Phase Control In Mixed Halide Methylammonium Lead Perovskites

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C: Physical Processes in Nanomaterials and Nanostructures

Phase Control In Mixed Halide Methylammonium Lead Perovskites Using Silicon Nanotube Templates Roberto Gonzalez-Rodriguez, Neta Arad-Vosk, Amir Sa'ar, and Jeffery Lee Coffer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06824 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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The Journal of Physical Chemistry

Phase Control In Mixed Halide Methylammonium Lead Perovskites Using Silicon Nanotube Templates

Roberto Gonzalez-Rodriguez,1 Neta Arad-Vosk,2 Amir Sa’ar,2 Jeffery L. Coffer1* 1

Department of Chemistry and Biochemistry, Texas Christian University, Fort Worth, 7129,

USA 2

Racah Institute of Physics and the Harvey M. Kruger Family Center for Nanoscience and

Nanotechnology, Hebrew University of Jerusalem, Jerusalem 91904, Israel AUTHOR INFORMATION Corresponding Author Prof. Jeffery L. Coffer Phone: (817) 257-6223 *[email protected]

ACS Paragon Plus Environment

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Mixed halide perovskite nanostructures of composition CH3NH3PbI3-xBrx (x = 0, 0.25, 0.50, 0.75, 3) and inner diameter, d (d = 30,70,200 nm) using silicon nanotube (SiNTs) templates are described. For a given composition, the photoluminescence of these mixed halide perovskites is compared to bulk perovskite microwires/microcubes. Interestingly, for CH3NH3PbI2.25Br0.75 formed within these nanotubes, precise control of the cubic phase of this perovskite is maintained over time, as monitored by x-ray diffraction up to 21 days; this behavior is in stark contrast to the microwire, non-templated structure of the same composition, which reverts to the tetragonal phase during this period. For the 70 and 200 nm inner diameter templates, these nanotubes also provide some stabilization and effective packaging of the perovksite with regard to crystallinity and PL intensity over this time period.


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Introduction Significant fundamental and application-relevant properties of lead halide perovskite phases (ABX3) continue to be reported at an impressive pace, notably in the areas of photovoltaics (PV)1-4 and optoelectronics;5-10 layered device platforms based on such perovskite phases now report solar cell efficiencies in excess of 20%4 and for light emission, external quantum efficiencies beyond 3%.11 While most of these structures utilize cationic (A) centers composed of organoammonium species, increasing reports involve the incorporation of cesium (Cs) at the appropriate lattice positions, notably in nanocrystal/nanostructure form.12-17 In this class of materials, bandgap tunability is most commonly achieved by variation of halide identity (X = Cl,Br,I) and relative composition of each. For the series, ABX3, where A = Methylammmonium18 or formamidinium,19, mixed I/Br halide perovskites (CH3NH3PbI3-xBrx) offer a tunable band gap from the visible to the near-infrared.18,19 Specifically, the band gap of CH3NH3PbI3-xBrx ranges from 1.6 to 2.3 eV,20 making this subset of perovskites ideal candidates in tandem solar cell designs as a top layer absorber.21 While such mixed halide perovskite systems are most commonly reported in thin film form, control of perovskite feature size remains a challenge in general. There are some emerging reports regarding control of perovskite size using templates, such as aluminum oxide,22 mesoporous TiO2 or ZrO2,23 mesoporous silicon films,24,25 and silicon nanotubes (SiNTs).26 Surface passivation routes based on cesium-compositions are also an option in the creation of nanosized perovskites, such as those in the form of nanocrystals.17 Considering these possible choices, silicon in nanotube form is one of the most promising templates because: (a) unlike the above porous oxides, it has a tunable semiconducting character (and associated band level


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energetics with doping), a sensitive control of template dimensions (one dimensional tubes with tunable inner diameter, shell thickness, and length), and ultimately in terms of processing, a compatibility with current PV technology. In this work, we report the formation of tunable bandgap CH3NH3PbI3-xBrx nanostructures (x = 0, 0.25, 0.50, 0.75, and 3) using SiNTs as a template, e.g. a reaction vessel for the controlled formation of these perovskites. To facilitate formation, we selected SiNTs with a wall thickness of 10 nm, an average length ~2 µm, and a varying inner diameter (ID) of 30 nm, 70 nm, and 200 nm. Such porous sidewalls are necessary for effective infiltration of reactants inside the nanotube. In these experiments, for a given composition, we contrast the structure and photoluminescence (PL) properties of perovskites formed inside SiNTs with those of bulk-like CH3NH3PbI3-xBrx microstructures. We find that in the case of x = 0.75, a unique stabilization of the cubic phase of perovskite is achieved in the smaller (70 and 30 nm ID) SiNT templates. Methods Materials Characterization. SEM imaging was achieved with the use of a JEOL-JSM-7100F; TEM, using a JEOL JEM-2100. In situ compositional information was provided in each instrument with energy dispersive x-ray analysis. ZnO Nanowire Array (ZnO NWA) Fabrication. ZnO NWA templates were prepared on a given substrate (FTO or Si wafer) that was previously seeded with ZnO nanocrystals by placing in a mixture (1:1 v:v) of 0.005 M Zn(NO3)2 and 0.005 M hexamethylenetetramine at 92°C for 3h to obtain 30 nm diameter nanocrystals; to obtain 70 nm diameter nanocrystals, (1:1 v:v) of 0.02 M Zn(NO3)2 and 0.02 M hexamethylenetetramine at 92°C for 9h was used; and for 200 nm


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nanocrystals, a (1:1 v:v) of 0.1 M Zn(NO3)2 and 0.1 M hexamethylenetetramine at 92°C for 1 h was employed. Silicon Nanotube Fabrication. There are two components to this process. On the first part, a ZnO NW array sample was inserted into a quartz tube reactor, and Si deposition on the ZnO NWA was achieved through the use of silane (20 sccm, 0.5% in He, Praxair) mixed with He carrier gas (200 sccm, UHP grade, Praxair) that was passed through a furnace operating at 530 °C for 5 min. In the second step, etching of the ZnO NW cores occurs by placing these Si-coated ZnO NW samples in another quartz reactor and heated to 450 °C; NH4Cl was loaded in an alumina boat located upstream and heated to 350 °C. The gaseous etchant was transported via He gas downstream (170 sccm) to the furnace for 1 hr for removal of the ZnO NWA template. Perovskite Formation. In a typical perovskite formation process, exemplified here by the loading of the SiNTs with CH3NH3PbI3, a 1:1 solution of CH3NH3I and PbI2 at a concentration of 2.06 × 10

−3

M is prepared in DMF. SiNTs arrays (on FTO glass or Si wafer pieces) are

soaked into the solution for 2 h at 60 °C, then spun at 6000 for 40 s, followed by baking the sample for 30 min at 95 °C. As controls, perovskite microwires (or microcubes, in the case of bromide-containing species) are prepared by placing 1 drop of a solution 1:1 of CH3NH3X and PbX2 (6.18 × 10 −3 M each) in DMF. After air drying, the sample is heated for 30 min at 95 °C. Results and Discussion The process for fabrication of SiNTs follows that reported previously by our research group.27 It consists of the deposition of Si using dilute silane at 530°C on a preformed ZnO nanowire array template, followed by removal of the ZnO template using a NH3/HCl etch.


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Perovskite nanostructure formation inside a given SiNT involves the preparation of different ratios of PbI2:CH3NH3I:CH3NH3Br, followed by the immersion of SiNTs in a given solution, and finally a spin coating step (6000 rpm, to remove excess reactant solution) and baking at 95°C for 30 min. Perovskite microstructures were prepared using the same PbI2:CH3NH3I:CH3NH3Br ratios (See Supporting Information, Table S1) Employing the above reaction conditions in a given nanotube template yields small nanorods of a given perovskite composition. In Figure 1, TEM images of CH3NH3PbI3-xBrx (reactant ratios with x = 0.75) formed at the tips of SiNTs with 200, 70, and 30 nm ID are shown; perovskite diameter is restricted by Si NTs diameter and have an average length of ~350 nm. As

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Figure 1. TEM images of perovskite nanorods formed in SiNTs: A) 200 nm ID Si NTs with C CH3NH3PbI2.25Br0.75; B) 70 nm Si NTs with CH3NH3PbI2.25Br0.75; and C) 30 nm ID Si NTs with CH3NH3PbI2.25Br0.75. a control, microstructures were also prepared for a given perovskite composition in the absence of a nanotube template. Morphologically, these typically take the form of wires; under the reactant concentrations employed here, an average diameter of several microns and hundreds of microns length is found. Fig 2A shows a representative SEM of CH3NH3PbI3 microwires of


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average diameter of approximately 7 µm. Such morphology is retained when x= 0.25, 0.50, and 0.75; however when x=3 (the pure bromide system), a distinct cube-like morphology is observed (Fig 2B).

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Figure 2. FE SEM images of: A) CH3NH3PbI3 microwires, B) CH3NH3PbBr3 microcubes. Such structures are strongly luminescent, with uniform emission observed in the large area fluorescence images for a given film (Supplementary Fig S1). Note that in the larger (200 nm ID) nanotube templates, emission from discrete nanotubes can be imaged via confocal microscopy (green emission in the case of CH3NH3PbBr3; Fig S1). In each SiNT-templated perovskite system, as well as the microwire/microcube controls, there is a clear shift in photoluminescence (PL) maximum to shorter wavelength with increasing bromide content (Supplementary Fig S2). Such shifts are expected on the basis of computational studies noting that increasing amounts of bromide in a CH3NH3PbI3-xBrx system affects the bandgap mainly as a result of lowering the valence band maximum (VBM) of this perovskite rather than raising the conduction band minimum (CBM).28 This is likely a consequence of the fact that the CBM is comprised mainly of Pb-6p orbitals.


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In Figure 3, we compare the evolution of the room temperature PL spectra for the series CH3NH3PbI3-xBrx for the case of microwire morphology versus that of perovskite nanorods formed within the 30 nm ID SiNTs. Interestingly, a distinct difference is noted in the observed linewidth of the microwire sample for the composition CH3NH3PbI2.25Br0.75 relative to those templated by the SiNTs, where the rather extreme broadness of this peak infers the presence of multiple species in the microwire sample.

A

B (B)

Figure 3. Room temperature PL spectra for CH3NH3PbI3-xBrx structures: A) microwires (x = 0, 0.25, 0.50, 0.75) and microcubes (x = 3); B) nanorods formed inside 30 nm ID SiNTs (x = 0, 0.25, 0.50, 0.75, and 3). All spectra were acquired using an excitation wavelength of 370 nm and are normalized to a common intensity to emphasize differences in emission maxima. To analyze these observations in greater detail, it is recognized that that such mixed halide compositions at x > 0.6 can exist in the cubic phase,29-31 and the likelihood of halide exchange occurring upon sample irradiation in such structures is also strong.32,33 Thus, the timebased spectral evolution of microwires of CH3NH3PbI2.25Br0.75 was evaluated and these results


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compared with that of CH3NH3PbI2.25Br0.75 formed within SiNTs of varying inner diameter (Fig 4). At day 0, the PL spectrum of CH3NH3PbI2.25Br0.75 microwires exhibit a PL maximum of 742 nm, a rather broad shoulder centered around 675 nm, and an associated full width at half maximum (FWHM) of ~100 nm. Over a 21-day period, this broad peak narrows considerably to a value of 50% of the original. Interestingly, perovskite nanostructures formed with this composition within the 70 nm and 30 nm ID SiNT templates demonstrate FWHM values of ~5055 nm that do not change upon sample aging. CH3NH3PbI2.25Br0.75 nanorods formed within the somewhat larger 200 nm ID SiNTs show a slightly broader emission peak at day 0 (~60 nm), but this narrows to the steady state value of 50 nm relatively quickly (Fig 4b).


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Figure 4. Evolution of PL spectra (up to 21 days) for CH3NH3PbI2.25Br0.75 structures: a) perovskite microwires; b) perovskite nanorods formed within 200 nm ID Si NTs; c) perovskite nanorods formed within 70 nm ID Si NTs; and d) perovskite nanorods formed within 30 nm ID Si NTs. All spectra were acquired at room temperature using an excitation wavelength of 370 nm.


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The associated structural evolution, or lack thereof, of these samples as a function of time can most sensitively be probed via X-ray diffraction (XRD). We have analyzed, for a given perovskite structure formed with the ideal composition of CH3NH3PbI2.25Br0.75, the reflections for perovksite microwires and perovskite nanorods formed within SiNTs on fluorine-doped tin oxide (FTO) substrates in the range of 2Ɵ from 10° to 60° (Supplementary Fig S3). The focus here is with regard to the behavior of the (110) and (220) reflections over the same timeframe as the PL measurements. The (110) reflection for tetragonal CH3NH3PbI3-xBrx (near 14o) is quite sensitive to perovskite composition (Fig 5).29-31 For perovskite nanorods formed within the 200 and 70 nm ID SiNTs, the observed reflection at 14.25° is very close to the known value for films of composition CH3NH3PbI2.25Br0.75 .29 In the case of perovskite formed within the smallest 30 nm ID SiNTs, the value of the (110) peak at 14.40° corresponds more closely to a composition of CH3NH3PbI2Br1, a value consistent with SEM-EDX data suggesting a higher Br content in the sample (Table S2). The linewidth of the peak in this sample is measurably broader than other perovskites formed within the nanotubes, consistent with a smaller crystal size. It is very significant to note that the maximum of this (110) refection in perovskite samples formed within SiNTs remains unchanged over time. However, the case of the microwire morphology, an initial value at 14.25° (with a distinct lower wavelength shoulder) is consistent with the presence of more than one phase, as suggested by the PL data. Over time, a splitting of this broad peak occurs, with an eventual shift to the lower 2Θ value and diminution of the 14.25o peak, suggesting significant halide migration in the non-templated microwire structure. We next turn our attention to the 2Θ range of 27.5 o to 31o (Figure 5b) where the (220) tetragonal phase and (200) cubic phase reflections appear. Significantly, the XRD of microwires with x=0.75 show a clear phase transition from cubic to tetragonal upon aging, with the most


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significant difference observed in the first 7 days. In contrast, this perovskite composition, when formed within Si NTs, remains a cubic phase the entire 21 day observation period. Also, as a point of reference, it should be noted that the parent CH3NH3PbI3 perovskite in microwire form does not evolve with respect to either PL lineshape (Supplementary Fig S4) nor XRD intensity (Supplementary Fig S5; retaining tetragonal symmetry) over the entire 21 day window. An associated relevant issue to address is the evolution of intensity of these perovskite xray reflections (as well as PL) over time as a function of nanotube host dimensions, providing complementary information regarding the ability of a given nanotube template to stabilize this perovksite phase. For the (200) and (110) reflections of CH3NH3PbI2.25Br0.75 in microwire (nontemplated, un-stabilized) form, there is nearly completely loss of detectable crystallinity over the three week period, with an accompanying extreme degradation of PL intensity (> 85%, Supplementary Figure S6). This is in contrast to the relative stabilization of this perovskite composition provided by the 70 nm ID and 200 nm ID SiNT templates, where roughly half of the original X-ray intensity remains over the same three week period, and a proportional amount of emission intensity is retained (50%, Supplementary Figure S7). There is, however, clearly some rather significant loss of x-ray intensity of these reflections by day 21 for CH3NH3PbI2.25Br0.75 stabilized by the relatively small 30 nm ID SiNT framework; this is believed to be a consequence of defects (voids) in the perovskite morphology formed within this specific small inner diameter nanotube, as evidenced by high resolution TEM imaging (Supplementary Figure S8).


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A

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Figure 5. XRD spectra of CH3NH3PbI2.25Br0.75 perovskite microwires and nanorods formed within SiNTs: A) reflections from 2Ɵ = 13 to 17o; B) reflections from 2Ɵ = 27.5 to 31o. The relatively long-term stabilization of the cubic phase of this specific perovskite composition is presumably influenced by the perovskite grain boundaries of the nanorods formed within the nanotube as well as interfacial strain generated at the perovskite – nanotube interface. The surface free energy change associated with lattice reorganization, and generation of an elongated axis upon transition to the tetragonal from cubic is prohibitively too high energetically for these perovskites embedded within the silicon nanotube host. This nanotube-influenced phase stabilization is consistent with properties observed earlier for CH3NH3PbI3 nanorods formed within SiNTs, where an inhibition of the phase transition from tetragonal to orthorhombic (at ca. 160K) was clearly inhibited in the case of smaller inner diameter (30 nm and 70 nm)


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nanotubes.34

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The extreme strain created at the interface between CH3NH3PbI2.25Br0.75

nanostructures and the 30 nm ID SiNT is likely responsible for the metastability of the perovskite formed within this particularly sized template. In general, the retention of some X-ray and PL intensity for CH3NH3PbI2.25Br0.75 formed in the cubic phase within a given nanotube is a reflection of the ability of these Si nanostructures to provide stabilization of the perovskite against moisture and oxidation. Conclusions In summary, we have reported precise phase control of mixed-halide perovskites using SiNTs as a template. The data suggest SiNTs can stabilize the initial formation of a cubic phase for perovskites of composition CH3NH3PbI2.25Br0.75 in nanorod form, unlike unstabilized microrods and bulk perovskites of this composition. The long-term implications for the presence of such a cubic structure in a photovoltaic platform remain to be investigated. Recent optical measurements, along with molecular dynamics simulations of the high temperature cubic phase of CH3NH3PbI3 suggest that local tetragonal distortions, on a sub-picosecond timescale, remain in place for such a phase, and the effect on bandgap is minimal.35 However, the impact on such perovksites in nanoscale form warrants further exploration. ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge on the ACS Publications website at DOI: The following file is available free of charge (PDF). Experimental details (sample characterization, materials fabrication); Table S1 (Relative Molar


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Ratios of Lead(II) Halide and Methyl Ammonium Halide Precursors Used); Table S2 (Elemental Composition Data (from SEM-EDX) of mixed halide perovskites); Fig S1 (Fluorescence Microscopy images); Fig S2 (PL spectra as a function of increasing bromide concentration); Fig S3 (Typical XRD patterns of CH3NH3PbI2.25Br0.75); Fig S4 (Evolution of Evolution of PL of CH3NH3PbI3 microwire samples as a function of time); Fig S5 (Evolution of XRD of CH3NH3PbI3 microwire (uw) samples as a function of time); Fig S6 (Normalized PL Intensity of CH3NH3PbI2.25Br0.75 in various nanotube templates as a function of time); Fig S7 ((Normalized X-Ray Intensity of the (200) reflection of CH3NH3PbI2.25Br0.75 in various templates as a function of time); Fig S8 (High resolution TEM image of formed within 30 nm ID SiNT templates). AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by the Robert A. Welch Foundation (Grant P-1212) and the TCU Invests in Scholarship (TCU IS) Initiative. REFERENCES (1)

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TOC GRAPHICS

Emission

CH3NH3PbI2.25Br0.75 in 200 nm SiNTs

XRD


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Phase Control In Mixed Halide Methylammonium Lead Perovskites

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