Abstract
Metal halide perovskite crystal structures have emerged as a class of optoelectronic materials, which combine the ease of solution processability with excellent optical absorption and emission qualities. Restricting the physical dimensions of the perovskite crystallites to a few nanometers can also unlock spatial confinement effects, which allow large spectral tunability and high luminescence quantum yields at low excitation densities. However, the most promising perovskite structures rely on lead as a cationic species, thereby hindering commercial application. The replacement of lead with nontoxic alternatives such as tin has been demonstrated in bulk films, but not in spatially confined nanocrystals. Here, we synthesize CsSnX3 (X = Cl, Cl0.5Br0.5, Br, Br0.5I0.5, I) perovskite nanocrystals and provide evidence of their spectral tunability through both quantum confinement effects and control of the anionic composition. We show that luminescence from Sn-based perovskite nanocrystals occurs on pico- to nanosecond time scales via two spectrally distinct radiative decay processes, which we assign to band-to-band emission and radiative recombination at shallow intrinsic defect sites.
Highlights
Metal halide perovskite crystal structures have emerged as a class of optoelectronic materials, which combine the ease of solution processability with excellent optical absorption and emission qualities
While the first effect relies on the different ionization potentials of the various halide components, the second phenomenon is unlocked in quantum-confined nanostructures where the optical bandgap increases as the size is reduced.[11,12]
At modest excitation densities the photoluminescence quantum efficiency (PLQE) for perovskite nanocrystals can be much higher than for bulk films of the same material,[4,9] which is advantageous for achieving low lasing thresholds[5] and highly efficient light-emitting diodes (LEDs).[13]
Summary
Mixed halide perovskite particles due to less intense and broader reflections (see Figure 1a). As is observed for the bulk materials, the bandgaps of tin-containing perovskite nanocrystals are red-shifted compared to analogous lead-based particles,[23] most likely due to the higher electronegativity of the tin ion occupying the ‘B’ site in the ABX3 perovskite structure.[16] Compositional changes of the halide component allowed tuning of the optical bandgap, as has been shown for APbX3 perovskite nanoparticles (see Figure 1b).[8,9,24] In addition to varying the halide ratio of the SnX2 precursor during synthesis, the halide composition can be adjusted postsynthetically via an anion-exchange reaction using different, pure-halide nanocrystals. We interpret our observation of an accelerated fast PL decay component for particles stored under inert atmosphere for 1 week (see Figure 3b) and the accompanied quench in PLQE as the consequence of nonradiative decay at deep acceptor states which have been developed slowly after synthesis The formation of such deep acceptor sites can be produced by very subtle structural rearrangements[33] which may not be detectable in our XPS measurements.
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