Lead-based perovskite nanocrystals (NCs) have outstanding optical properties and cheap synthesis conferring them a tremendous potential in the field of optoelectronic devices. However, two critical problems are still unresolved and hindering their commercial applications: one is the fact of being lead-based and the other is the poor stability. Lead-free all-inorganic perovskite Cs Bi X (X=Cl, Br, I) NCs are synthesized with emission wavelength ranging from 400 to 560 nm synthesized by a facile room temperature reaction. The ligand-free Cs Bi Br NCs exhibit blue emission with photoluminescence quantum efficiency (PLQE) about 0.2 %. The PLQE can be increased to 4.5 % when extra surfactant (oleic acid) is added during the synthesis processes. This improvement stems from passivation of the fast trapping process (2-20 ps). Notably, the trap states can also be passivated under humid conditions, and the NCs exhibited high stability towards air exposure exceeding 30 days.

Lead-free double-perovskite nanocrystals (NCs), that is, Cs2AgIn x Bi1–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.

Lead-free perovskite nanocrystals (NCs) were obtained mainly by substituting a Pb cation with a divalent cation or substituting three Pb cations with two trivalent cations. The substitution of two Pb cations with one monovalent Ag and one trivalent Bi cations was used to synthesize Cs AgBiX (X=Cl, Br, I) double perovskite NCs. Using femtosecond transient absorption spectroscopy, the charge carrier relaxation mechanism was elucidated in the double perovskite NCs. The Cs AgBiBr NCs exhibit ultrafast hot-carrier cooling (<1 ps), which competes with the carrier trapping processes (mainly originate from the surface defects). Notably, the photoluminescence can be increased by 100 times with surfactant (oleic acid) added to passivate the defects in Cs AgBiCl NCs. These results suggest that the double perovskite NCs could be potential materials for optoelectronic applications by better controlling the surface defects.