Formation of Perovskite Heterostructures by Ion Exchange - ACS

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Formation of Perovskite Heterostructures by Ion Exchange Nathan T. Shewmon,† Hyeonggeun Yu,† Iordania Constantinou,† Erik Klump,‡ and Franky So*,† †

Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695, United States Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, United States

‡

S Supporting Information *

ABSTRACT: Thin-film optoelectronic devices based on polycrystalline organolead-halide perovskites have recently become a topic of intense research. Single crystals of these materials have been grown from solution with electrical properties superior to those of polycrystalline films. In order to enable the development of more complex device architectures based on organoleadhalide perovskite single crystals, we developed a process to form epitaxial layers of methylammonium lead iodide (MAPbI3) on methylammonium lead bromide (MAPbBr3) single crystals. The formation of the MAPbI3 layer is found to be dominated by the diffusion of halide ions, leading to a shift in the photoluminescence and absorption spectra. X-ray diffraction measurements confirm the single-crystal nature of the MAPbI3 layer, while carrier transport measurements show that the converted layer retains the high carrier mobility typical of single-crystal perovskite materials. Such heterostructures on perovskite single crystals open possibilities for new types of devices. KEYWORDS: hybrid perovskite, single crystal, solution processing, heterojunction, ion exchange

1. INTRODUCTION Organolead-halide perovskite materials are an interesting new class of solution-processable semiconductors with promising properties for a variety of optoelectronic applications including photovoltaics, 1,2 photodetectors, 3,4 LEDs,5 and lasers. 6 Although most device reports have focused on spin-coated polycrystalline thin films of methylammonium lead halides MAPbI3 and MAPbBr3, recent studies have shown that single crystals of these materials have greatly improved carrier mobilities and carrier diffusion lengths relative to the polycrystalline materials.7−12 While a number of different single crystal device architectures have been studied with lateral conduction paths along the surface9 as well as through the (millimeters thick) bulk,10,13 the different types of devices that can be fabricated using single crystals are limited. If heterostructures can be formed on perovskite single crystals, new types of devices such as double heterojunction LEDs, lasers, and quantum well devices can be realized. A number of methods have been developed recently for the growth of millimeter-size single crystals of organolead-halide perovskites. Typically nucleation and subsequent crystal growth are initiated when the perovskite solution becomes super© 2016 American Chemical Society

saturated which can be achieved by changing the solution temperature7,8,10,14−16 or by adding a poor solvent.12,17 Additionally, while some researchers have used seed crystals or substrates to initiate nucleation,9,10 others rely on homogeneous nucleation to form free-standing crystals.7,14 In this work, we use the inverse solubility method developed by Saidaminov et al.7 to grow high quality single crystals of MAPbBr3 which we then immerse in a MAPbI3 solution to form a heterostructure. Results from structural characterization by scanning electron microscopy (SEM) and X-ray diffraction (XRD) confirm the single-crystal nature of the MAPbI3 layers formed by this method, while photoconductor device characterization shows that the MAPbI3 layers have a charge carrier mobility similar to that of the bulk single crystal material. To our knowledge, this is the first report of high quality, singlecrystal heterojunction formation in organolead-halide perovskite materials. Received: August 10, 2016 Accepted: November 10, 2016 Published: November 14, 2016 33273

DOI: 10.1021/acsami.6b10034 ACS Appl. Mater. Interfaces 2016, 8, 33273−33279

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ACS Applied Materials & Interfaces

2. EXPERIMENTAL SECTION 2.1. Crystal Growth and Ion Exchange. The bulk crystal growth method used in this work has been reported in detail elsewhere.7 For the ion exchange process, MAPbBr3 crystals were dipped in an aqueous solution of lead acetate trihydrate, hydriodic acid, and methylamine with a 1:1:12 molar ratio of methylamine:lead:iodine. The aqueous dipping solution was stirred for 24 h at room temperature and filtered before use. 2.2. Sample Characterization. Cleaved samples were characterized using a FEI Verios 460L field emission scanning electron microscope for both electron imaging and EDX composition mapping. Crystal structure and orientation were characterized using a Rigaku SmartLab X-ray diffractometer using a Cu Kα X-ray source in 2θ−ω mode. Photoluminescence spectra were measured using an Edinburgh FL/FS920 spectrometer with an excitation at 500 nm. Absorption spectra were measured using an in-house setup consisting of a xenon dc arc lamp, an ORIEL 74125 monochromator, a Keithley 428 current amplifier, a SR 540 chopper system, an SR830 DSP lock-in amplifier from SRS, and a calibrated silicon photodiode. 2.3. Device Fabrication and Characterization. Photoconductor devices were fabricated by evaporating 100 nm Au electrodes through a shadow mask, resulting in a 170 μm channel between the symmetric electrodes as measured by a calibrated optical microscope. Responsivity measurements were done using an in-house setup consisting of a xenon dc arc lamp, an ORIEL 74125 monochromator, a Keithley 428 current amplifier, a SR 540 chopper system, an SR830 DSP lock-in amplifier from SRS, and a calibrated silicon photodiode. Carrier lifetime measurements were carried out by exciting the photoconductor device using a LASER-EXPORT DTL-319QT (527 nm 5 ns, 60 μJ/pulse) laser while applying a 5 V dc bias and reading the transient response with a Tektronix MDO3014 mixed domain oscilloscope set to a 75 Ω input impedance.

Figure 1. Models of the crystal structure of cubic MAPbBr3 and tetragonal MAPbI3. (a) Single unit cell of MAPbBr3 viewed along the [010] direction, (b) single unit cell of MAPbI3 viewed along the [001] direction, and (c) alignment of the MAPbBr3 [100] crystal direction with the MAPbI3 [110] crystal direction. (d) and (e) show pictures of as-grown single crystals of MAPbBr3 and MAPbI3, respectively.

indicating that diffusive ion exchange is the dominant growth mechanism of the iodide layer. To examine the formation of the MAPbI3 layer, samples were cleaved and the cross-section of the cleaved surface was examined (see Figure 2a). Figures 2b−d show the optical microscope (OM) as well as SEM images of the cleaved surface of a crystal that was immersed in a MAPbI3 solution at 80 °C for 120 s. A darker colored surface layer approximately 10 μm thick is visible in the OM image (Figure 2b), while in the SEM image of the same sample (Figure 2c) the surface layer looks monocrystalline, with no visible grain boundaries. Energydispersive X-ray spectroscopy (EDX) mapping (Figure 2d) shows that the surface layer is iodine-rich, confirming that a heterostructure is formed. We note that only saturated solutions of lead acetate, methylamine, and hydriodic acid gave this relatively uniform surface layer, while less concentrated solutions either led to excessive porosity (Figure S3a) or second phase formation (Figure S3b). When the MAPbI3 solution is not fully saturated, it has some solvating power leading to etching of the MAPbBr3 crystal surface. Additionally, attempts were made to carry out the reverse process wherein MAPbI3 single crystals were dipped in MAPbBr3 growth solution. All MAPbBr3 solutions tested were found to etch the MAPbI3 surface layer, and therefore reverse formation of MAPbBr3 on top of MAPbI3 was not possible. The thickness of the MAPbI3 layer formed on a MAPbBr3 single crystal was found to be a function of the dipping time, presumably as a result of the diffusion of iodine ions into the crystal. Figure 3 compares composite EDX maps of the bromine and iodine concentration at the edge of the cleaved sample surface immersed in the MAPbI3 solution at 80 °C for 5, 30, or 120 s. Line scan data from these EDX maps are also presented in Figure S2. Clearly, the thickness of the iodine rich surface layer increases with dipping time and roughly follows

3. RESULTS AND DISCUSSION Examples of bulk single crystals of MAPbBr3 and MAPbI3 we grew using the inverse solubility method7 are shown in Figure 1d,e, and the corresponding single crystal X-ray diffraction (XRD) patterns taken normal to their top (100) faces are shown in Figure S1. The orange MAPbBr3 crystal has a cubic unit cell18 while the black MAPbI3 crystal has a tetragonal unit cell due to the tilting of the PbI6 octahedra.19 This difference in crystal structure leads to the difference in the faceted shape of these two crystals, with the MAPbBr3 crystal having a squareshaped top facet and the MAPbI3 crystal having a diamondshaped top facet. For the case of epitaxy between these two materials, one might expect that the lead-centered octahedra would align across the interface, as shown in Figure 1c. In this case, the (100) direction normal to the MAPbBr3 crystal surface is aligned with the (110) direction in the MAPbI3 crystal with a lattice mismatch of only ∼5%. To form a MAPbI3 layer on a single crystal of MAPbBr3, we immerse the MAPbBr3 crystal into a MAPbI3 solution which is an aqueous solution consisting lead acetate, methylamine, and hydriodic acid. Upon immersion, the surface of the orange MAPbBr3 crystal becomes opaque and black within 2−3 s as shown in Figure 2a. Since the absorption depth for visible light in MAPbI3 is around 100−500 nm,20,21 the rapid color change indicates that the thickness of the MAPbI3 layer grows at an initial rate of several hundred nm/s. We note that for these samples it is possible that the iodide crystal is either grown outward from the bromide crystal surface or by ion exchange with iodide ion diffusion into the surface. However, as shown in Figure S2, the interface between the two different perovskites is not abrupt with a composition gradient spreading over several micrometers between the MAPbI3 and MAPbBr3 crystals, 33274


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Figure 3. (a) EDX mapping of the edge of the cleaved surface of samples dipped at 80 °C for 5, 30, or 120 s. Cracks in the 30 s sample resulted from electron beam damage. (b) Measured surface layer depth vs dipping time. The black line is a fit to the simplified diffusion equation, yielding Deff = 6 × 10−10 cm2/s.

We believe that the apparently dominant effect of ion exchange over crystal growth for the samples discussed in this work is due to the high ionic mobility of organolead-halide perovskite materials, along with a high intersolubility of MAPbI3 with MAPbBr3.25 The mobility of halide ions in organolead-halide perovskite materials has been studied both experimentally and by simulation and is often implicated in the poor stability of perovskite devices.24,26−30 Clearly, reduced intersolubility as well as reduced ionic mobilities would be prerequisites for achieving a sharp heterointerface between different perovskite compositions. To further investigate the crystallinity and orientation of these MAPbI3 layers, we used X-ray diffraction (XRD) measurements. Figure 4 shows the 2θ−ω XRD scans of a single crystal of MAPbBr3 before and after dipping in the MAPbI3 solution. In addition to the bromide diffraction peaks, a second set of peaks appear at smaller 2θ angles in the dipped sample corresponding to the expected diffraction peak positions of the (110) planes for MAPbI3.7 The fact that the (110) MAPbI3 peaks appear along with the (100) MAPbBr3 peaks from the substrate in the same scan at the same ω angle and without the presence of other diffraction peaks indicates that the MAPbI3 layer is single-crystalline with crystal orientation dictated by the underlying MAPbBr3 substrate. Further investigation of the XRD pattern at around 2θ = 30° (Figure 4b) reveals continuous diffracted intensities between 2θ = 28.35° (pure MAPbI3 (220) peak) and 2θ = 30.11° (pure MAPbBr3 (200) peak), confirming the graded composition at the heterointerface between the two crystals. We note that the converted MAPbI3 layer shows a peak at 2θ = 28.50°, which we attribute to the formation of an alloy within the graded composition region, along with a shoulder at 2θ = 28.35°, corresponding to pure MAPbI3 material at the surface. The fact that the peak at 2θ = 28.50° has higher intensity than the pure MAPbI3 peak at 2θ = 28.35° indicates that the layer of graded

Figure 2. (a) Images of a MAPbBr3 after several seconds of dipping halfway in MAPbI3 solution. Cleaving the crystal reveals the unchanged interior, and the cross section of the thin surface layer. (b−d) Images of the edge of the cleaved surface of a crystal after dipping in MAPbI3 solution, showing the thickness and morphology of the surface layer using (b) optical microscopy, (c) SEM, and (d) EDX mapping of iodine.

the solution to Fick’s second law for diffusion with infinite concentration at the surface:22 x = 2(Defft)1/2, where x is the diffusion depth taken as the depth at which the iodine and bromine concentrations are equal, Deff is the effective diffusion coefficient, and t is the diffusion time. Fitting to this equation gives an effective diffusion coefficient of ∼6 × 10−10 cm2/s (Figure 3). We use the term effective diffusion coefficient here because the above equation is a simplified version of what is actually happening in this casethere are two different sized ions, Br− and I−, diffusing in opposite directions at different rates, and in this case Fick’s second law is no longer analytically solvable. 23 For the simplified equation above, D eff is approximately equal to the average of the diffusion coefficients for Br− and I− ions.23 We note that our measured Deff value at 80 °C is slightly higher than the diffusion coefficient for I− ions in MAPbI3 of D < 10−11 cm2/s predicted by simulations.24 The higher apparent diffusion coefficient may indicate that the thickness of the MAPbI3 layer actually results from a combination of both ion diffusion into the crystal and film growth outward from the bromide crystal surface, yielding a faster than expected MAPbI3 layer growth rate. 33275


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Figure 4. 2θ−ω XRD scans from the top surface of single crystal samples before and after ion exchange: (a) shows the full scan range with MAPbBr3 peaks indexed in red and MAPbI3 peaks indexed in black; (b) shows the same data in the MAPbI3 (200)−MAPbBr3 (200) range, with expected peak positions for the two pure materials marked with a thin vertical line.

composition in these samples is thicker than the pure MAPbI3 layer at the surface. To evaluate the optical and electronic properties of the MAPbI3 surface layer, we measured its photoluminescence (PL) and absorption spectra for different dipping temperatures and times. The peak PL wavelength shifts from 580 to 780 nm after 5 s of dipping time at room temperature (Figure 5a) and then stays nearly constant for longer dipping times, while the absorption edge monotonically red-shifts with longer dipping times. The MAPbI3 PL peak strongly overlapping with the absorption spectrum is typical for single crystal samples.17,20 The 780 nm PL peak indicates that the surface composition is pure MAPbI3 irrespective of the dipping time, with photoexcited carriers in the higher bandgap bromide crystal migrating to the lower band gap iodide surface layer before radiatively recombining. On the other hand, the monotonic red-shift of the absorption edge with increased dipping time shown in Figure 5b is attributed to a monotonic increase in the surface layer thickness, resulting in increased absorption in the MAPbI3 crystal between 800 and 850 nm. Figure 5c shows the quenching of the peak PL intensity with increased dipping times and temperatures. We believe that during the heterojunction formation process defects accumulate at the crystal surface, resulting in nonradiative recombination. However, for samples immersed in MAPbI3 solution for less than 30 s at room temperature, the PL intensity is still similar to or higher than the pure MAPbBr3 crystal before dipping, indicating the high quality of this MAPbI3 surface layer. To probe the electrical properties of the MAPbI3 surface layer, photoconductor devices were fabricated using the structure shown in Figure 6a. To fabricate the photoconductor devices, we deposit two gold electrodes on the crystal surface for ohmic contacts,31 with a 170 ± 5 μm long channel between the electrodes defined using a shadow mask. For this device

Figure 5. PL and absorption spectra of single-crystal MAPbI3 layers: (a) normalized PL before and after ion exchange, (b) absorption spectra of samples dipped for different times compared to absorption for MAPbBr3 and MAPbI3 single crystals, and (c) peak PL intensity of samples dipped for various times and temperatures.

with symmetric electrodes, we expect the photocurrent to change linearly with applied voltage according to eq 1:32 GoptτμqV ΔnμqV = (1) L L where Jphoto is the increase in current density upon illumination, E is the applied field, Δσ and Δn are the change in conductivity and carrier concentration upon illumination, respectively, L is the channel length, μ is the carrier mobility, q is the electron charge, Gopt is the optically generated carrier concentration, and τ is the carrier lifetime. These photoconductor devices can be used to evaluate the carrier transport properties across the lateral channel through the surface layer using eq 1. In Figure 6b we compare the normalized responsivity spectra for photoconductor devices fabricated on the surface of a MAPbBr3 single crystal, a MAPbI3 layer grown on a MAPbBr3 crystal (dipped for 30 s at room temperature), and a spincoated polycrystalline MAPbI3 film (∼1.1 μm thick). In agreement with optical absorption data, the photoresponse of the converted MAPbI3 device is extended out to 800 nm. The 1.1 μm thick polycrystalline MAPbI3 device shows a further red-shifted photoresponse cutoff close to 830 nm. This additional long wavelength responsivity in the spin-coated Jphoto = ΔσE =

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summarized in Table 1. We find that the MAPbBr3 single crystal device has a carrier lifetime of 700 ± 20 ns, in agreement Table 1. Summary of Carrier Lifetimes (τ) and Carrier Mobilities (μ) Determined through Photoconductor Device Characterization device

τ (ns)

μ (cm2/(V s))

MAPbBr3 single crystal dip 30 s polycrystalline MAPbI3

700 ± 20 260 ± 10 220 ± 10

70 ± 10 40 ± 10 (3 ± 1) × 10−2

with previously published results.13,21,35,36 In the converted MAPbI3 sample, the carrier lifetime is reduced to 260 ± 10 ns, indicating the formation of trap states from the crystal defects formed during ion exchange. This is in agreement with the above data showing PL quenching for longer dipping times, and we believe that with further optimization of the ion exchange process this trap concentration could be reduced. For example, it has been observed that small changes in the stoichiometry of organolead-halide perovskites can strongly affect recombination by passivating surface and interface defects.37,38 Additionally, further lowering the temperature during ion exchange to slow the process might be another strategy for improving film quality and reducing defect concentration. The spin-coated polycrystalline MAPbI3 device shows an even shorter carrier lifetime of 220 ± 10 ns, presumably resulting from defects at grain boundaries. Using these values for carrier lifetime along with the slopes of the responsivity vs voltage data above, we use eq 1 to calculate the charge carrier mobilities for the different samples, and the results are summarized in Table 1. The carrier mobility in the MAPbI3 surface layer is only slightly lower than the pure MAPbBr3 single crystal, while in the polycrystalline film the mobility is about 3 orders of magnitude lower. This further confirms the high quality of the single-crystal MAPbI3 layer. To compare the quality of the film made by ion-exchange with a MAPbI3 bulk crystal, we fabricated a photoconductor device on the (100) facet of a MAPbI3 bulk crystal. We found that the photoresponse of the bulk crystal sample is nonlinear with applied bias (Figure S5) in contrast to the data for the ionexchanged film. This nonlinear photoresponse was also observed in a previous report with a similar device structure measured in air ambient conditions.9 It should be noted that a linear response was obtained when the measurements were done in vacuum.39 Additionally, unlike the ion-exchanged sample reported above, the transient photoresponse of the MAPbI3 crystal device (Figure S4b) showed a biexponential decay, with time constants of 90 ± 10 and 810 ± 20 ns, indicating the presence of separate recombination pathways due to surface defects. Nevertheless, by fitting the slope of the responsivity vs voltage curve, we estimate the mobility value is in the range between 5 and 40 cm2/(V s), which is close to what has been found in the literature.11 From these data, we conclude that the crystal quality of the ion-exchanged film should be comparable to the bulk MAPbI3 crystal.

Figure 6. Photoconductor device structure and characterization data: (a) device structure for the three devices compared here, (b) normalized responsivity spectra taken at an applied bias of 5 V, and (c) responsivity at λ = 500 nm as a function of voltage; the inset shows the same data in the log scale.

film indicates that the polycrystalline film is thicker than the pure MAPbI3 surface region in the single-crystal surface layer. Figure 6c shows the responsivities for the three devices at 500 nm incident wavelength and biased at 5 V. As predicted in the above equation, the responsivity increases linearly with increasing voltage. The slope of the responsivity vs voltage data, which is proportional to the μτ product, is notably higher for the pure MAPbBr3 crystal compared to the converted MAPbI3 device. However, the inset of Figure 6c, which shows the same data in the log scale, reveals that the responsivity of the polycrystalline film device is more than 3 orders of magnitude lower than that of the single-crystal iodide layer, indicating that the converted MAPbI3 material has superior transport properties resulting from its single crystal nature. We note that others have reported significantly higher responsivity for photoconductor devices based on polycrystalline MAPbI3 with similar device structures.33,34 The discrepancy between our results and those previously reported is most likely due to the fact that our devices were unencapsulated and measured in air as well as the relatively large 170 μm electrode spacing. Photocurrent transients for the above photoconductor devices biased at 5 V and illuminated with a nanosecond pulsed 532 nm laser are shown in Figures S4a,c,d. These transients show a monoexponential decay, with fitting results

4. CONCLUSIONS In conclusion, we have developed a solution-based method to form high quality single-crystal MAPbI3 layers on single crystals of MAPbBr3. We find that after dipping in MAPbI3 growth solution, a single-crystal layer of MAPbI3 is formed at the surface with a graded layer at the interface due to the 33277


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ACS Applied Materials & Interfaces interdiffusion of I− and Br− ions. SEM and EDX data show that the iodine-rich surface layer thickness increases with dipping time, resulting in stronger absorption close to the band edge. However, for longer dipping times or higher temperatures, the PL intensity is strongly suppressed. For MAPbI3 layers formed after dipping for 30 s at room temperature, a strong PL intensity is maintained, and the carrier mobility in the surface MAPbI3 layer is comparable to that of the original MAPbBr3 single crystal material. The formation of high quality organolead-halide perovskite single-crystal heterostructures like the ones presented in this work opens doors to novel optoelectronic devices using single crystal materials.

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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.6b10034. Single crystal XRD spectra; EDX depth profiles of iodine and bromine concentration for varying dipping times; example optical microscope images from crystals dipped in iodine solution that was not sufficiently concentrated; photocurrent transients from photoconductor devices (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.S.). ORCID

Franky So: 0000-0002-8310-677X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support of Office of Naval Research (Award N00014-14-1-0173). This work was performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (Award ECCS-1542015). The AIF is a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).

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REFERENCES

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