Controlling Solvate Intermediate Growth for Phase-Pure Organic Lead Iodide Ruddlesden–Popper (C₄H₉NH₃)₂(CH₃NH₃)_n−1Pb_nI_3n+1 Perovskite Thin Films

Abstract

The growth of Ruddlesden–Popper perovskite thin films of organic lead halides is complicated by the existence of multiple crystallization pathways available to precursors in solution. During thin-film growth processes, such as spin-coating or blade-coating, solvents can evaporate too quickly to clearly resolve different reaction intermediates and products that form during crystallization. Here, we resolve multiple reaction products and intermediates that form during growth of (C4H9NH3)2(CH3NH3) n−1Pb n I3n+1 Ruddlesden–Popper compounds by studying drop-cast precursor solutions through the evolution of X-ray diffraction, photoluminescence, and optical micrographs in situ over long timescales in a thin-film geometry. We found that methylammonium-rich solvate intermediates play a crucial role in directing the bulk optical properties of the films and form simultaneously with smaller regions of Ruddlesden–Popper phases during growth. The microstructure and optical properties of these sub-phases were characterized during growth and after annealing, revealing that discrepancies between thin-film and single-crystal optical properties originate from solvate intermediates. These lower-band-gap minority phases dominate the optical emission spectrum by means of rapid energy migration and contribute to sub-band-gap electronic states in photovoltaic devices. Processing routes to yield thin films with optical properties similar to single crystals of Ruddlesden–Popper phases were developed by tuning the precursor stoichiometry and deposition kinetics.

Interfaces in metal halide perovskites probed by solid-state NMR spectroscopy

“…Layered MHPs with a nominal stoichiometry of n greater than 2 typically segregate into regions of higher and lower n-values due to solution-processing kinetics. 5,[235][236][237][238][239][240][241] These polycrystalline materials have optical and transport properties characteristic of bulk heterojunctions. 239,[242][243][244][245][246] The desirable stability of layered MHPs have inspired efforts to incorporate these phases within 3D MHPs, or as capping and passivation layers, for high-performing photovoltaic devices.…”

Optimization of Composition with Reduced Phase Impurity in Quasi‐2D Perovskite for Electroluminescence

“…[ 13,16 ] Higher phase impurity can result in inefficient energy transfer, and emission comes from individual phases rather than only the lowest bandgap phase. [ 17–19 ] Phase impurity can be controlled via having slight modulation of stoichiometric composition, [ 9 ] non‐stoichiometric compositions, [ 20 ] use of additive, [ 14 ] change of processing environment, e.g., ambient temperature variation, [ 21 ] thickness variation of the active layer, spin speed, change of host solvent, [ 22 ] and method of deposition. [ 23–25 ]…”

Mixed Spacer Cation Stabilization of Blue‐Emitting n = 2 Ruddlesden–Popper Organic–Inorganic Halide Perovskite Films

“…However, luminescence intensity is also dependent on the native defects in the films, which are affected by the deposition conditions. Therefore, we performed comprehensive optimization of the film deposition conditions for (BA 0.5 PEA 0.5 ) 2 MAPb 2 Br 7 and BA 2 MAPb 2 Br 7 films, including the investigation of the factors that known to affect crystallization and/or optical quality, such as the effects of precursor purity, solvents and/or additives used, and the use of antisolvents . The details are given in Note S1 and Figures S6–S12 of the Supporting Information.…”

“…RPPs have a general formula R 2 A n −1 B n X 3 n +1 , where R + is the bulky amine spacer cation, A + is an organic cation, B 2+ is a metal cation, X − is a halide anion, and n is the number of perovskite layers between the spacer cations. RPPs have been extensively studied since the choices of R and n enable wider tunability of properties compared to 3D perovskites ABX 3 . Emission tuning by changing n instead of using halide mixtures enables improved color stability compared to mixed halide LEDs which commonly exhibit peak shifts with time and/or bias due to phase segregation .…”

“…The existence of multiple n phases is undesirable for LED applications since it can lead to emission peak shift with increasing bias . Thus, considerable efforts have been made to control the phase composition and layer orientation in RPP thin films, especially those containing common spacer cations such as butylammonium (BA) and phenethylammonium (PEA) . Despite these efforts, the differences in behavior (processability, stability, and emission efficiency) of the RPP with BA and PEA spacer cations are not well understood, and the blue‐emitting RPPs typically contain multiple peaks in either absorption or emission spectra, indicating that phase purity has not been achieved.…”