Lead-Free Direct Band Gap Double-Perovskite Nanocrystals with Bright Dual-Color Emission

Lead-free double-perovskite nanocrystals (NCs), that is, Cs₂AgIn _xBi_{1- x}Cl₆ ( x = 0, 0.25, 0.5, 0.75, and 0.9), that can be tuned from indirect band gap ( x = 0, 0.25, and 0.5) to direct band gap ( x = 0.75 and 0.9) are designed. Direct band gap NCs exhibit 3 times greater absorption cross section, lower sub-band gap trap states, and >5 times photoluminescence quantum efficiency (PLQE) compared to those observed for indirect band gap NCs (Cs₂AgBiCl₆). A PLQE of 36.6% for direct band gap NCs is comparable to those observed for lead perovskite NCs in the violet region. Besides the band edge violet emission, the direct band gap NCs exhibit bright orange (570 nm) emission. Density functional theory calculations suggesting forbidden transition is responsible for the orange emission, which is supported by time-resolved PL and PL excitation spectra. The successful design of lead-free direct band gap perovskite NCs with superior optical properties opens the door for high-performance lead-free perovskite optoelectronic devices.