Nanosize and Interfacial Strain Induced Crystal Phase Selectivity in Cesium Lead Iodide
Authors:   , Chakrabarti Arkita , Fafarman Aaron T. 
Published: September 2023

Perovskite phase cesium lead iodide (CsPbI3) nicknamed as "black phase" makes a worthy candidate as an absorber layer in photovoltaic devices owing to their excellent optoelectronic properties such as high absorptivity, bulk defect tolerance, high charger carrier lifetime and a suitable band gap. However, the low density, cubic, perovskite phase is thermodynamically favored only at temperatures above 320Ëš C, below which, after undergoing a series of octahedral tilts it transforms into a high density, edge sharing, orthorhombic structure called the "yellow phase" with a non-functional band gap. Several reports have achieved lowered phase transition temperatures and increased metastability of perovskite-phase CsPbI3 and attributed the effect variably to reduced size, surface-induced strain, and/or external additives. While theory supports that both reduced size and substrate-induced strain can tilt the competition between surface and bulk energies to favor the perovskite phase, there has not been a study of the relative influence of size and interfacial interactions on these dynamics. In this dissertation, we ask the question: how do fundamental parameters such as size, surface chemistry and surface strain alter the phase transition thermodynamics of CsPbI3? In the first study, we selectively varied crystalline domain size by embedding CsPbI3 in open nanoporous titania scaffolds composed of particles ranging from 20 to 200 nm and measured the influence on phase transition behavior. We modified the surface of the scaffold using polar, ionizable, and nonpolar silane self-assembled monolayers (SAMs) to systematically test the extent to which the chemical identity of the interface impacts the surface energy contribution to the phase stability in the perovskite-scaffold composite. While structural and functional consequences were observed for different interfacial chemistries, there was no significant influence on phase equilibrium, and the disruption of long-range crystalline order alone was found to be sufficient to depress the phase transition temperature by more than 250 °C compared to bulk. In the second study of this dissertation, we tested the effect of interfacial strain in thermodynamically altering the crystalline structure of CsPbI3. Tensile strain generated due to the mismatch in thermal expansion coefficients between thin-film CsPbI3 and its substrate (so called substrate-clamping) has been shown to impart metastability of the otherwise unstable perovskite phase at room temperature. However, such substrate-induced stress is biaxial, resulting in an opposing strain in the third, orthogonal axis. Herein, we extend such engineered interfacial strain to all three dimensions to approximate the condition of 'negative pressure,' i.e., 3D tensile stress. Using a generalizable approach, we show negative pressure stabilizes the symmetric, low density, cubic perovskite phase of CsPbI3. In this work, we crystallized CsPbI3 in the perovskite phase inside rigid oxide scaffolds at elevated temperatures and quenched them. The three-dimensional tensile strain generated by this thermally induced stress differs from its biaxial counterpart in that it is uniquely capable of imparting thermodynamic stability to higher symmetry crystal phases within the perovskite family of polymorphs. Higher symmetry is associated with increased bond-overlap in the octahedral network, which is desirable for reducing the bandgap and increasing the absorptivity. We used X-ray diffraction and high-resolution transmission electron microscopy to identify the phase and estimate lattice expansion as a function of thermal excursions imposed on the CsPbI3-scaffold composite. Photoluminescence studies showed bandgap tunability as a consequence of negative pressure.