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

Lanthanide ions (Yb3+ or Er3+) alloying of CsPb(Cl1-xBrx)(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(Cl1-xBrx)(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.