Stabilization of the Metastable Lead Iodide Perovskite Phase via

Jun 8, 2017 - Similar growth behaviors were also observed when 4-fluorophenylethylammonium (4-FC6H5CH2CH2NH3+, FPEA) iodide was used (Figure S8). ...
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Letter pubs.acs.org/NanoLett

Stabilization of the Metastable Lead Iodide Perovskite Phase via Surface Functionalization Yongping Fu,† Tao Wu,‡ Jue Wang,§ Jianyuan Zhai,† Melinda J. Shearer,† Yuzhou Zhao,† Robert J. Hamers,† Erjun Kan,‡ Kaiming Deng,‡ X.-Y. Zhu,§ and Song Jin*,† †

Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, Wisconsin 53706, United States Department of Applied Physics, Nanjing University of Science and Technology, Nanjing, 210094, P. R. China § Department of Chemistry, Columbia University, New York, New York 10027, United State ‡

S Supporting Information *

ABSTRACT: Metastable structural polymorphs can have superior properties and applications to their thermodynamically stable phases, but the rational synthesis of metastable phases is a challenge. Here, a new strategy for stabilizing metastable phases using surface functionalization is demonstrated using the example of formamidinium lead iodide (FAPbI3) perovskite, which is metastable at room temperature (RT) but holds great promises in solar and light-emitting applications. We show that, through surface ligand functionalization during direct solution growth at RT, pure FAPbI3 in the cubic perovskite phase can be stabilized in nanostructures and thin films at RT without cation or anion alloying. Surface characterizations reveal that longchain alkyl or aromatic ammonium (LA) cations bind to the surface of perovskite structure. Calculations show that such functionalization reduces the surface energy and plays a dominant role in stabilizing the metastable perovskite phase. Excellent photophysics and optically pumped lasing from the stabilized single-crystal FAPbI3 nanoplates with low thresholds were demonstrated. High-performance solar cells can be fabricated with such directly synthesized stabilized phase-pure FAPbI3 with a lower bandgap. Our results offer new insights on the surface chemistry of perovskite materials and provide a new strategy for stabilizing metastable perovskites and metastable polymorphs of solid materials in general. KEYWORDS: Metastable polymorphs, lead halide perovskites, surface functionalization, perovskite photovoltaics, nanolasers, photophysics

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energies can potentially stabilize the metastable polymorphs in a thermodynamic sense. Many structural polymorphs and complex phase behaviors exist in hybrid organic−inorganic lead halide perovskites8,9 which have emerged as a new class of solution-processed semiconductors for high-performance optoelectronic applications.10−12 They have a general formula of APbX3, where A is an organic cation such as methylammonium (MA, CH3NH3+) or formamidinium [FA, CH(NH2)2+] and X is a halide anion. Despite the exciting progress, the commercialization of perovskite solar cells is hindered by the intrinsic instabilities of perovskite materials to heat, moisture, and even light. Particularly, the MAPbI3 is inherently thermally unstable at temperatures above 85 °C.13,14 The perovskite phase of FAPbI3 (α-phase, cubic, space group Pm3̅m, a = 6.3621 Å) may be a better solar material than MAPbI3, due to the improved thermal stability and a smaller bandgap.15−17 However, below 120 °C, FAPbI3 preferably crystallizes in a nonperovskite hexagonal

etastable structural polymorphs share the same chemical composition but different crystal structures from their corresponding thermodynamically stable phases and therefore can have different and sometimes superior electronic, optical, and other physical properties due to the different structures. For examples, diamond is the metastable polymorph of graphite, which is the thermodynamically more stable phase under ambient conditions; the metastable 1T-phase MoS2 polymorph is metallic and exhibits dramatically enhanced catalytic activity for the hydrogen evolution reaction than the semiconducting 2H-phase of MoS2.1 Hence, stabilizing crystal structures that are metastable in bulk is interesting for functional materials and may open up new applications.2−5 Metastable polymorphs often can be accessed by using high temperatures and/or pressures but may also spontaneously convert back to the thermodynamically stable phase under ambient conditions.4 Interestingly, the surface energies of these metastable polymorphs are often lower than that of the thermodynamically stable phase, such as in the cases of γ-Al2O3 vs α-Al2O3 and anatase vs rutile.6,7 When the energy contribution from the surface outweighs that from the bulk, especially at the nanoscale dimension, such different surface © 2017 American Chemical Society

Received: April 10, 2017 Revised: May 11, 2017 Published: June 8, 2017 4405

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Figure 1. Schematic illustration and structural characterizations of the different perovskite products grown under the presence of LA with various LA/FA molar ratios. (a) Different products are grown from lead precursor films (such as PbI2 or PbAc2) in precursor solutions with different αLA/FA molar ratios. The crystal structures from left to right are hexagonal phase of FAPbI3, cubic perovskite phase of FAPbI3, and layered perovskite of (LA)2(FA)Pb2I7, viewed along the [001], [111], and [100] directions, respectively. (b, c, d) SEM images of the hexagonal FAPbI3, cubic FAPbI3, and layered perovskite (BA)2(FA)Pb2I7 products grown from PbI2 films in a FAI solution, a mixed BAI/FAI solution with a αBA/FA of 0.86, and a mixed BAI/FAI solution with a αBA/FA of 2.57, respectively. The insets are the photographs of the as-grown samples, displaying yellow, black, and red color indicative of the respective phase. (e) Corresponding PXRD patterns together with that of layered perovskite (BA)2PbI4 and the standard PXRD pattern of cubic FAPbI3. The peak indicated by * is from the unreacted PbI2.

structure (δ-phase, space group P63mc, a = 0.868 nm, c = 0.793 nm) that is thermodynamically stable but shows poor photovoltaic performance. The hexagonal δ-phase polymorph consists of face-sharing PbI6 octahedra form one-dimensional PbI3− chains along the [001] direction, with the FA cations occupying the space between the chains to balance the charge (see Figure 1a for crystal structure).15 The perovskite α-phase of FAPbI3 can be accessed at higher temperatures by either thermal annealing of the hexagonal FAPbI3 or crystallization from solution at high temperatures (e.g., 120 °C),15 but it spontaneously transitions into the hexagonal phase upon cooling,18 which rules out the possibility for practical use. In contrast, the metastable α-phase polymorph of FAPbI3 adopts a cubic perovskite structure, in which a three-dimensional (3D) PbI3− network consists of corner-sharing PbI6 octahedra with the FA cations filling in the octahedral holes19 (see Figure 1a for crystal structure). Tremendous efforts have been made to stabilize the perovskite lattice of FAPbI3 through solid-state alloying with other ions,20−24 such as Br anions, smaller-sized MA, cesium (Cs), or rubidium (Rb) cations. Indeed, the materials employed in the most efficient solar cells have shifted recently from MAPbI3 to these stabilized FAPbI3 alloys.25 However, this alloying approach also leads to an undesired increase in the band gap, resulting in less photon absorption in the near-IR spectral region as compared to pure FAPbI3. Most recently, phase retention of pure FAPbI3 perovskite was observed when FAPbI3 was encapsulated in mesoporous TiO2 film26 or by controlling FAI/PbI2 precursor ratio,27 and a kinetic trapping of the FAPbI3 perovskite phase by thermal quenching was proposed;28 however, these methods still require annealing above RT to obtain the perovskite phase and the resulting kinetically trapped perovskite FAPbI3 phase does not have good long-term stability.

Here, we describe a new chemical strategy to stabilize metastable phases using surface functionalization. We directly synthesize at RT pure FAPbI3 in the cubic perovskite phase (i.e., without cation or anion alloying) that shows phase stability over months at RT. The key to this phase stabilization is the surface functionalization by long-chain alkyl or aromatic ammonium (LA) cations. Specifically, single-crystal nanowires (NWs), nanoplates, and thin films of cubic FAPbI3 perovskite with well-defined (100) facets are directly synthesized at RT under the presence of LA cations, but LA cations are not incorporated into the crystal lattice of the products. Surface analysis and density functional theory (DFT) calculations confirm the surface functionalization decreases the surface energy of the perovskite phase, which reverses the thermodynamic stability of the cubic perovskite phase versus the hexagonal phase, especially in the nanostructures. These stabilized FAPbI3 nanostructures display excellent photophysical properties, including low carrier recombination rates similar to those of MAPbI3. We further demonstrated highperformance optically pumped lasing and efficient photovoltaic devices with power conversion efficiency of ∼15.5%. Synthesis and Structural Characterizations of Cubic FAPbI3 Nanostructures. The hexagonal phase FAPbI3 is always the thermodynamic product when reacting a lead iodide (PbI2) film with a FAI precursor solution if the temperature is below 120 °C (Figure 1a). Interestingly, we discovered that, in the presence of long-chain alkyl or aromatic ammonium (LA) cations with proper concentration ratios to the FA cation in the precursor solution (see Experimental Section in the Supporting Information), the cubic phase of FAPbI3 nanostructures (i.e., nanowires, nanorods, nanoplates, and nanocubes) can directly grow from the solution at RT (Figure 1a), which was not previously possible. As explained in detail below, we found that 4406

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Figure 2. Structural characterizations of LA-stabilized cubic perovskite FAPbI3 nanostructures. (a) Optical and (b) SEM images of the cubic FAPbI3 nanostructures grown from a PbI2 thin film in a mixed precursor solution of FAI (20 mg/mL) and BAI (20 mg/mL) after a reaction time of ∼20 h. The inset shows rectangular facet of a NW. (c) PXRD patterns of the FAPbI3 nanostructures grown from PbI2 films in FAI precursor solution with the presence of various LA (FPEA, PEA, and BA) cations, showing only cubic phase FAPbI3. Low-resolution TEM images of a nanoplate (d) and a NW (e) and their corresponding selected-area electron diffraction (SAED) patterns along the [001] zone axis (ZA) of the cubic phase. (f) Confocal PL spectra of individual LA-stabilized FAPbI3 nanostructures, in comparison with the thermally converted cubic FAPbI3.

size of the nanostructure increased progressively with the reaction time. The optical (Figure 2a) and SEM (Figure 2b) imaging revealed rectangular faceted NWs and nanoplates after ∼20 h growth that suggested the cubic FAPbI3 perovskite structure. The PXRD pattern of the corresponding product shows a set of diffraction peaks that can be indexed exclusively to the cubic phase of FAPbI3 (Figure 2c, red curve) without any additional peaks from hexagonal FAPbI3 or layered perovskites. The NW typically were up to 10 μm in length, with flat rectangular end facets (inset in Figure 2b). The width of the rectangular cross-section varied from NW to NW and was typically a few hundred nanometers. The nanoplates were ∼10 μm in lateral dimension and several hundreds of nanometers in thickness. Such dimensions make them ideal building blocks for nanophotonic applications. To demonstrate the generality of surface ligand-induced phase stabilization of perovskite FAPbI3, we also successfully synthesized cubic phase FAPbI3 by using other aromatic ammonium cations (e.g., phenylethylammonium, 4-fluorophenylethylammonium) with some modifications of the growth conditions (Figure 2c, blue and green curves). Figure S4 provides the time-dependent PXRD of the products grown from a PbI2 film in a mixed precursor solution of FAI and phenylethylammonium (C6H5CH2CH2NH3+, PEA) iodide with various PEA/FA ratios at room temperature. The general trend of phase evolution of the products with increasing reaction time was similar to the case of BA/FA; however it was more difficult to optimize a proper αPEA/FA region to grow only cubic phase FAPbI3 in high yield with dimensions up to several micrometers (Figure S5). The hexagonal FAPbI3 and layered perovskite (PEA)2FAPb2I7 could not be sufficiently suppressed; either one often emerged at early time and then grew quickly. This issue can be partially addressed by either replacing PbI2 with PbAc2 precursor at RT or increasing the reaction temperature to 50 °C. We compared the phase and

the phases of the products were determined by several experimental factors such as the molar ratio of LA/FA (αLA/FA), the choice of LA, reaction time, and lead precursors. We first use n-butylammonium (n-C4H9NH3+, BA) cation as an example to demonstrate the effect of the BA/FA molar ratio (αBA/FA) on the phase selectivity. The phase identity of the products was determined using powder X-ray diffraction (PXRD). Figure 1b−d shows representative scanning electron images (SEM), and Figure 1e shows the PXRD patterns of the products synthesized using different αBA/FA ratios at a reaction time of 30 min. The concentration of iodide ions was high enough to enable the dissolution of PbI2 solid into PbI42− complex ions.29 The SEM image (Figure 1b) of the product grown from pure FAI solution shows hexagonal NWs, as expected for the hexagonal crystal habit of FAPbI3. However, at a low αBA/FA of 0.29, the initial product was found to be a mixture of the hexagonal and cubic phases of FAPbI3. With an increase in the reaction time, the product eventually became the hexagonal phase of FAPbI3 (see Figure S1 for time-dependent PXRD and SEM images). At a high αBA/FA of 2.57, the product was a mixture of cubic phase FAPbI3 and layered perovskite (BA)2FAPb2I7 microflakes (Figure 1d) initially but the majority eventually turned into (BA)2FAPb2I7 (see Figure S2 for timedependent PXRD). Interestingly, with an intermediate αBA/FA of 0.86, the growth of both hexagonal phase FAPbI3 and layered perovskite (BA)2FAPb2I7 was inhibited, leading to the formation of only cubic phase FAPbI3 (Figure 1c). The color of these substrates, yellow, black, and red (insets of Figure 1b−d), also clearly revealed the different phases of the products. To gain more information about the formation process of the cubic FAPbI3 nanostructures, the morphology evolution was monitored by SEM as the reaction time increased (Figure S3). Similar to what has been observed in MAPbI3,29,30 nanocrystals of cubic FAPbI3 immediately formed on the surface and served as “seeds” to initiate the growth of these nanostructures. The 4407

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Figure 3. Surface characterization and surface chemistry of the (001) facet of cubic FAPbI3. (a) High-resolution XPS of F 1s region for FPEAstabilized FAPbI3 and thermally converted FAPbI3. (b) Fitted high-resolution XPS of N 1s region for FPEA-stabilized, PEA-stabilized, and BAstabilized FAPbI3, in comparison with that for thermally converted FAPbI3 nanostructures. (c) Schematic illustration of the cubic FAPbI3 crystal lattice cut along the (001) plane. (d, e) Schematic illustration of the (001) surface of cubic FAPbI3 terminated by FA cations and PEA cations, respectively.

Surface Characterizations. The PXRD patterns (Figure 2c) of the stabilized perovskite samples grown under optimized conditions show diffraction peaks identical to that of the cubic FAPbI3, which indicates that the LA cations are not incorporated into the bulk perovskite lattice. Further, lattice incorporation of LA cations would have led to the formation of layered perovskites, which have different PL emission peaks and would have been readily detected (an example is shown in Figure S8c). However, surface analysis by X-ray photoelectron spectroscopy (XPS) confirmed the presence of LA cations on their surfaces. We performed XPS characterization on the three FAPbI3 samples grown in the presence of 3 LA cations (but underwent washing to remove free cations) in comparison with the thermally converted FAPbI3.32 Using the F-containing FPEA as a marker, a distinct F 1s peak was clearly observed for the as-grown FAPbI3 but not present in converted FAPbI3, confirming that indeed FPEA is bound to the surface (Figure 3a). Moreover, when investigating the N 1s spectra (Figure 3b), the peak deconvolution of the various LA-stabilized FAPbI3 nanostructures reveals the presence of two N states, which is evidenced by a shoulder observed on the main peak at 399.4 eV. In contrast, the thermally converted FAPbI3 shows only one peak. The major peak is assigned to the N atoms in the FA cations, and the shoulder peaks at a higher (∼1.8 eV) binding energy are due to the N atoms in the surface bound LA cations. The lower binding energy of the N states in FA is due to the partial C−N double bond as compared to the C−N single bond in LA, and a similar difference has been reported for XPS of FA and CH3NH3+ cations in a stabilized perovskite alloy.33 Further evidence for the existence of the LA cations comes from the mass spectrometry analysis (Figure S10) of the redissolved samples showing the molecular ions with m/z at 122.0964 and 74.0964 corresponding to PEA and BA, respectively. Mechanism of Phase Stabilization. We propose that such an unprecedented phase stability of the perovskite phase of FAPbI3 can be attributed to (1) size effect and (2) surface functionalization of perovskite with LA cations. The thermodynamic stability of different polymorphs can be changed due to the different surface energies, especially in nanoscale crystals where the contribution of surface energy significantly increases

morphology of the products grown using different lead precursors or reaction temperatures, while the αPEA/FA was fixed at 0.69. When the PbAc2 film was used at room temperature, the surface quickly converted into a uniform and highly crystalline layer of cubic FAPbI3 nanocrystals, but the (PEA)2FAPb2I7 was kinetically suppressed at the early time. Only a few (PEA)2FAPb2I7 seeds emerged and in a much lower density than for the PbI2 precursor, for which the abundant (PEA)2FAPb2I7 seeds led to dominant growth of layered perovskites (Figures S5b and S6g). As a result, the soluble ions of PbI42− and FA will more likely crystallize on the cubic FAPbI3 nanocrystals as “seeds” to yield high-density cubic FAPbI3 within 6 h (Figure S6). In the latter approach, we observed that the formation of (PEA)2FAPb2I7 could be inhibited as the reaction temperature increased, which expanded the αPEA/FA region that led to the formation of pure cubic phase FAPbI3 (Figure S7). Similar growth behaviors were also observed when 4-fluorophenylethylammonium (4FC6H5CH2CH2NH3+, FPEA) iodide was used (Figure S8). Transmission electron microscope (TEM) analysis on the stabilized FAPbI3 nanoplates and NWs (Figure 2d and e) further confirmed the phase and revealed the crystal orientation. The corresponding selected-area electron diffraction (SAED) patterns (insets in Figure 2d and e) show a set of sharp diffraction spots that can be indexed to a cubic perovskite structure with zone axes of [001]. Together with the rectangular shape, we can conclude that all exposed surfaces of the NWs and nanoplates are the (001) facet, which indeed has been theoretically predicted to be the most stable facet in a cubic perovskite structure.31 The phase identity of individual nanostructures was also confirmed by the confocal photoluminescence (PL) measurement and energy-dispersive X-ray spectroscopy (EDS). The representative PL spectra of single FAPbI3 nanostructures (Figure 2f) stabilized by the BA, PEA, and FPEA ligands show a band-edge emission peak of 814 nm with a full-width at half-maximum (fwhm) of 50 nm, which is almost identical to the thermally converted cubic FAPbI3.32 Quantitative elemental analysis from an individual nanostructure yields an expected I/Pb ratio of ∼3 (Figure S9). 4408

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Figure 4. Structures of the (a) (FA)2(FA)n−1PbnI3n+1 and (b) (PEA)2(FA)n−1PbnI3n+1, for the specific case of n = 3; and (c) the calculated formation energies of (FA)2(FA)n−1PbnI3n+1 and (PEA)2(FA)n−1PbnI3n+1 as a function of n.

with a reduced crystal size.6,34 We found that, when a drop of PbAc2 solution in methanol reacted with FAI/IPA solution, the products immediately formed were usually nanoparticles in the cubic phase (Figure S11), but the products quickly turned into the hexagonal phase due to the later growth and aggregation. Similarly, bulk CsPbI3 preferably crystallizes in a nonperovskite orthorhombic phase that is thermodynamically stable at RT, but colloidal nanocrystals of CsPbI3 are stabilized in the perovskite phase at RT.35,36 This is similar to what has been commonly observed in many metal oxide nanoparticles.7 Such crossovers of polymorph stability at the nanoscale suggest that the surface energy can significantly influence the total free energy of FAPbI3 nanocrystals. However, as the nanocrystals grow, the phase stability is expected to follow that of the bulk phase due to the smaller contribution from surface energy. The free energy gain from size effect is often not sufficient to allow the growth of large crystals with a micrometer size, which explain the why the phase transition still occurs later without adding LAs in the precursor solution (Figure S11). Previous studies also showed metastable nanomaterials could only be stabilized with a size typically less than tens of nanometers;7 therefore, the growth of the cubic FAPbI3 perovskite structures with dimensions up to 10 μm cannot be simply due to size effect. In light of the surface analysis above, we hypothesize that the LAs can bind to the surface of FAPbI3, which further reduces the surface energy and plays a dominant role in ultimately stabilizing the perovskite lattice. To explicitly illustrate our hypothesis, it is necessary to first understand the surface chemistry and bonding properties of the perovskite (001) facets. Previous calculations have shown that the electronic structures, particularly the surface states, are influenced by the surface termination.37 From the viewpoint of the stoichiometry of APbX3, under-coordinated Pb atoms (or I vacancies) should be present on the (001) surface as expected from the crystal cut along the (001) plane (Figure 3c), which has been suggested by previous studies.38,39 The percentage of under-coordinated Pb atoms can be substantial because of the high surface-to-volume ratio of the nanostructures. However, given that the iodide ion has four lone pairs of electrons and the surface under-coordinated Pb atoms have an empty 5d orbital, the acid−base interaction should facilitate the formation of coordinative Pb−I bonds on the surface. Therefore, because of the excess I− ions in the growth solution, we suggest that here all surface Pb atoms are coordinated by iodide, which in turn

creates excess negative charge on the surface. As a result, additional A-cations should be introduced to occupy the cuboctahedral sites at the surface to compensate the negative charges (Figure 3d). This is also supported by the theoretically calculated surface energies that showed that the MAIterminated surface is more thermodynamically stable than the iodide vacancies-terminated MAPbI3 under the equilibrium growth condition40 and a recent photoemission spectroscopy study on single-crystal MAPbBr3 (001) that confirmed the nature of MABr termination.41 Note that, when the additional surface ammonium cations are included, the exact stoichiometry of these nanocrystals will deviate from the perfect APbX3. When the LA and FA cations coexist, we suggest it is thermodynamically more favorable for the LA cations to occupy these surface sites (Figure 3e), as evidenced by the XPS measurements above, because the theoretical calculations below show LA binding results in a lower surface energy. DFT Calculations. We then conducted density functional theory (DFT) calculations to understand the origin of the surface LA-induced phase stabilization. To illustrate the contributions from the surface energies of different cation terminations, we compared the formation energies of two types of two-dimensional layered perovskite analogues with the chemical formula of (FA)2(FA)n−1PbnI3n+1 (Figure 4a for the case of n = 3) and (PEA)2(FA)n−1PbnI3n+1 (Figure 4b), whose surfaces are terminated by FA and PEA cations, respectively. The formation energy was calculated using eq 1:42 E=

E(PEAorFA)2 (FA)n − 1PbnI3n + 1 − nE PbI2 − (n − 1)E FAI − 2E PEAIorFAI n

(1)

The more negative the free energy, the more stable the perovskite. The DFT results show that the PEA-terminated compounds always have more negative formation energy than their FA-terminated counterparts (that has the same n value) regardless of the n value, ranging from 1 to 8 (Figure 4c), confirming that the substitution of surface FA with PEA cations is thermodynamically favorable. The smaller surface energy can be attributed to the stronger ionic interaction between positively charged LAs and the negatively charged PbI3− network surface and the additional van der Waals interactions between adjacent LAs. The positive charge is delocalized in the FA cations (see inset in Figure 4c), resulting in weaker ionic interaction with PbI3−. The calculated formation energy of the hexagonal FAPbI3 phase (−4.93 eV) and the cubic FAPbI3 4409

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Figure 5. (a) TRPL decay kinetics of an individual BA-stabilized single-crystal FAPbI3 nanoplate at different estimated injected carrier densities and their global fits. The global fitting yields k1 of 1.1 × 106 s−1 and k2 of 7.2 × 10−10 cm3 s−1. (b) Calculated carrier diffusion length as a function of carrier density. (c) Power-dependent PL emission spectra of the nanoplate showing stimulated emission with a lasing threshold of ∼25 μJ/cm2.

previously was achieved by confining the crystal size in the nanometer scale. However, our results revealed another important contributing factor for determining the surface energy which has not captured much attention previously: that surface functionalization can play a critical role in stabilizing metastable polymorphs. Although the kinetics of the growth process might still play some role here, we argue that the stabilization of the metastable polymorph is mainly caused by thermodynamic factors, particularly the surface energy. Such a chemical approach can be generally applicable for stabilizing other metastable perovskites, such as all inorganic CsPbI3 and CsSnI3, which have potentially good photovoltaic performance but also suffer from the complex phase stability issue. Photophysical Properties and Optically Pumped Lasers. We carried out time-resolved photoluminescence (TRPL) measurements and optically pumped lasing experiments on these optically thin, single-crystal nanoplates of stabilized FAPbI3 to demonstrate their excellent photophysical and optoelectronic properties. The exciton binding energy of FAPbI3 is ∼10 meV, which is significantly lower than thermal energy at room temperature,43 which suggests that bound excitons spontaneously dissociate into free carriers. Figure 5a shows the TRPL kinetics of a representative nanoplate under different injected carrier densities (N0). At low N0 (1018 cm−3). We then derived the carrier diffusion length (L) as a function of N by using L = μkBT /eR (Figure 5b), where μ is the carrier mobility (∼27 cm2 V−1 s−1),47 kB is the Boltzmann constant, T 4410

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Figure 6. (a) SEM images of the stabilized FAPbI3 film with ∼500 nm nanocubes grown on mesoporous TiO2 coated FTO glass. The inset shows the cross-section view of the same film. (b) PXRD patterns of the stabilized FAPbI3 film over time together with that of standard PXRD pattern for cubic phase FAPbI3. (c) Absorption and PL spectra of the stabilized FAPbI3 and MAPbI3 films. (d) Current−voltage curves for the optimized solar cells based on MAPbI3 and stabilized FAPbI3. The inset table summarizes the photovoltaic performance values.

perovskite thin film with a full surface coverage for device fabrication, a uniform PbI2 film was first deposited on mesoporous TiO2 nanoparticle-coated fluorine-doped tin oxide (FTO) substrate by spin-coating PbI2 solution in dimethylformamide (DMF), and then the substrate was dipped into a mixed solution of FAI and FPEAI in IPA to grow FAPbI3 (see Experimental Section in the Supporting Information for details). It should be note that without adding LA in the precursor solution, the as-converted film was found to be FAPbI3 in the hexagonal phase. To minimize the growth of NWs and nanoplates, we intentionally suppressed the dissolution and recrystallization process29 by decreasing both the concentration of iodide ions in the precursor solution and the reaction time. We choose fluorinated FPEA as the stabilizing ligand because the more hydrophobic nature of the FPEA could potentially improve the film’s moisture resistance.42,53,54 The top-view SEM image (Figure 6a) of the asgrown film reveals ∼500 nm large nanocrystals with square facets, which can be beneficial for light collection and charge transport.55 The cuboid morphology also suggests the surfaces are (001) facet, the same as in the cases of NWs and nanoplates. The PXRD of the film showed a group of peaks at 13.86°, 19.69°, 24.21° and 28.02° that can be indexed exclusively to cubic FAPbI3, although some PbI2 remained (Figure 6b). To further confirm the high phase purity on the microscopic scale, we have mapped the μ-PL over a large area stabilized FAPbI3 film of 160 μm × 160 μm with a step size of 2 μm, showing only single PL emission band corresponding to the cubic phase of FAPbI3 (Figure S15). More importantly, such films are stable in the perovskite phase for several months when stored in a desiccator (Figure 6b), which is much improved compared to the previously reported α-FAPbI3 in the

is the temperature, e is the elementary charge, and R = k1 + k2N is the total recombination rate. Under a typical solar illumination condition corresponding to an N of ∼1015 cm−3, a carrier diffusion length of 6.2 μm can be estimated, which is comparable to the reported values for high-quality MAPbI3 and FAPbI3 single crystals48−51 and larger than that of a thermally converted FAPbI3 thin film.47 Such a slow nonradiative recombination rate, slow bimolecular recombination rate, and long carrier diffusion length imply that the stabilized FAPbI3 should be an excellent semiconductor for optoelectronics. Furthermore, these well-defined nanoplates with subwavelength thicknesses can act as both whispering-gallery-mode (WGM) cavities and gain mediums for nanolasers.52 Figure 5c shows the power-dependent PL spectra. Coherent emission from the nanoplate was found at a pump fluence of 25 μJ cm−2, corresponding to an injected carrier density of 1 × 1018 cm−3. Below the lasing threshold, the stabilized FAPbI3 nanoplate shows broad PL spectra centered at ∼810 nm and a fwhm of ∼48 nm. Above the lasing threshold, a sharp red-shifted lasing peak at 837 nm emerges and increases in intensity quickly with pump power, while the intensity of the spontaneous emission region approaches saturation, indicating the occurrence of lasing. The fwhm of the lasing peak (δλ) is ∼0.49 nm, corresponding to a quality factor of ∼1700 (Q = λ/δλ, where λ is the peak wavelength). Further lasing evidence comes from the interference pattern shown in the fluorescence images above the lasing threshold (Figure S14d). More detailed lasing characterization is provided in Figure S14. Photovoltaic Performance. Encouraged by the promising photophysical properties, we further fabricated solar cells using the stabilized cubic FAPbI3 perovskite, and they showed better device performance than MAPbI3. To create a smooth FAPbI3 4411

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Letter

Nano Letters

In summary, our results demonstrate that surface functionalization can enable the stabilization of the metastable perovskite phase in pure FAPbI3 without cation or anion alloying, providing a new chemical strategy to access various metastable perovskite phases with good optoelectronic properties which have been impossible to achieve previously. Excellent photophysical properties, high performance lasers, and solar cells have been demonstrated with the single-crystal stabilized perovskite FAPbI3 nanostructures or thin films that are stable at RT over several months. We further elucidate that the phase stabilization originates from a decrease in surface energy due to the long chain ammonium ligand functionalization and the synergistic decrease in nanocrystal size. These results reveal the importance of surface chemistry on the thermodynamic stability of perovskite materials and demonstrate a new strategy based on surface functionalization to stabilize other metastable perovskite materials, such as the all-inorganic CsPbI3 and CsSnI3, to enable their practical optoelectronic applications. Moreover, they provide new insights and open up a general strategy on the control of metastable structural polymorphs and manipulating the thermodynamic phase stability of solid state materials in general.

mesoporous TiO2 that showed detectable hexagonal phase after 24 h.26 Therefore, such improved phase stability is mainly caused by the surface ligand effect. The absorption and PL spectra of the as-grown FAPbI3 in comparison with those of the MAPbI3 thin film with similar morphology show the desired red-shifted band gap of the FAPbI3 perovskite (Figure 6c). The onset of the stabilized FAPbI3 absorption spectrum occurs at ∼820 nm, corresponding to a bandgap of 1.51 eV, thus allowing for wider light absorption than MAPbI3 (or alloyed FAPbI3). The PL spectrum of a stabilized FAPbI3 film shows a band-edge emission peak of 810 nm with a fwhm of ∼50 nm, which is ∼30 nm red-shifted from the PL peak of MAPbI3. The smaller band gap of FAPbI3 could lead to a potentially higher solar conversion efficiency. Starting from the stabilized FAPbI3 films grown on the mpTiO2, we then fabricated perovskite solar cells by sequentially depositing a hole transport layer of 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9-spirobifluorene (spiro-MeOTAD) and metal contact (Au) (see detailed fabrication procedures in the Supporting Information). The device structure was: FTO/compact TiO2/mp-TiO2/perovskite/spiro-MeOTAD/ Au. For comparison, perovskite solar cells employing MAPbI3 thin films with comparable morphology were also fabricated. Figure 6d shows the J−V curves for both the optimized FAPbI3 and MAPbI3 devices, and the device parameters are tabulated in the inset. The devices were measured with a scan rate of 100 mV s−1 in the reverse direction without any preconditioning, such as light soaking or forward voltage biasing (see Figures S16 and S17 for a typical device measured under different scan rates and directions). The optimized MAPbI3 device delivered a short-circuit current density (Jsc) of 22.4 mA/cm2, an opencircuit voltage (Voc) of 936 mV, a fill factor (FF) of 0.644, and a power conversion efficiency (PCE) of 13.5%. These values are similar to those for previously reported MAPbI3 solar cells with similar film morphology prepared by two-step solution conversion.56 The solar cells based on stabilized FAPbI3 had a better photovoltaic performance with a Jsc of 24.1 mA/cm2, a Voc of 951 mV, a FF of 0.677, and a PCE of 15.5%. A steadystate current measurement of the same device gave a stabilized PCE of ∼14.5% (Figure S18). The stabilized FAPbI3 has higher Jsc that can be attributed to the smaller bandgap and better light absorption of FAPbI3. The optical characterization demonstrates the excellent photophysical properties of pure FAPbI3 perovskite similar to what have been observed in MAPbI3 or stabilized mixed cation FA1−xMAxPbI3 alloys,57 indicating that the stabilized FAPbI3 could be an alternative but more stable perovskite for photovoltaics and lasers. Similarly, a comparative study of the carrier dynamics in APbBr3 with different A-cations has suggested the charge carriers in the perovskites are protected as large polarons and the remarkable photophysical properties are independent of the type of cations.46,58 Therefore, we could expect that superior photovoltaic performance of the stabilized FAPbI3 can be realized primarily through the increase of Jsc as compared to MAPbI3. While the PCE reported here is lower than the state-of-the-art devices based on carefully engineered mixed cation/anion stabilized (FA,MA,Cs)Pb(I,Br)3 alloys with larger bandgaps,21 we believe that more refined control of the film morphology and new techniques to deposit these LAstabilized FAPbI3 that is not based on two-step conversion method can further improve the device performance by increasing the Voc and FF.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b01500. Experimental section, addition SEM images, optical images, and PXRD patterns of various products, mass spectra of LAs, optically pumped lasing characterization of a nanoplate, schematic illustrations of the proposed growth processes, photophysical fitting data, and J−V curves of photovoltaic devices (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yongping Fu: 0000-0003-3362-2474 Melinda J. Shearer: 0000-0001-6121-3614 Robert J. Hamers: 0000-0003-3821-9625 Erjun Kan: 0000-0003-0433-4190 X.-Y. Zhu: 0000-0002-2090-8484 Song Jin: 0000-0001-8693-7010 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.F., J.Z., M.J.S., Y.Z., R.J.H., and S.J. acknowledge support by the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under award DE-FG02-09ER46664 for synthesis and characterization of the perovskite materials and device fabrication. J.W. and X.-Y.Z. acknowledges support by the Department of Energy under grant no. DE-SC0010692-02 for lasing and photophysical measurements. T.W., E.K., and K.D. are supported by the NSFC (1134160, 51522206, 11574151). M.J.S. thanks the NSF Graduate Research Fellowship for support. 4412

DOI: 10.1021/acs.nanolett.7b01500 Nano Lett. 2017, 17, 4405−4414

Letter

Nano Letters

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