Vacuum-deposited Rb <subscript>3</subscript> CeI <subscript>6</subscript> for deep-blue-light-emitting diodes.

Recently, perovskite light-emitting diodes (PeLEDs) have exhibited outstanding performance in next-generation high-definition display applications. However, compared with green and red PeLEDs, the development of efficient and stable blue PeLEDs to meet the requirement for a wide color gamut has been a challenge. Herein, we vacuum thermally deposited a film of the lead-free rare earth halide Rb 3 CeI 6 , which shows deep blue emission with peaks at 427 nm and 468 nm. Due to the parity-allowed 5d-4f transition of Ce(III), the excited-state lifetime is as short as 22.3 ns (427 nm) and 25 ns (468 nm), respectively. The photoluminescence quantum yield (PLQY) is optimized to 51% by regulating the nucleation and growth of Rb 3 CeI 6 grains. In a prototype rare earth light-emitting diode (ReLED) device, a thin insulating Al 2 O 3 layer (5 nm) is inserted between the electron transport layer (ETL) and the emitting layer (EML, Rb 3 CeI 6 ) to balance the carriers and reduce the dark current. The device shows a maximum luminance and EQE of 98 cd m -2 and 0.67%, respectively, and the electroluminescence (EL) spectrum maintains stability with changes in the operating voltage. In addition, the corresponding CIE coordinate is (0.15, 0.06), which closely matches the Rec. 2020 standard (0.131, 0.046).