Directing nucleation and growth kinetics in solution-processed hybrid perovskite thin-films

Abstract

A heightened understanding of nucleation and growth mechanisms is paramount if effective solution processing of organic-inorganic perovskite thin-films for optoelectronic applications is to be achieved. Many fabrication techniques have been utilized previously to develop high-performance perovskite layers but there remains an absence of a unifying model that describes accurately the formation of these materials from solution. The present study provides a thorough analysis of nucleation and growth kinetics underpinning the development of hybrid organic-inorganic perovskite thin-films. Through precise control of the perovskite growth conditions the spacing of heterogeneous nucleation sites was varied successfully from several hundred nanometers to several hundred microns. The crystalline regions surrounding these nuclei were found to comprise clusters of highly-oriented crystal domains exceeding 100 μm in diameter. However, no beneficial correlation was found between the size of these well-oriented grain-clusters and the optoelectronic performance. The formation of the perovskite microstructure features characteristics of both classical and non-classical growth mechanisms. The insights into perovskite thin-film growth developed by the present study provide clear implications for the development of future hybrid perovskite microstructures.

Engineering Halide Perovskite Crystals through Precursor Chemistry

“…In fact, perovskite thin films with millimeter‐scale crystalline grains have been reported by spin‐coating the hot precursor solution (70 °C) onto the heated substrates maintained at 190 °C (see Figure a,b) . The resulting thin films exhibit lower nucleation density at first glance, while further study revealed that the millimeter scale grains were also composed of polycrystalline microstructure with a grain size of <500 nm, instead of single‐crystalline domains as shown in Figure c 45a…”

“…Nonclassical growth theories, such as Volmer–Weber growth (island growth) behavior, Ostwald ripening, and oriented‐attachment45a have been used to try to describe the growth mechanism of perovskite crystals. Briefly, in Volmer–Weber growth, the precursor species favors mutual binding rather than binding to the substrate, the small clusters that do nucleate on the substrates surface form central nucleation sites, then islands initially coalesce from the pre‐existed nuclei until the growth is interrupted by a neighboring island.…”

“…In the case of oriented‐attachment theory, a single domain can be self‐assembled by joining small particles at a planar interface, which shares a common crystallographic orientation to minimize the surface free energy . Despite the absence of strong evidence to suggest that particles agglomerates exist within the precursor prior to film formation,45a the oriented‐attachment growth behavior is consistent with the results that the presence of defects, such as misfit dislocations, twin structures, and super‐lattices are formed at the interface 21,45a,72. In addition, Hu et al proposed the following nanoassemble model of the formation of perovskite polycrystalline thin films from precursor solutions that contains Pb(Ac) 2 and MAI: 1) formation of solvate intermediate phase 1: the stable Pb‐centered ion‐cage (MAI) x ‐PbI 2 ‐(DMF) y is formed on removal of the solvent, which is stable at room temperature and commonly reported in the literatures; 2) conversion to intermediate phase 2: when the solvent is further removed, the solvate intermediate phase (MAI) x ‐PbI 2 ‐(DMF) y are forced into contact and transform into more stable perovskite crystals with PbI 2 , MAI and another intermediate phase 2 (MAI) m ‐PbI 2 ‐(DMF) n , which should be centered with a [PbI 6 ] 4− octahedron and coordinated with MAI and DMF shells; 3) formation of perovskite: the solvate intermediate phase (MAI) x ‐PbI 2 ‐(DMF) y undergo rapid decomposition at elevated temperature, leading to the formation of perovskite crystals, in this case, the emergence of intermediate (MAI) x ‐PbI 2 ‐(DMF) y is suppressed 28b…”

Two-dimensional organic-inorganic hybrid perovskite: from material properties to device applications

“…The crystal growth kinetics are closely related to surface property of substrate, composition and concentration of precursors, additives, annealing temperature, and atmosphere condition [19][20][21][22]. Controlling the procedure of nucleation and growth during film deposition and annealing is a promising route to morphology management and device optimization [23,24]. For large-area preparation, it is generally difficult to obtain high-quality perovskite films, leading to moderate device efficiency.…”

Performance Enhancement of Perovskite Solar Cells Induced by Lead Acetate as an Additive

“…The perovskite crystallization process consists of supersaturation, nucleation and crystal growth. When the film reaches supersaturation due to the evaporation of solvent, nuclei start to form and grow to bigger grains . The nucleation density or rate is critical to the grain size and morphology of perovskite films.…”