Amino-Deliquescence and Amino-Efflorescence of Methylammonium Lead Iodide

Abstract

Driven by the exceptional optoelectronic performance and prospective applications of organic−inorganic hybrid perovskites (HPs), an array of methods to synthesize and process HPs has been developed. Although most studies focus on solution processing, a number of reports have examined vapor-phase effects, such as the unusual liquefaction of HP films when exposed to methylamine (MA 0 ) vapor. Here, using in situ spectroscopy and microscopy, we examine the thermodynamics and kinetics of the liquefaction and recrystallization of methylammonium lead iodide (MAPbI 3 ) films with MA 0 and find that the phenomena are best described as amino-deliquescence and amino-efflorescence, respectively. By constructing a quantitative phase diagram, we show that aminodeliquescence is driven by the highly exothermic dissolution of MAPbI 3 by MA 0 with a heat of solution of approximately −96 kJ mol −1 , which drives the condensation of MA 0 at a pressure more than two orders of magnitude below the equilibrium vapor pressure. Surprisingly, the dissolution is accompanied by a decrease in entropy of ∼173 J mol −1 K −1 , suggesting the formation of a liquid state with the semi-ordered MA 0 solvent. Kinetic analysis of aminoefflorescence reveals nucleation and growth rates that decrease and increase, respectively, with increasing temperature, which together yield thin-film grain sizes that increase exponentially with temperature to produce millimeter-sized grains. The findings reveal amino-deliquescence as a highly driven thermodynamic process that is potentially a general effect for HP materials in the presence of amines. The apparently ordered nature of the liquid and large grain size after amino-efflorescence may provide a further pathway for control over morphology, crystallinity, and composition of HP systems.

Lithographically Patterning Hybrid Perovskite Single Crystals by Surface-Engineered Amino-Deliquescence/Efflorescence

“…We recently showed that the nucleation and growth rates during amino-efflorescence show an exponentially decreasing and increasing dependence on increasing temperature, respectively. 37 As shown by the red dashed line in Figure 1a, this dependence caused the apparent grain size of thin films to vary from microns at room temperature to millimeters at temperatures near 100 °C. Here, we more thoroughly analyze the crystallinity and orientation of MAPbI 3 thin film samples spin-cast on FTO-coated glass substrates after an aminodeliquescence/efflorescence cycle at a controlled p MA0 and temperature inside a hot-walled quartz tube vacuum reactor.…”

“…A full list of substrates and their preparation is included in Table S1. For reference, our prior study 37 of amino-deliquescence/efflorescence was performed using spin-cast thin films on fluorinedoped tin oxide (FTO) glass substrates, having a MA 0 (MAPbI 3 ) n (l) contact angle of ∼32°.…”

“…MAPbI 3 thin films were spin-cast on FTO-coated glass slides using a toluene antisolvent method as described previously. 37 Microscopy. Optical brightfield imaging was performed using a CMOS camera fitted on a zoom lens tube, backlit with a fiber-coupled white light source with light-diffusing material between the fiber output and the sample.…”

“…36 We recently examined these phenomena under controlled temperature and pressure conditions to construct the phase diagram shown in Figure 1a, and we found through thermodynamic and kinetic analyses that the processes are best described as amino-deliquescence and amino-efflorescence. 37 Amino-deliquescence occurs when the atmospheric MA 0 pressure (p MA0 ) exceeds a critical value (p del ), corresponding to the vapor pressure of MA 0 in a droplet of MA 0 (MAPbI 3 ) n (l), where n denotes the stoichiometry of the saturated solution. When p MA0 > p del , there is a thermodynamic driving force for liquefaction into MA 0 (MAPbI 3 ) n (l).…”

“…The reverse process, efflorescence, occurs when the relative humidity drops below the critical value, causing recrystallization. − In the presence of methylamine (MA 0 ) gas, MAPbI 3 thin films undergo similar phenomena, liquefying in the presence of the MA 0 vapor and recrystallizing when it is removed . We recently examined these phenomena under controlled temperature and pressure conditions to construct the phase diagram shown in Figure a, and we found through thermodynamic and kinetic analyses that the processes are best described as amino-deliquescence and amino-efflorescence . Amino-deliquescence occurs when the atmospheric MA 0 pressure ( p MA0 ) exceeds a critical value ( p del ), corresponding to the vapor pressure of MA 0 in a droplet of MA 0 (MAPbI 3 ) n ( l ), where n denotes the stoichiometry of the saturated solution.…”