Research Article www.acsami.org
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 S Supporting Information *
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 postbaking 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(3,4ethylenedioxythiophene):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
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-eth ylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is commercially available and one of the most widely used holeinjection 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 WO x 13,21,27,31,32 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© 2016 American Chemical Society
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 O2plasma 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 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 Received: June 6, 2016 Accepted: July 26, 2016 Published: July 26, 2016 20946
DOI: 10.1021/acsami.6b06723 ACS Appl. Mater. Interfaces 2016, 8, 20946−20954
Research Article
ACS Applied Materials & Interfaces
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.
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 solutionprocessed 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 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
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 (
