutron beam incident on the top organic layer. The luminescence of the films was monitored with an Ocean Optics USB2000 spectrometer, and the films were excited using a Nichia UV-LED 365 nm excitation source. The aluminum block heating stage was isolated from the neutron cell by a ceramic stand and was heated (1 or 10 °C min−1 during ramping) with two cartridge heaters, with the temperature controlled using a Lake Shore 340 temperature controller. The cell was kept under coarse vacuum for all experiments. Reduction and analysis of the reflectivity profiles were performed using the Motofit reflectometry analysis program.45,46 All of the NR fits included a 12 Å oxide layer (SLD = 3.47 × 10−6 Å−2) on the surface of the silicon substrate (SLD = 2.07 × 10−6 Å−2).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b01450. Syntheses and characterization of d13-Alq3 and d32-NAD, reflectometry profiles, and PL spectra (PDF) 14160


Research Article


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Dependence of Organic Interlayer Diffusion on Glass-Transition

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