Bypassing Solid-State Intermediates by Solvent Engineering the Crystallization Pathway in Hybrid Organic–Inorganic Perovskites

Abstract

Hybrid organic–inorganic perovskites (HOIPs), such as CH3NH3PbI3 (MAPbI3), are attractive for inexpensive, high-performance solar cells. Controlling HOIP thin-film quality and morphology, which is essential to achieve consistent solar-cell efficiencies, requires a fundamental understanding of the link between solution chemistry and crystallization pathways. To elucidate the effect of solvent choice and solution speciation on the crystallization pathway, we combined computational modeling of molecular-level solvent–solute interactions in HOIP growth solutions with experimental monitoring of film phase evolution. Using density functional theory calculations and a Bayesian optimization-based approach (PAL), we exhaustively searched the HOIP/solvent combinatorial space to obtain a ranked list of increasing HOIP/solvent intermolecular binding energy. Then, using in situ X-ray diffraction, we tested solvents of varying coordinating abilities with MAPbI3 to correlate the PAL-generated ranking with the crystallization pathway. Weakly coordinating solvents (e.g., N-methyl-2-pyrrolidone) formed the perovskite via a two-step crystallization pathway, where a crystalline intermediate formed and was subsequently transformed to perovskite with heating, whereas strongly coordinating solvents (e.g., tetrahydrothiophene 1-oxide) formed the perovskite directly from solution. We propose that the solution coordination chemistry determines the crystallization pathway. Our integrated experimental–computational approach could be applied to study the interplay between solution chemistry and crystallization pathways for other solution-grown materials.

Hybrid perovskite crystallization from binary solvent mixtures: interplay of evaporation rate and binding strength of solvents

“…As a result of this reorganization, the film approaches supersaturation and crystalline intermediate phases can be formed. 27,28 The common failure to observe the diffraction pattern of the GBL-MAPbI3 intermediate phase during crystallization can be attributed to the presence of water ( Figure S1). Nevertheless, comparing the molecular sizes of DMF and GBL, we find that their molecular dimensions are similar, with the longest dimension of the GBL molecule being only 0.07 Å larger than DMF.…”

“…30 For NMP, the formation of a crystalline intermediate phase was indicated by a diffraction peak at 2θ = 8.2°, similarly to previously reported. 27,31,32 The diffraction patterns of the intermediate phases in the extended 2θ range are shown in Figure S3. To further quantify the difference in the crystallization onsets, we have calculated the evaporation rates, REvap, for these solvent systems as a function of time using an evaporation model described in detail in the Supporting Information (SI).…”

“…This peak then evolves into the peak at 8.18°, typical for the pure NMP intermediate structure ( Figure S4). 27,31,32 NMP mixture at 40 °C. These vast differences in the crystallization onset can be rationalized through the calculation of the total vapor pressure and evaporation rates of the binary mixtures.…”

Formation of Lead Halide Perovskite Precursors in Solution: Insight from Electronic‐Structure Theory

“…[15][16][17] Many studies have contributed to clarify the role of solventÀsolute interactions in determining the crystal structure and the properties of the grown films. [18][19][20][21][22][23][24][25][26][27][28] An intricate interplay between solubility, polarity, dielectric screening, and coordinating ability has been accounted responsible for the efficacy of different solvents to form complexes with LHPs. [20,21,29] Furthermore, the choice of the solvent turned out to be a critical parameter for the formation of intermediate phases [19,[30][31][32][33][34] and even for the creation of defects in the final products.…”

Intermediates transformation for efficient perovskite solar cells

“…Solvents and solution speciation are linked to perovskite quality. Weakly coordinating solvents formed an intermediate and transformed to perovskite after annealing, while strongly coordinating solvents formed perovskite directly [31]. Typically, PbI 2 and other solvents are selected as the intermediates, because ionic bonding in I-Pb-I layer is strong enough, and Van der Waals interaction appears between adjacent sandwiched layers, making PbI 2 possible to be intercalated by different guest molecules [32].…”