Ligand-Free, Quantum-Confined Cs₂SnI₆ Perovskite Nanocrystals

Abstract

Tin-halide\nperovskite nanocrystals are a viable precursor for lead-free,\nhigh-efficiency active layers for photovoltaic cells. We describe\na new synthetic procedure for quantum-confined Cs₂SnI₆ nanocrystals with size-dependent band gaps in the long-visible\nto near-infrared (1.38–1.47 eV). Hot injection synthesis produces\nparticles with no organic capping ligands, with average diameters\nthat increase from 12 ± 2.8 nm to 38 ± 4.1 nm with increasing\nreaction temperature. The band gap, energies of the first excitonic\npeak, ground-state bleach peak (in the transient absorption spectrum),\nand photoluminescence peak depend linearly on the inverse square of\ndiameter, consistent with quantum-confined excitons with an effective\nmass of (0.12 ± 0.02)<i>m</i>₀, where <i>m</i>₀ is the mass of an electron, a factor of 4.6\nsmaller than that in the bulk material. Transient absorption measurements\nshow that approximately 90% of the bleach amplitude decays within\n30 ps, probably because of carrier trapping on unpassivated surface\nsites. The films made by simple drop-casting of Cs₂SnI₆ nanocrystal solutions, with no postsynthetic ligand exchange\nor removal, are smooth and uniform, resist delamination, and have\nno visible gaps at the film–substrate interface.