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

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(3,4-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.