Phase Stability of CsPbI3 Perovskite Nanocrystals

All-inorganic Pb-halide perovskite materials are of immense interest for the optoelectronics and photo-voltaics applications. One prominent member of this group is CsPbI3. However, the cubic phase of CsPbI3, one with most desirable bandgap for photo-voltaic (PV) application, is stable at high temperature of over 320 C. While different groups have explored this compound as PV materials with improved compositional stability, the phase stability remains a concern. Therefore, its adoption within the community for PV application is limited.

In this work stabilization of cubic phase of CsPbI3 is attempted by means of producing nanocrystals. Nanocrystalline CsPbI3 is produced by colloidal method. Rietveld refinement of XRD data shows stability of cubic phase even at room temperature. It is also shown that less than 50 nm sized nanocrystal of CSPbI3 remains cubic at room temperature. This arises mainly due to surface and strain energy effect which are quite significant for nanometric sizes but their contributions are generally vanishingly small for bulk sizes. It is likely that such surface and strain energy effect at small sizes ( 50 nm) is rendering the cubic structure metastable at room temperature for CsPbI3 nanocrystals. Monotonic increase in lattice constant with decreasing particle size in nanocrystalline CsPbI3 is also observed. It has been suggested that the lattice expansion is due to the of electrostatic relaxation as a result of size confinement, reduced ionic valences and consequent decrease in electrostatic force at small sizes. It is possible that such lattice expansion with decreasing particle size is helping in stabilizing the cubic phase at room temperature. Perovskite quantum dot (QD) solar cells of CsPbI3 showed power conversion efficiency of 10.77 %. This devices also function as light emitting diodes (LEDs) with low turn-on voltage and tunable emission. The synthesis of normally unstable material phases stabilized through colloidal QD synthesis provides another mechanism for materials design for photovoltaics, LEDs, and other applications. The photoluminescence behaviors of CsPbI3 perovskite NCs are advantageous over those of traditional CdSe based cQDs, and therefore are better candidate for optoelectronics application.