Ligand-Free, Quantum-Confined Cs₂SnI₆ Perovskite Nanocrystals

Tin-halide perovskite nanocrystals are a viable precursor for lead-free, high-efficiency active layers for photovoltaic cells. We describe a new synthetic procedure for quantum-confined Cs₂SnI₆ nanocrystals with size-dependent band gaps in the long-visible to near-infrared (1.38-1.47 eV). Hot injection synthesis produces particles with no organic capping ligands, with average diameters that increase from 12 ± 2.8 nm to 38 ± 4.1 nm with increasing reaction temperature. The band gap, energies of the first excitonic peak, ground-state bleach peak (in the transient absorption spectrum), and photoluminescence peak depend linearly on the inverse square of diameter, consistent with quantum-confined excitons with an effective mass of (0.12 ± 0.02)m₀, where m₀ is the mass of an electron, a factor of 4.6 smaller than that in the bulk material. Transient absorption measurements show that approximately 90% of the bleach amplitude decays within 30 ps, probably because of carrier trapping on unpassivated surface sites. The films made by simple drop-casting of Cs₂SnI₆ nanocrystal solutions, with no postsynthetic ligand exchange or removal, are smooth and uniform, resist delamination, and have no visible gaps at the film-substrate interface.