Passivation Layer of Potassium Iodide Yielding High Efficiency and Stable Deep Red Perovskite Light-Emitting Diodes

Perovskite light-emitting diodes (PeLEDs) are promising candidates used for superthin emissive displays with high resolution, high brightness, and wide color gamut, but the CsPbI₃ nanocrystal (NC) based ones usually have an external quantum efficiency (EQE) of less than 20%, which needs further enhancement to minimize the gap between their counterparts. Herein, we propose to improve optical properties of the CsPbI₃:Sr emissive layer (EML) by inserting an additional potassium iodide (KI) passivation layer between the hole transport layer and EML to increase the film quality, photoluminescence quantum yield, and thermal stability of the EML. The KI layer can also increase the carrier mobility to balance the charge injection in PeLEDs, leading to a reduction in Auger recombination and Joule heating. An interesting deep-red-emitting PeLED (λ_em = 687 nm) with a record EQE of 21.8% and a lifetime T₅₀ of 69 min is obtained by applying the additional KI passivation layer. Moreover, a flexible PeLED consisting of the KI layer is also demonstrated to have a record EQE of 12.7%. These results indicate that the use of a functional KI layer is a feasible way to develop high-performance electroluminescent devices.