The perovskite phase of cesium lead iodide (α-CsPbI or "black" phase) possesses favorable optoelectronic properties for photovoltaic applications. However, the stable phase at room temperature is a nonfunctional "yellow" phase (δ-CsPbI). Black-phase polycrystalline thin films are synthesized above 330 °C and rapidly quenched to room temperature, retaining their phase in a metastable state. Using differential scanning calorimetry, it is shown herein that the metastable state is maintained in the absence of moisture, up to a temperature of 100 °C, and a reversible phase-change enthalpy of 14.2 (±0.5) kJ/mol is observed. The presence of atmospheric moisture hastens the black-to-yellow conversion kinetics without significantly changing the enthalpy of the transition, indicating a catalytic effect, rather than a change in equilibrium due to water adduct formation. These results delineate the conditions for trapping the desired phase and highlight the significant magnitude of the entropic stabilization of this phase.

Synthesis and purification of CsPbX3 nanocrystals: The synthesis followed the procedure of Protesescu et al. 1 with only slight modifications. 0.204 g of Cs2CO3 (Aldrich, 99.9%), 10 mL 1-octadecene (ODE, Sigma-Aldrich, 90%) and 0.63 mL of Oleic acid (OA, Sigma-Aldrich, 90%) were charged in a 3-neck flask and degassed at 120 °C for 1 h. The mixture temperature was raised to 150 °C under a N2 atmosphere and held there until the formation of Cs-oleate was observed by the complete dissolution of the solid. The solution temperature was maintained at 100 °C to avoid any precipitation. To synthesize the nanocrystals, 0.75 mmol of PbX2 (PbI2: 0.346 g, Strem Chemicals, 99.999%; PbCl2: 0.209 g, Aldrich, 99.999%), along with 20 ml of ODE, were combined in a 3-neck flask and degassed for 1 h at 120 °C. 4 mL of dried OA and 4 mL of dried Oleylamine (OLA, Aldrich, 70%) were injected at 120 °C under N2. For the PbCl2, 4 mL of tri-n-octylphosphine (TOP, Strem, 97%) was also added. Once PbX2 dissolved completely the mixture temperature was increased to 165 °C and 1.6 mL of Cs-oleate solution was injected rapidly. After a short wait (10 s), the synthesis product was quenched by an ice-water bath. After the synthesis, the reaction mixture was treated with an anti-solvent to flocculate the nanocrystals,

Hybrid organic/inorganic lead iodide perovskites of the formula APbI3, where A is a molecular cation such as methylammonium, exhibit remarkably slow photoinduced charge carrier recombination rates, for reasons that remain uncertain. Prevalent hypotheses credit this behavior to the unique dipolar nature of the molecular cation. Herein, transient terahertz spectroscopy is applied to solution-processed, all-inorganic, perovskite-phase cesium lead iodide (CsPbI3) thin films, which lack such a dipole. The recombination kinetics are studied as a function of the initial photoinduced carrier concentration and the wavelength of excitation. A kinetic model combining diffusion and recombination is fit to the data, from which the rate constants are determined, revealing a bimolecular recombination rate of 10–10 cm3 s–1, comparable to high-quality, single-crystal, direct-gap semiconductors. This rate, as well as a charge carrier mobility > 30 cm2 V–1 s–1 measured herein for CsPbI3, are similar to values reported for the hybrid perovskites, strongly suggesting that the organic cation does not confer a fundamental advantage.

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