A Solution-Processed Heteropoly Acid Containing MoO3 Units as a

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A Solution-Processed Heteropoly Acid Containing MoO3 Units as a Hole-Injection Material for Highly Stable Organic Light-Emitting Devices Satoru Ohisa, Sho Kagami, Yong-Jin Pu, Takayuki Chiba, and Junji Kido ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06723 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 27, 2016

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ACS Applied Materials & Interfaces

A Solution-Processed Heteropoly Acid Containing MoO3 Units as a Hole-Injection Material for Highly Stable Organic Light-Emitting Devices

Satoru Ohisa, Sho Kagami, Yong-Jin Pu,* Takayuki Chiba, and Junji Kido* Graduate School of Organic Materials Science, Yamagata University 4-3-16 Johnan, Yonezawa, Yamagata 992-8510, Japan. E-mail: [email protected]; [email protected]. ac.jp Tel & Fax: +81-238-26-3595

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Abstract We report hole-injection layers (HILs) comprising a heteropoly acid containing MoO3 units, phosphomolybdic acid (PMA), in organic light-emitting devices (OLEDs). PMA possesses outstanding properties, such as high solubility in organic solvents, very low surface roughness in the film state, high transparency in the visible region, and an appropriate work function (WF), that make it suitable for HILs. We also found that these properties were dependent on the post-baking atmosphere and temperature after film formation. When the PMA film was baked in N2, the Mo in the PMA was reduced to Mo(V), whereas baking in air had no influence on the Mo valence state. Consequently, different baking atmospheres yielded different WF values. OLEDs with PMA HILs were fabricated and evaluated. OLEDs with PMA baked under appropriate conditions exhibited comparably low driving voltages and higher driving stability compared with OLEDs employing conventional

hole-injection

materials

(HIMs),

poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate), and evaporated MoO3, which clearly shows the high suitability of PMA HILs for OLEDs. PMA is also a commercially available and very cheap material, leading to the widespread use of PMA as a standard HIM.

Keywords: heteropoly acid, polyoxometalate, MoO3, gap state, solution process


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1. INTRODUCTION The incorporation of a hole-injection layer (HIL) is very effective in improving electric characteristics and driving the stability of organic electronic devices.1-9 In particular, in organic light-emitting devices (OLEDs), solution-processed HILs covering electrodes such as indium tin oxide (ITO) suppress leakage currents in devices, which boosts the efficiency of a device. To date, there

have

been

various

reports

of

solution-processed

HILs.

Poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is commercially available and one of the most widely used hole-injection materials (HIMs).4,10 However, the driving stability of devices employing PEDOT:PSS is very poor. On the other hand, stable and solution-processable HIMs have been developed by some manufacturers and are used in industry;5,11 they are very useful for improving device characteristics. However, the manufacturers have not disclosed the details of the HIMs and we cannot know their details such as molecular structure. Hence, the commercially available, cheap, and stable HIM has been highly desired. Transition metal oxides such as MoOx,12-27 VOx,13,14,21,26-30, and WOx13,21,27,31,32 as n-type semiconductors and NiOx13,21,27,33-38 as a p-type semiconductor have been used in OLEDs and organic photovoltaic cells. Some of these oxides have been applied in solution-processed HILs. Solution-processed MoOx HILs are the most studied because thermally evaporated MoO3 (e-MoO3) HILs have been the most widely used in conventional evaporation-processed OLEDs due to their superior electrical characteristics and high stability. Solution-processed MoOx films have been formed using nanoparticle (NP) suspensions12,18,22,23 and sol–gel methods14-17,19,20,24,25 because of the low solubility of bulk MoOx powder in typical solvents. These methods require preliminary syntheses of NPs or post-treatments after film formation such as atmospheric oxidation, high-temperature sintering, and O2-plasma oxidation. For example, Meyer et al. reported MoOx films spin-coated from a suspension of NPs.23 The NPs were dressed with a polymeric dispersing material that prevented agglomeration due to van der Waals interactions, which enabled uniform film 3 ACS Paragon Plus Environment


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formation. The excess polymeric dispersing material inhibits smooth charge transport; therefore, O2-plasma treatment was performed to remove the polymeric dispersing material after film formation. OLEDs fabricated with the treated MoOx film exhibited nearly the same characteristics as those with e-MoO3 films. Although the characteristics of the MoOx films are remarkable, the extra complex processing, such as O2-plasma treatment, clearly hinders their utilization as standard HIMs. If possible, the extra processes should be avoided to simplify the fabrication process. Phosphomolybdic acid (PMA) is a commercially available heteropoly acid compound containing MoO3 units. PMA is widely used as a solid acid catalyst and as a color reagent for the detection of compounds on thin-layer chromatography plates.39-49 PMA is a commonly used material and can be purchased at a very low price (< 1$ per gram). PMA comprises a phosphorus anion and twelve molybdenum oxyanions, which form a Keggin-type structure (H3PMo12O40) (Figure S1), including three protons attached to a water cluster per Keggin-type structural unit. PMA can easily dissolve in polar solvents such as water, and alcohol, which enables simple film formation from solutions without extra processes such as preliminary syntheses of NPs; thus, PMA is a promising material for solution processing. PMA has already been applied in some organic electronic devices.50-57 Vasilopoulou et al. used several heteropoly acids including PMA as electron-injection layers in OLEDs, and as electron-extraction layers in organic photovoltaic cells (OPVs).50-52,56-58 Zhu et al. reported the application of PMA as a hole-extraction layer in OPVs.54 OPVs with PMA showed nearly the same device characteristics as those with e-MoO3. Pu et al. reported solution-processed tandem-OLEDs comprising several stacked light-emitting units interconnected by charge-generation layers containing PMA.58 However, so far, the application of PMA as a hole-injection layer in OLEDs has not been reported. In this work, we report the application of PMA in solution-processed HILs for OLEDs. We verified the suitability of PMA for an HIM by evaluating its solubility, thermal characteristics, and film properties, including surface morphology, optical characteristics, and work function (WF). 4 ACS Paragon Plus Environment

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These film properties are usually sensitive to post-baking conditions in the film drying process. Hence, we evaluated the influence of post-baking temperature and atmosphere, N2 or air, on these film properties. We fabricated OLEDs with PMA HILs and evaluated their characteristics, which were found to depend on the post-baking conditions of the PMA films. The PMA HILs baked under appropriate conditions realized comparably low driving voltages compared with conventional PEDOT:PSS, and driving stabilities as high as e-MoO3, which clearly shows the high suitability of PMA HILs for OLEDs.

2. RESULTS AND DISCUSSION 2.1 Characterization of PMA properties 2.1.1. Solubility test First, we evaluated the solubility of PMA in several solvents: water, methanol, ethanol, 2-propanol, 2-ethoxyethanol, acetone, n-butyl acetate, acetonitrile, tetrahydrofuran (THF), toluene, and p-xylene. We used PMA n-hydrate as the reagent. PMA was added to these solvents at room temperature at a concentration of 10 mg/ml. Photographs of the PMA solutions are shown in Figure S2. PMA dissolved well into all of these solvents except for toluene and p-xylene. The PMA solutions showed yellow or bluish-yellow color. Polar solvents are preferable for dissolving PMA. It is noteworthy that PMA could not dissolve in toluene and p-xylene. In solution-processed OLEDs, hole-transporting materials are stacked onto HILs by solution processing. HIMs are required to be insoluble in solvents such as toluene and p-xylene, which are used to dissolve the hole-transporting materials. PMA could dissolve in the polar solvents but not in the aromatic solvents, which indicates that the hole-transport materials can be stacked onto the PMA HIL without dissolution of PMA. Next, we evaluated the stability of the solutions in an N2 atmosphere. Heteropoly acids such as PMA have been used as solid acid catalysts in reactions such as the etheration and dehydration of alcohols, polymerization of THF, and hydration of olefins. PMA works as a strong oxidizing agent. If PMA is 5 ACS Paragon Plus Environment


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reduced by reactions with these solvents, the solutions give a blue color known as molybdenum blue.47 This reaction does not only occur during solution storage, but also in the film baking process, during which residual solvent in the as-coated films can react with the PMA. Therefore, the selection of stable solvents for PMA is important for achieving robust expression of the electrical characteristics of PMA. After storing for 6 months, most solutions gave a blue color, indicating the reduction of PMA, whereas the PMA solutions in water, 2-propanol, and acetonitrile did not show any color change. These three solvents are good solvents for PMA.

2.1.2. Thermal characteristics The thermal characteristics of PMA were investigated by thermogravimetry–differential thermal analysis (TG–DTA) under flowing N2. We evaluated three types of PMA powders. One was immediately measured after being taken from a reagent bottle. The others were measured after storage in N2 or air for 2 weeks. Figure 1 shows the TG–DTA curves. The weight loss up to about 150 °C is mainly attributable to the desorption of the water of hydration (nH2O). The amount of weight loss depended on the storage conditions. The sample stored in N2 showed the least weight loss, whereas the sample stored in air showed the largest weight loss. These observations indicate that the adsorption and desorption of water on PMA arose during storage. During the storage in N2, the water derived from hydration on PMA desorbs from the PMA surface, resulting in less weight loss in TGA analysis. During the storage in Air, H2O in air adsorbs on the PMA surface, resulting in larger weight loss in TGA analysis. According to previous studies, the gradual weight loss up to 450 °C is probably due to the loss of water generated from two protons and one oxygen atom extracted from the Keggin-type structure.49, 59 A strong exothermic peak was found at 437 °C in the DTA curve, which can be assigned to the formation of a mixture of phosphorus and molybdenum oxides.


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Figure 1. TG–DTA curves. a) TGA curves of three types of PMA powders: immediately measured after being taken out from a reagent bottle, or measured after storage in N2 or air for 2 weeks. b) TG– DTA curve of PMA measured immediately after being taken out from a reagent bottle.

Desorption of the hydrated water was also verified by attenuated total reflectance–Fourier transform infrared spectroscopy (ATR–FTIR). PMA powders were baked in N2 at various temperatures for 10 min. The infrared spectra of these samples are shown in Figure S3. Unbaked PMA showed strong absorptions attributable to water of hydration at around 1600 cm−1 and 3200 cm−1. These peaks completely disappeared after baking at 150 °C, indicating the complete desorption of the water of hydration.

2.1.3. Surface morphology We investigated the surface morphology of PMA films using atomic force microscopy (AFM). PMA was spin-coated onto ITO substrates and baked in N2 or air atmospheres at various temperatures up to 300 °C. For comparison, we also fabricated an e-MoO3 film on an ITO substrate. Figure S4 shows the AFM results for the films. The PMA films showed very small surface roughness values (Ra), for example, 0.173 nm for the film baked at 100 °C in N2, indicating that the PMA films are suitable for HILs, which are required to be flat and smooth. The e-MoO3 film showed slightly larger Ra value of 0.446 nm. The Ra values of the PMA films baked in N2 were smaller than those of the unbaked films, which suggests that the film morphologies were affected by baking. On the other hand, the Ra values of the films baked in air were nearly independent of the baking 7 ACS Paragon Plus Environment


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temperature. This is probably because the existence of water and oxygen molecules in the air atmosphere affects the constancy of the surface morphology. The detailed reason why different baking atmospheres cause different surface morphologies is currently under investigation.

2.1.4. Optical characteristics We evaluated the optical characteristics of PMA films spin-coated onto quartz substrates from acetonitrile solutions to give films with a thickness of 30 nm. The PMA films were baked in N2 or air atmospheres at various temperatures up to 300 °C. Figure 2 shows the UV–Vis–NIR absorption spectra of the films, all of which show strong absorption peaks in the UV region, which are characteristic of the Keggin-type structure. In the visible region, no absorption bands were observed in the unbaked film or the films baked in air, which exhibit the highest transparency of these films (Figure S5). However, the films baked in N2 showed additional absorption in the red to near-infrared region, which gradually increased with increasing baking temperature. As shown in Figure S5, the transmittance of the baked films in N2 slightly decreased compared with the unbaked film. This absorption is attributable to the reduction of Mo resulting from the loss of oxygen, which leads to changes in chemical composition. The reason why baking in air did not produce additional absorption is probably that oxygen molecules in air compensate for the generated oxygen vacancies in the PMA films.


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Figure 2. UV–Vis–NIR spectra of PMA films baked at different temperatures in a) N2 and b) air.

2.1.5. Valence states of Mo The valence states of Mo in PMA were evaluated by X-ray photoelectron spectroscopy (XPS). The obtained Mo 3d3/2 and 3d5/2 characteristic peaks of the PMA films are shown in Figure 3. The binding energy was calibrated by using the C 1s peak position. PMA was spin-coated onto Si substrates in N2 or air atmospheres to give films with a thickness of 30 nm. The unbaked PMA film, and an e-MoO3 film deposited for comparison, showed only peaks corresponding to the Mo(VI) state. Generally speaking, solution-processed MoOx films contain Mo(IV) and Mo(V), which deteriorate hole-injection characteristics in many cases and, thus, require further oxidation treatments to convert the Mo to highly oxidized Mo. In PMA, Mo(VI) is stable in the Keggin-type structure and does not require additional oxidation treatment. PMA films were also baked at different temperatures in N2 or air. The XPS spectra of the PMA films baked in N2 showed a gradual reduction from Mo(VI) to Mo(V) with increasing baking temperature, whereas the spectrum of the PMA film baked at 300 °C in air showed only peaks corresponding to Mo(VI). These results were consistent with the UV–Vis spectral results.



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Figure 3. XPS spectra of a) PMA films baked at different temperatures in N2 and b) unbaked PMA, PMA baked at 300 °C in air, and evaporated MoO3 (e-MoO3) films.

2.1.6. Work function Chemical composition changes influence on the electronic characteristics of PMA films. We evaluated the WF of the PMA films by photoelectron yield spectroscopy (PYS) under vacuum. PMA was spin-coated onto the indium–tin oxide (ITO) films on glass substrates to give PMA films with a thickness of 30 nm, which were then baked at various temperatures in N2 or air. The films baked in N2 were then handled without exposure to air. For comparison, an e-MoO3 film was also fabricated. The photoelectron yield often takes the functional form (hν − Ip)1/n in the neighborhood of the photoelectron emission threshold.60 Here, we adopted an n value of 2, which is often used for common semiconductors. The WF values obtained by the PYS measurements are shown in Figure 4. The unbaked PMA and the e-MoO3 showed nearly the same WF value of 6.4 eV, suggesting that PMA is a suitable HIL. In previous studies, the WF values of sol–gel-derived MoOx were reported to be much lower than those of e-MoO3 due to the low valence state of Mo along with the oxygen vacancies.12,14,16,18-20,24 However, here PMA showed a high WF due to it being in a highly oxidized state. The WF of e-MoO3 has also been reported to range from 6.6 eV to 6.9 eV,27 as measured by ultraviolet photoelectron spectroscopy (UPS), which is close to the WF of e-MoO3 obtained in this 10 ACS Paragon Plus Environment

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work. Next, PMA films were baked at different temperatures in N2 or air. Baking in N2 gradually decreased the WF of the PMA films with increasing baking temperature, along with the gradual reduction of Mo(VI), and the PMA baked at 300 °C showed the lowest WF of 5.5 eV. In contrast, all of the films baked in air, which did not show the reduction of Mo(VI), showed nearly the same WF of ca. 6.4 eV, equivalent to that of the unbaked film. The reduction of Mo(VI) along with the generation of oxygen vacancies clearly correlated with the WF values. The PMA with more reduced Mo exhibited smaller WF values. The constant WF value of the films baked in air is ascribable to the stability of the Mo valence state during baking in air. Compensation of the oxygen vacancies with oxygen molecules from the air probably plays a role in stabilizing the Mo valence state in the films baked in air.

Figure 4. Plots of work function vs. baking temperature for PMA films baked in N2 or air atmospheres.

2.2. OLEDs We fabricated OLEDs containing PMA as an HIL. Evaporation-processed OLEDs with the structures

[ITO/HIL/4,4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl

nm)/4,4’-bis(N-carbazolyl)-1,1’-biphenyl (Ir(ppy)3)

(30

(CBP):8wt.%

(α-NPD)

(30

tris(2-phenylpyridinato)iridium(III)

nm)/bis(2-methyl-8-quinolinolate)-4-(phenylphenolato)aluminum

(BAlq)

(10

nm)/tris-(8-hydroxyquinoline)aluminum (Alq3) (30 nm)/8-hydroxyquinolatolithium (Liq) (1 nm)/Al 11 ACS Paragon Plus Environment


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(100 nm)] were fabricated. Except for the HIL, each layer was formed by evaporation processing under high vacuum. PMA was dissolved in acetonitrile and the solution was spin-coated onto ITO substrates to give films with thicknesses of 10 nm. The PMA films were baked in N2 or air at various temperatures. For comparison, PEDOT:PSS (30 nm) coated in air was also adopted as an HIL. The energy diagram of the OLEDs are shown in Figure S6. The current density–voltage (J–V) and luminance–voltage (L–V) characteristics of the devices are shown in Figure 5 and Figure S7, respectively. All of the devices employing baked PMA showed lower driving voltages than the device employing unbaked PMA. For PMA baked in N2, the driving voltages showed a dependence on the baking temperature, and baking at temperatures above 150 °C greatly improved the driving voltages, which were comparable to that of the device with PEDOT:PSS. PMA did not require additional atmospheric oxidation to express its electronic properties. In the devices employing PMA baked in air, all of the devices showed nearly the same driving voltage.

Figure 5. Current density–voltage characteristics of OLEDs fabricated with PMA or PEDOT:PSS HILs. The PMA film was either unbaked or baked at various temperatures in a) N2 or b) air.

To verify that this behavior was derived from the hole-injection characteristics from PMA to α-NPD, we fabricated hole-only devices (HODs) with the structure [ITO/PMA (10 nm)/α-NPD (90 nm)/e-MoO3 (5 nm)/Al (100 nm)] under the same PMA baking conditions used above. Here, the ITO electrode is an anode and the Al electrode is a cathode. The e-MoO3 layer was inserted to block 12 ACS Paragon Plus Environment

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electron injection from Al to α-NPD. Figure S8 shows the J–V characteristics of the HODs. As is the case for the OLEDs, baking in N2 above 150 °C greatly improved the driving voltages. After baking in air, all of the devices showed nearly the same driving voltage. This behavior is consistent with that of the OLEDs. We consider that four factors determine the driving voltage: the existence of water of hydration, the film morphology, the WF values, and gap state formation in the PMA films. First, the water of hydration surrounding the PMA molecules physically separates PMA from α-NPD, which could inhibit hole injection from PMA to α-NPD. Thus, removal of the water of hydration is essential for obtaining good electronic characteristics. According to the TG and FTIR results, PMA requires baking temperatures above 150 °C to remove the water of hydration completely. Hence, the improvement in the J–V characteristics after baking at 100 °C and 150 °C in both atmospheres is mainly attributable to the removal of the water of hydration. In addition, water could have adsorbed onto the PMA when the films were handled in air. This adsorption of water must have influenced the device characteristics. Second, according to the AFM results, baking above 100 °C in N2 clearly flattened the PMA film surface, probably due to the removal of the water of hydration and/or chemical composition changes. This flattening could influence the device characteristics. In contrast, baking in air had almost no influence on the film morphology. The difference in surface morphology between films baked in N2 and air could result in different device characteristics. As mentioned above, the detailed reason for the surface morphology difference is currently under investigation. Third, the WF values can also influence the hole-injection characteristics. In this work, we used a charge-generation layer comprising PMA and α-NPD. Here, PMA works as an electron acceptor, and α-NPD works as an electron donor. α-NPD has an Ip value of 5.5 eV. The smallest WF of PMA in this work is 5.5 eV, obtained by baking at 300 °C in N2, which is comparable to the Ip value of α-NPD. Thus, all of the PMA films used in this work satisfy the requirement of charge generation, which suggests that the influence of WF differences on device characteristics is not large in this work. However, strictly speaking, differences in the vacuum level positions of the respective PMA films 13 ACS Paragon Plus Environment


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should be taken into account in the above discussion. Unfortunately, in this work, the differences were not taken into account due to the needed data shortage. Fourth, gap states in the PMA films can influence the device characteristics. As the UV–Vis–NIR spectra show, additional absorption was generated in the long-wavelength region by baking in N2, in addition to the characteristic ultraviolet absorption of the PMA Keggin-type structure, which implies the formation of gap states between the valence and conduction bands. Wang et al. observed gap state formation in e-MoO3 by baking in vacuum after deposition using UPS and XPS measurements.61 The UPS spectra showed that the e-MoO3 films baked at higher temperature contained more filled Mo 4d states, located between the valence band and the Fermi energy, and the WF values were reduced. The XPS spectra showed that higher-temperature baking partially reduced Mo(VI) to Mo(V). According to the literature, these results indicate that Mo–Mo dimerization occurs due to the increase of oxygen vacancies.62 The formation

of

gap

states

improved

the

hole-injection

efficiency

from

ITO

to

1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC), which is ascribable to a more energetically favorable band alignment. In this work, gap state formation is also considered to have improved the hole-injection efficiency. This should be investigated by photoelectron spectroscopy measurements in future work. Figure 6 shows lifetime, relative luminance–driving time, and driving voltage–driving time curves of the fabricated OLEDs. Here, all of the devices were driven with a constant current of 6.75 mA/cm2 for the lifetime tests. All of the devices employing PMA showed much higher stability and smaller driving voltage increases compared with those employing PEDOT:PSS. For the PMA films baked in N2, the device lifetimes depended on the baking temperature, whereas for air baking, the device lifetimes did not depend on the baking temperature. Here, we consider that the device lifetimes were also dependent on the existence of water of hydration, film morphology, WF, and gap state formation. The lack of a dependence of lifetime on baking temperature in air suggests that the existence of water of hydration had almost no effect on the device lifetimes. This is a surprising 14 ACS Paragon Plus Environment

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result because, generally speaking, the existence of water molecules heavily reduces the device lifetime. The film morphology also seems to have little relation with the device lifetime according to the results for the devices employing PMA baked in N2. The existence of Mo(V) derived from baking in N2, which relates to the WF and gap state formation, probably controlled the device lifetimes.

Figure 6. Lifetime curves of OLEDs. Relative luminance–driving time curves of the devices baked at various temperatures in a) N2 and b) air. Driving voltage–driving time curves of the devices baked at various temperatures in c) N2 and d) air.

The length of lifetimes for devices employing PMA HILs compared with highly stable HIMs such as e-MoO3 is an important point. We also fabricated OLEDs, with the same structure described above, using PMA HILs baked in N2 or air at 150 °C, and using e-MoO3. In the lifetime tests, devices were driven at 6.75 mA/cm2. Device lifetime is sensitive to slight differences in the evaporation chamber environment.63 Therefore, these devices were fabricated in one evaporation 15 ACS Paragon Plus Environment


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batch. Figure 7 and S9 shows J–V, L–V, relative luminance–driving time, and driving voltage– driving time curves. The device with an e-MoO3 HIL showed large leakage currents at low driving voltage. On the other hand, the devices with PMA HILs showed almost no leakage current due to the suppression effect of the solution-processed PMA HIL. In general, solution-processed HILs have planarization effects of the surface and suppresses the leakage current. In the initial OLED testing, the device employing PMA baked in N2 showed comparable characteristics to that employing e-MoO3. Furthermore, the device employing PMA baked at 150 °C exhibited higher stability than the device employing e-MoO3. On the other hand, the device employing PMA baked in air showed higher driving voltages than the others, and nearly as high stability as the device with e-MoO3. These results clearly indicate that PMA is an excellent HIM in OLEDs. The lifetime of the devices with PMA shown in Figure 7 are longer than those shown in Figure 6, indicating the large influence of the device fabrication environments on the device lifetime. Probably, water amount in the evaporation chamber influenced on the device lifetime length.63

Figure 7. Lifetime curves of OLEDs employing PMA HIL baked at 150°C in N2 or air or e-MoO3 HIL. a) Relative luminance–driving time curves of the devices. b) Driving voltage–driving time curves of the devices.

We have evaluated the PMA HILs baked in N2 or air, which both show different characteristics in OLEDs. The PMA HILs baked in N2 or air have different advantages for application as HILs. The 16 ACS Paragon Plus Environment

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PMA films baked in N2 showed characteristics that depended on the baking temperature. Baking at the appropriate temperatures gave comparably low driving voltage and higher driving stability compared with devices employing the conventional HIMs PEDOT:PSS and e-MoO3. On the other hand, the PMA films baked in air showed characteristics that were independent of baking temperature. However, the device employing PMA baked in air showed a higher driving voltage and lower driving stability than the device employing PMA baked in N2 at an appropriate temperature. These seem to be demerits of the PMA films baked in air. However, the PMA films baked in air can give highly robust characteristics to OLEDs. In situations requiring that OLEDs possess highly robust characteristics, PMA films baked in air would be very useful. Finally, we also applied PMA HILs to OLEDs fabricated by solution processing, except for the electron-injection

layer

and

electrodes.

Solution-processed

OLEDs

[ITO/HIL/poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl)

with

the

structure

diphenylamine))

(TFB) (20 nm)/poly[(9,9-di-n-octylfluorenyl-2,7diyl)-alt-(benzo[2,1,3]-thiadiazol-4,8-diyl)] (F8BT) (80 nm)/Liq (1 nm)/Al (100 nm)] were fabricated. PMA (10 nm) and PEDOT:PSS (30 nm) were adopted as HILs. TFB, dissolved in p-xylene, was stacked onto the PMA layer without dissolution of the PMA. The device characteristics are shown in Figure S10. The OLED employing the PMA HIL showed a comparably lower driving voltage than the OLED employing a PEDOT:PSS HIL, which also demonstrates the high suitability of PMA HILs for use in solution-processed OLEDs.

3. CONCLUSION We demonstrated solution-processed PMA as the HIL in OLEDs. The PMA films were well suited for solution-processed HILs due to their solubility, film morphology, and optoelectronic characteristics. We also found that these properties were dependent on the post-baking atmosphere and temperature. When PMA was baked in N2, the reduction of Mo(VI) to Mo(V) occurred, whereas baking in air did not affect the Mo valence state. The OLEDs employing PMA baked under 17 ACS Paragon Plus Environment


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appropriate conditions exhibited comparably low driving voltages, and higher driving stability, compared with OLEDs employing the conventional HIMs PEDOT:PSS and e-MoO3, which clearly proves the high suitability of PMA for use as an HIL. PMA is a commercially available, very cheap material, which could lead to the widespread use of PMA as a standard HIM. By using PMA HILs, further progress in developing highly efficient, stable OLEDs is expected.

4. EXPERIMENTAL SECTION Materials: PMA n-hydrate was purchased from Kanto Chemical Co., Inc. All solvents were purchased from commercial sources and used as received. PEDOT:PSS (CleviosTM P VP CH 8000) was purchased from Heraeus Materials Technology. α-NPD, CBP, Ir(ppy)3, BAlq, Alq3, and Liq were purchased from e-Ray Optoelectronics Technology Co., Ltd. and used after sublimation purification. MoO3 was purchased from Furuuchi Chemical Co. F8BT was provided by Sumitomo Chemical Co., Ltd.

General procedures: For the solubility test, PMA n-hydrate was added to water, methanol, ethanol, 2-propanol, 2-ethoxy-2-ethanol, acetone, butyl acetate, acetonitrile, THF, toluene, or p-xylene at a concentration of 10 mg/ml at room temperature in an N2 atmosphere. The solutions were sealed and stored in an N2-purged glove box for 6 months. For all film formations, PMA was dissolved in acetonitrile at a concentration of 10 mg/ml, and the solution was spin-coated onto an ITO substrate (Asahi Glass Co., Ltd) and baked at 100 °C, 150 °C, 200 °C, or 300 °C for 10 min in N2 or air. Surface profiles were measured by AFM (Veeco Dimension icon) with scanning areas of 2.0 µm × 2.0 µm. TG–DTA measurements were performed using a SEIKO EXSTAR 6000 TG/DTA 6200 unit under an N2 flow at a heating rate of 10 °C min−1. UV–Vis–NIR spectra were measured using a Shimadzu UV-3150 UV–Vis–NIR spectrophotometer. ATR–FTIR spectra were measured using a 18 ACS Paragon Plus Environment

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Shimadzu IR Prestige-21 Fourier transform spectrophotometer. WF values were determined by PYS under vacuum (∼10−3 Pa). The valence states of Mo were evaluated using XPS measurements.

Fabrication of OLEDs and HODs: OLEDs and HODs were fabricated on pre-cleaned ITO substrates (Asahi Glass Co., Ltd.). Evaporation-processed OLEDs with the structure [ITO/HIL/α-NPD (30 nm)/CBP:8wt.% Ir(ppy)3 (30 nm)/BAlq (10 nm)/Alq3 (30 nm)/Liq (1 nm)/Al (100 nm)] were fabricated. Except for the HIL, each layer was formed by evaporation processing under high vacuum. PMA was spin-coated to give a film with a thickness of 10 nm and baked at 100 °C, 150 °C, 200 °C, or 300 °C for 10 min in an N2 or air atmosphere. PEDOT:PSS and MoO3 were spin-coated and evaporated onto ITO substrates, respectively. Solution-processed OLEDs with the structure [ITO/HIL/TFB (20 nm)/F8BT (80 nm)/Liq (1 nm)/Al (100 nm)] were fabricated. PMA and PEDOT:PSS were adopted as HILs. PMA was spin-coated to give a film with a thickness of 10 nm and baked at 150 °C for 10 min in an N2 atmosphere. TFB and F8BT were spin-coated from p-xylene solutions. HODs with the structure [ITO/PMA (10 nm)/α-NPD (90 nm)/e-MoO3 (5 nm)/Al (100 nm)] were fabricated. PMA was spin-coated to give a film with a thickness of 10 nm and baked at 100 °C, 150 °C, 200 °C, or 300 °C for 10 min in an N2 or air atmosphere. α-NPD, e-MoO3, and Al were evaporated under high vacuum. The J–V–L characteristics of the OLEDs, and the J–V characteristics of the HODs, were measured using a current source (Keithley 2400) and a luminance meter (Konica Minolta CS-200), respectively. EL spectra were measured using a spectral radiance meter (Konica Minolta CS-2000). Current efficiencies were calculated using the Lambertian assumption.

ACKNOWLEDGEMENTS



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We would like to thank Masanori Miura, and Prof. Hirose for XPS measurements, and the “Strategic Promotion of Innovative R&D Program” of Japan Science and Technology Agency (JST) for financial support.

SUPPORTING INFORMATION Keggin-type structure of PMA, Solubility test results, FTIR spectra, AFM data, trasmittance spectra, and OLED and HOD characteristics are available in supporting information.

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Table of Contents Graphic


A Solution-Processed Heteropoly Acid Containing MoO3 Units as a

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