Monovalent Charge Compensation Enables Efficient Lanthanide‐Based Near‐Infrared Perovskite LEDs

Abstract Lanthanide ions (Yb³⁺ or Er³⁺) alloying of CsPb(Cl_1‐xBr_x)₃ quantum dots (QDs) to emit approaching 1000 nm show promise in near‐infrared light‐emitting diodes (NIR‐LEDs). High Yb³⁺ alloying ratio increases the electroluminance efficiency of emission at 990 nm and enables high external quantum efficiency (EQE) of NIR‐LEDs, however, the high alloying ratio also results in inferior material stability and PLQY drop because of Yb³⁺‐induced nanocrystal precipitation. This study finds that the heavy alloying of Yb³⁺ ions causes lattice distortion and coherent energy reduction of Yb³⁺: CsPb(Cl_1‐xBr_x)₃ QDs, induced by two Yb³⁺ ions replacing three Pb²⁺, which leads to the collapse of the octahedral structure in ambient conditions. It posits that spontaneous monovalent ion (Na⁺) alloying can address the trade‐off between material stability and emission intensity. The Na⁺ occupies the vacancy of Pb²⁺ ions, relaxing the distortion in the lattice and improving the phase stability of octahedral structure, and this optimized structure in turn allows a higher Yb³⁺ alloying ratio. Stability measurements show that the Na⁺/Yb³⁺ co‐alloyed films show ten‐fold higher material stability and 2.0‐fold emission efficiency related to controls. It reports that as a result Na⁺/Yb³⁺ co‐alloyed NIR‐LEDs have an EQE of 6.4% at 990 nm, which is among the highest perovskite NIR‐LEDs beyond 950 nm.