A solution-processed phosphomolybdic acid hole injection layer for high-performance quantum dot light-emitting diodes

In this work, we report high-performance quantum dot light-emitting diodes (QLEDs) employing a solution-processed phosphomolybdic acid (PMA) hole injection layer (HIL). We demonstrate that the chemical composition of PMA strongly depends on the baking atmosphere: baking in N₂ partially reduces Mo⁶⁺ to Mo⁵⁺, resulting in a reduced PMA phase (rPMA). The Mo⁶⁺ state in PMA yields a higher work function, which facilitates efficient hole injection at the HIL/hole transport layer interface. In contrast, the Mo⁵⁺ species in the rPMA film introduces intragap states that act as non-radiative recombination centers, leading to photoluminescence quenching. Consequently, the QLED incorporating PMA achieves a maximum external quantum efficiency of 24.9% and an extended operational lifetime (T₉₅ = 1430 h@1000 cd m^-2). This work highlights the critical role of chemical state control and provides a versatile, solution-processable material for high-performance QLEDs.