Phosphotungstic and phosphomolybdic acid solid-solutions: a new approach for QLED hole injection layers

Abstract

The organic hole injection layer (HIL) PEDOT:PSS used in QLEDs has issues with bottom electrode corrosion and charge imbalance with electrons. To address these issues, we developed a solid-solution inorganic hole injection layer by alloying the phosphomolybdic acid (PMA), which has high charge mobility, and phosphotungstic acid (PWA), which has high stability. By varying the W–Mo ratio, we could adjust the energy band alignment of the hole injection layer. As a result, the efficiency of the device was improved by preventing hole accumulation caused by the large energy barrier at the interfaces in the HIL when holes are injected from the ITO electrode. By adjusting the Mo ratio in PWA–PMA, the conductivity of the hole injection layer can be easily tuned. The enhanced hole mobility resolved the charge imbalance issue, leading to higher device efficiency and a reduction in turn-on voltage. This work enables the use of PWA–PMA as a hole-injection layer in QLEDs for the first time.