Lead iodide perovskite (LIP) nanocrystals are promising materials for optoelectronic applications including solar cells and red-light-emitting diodes because of their narrow band gaps and strong emission in the red-to-near infrared range. However, stabilizing the LIP nanocrystals in the photoactive perovskite phase is still challenging. In our prior studies, we compared the thermal-induced structural evolutions of MAPbI₃ nanocrystals and CsPbI₃ nanocrystals using in-situ wide- and small-angle X-ray scattering (GIWAXS/GISAXS). We found that MAPbI₃ nanocrystals underwent a phase transformation from tetragonal to cubic perovskite phase at 60 ^oC, followed by chemical decomposition into PbI₂ and volatile species at 90 ^oC. The poor thermal stability of MAPbI₃ nanocrystals originates from the volatile MA cations. All-inorganic CsPbI₃ nanocrystals are more stable, which transformed from the metastable black perovskite γ-phase to non-perovskite photo-inactive δ-phase at relatively high temperatures (190-200 ^oC). These thermal-induced phase transformations are not reversible and led to the degradation of photoluminescence.
To modulate the thermal stability of LIP nanocrystals, we designed Cs_1-xMA_xPbI₃ nanocrystals using the A-site compositional alloying strategy. The direct synthesis of Cs_1-xMA_xPbI₃ nanocrystals has suffered from a limited content of Cs incorporation (<0.2). To overcome this substitution limit, we developed a post-synthetic A-site cation cross-exchange at room temperature in a mixture of phase-pure CsPbI₃ and MAPbI₃ nanocrystals. The resulting Cs_1-xMA_xPbI₃ nanocrystals achieved a wide range of cation compositions with tunable photoluminescence, and remained in the photoactive perovskite phase with incorporated Cs content as high as 0.74. Then, we monitored the kinetics of thermal-induced phase transformation processes in Cs_1-xMA_xPbI₃ nanocrystals compared with CsPbI₃ and MAPbI₃ nanocrystals. The rate constant of phase transformation is determined from the Avrami modeling, and Cs_1-xMA_xPbI₃ nanocrystals showed one order of magnitude smaller rate constant than that of CsPbI₃ or MAPbI₃ nanocrystals. This work not only expands the composition range of phase-pure and red-emitting LIP perovskite nanocrystals, but also points out that A-site compositional alloying is a promising approach to enhancing the stability of perovskite nanocrystals.

Brian Korgel

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