Letter www.acsami.org
Electrodeposition of Epitaxial Lead Iodide and Conversion to Textured Methylammonium Lead Iodide Perovskite James C. Hill, Jakub A. Koza, and Jay A. Switzer* Department of Chemistry and Graduate Center for Materials Research, Missouri University of Science and Technology, Rolla, Missouri 65409-1170, United States S Supporting Information *
ABSTRACT: Applications for lead iodide, such as lasing, luminescence, radiation detection, and as a precursor for methylammonium lead iodide perovskite photovoltaic cells, require highly ordered crystalline thin films. Here, an electrochemical synthesis route is introduced that yields textured and epitaxial films of lead iodide at room temperature by reducing molecular iodine to iodide ions in the presence of lead ions. Lead iodide grows with a [0001] fiber texture on polycrystalline substrates such as fluorine-doped tin oxide. On single-crystal Au(100), Au(111), and Au(110) the out-of-plane orientation of lead iodide is also [0001], but the in-plane orientation is controlled by the single-crystal substrate. The epitaxial lead iodide on single-crystal gold is converted to textured methylammonium lead iodide perovskite with a preferred [110] orientation via methylammonium iodide vapor-assisted chemical transformation of the solid. KEYWORDS: lead iodide, PbI2, epitaxy, interface, electrodeposition, electrochemical synthesis, methylammonium lead iodide, perovskite
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Here, an electrochemical synthesis route is introduced for producing epitaxial thin films of PbI2, and the epitaxial PbI2 is converted to textured methylammonium lead iodide perovskite thin films. PbI2 will readily precipitate when iodide anions are introduced to a solution containing lead cations. To selectively grow PbI2 thin films on a substrate via electrodeposition, one of these species needs to be electrochemically generated locally near the working electrode. Therefore, PbI2 was electrodeposited by reducing molecular iodine to iodide ions in the presence of lead cations, as shown in eqs 1 and 2 below.
ead iodide, PbI2, is a semiconductor with a direct band gap of 2.3−2.5 eV.1,2 It forms a layered structure with trigonal space group P3̅m1 (Figure S1).3 This layered structure causes lead iodide to possess unique optical and electronic properties, which enable it to be used as a photoluminescent material,4−6 a lasing material,7 and in X-ray and γ-ray radiation detectors.8−10 Recently, it has garnered extensive interest as a precursor for producing methylammonium lead iodide perovskite photovoltaic cells.11−17 A simple low-temperature, vapor-assisted conversion of PbI2 thin films to methylammonium lead iodide (MAPbI3) thin films was recently demonstrated.18 A facile synthesis method for producing inexpensive, highly ordered thin films of MAPbI3 would be of significant interest because it has recently been shown that large-grained materials have lower trap densities, longer diffusion lengths, enhanced photoluminescence, and higher conversion efficiencies in photovoltaic cells.15,16 Electrodeposition is a cost-effective and scalable synthesis method that allows for a high degree of fine-tuning parameters such as potential, pH, temperature, solvent, and concentration to synthesize thin films.19 We have previously reported the epitaxial electrodeposition of materials such as Cu2O, ZnO, δBi2O3, Fe3O4, CuO, and SnS onto single-crystal gold substrates.20−25 Epitaxial films can reduce the number of defect sites at interfaces and grain boundaries. The reduction of defect sites leads to better device performance in radiation detectors and lasers, and higher conversion efficiencies in photovoltaic cells.26 © XXXX American Chemical Society
I 2(aq) + 2e− ⇆ 2I−(aq)
(1)
Pb2 +(aq) + 2I−(aq) → PbI 2(s)
(2)
The electrodeposition solution was prepared by dissolving lead nitrate and sodium nitrate in deionized (DI) water, adjusting the pH to 2 with nitric acid, and subsequently combining this with a solution of iodine dissolved in ethanol for final concentrations of 5 mM lead nitrate, 10 mM iodine, and 100 mM sodium nitrate in 66% ethanol. Iodine begins to be reduced at about +0.3 V vs Ag/AgCl and lead ions are reduced to lead metal at about −0.4 V vs Ag/AgCl (Figure S2). Therefore, lead iodide can be electrodeposited in the potential range of −0.3 to +0.3 V vs Ag/AgCl. The Faradaic efficiency of the reaction was measured with an electrochemical quartz Received: August 5, 2015 Accepted: November 13, 2015
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DOI: 10.1021/acsami.5b07222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces crystal microbalance (EQCM), revealing that the reaction has an initial Faradaic efficiency above 65%, which decreases below 10% by the time 0.5 C/cm2 of charge is passed. Iodine continues to be reduced, but PbI2 deposits at a slower rate because of lead cation depletion due to the low lead concentration in the solution (Figure S3). The X-ray diffraction (XRD) pattern of PbI2 electrodeposited onto fluorine-doped tin oxide (FTO) matches Joint Committee on Power Diffraction Standards (JCPDS) card #07−0235, which corresponds to PbI2 with space group P3̅m1 (Figure S4). The electrodeposited PbI2 grows with a preferential [0001] orientation with the c-axis perpendicular to the substrate, even on a polycrystalline substrate such as FTO. Scanning electron microscopy (SEM) images of a PbI2 film grown on FTO until 0.5 and 2.0 C/cm2 of charge were passed show the surface morphology of the preferential growth as the PbI2 initially forms triangular shapes that expand to cover the entire substrate (Figure S5). Lead iodide was deposited on single-crystal gold substrates with [100], [110], and [111] orientations. The Au/PbI2 system is interesting as an epitaxial interface because there is a large lattice mismatch and difference in symmetry between trigonal PbI2 with a = 0.4557 nm and c = 0.6979 nm and the facecentered cubic Au substrate with a = 0.4079 nm. Despite possessing very different crystal structures, the XRD patterns all exhibit out-of-plane orientations along the [0001] axis on each of the ordered gold substrates (Figure 1a). The thin films have such a high degree of order in [0001] that the (0007) peak at 101.2° is clearly visible. Williamson−Hall plots of the {0001} family of planes revealed that size (i.e., X-ray coherence length) broadening is a much more significant component to line broadening than nonuniform strain because the Williamson− Hall plots have a nearly horizontal line (Figure S6).27 The line broadening of PbI2 on Au(100) had a strain component of 0.028% and a size component of 39.2 nm, of PbI2 on Au(110) had a strain component of 0.031% and a size component of 40.2 nm, and of PbI2 on Au(111) had a strain component of 0.034% and a size component of 40.4 nm. The X-ray coherence length is significantly smaller than the film thickness, suggesting that epitaxial films are not entirely composed of a series of vertical single crystals. The corresponding SEM images from Au(100) (Figure 1b), Au(110) (Figure 1c), and Au(111) (Figure 1d) show the triangular PbI2 crystals with in-plane orientation. Higher magnification SEM images more clearly show the 3-fold symmetry of the PbI2 (Figure S7). A cross-sectional highresolution transmission electron microscopy (TEM) image and selected area diffraction (SAD) patterns of the epitaxial PbI2 on Au(110) are shown in Figure 1e. The d-spacing between the layers is 0.7 nm, which matches the expected (0001) d-spacing along the c-axis between the I−Pb−I hexagonally close-packed layers. Prior to collecting the SAD patterns, the sample was tilted to Au [11̅0] and PbI2 [112̅0] viewing directions parallel to the Au/PbI2 interface. The lattice fringes of the gold substrate are not visible because of the thickness of the gold layer, which etches at a slower rate than PbI2 during thinning of the sample. A low-magnification SEM image and a lowmagnification cross-section TEM image show that the PbI2 layer is approximately 670 nm thick after it coalesces into a continuous film after 2.0 C/cm2 of charge is passed (Figure S8). The in-plane orientation of the films was confirmed by X-ray pole figures. A diffraction angle (2θ) was chosen that is not
Figure 1. (a) XRD pattern of PbI2 electrodeposited onto single-crystal Au(100), Au(110), and Au(111) substrates until 1.0 C/cm2 of charge were passed. The asterisks indicate gold substrate peaks. Scanning electron microscopy images of PbI2 electrodeposited onto singlecrystal (b) Au(100), (c) Au(110), and (d) Au(111) until 0.5 C/cm2 of charge was passed. (e) Cross-sectional high-resolution TEM of PbI2 on Au(110) viewed along Au [11̅0] and PbI2 [112̅0].
parallel to the surface of the material, the tilt angle (χ) was varied from 0° to 90°, and the azimuthal angle (ϕ) was varied from 0° to 360°. A diffraction angle of 56.49° was chosen for the pole figures. This diffraction angle corresponds to the (1123̅ ), (121̅ 3), and (211̅ 3̅ ) planes of PbI2, which have equivalent d-spacings and produce a calculated pole figure with 6 spots (Figure S9). Each pole figure displays 12 spots at a tilt angle of 45.6°. The pole figure of the PbI2 with [0001] out-ofplane orientation electrodeposited on Au(100) (Figure 2a) exhibits 12 equidistant spots at Δϕ = 30°. This is the result of 4 domains rotated 90° apart due to the 4-fold symmetry of the Au(100) surface. Two of the domains form one set of 6 higher intensity spots at Δϕ 60° that are equivalent and more energetically favorable than the other 2 domains and 6 spots, which are less energetically favorable. The {0001} PbI2 planes are parallel to the {100} Au planes in the out-of-plane direction, and the [110̅ 0] PbI2 and ⟨001⟩̅ Au directions are parallel inplane. Therefore, the epitaxial relationship is PbI2(0001)[11̅00]//Au(100) ⟨001̅⟩. The PbI2 electrodeposited on Au(110) (Figure 2b) produces a nearly identical pole figure compared to the Au(100) substrate. It has 2 domains due to B
DOI: 10.1021/acsami.5b07222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
The PbI2 electrodeposited on Au(111) (Figure 2c) has a unique spot pattern with one set of 6 spots at Δϕ = 60°, and a second set of 6 spots rotated Δϕ = 6° in relation to the first set. It has an epitaxial relationship of PbI2(0001)[110̅ 0 ± 3°]// Au(111) ⟨11̅0⟩. The PbI2 [11̅00] direction is not directly parallel with the Au ⟨11̅0⟩ directions, but rotated ±3°ϕ, suggesting that this slight rotation is more energetically favorable. The corresponding pole figures for each of the single-crystal gold substrates are shown in Figure S10. The full width at half-maximum (fwhm) of the peaks in the azimuthal scans is a direct measure of the in-plane order. The average fwhm was calculated from the corresponding azimuthal scans to be 9.4° (ϕ) for the PbI2 and 5.9° (ϕ) for the Au on the Au(100) substrate, 5.9° (ϕ) for the PbI2, and 4.6° (ϕ) for the Au on the Au(110) substrate, and 11.2° (ϕ) for the PbI2 doublet and 5.3° (ϕ) for the Au on the Au(111) substrate; suggesting that the PbI2 on the single-crystal gold Au(110) has the highest in-plane order (Figure S11 ). Interface models for each of the lead iodide and single-crystal gold substrate epitaxial relationships were prepared to calculate the mismatch. PbI2(0001)[11̅00]//Au(100) ⟨001̅⟩ has a lattice mismatch of −1.59% along Au[041̅] and PbI2[527̅0], and −1.32% along Au[001̅ ] and PbI 2 [11̅ 0 0] (Figure 3a). PbI2(0001)[11̅00]//Au(110) ⟨001⟩ has a lattice mismatch of −0.75% along Au[22̅1] and PbI2[415̅0], and −2.54% along Au[001] and PbI2[11̅00] (Figure 3b). PbI2(0001)[11̅00 ± 3°]//Au(111) ⟨11̅0⟩ was unique because the [11̅00] direction of (0001) PbI2 was rotated ±3° in relation to Au ⟨11̅0⟩. The mismatch calculated from the coincidence lattice shows why this arrangement is more energetically favorable. If the PbI2 is rotated ±3°, the lattice mismatch is only −0.10% along the Au[112̅] and PbI2[112̅0] direction, and −0.10% along the Au[211̅ ]̅ and PbI2[211̅ 0̅ ] direction (Figure 3c). However, if the PbI2 is not rotated it has a larger coincidence lattice and a larger lattice mismatch of −0.24% along the Au[101̅] and PbI2[101̅0] direction, and −0.24% along the Au[11̅0] and PbI2[11̅00] direction. (Figure S12). The electrodeposited epitaxial PbI2 was converted to MAPbI3 by exposing the PbI2 to methylammonium iodide vapors in an inert atmosphere. This low-temperature, vaporassisted chemical transformation of a solid process led to
Figure 2. Pole figures of PbI2 electrodeposited onto single-crystal (a) Au(100), (b) Au(110), and (c) Au(111) until 2.0 C/cm2 of charge was passed and measured at 56.49° 2θ. The radial gridlines are at 30° tilt angle intervals.
the two-fold symmetry of the Au(110) surface, producing 12 equidistant spots at Δϕ = 30° with 1 domain more energetically favorable than the other domain, and has an epitaxial relationship of PbI2(0001)[110̅ 0]//Au(110) ⟨001⟩.
Figure 3. Interface models of the electrodeposited epitaxial lead iodide on single-crystal gold for (a) PbI2(0001)[110̅ 0]//Au(100) ⟨001⟩̅ , (b) PbI2(0001)[11̅00]//Au(110) ⟨001⟩, and (c) PbI2(0001)[11̅00 ± 3°]//Au(111) ⟨11̅0⟩ with corresponding calculated mismatch for the indicated coincidence lattices. Gold atoms are blue and lead atoms are red. C
DOI: 10.1021/acsami.5b07222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. (a) X-ray diffraction pattern of Au(110)/MAPbI3(110) converted from Au(110)/PbI2(0001) by annealing at 180 °C for 2 h in the presence of methylammonium iodide vapor, and (b) the corresponding X-ray pole figure measured at 40.52° 2θ. The radial gridlines are at 30° tilt angle intervals. The PbI2 was electrodeposited until 0.5 C/cm2 of charge was passed.
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tetragonal MAPbI3 with preferred [110] out-of-plane orientation (Figure 4a). The corresponding X-ray pole figure was collected at a 40.52° tilt angle which corresponds to the (224) MAPbI3 pole. The pole figure reveals a ring with 12 low intensity spots at the expected tilt angle of 44.7° for MAPbI3 (110) peaks, suggesting that the MAPbI3 thin film has a fiber texture with a small degree of in-plane ordering (Figure 4b). The two additional spots at a 35.3° tilt angle are from the Au substrate (as indicated on Figure 4b). The (224) MAPbI3 pole at 40.52° is close enough to the (111) Au peak at 38.18° that the Au(110) substrate peak is visible at a tilt angle of 35.3°. The epitaxial relationship of the film is MAPbI3(110)[230]// Au(110) ⟨001⟩. In summary, an electrochemical synthesis route was introduced for lead iodide. Thin films of PbI2 with preferred orientations were grown on various substrates. Epitaxial thin films of PbI2 with a [0001] out-of-plane orientation were electrochemically grown on single-crystal gold substrates with [100], [110], and [111] orientations, and the coincidence lattice mismatch was calculated for each sample. The epitaxial PbI2 was converted to fiber textured MAPbI3 with a [110] outof-plane orientation via vapor-assisted chemical transformation of solid PbI2. These textured thin films of MAPbI3 should have higher photoluminescence intensity, lower trap state density, and higher photovoltaic efficiencies than polycrystalline films.15,16
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation under Grant DMR-1104801 (epitaxial electrodeposition of PbI2), and the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Grant DE-FG02-08ER46518 (conversion of PbI2 to perovskite).
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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/acsami.5b07222. Experimental methods, crystal structure of PbI2, linear sweep voltammetry, EQCM data, XRD of FTO/PbI2, SEM of FTO/PbI2, Williamson-Hall plots of epitaxial PbI2 on single-crystal Au, SEM of electrodeposited epitaxial PbI2 on single-crystal Au, TEM of electrodeposited epitaxial PbI2 on single-crystal Au, calculated X-ray pole figure for PbI2, single-crystal Au substrate pole figures, single-crystal Au and electrodeposited epitaxial PbI2 azimuthal scans, and the coincidence lattice for unrotated Au(111)/PbI2(0001) (PDF) D
DOI: 10.1021/acsami.5b07222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.5b07222 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
