Alkalis-doping of mixed tin-lead perovskites for efficient near-infrared light-emitting diodes

Substitution of lead (Pb) with tin (Sn) is a very important way to reduce the bandgap of metal halide perovskite for applications in solar cells, and near infrared (NIR) light-emitting diodes (LEDs), etc. However, mixed Pb/Sn perovskite becomes very disordered with high trap density when the Sn molar ratio is less than 20%. This limits the applications of mixed Pb/Sn perovskites in optoelectronic devices such as wavelength tunable NIR perovskite LEDs (PeLEDs). In this work, we demonstrate that alkali cations doping can release the microstrain and passivate the traps in mixed Pb/Sn perovskites with Sn molar ratios of less than 20%, leading to higher carrier lifetime and photoluminescence quantum yield (PLQY). The external quantum efficiency (EQE) of Sn_0.2Pb_0.8-based NIR PeLEDs is dramatically enhanced from 0.1% to a record value of 9.6% (emission wavelength: 868 nm). This work provides a way of making high quality mixed Pb/Sn optoelectronic devices with small Sn molar ratios.