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Exceptional Grain-Growth in Formamidinium Lead Iodide Perovskite Thin Films Induced by the #-to-# Phase Transformation Srinivas K. Yadavalli, Yuanyuan Zhou, and Nitin P Padture ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01150 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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ACS Energy Letters

Exceptional Grain-Growth in Foramidinium Lead Iodide Perovskite Thin Films Induced by the δ-to-α α Phase Transformation Srinivas K. Yadavalli, Yuanyuan Zhou and Nitin P. Padture* School of Engineering, Brown University, 184 Hope Street, Providence, RI 02912, United States

Abstract The annealing of fine-grained (~0.175 µm) ‘yellow’ δ-FAPbI3 non-perovskite thin films at 100 ˚C in DMSO vapor for a few minutes results in exceptional grain-growth (~5 µm) induced by their transformation to the desirable ‘black’ α-FAPbI3 perovskite phase. Mechanistic insights into this unprecedented phenomenon are provided.

Graphical Abstract

Organic-inorganic halide perovskites (OIHPs) light-absorbers are at the heart of the new thin-film perovskite solar cells (PSCs).1 While methylammonium lead triiodide (MAPbI3) light-absorber (bandgap ~1.55 eV) is the most widely studied, there has been growing interest in formamidinium lead iodide (FAPbI3) due its more suitable band gap (~1.45 eV) and inherently superior thermal stability.2-6 However, the formation of

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photoinactive ‘yellow’ δ-FAPbI3 phase (hexagonal, space group P63/mmc) during the solution-processing of photoactive ‘black’ α-FAPbI3 (cubic, space group Pm-3m) OIHP polycrystalline thin films using established protocols has been a major impediment.4 Composition-invariant δ→α polymorphic transformation in FAPbI3 can be achieved, but it requires prolonged heat-treatments at elevated temperatures, which can also end up degrading the films.7 Moreover, most of the solution-processing methods proposed so far produce finegrained α-FAPbI3 thin films with a high-density grain-boundary network, which is detrimental to the PSCs performance: grain boundaries scatter photocarriers, serve as recombination sites, and they also allow facile ion-migration and moisture ingression.8-10 Thus, for PSCs application, it is highly desirable to have large OIHP grains that are several times the film thickness.11 In this context, we have discovered a new phenomenon, where fine-grained (~175 nm) δ-FAPbI3 thin films (~300 nm thickness) transform rapidly to phase-pure α-FAPbI3 OIHP thin films with ultra-large grain size exceeding an unprecedented ~5 µm. This is realized by annealing the δ-FAPbI3 thin films at a relatively low temperature of 100 ˚C in a dimethyl sulfoxide (DMSO) solvent-vapor environment for just a few minutes. Figure 1A is the top-surface SEM image of the initial δ-FAPbI3 thin film, and Figs. 1B and 1C are of those DMSO-vapor-annealed for 3 and 15 min, respectively. The corresponding X-ray diffraction (XRD) patterns are presented in Figs. S1 and S2 in the Supporting Information. The δ→α transformation proceeds through the nucleation and growth of the dark clusters of the product phase (Fig. 1B) within the δ-FAPbI3 matrix. (The dark contrast is due to its higher electrical conductivity and the attendant reduction in the ‘charging’ effect in the SEM.) Electronbackscatter electron diffraction (EBSD) from two different areas within the cluster show similar Kikuchi patterns in Fig. 1B, indicating the same crystallographic orientation of the entire cluster, i.e. each dark cluster is a true single-crystal, albeit with a rough ‘faceted’ 2 Environment ACS Paragon Plus

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ACS Energy Letters

surface morphology (inset). Similarly, the SEM and the EBSD results in Fig. 1C confirm that the grains in the final microstructure of the fully-transformed α-FAPbI3 thin film are also true single-crystals. The inset in Fig. 2 is a cross-section of that final α-FAPbI3 thin film, with the adjacent vertical grain boundaries marked by arrows.

Figure 1. (A) Top-surface SEM image of the initial δ-FAPbI3 thin film. Top-surface SEM images, and corresponding EBSD Kikuchi patterns, of the initial thin film DMSO-vapor annealed at 100 °C for: (B) 3 min and (C) 15 min. Insets: higher-magnification SEM images, same magnification as (A).

Figure 2 plots the average grain sizes as a function of DMSO-vapor-annealing duration. The average size of the δ-FAPbI3 grains increased from 175±20 nm to 650±73 nm during the first three minutes of annealing. This grain growth is attributed to the lowering of the grain-boundary migration energy in the DMSO-vapor environment, which is similar to the solvent-vapor annealing of MAPbI3

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and (FA,Cs)Pb(I,Br)

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thin films. However,

beyond 2.5 min annealing, α-FAPbI3 clusters are found to nucleate (Fig. S3B) and grow rapidly as they consume the fine-grained parent δ-FAPbI3 matrix. At 5 min, the thin film is almost completely transformed to α-FAPbI3 (Fig. S1), and the average size of the α-FAPbI3 grains has jumped to 4.78±0.65 µm. Remnants of the surface ‘faceting’ are still visible on the top surface of that thin film (Fig. S3C). Upon further annealing for 15 min, the grain surfaces



become smoother, and the grains are better defined, but the grain growth is not appreciable. Control δ-FAPbI3 thin films annealed at 100 ˚C for 15 min in N2 atmosphere show no phase transformation or grain growth (Fig. S4). Prolonged N2-annealing for 120 h did result in some δ→α transformation. These results confirm the critical role of DMSO-vapor in the δ→α transformation in FAPbI3 thin films, and the concomitant exceptional grain-growth.

Figure 2. Average FAPbI3 grain size in the thin film as a function of DMSO-vapor annealing (100 °C) duration. Inset: Cross-sectional SEM image of thin film annealed for 15 min (arrows indicate adjacent vertical grain boundaries).

The δ→α transformation in FAPbI3 is reconstructive in nature, where the (PbI6)4octahedra are face- and corner-shared in δ-FAPbI3 and α-FAPbI3, respectively.14-15 Therefore, the FA+, Pb2+, and I- ions must undergo a complex combination of sliding and twisting, involving thermally-activated breakage and reformation of bonds. Since this is a composition-invariant polymorphic transformation, only short-range thermally-activated attachment and detachment of atomic/molecular species across the δ-α interface is needed. In this context, in situ neutron diffraction studies have shown that during slow heating, δFAPbI3 transforms to α-FAPbI3 at ~77 ˚C.14 Thus, the δ→α transformation is thermodynamically favorable above ~77 ˚C, but it is kinetically suppressed through an


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ACS Energy Letters

activation barrier. It is clear that DMSO-vapor molecules interact with the δ-FAPbI3 thin films in a unique way to facilitate their rapid transformation to α-FAPbI3 OIHP, indicating a significant lowering of the activation barrier in the presence of DMSO vapor. Vapor of other polar solvents also has a similar effect, but DMSO is found to be most effective due to its very high polarity index of 7.2. Since the annealing temperature (100 ˚C) is relatively low, the kinetics of α-FAPbI3 nucleation are expected to be slow.11 Thus, the few available singlecrystal α-FAPbI3 nuclei grow, while consuming the parent fine-grained δ-FAPbI3 matrix, resulting in the unprecedented ultra-large grain size of ~5 µm. This effect could be extended to other FA-based compositions as well. Thus, ultra-large-grained FA-based OIHP thin films approaching ‘single-crystals’ made by controlling the δ→α phase transformation kinetics are likely to have profound implications on the development of OIHP-based PSCs and other optoelectronic devices with enhanced performance.

ASSOCIATED CONTENT Supporting Information Experimental procedures and four figures (Figs. S1-S4).

AUTHOR INFORMATION Corresponding Author *

Email: [email protected] (N.P.P.).

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS



This research is supported by the Office of Naval Research (N00014-17-1-2232) and the National Science Foundation (OIA-1538893). Experimental assistance from Q. Williams is gratefully acknowledged. REFERENCES 1.

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