Probing Facet-Dependent Surface Defects in MAPbI3 Perovskite

May 22, 2019 - Recently, with the successful growth of single-crystal halide perovskites, which have relatively low trap densities,(13−15) it is exp...
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Article Cite This: J. Phys. Chem. C 2019, 123, 14144−14151

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Probing Facet-Dependent Surface Defects in MAPbI3 Perovskite Single Crystals Dohyung Kim,†,⊥ Jung-Ho Yun,‡,⊥ Miaoqiang Lyu,‡ Jincheol Kim,§ Sean Lim,∥ Jae Sung Yun,*,§ Lianzhou Wang,*,‡ and Jan Seidel*,†

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†

School of Materials Science and Engineering, §Australian Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable and Engineering, and ∥Electron Microscope Unit, University of New South Wales, Sydney, New South Wales 2052, Australia ‡ Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, Queensland 4072, Australia S Supporting Information *

ABSTRACT: Halide perovskites such as methylammonium lead iodide (MAPbI3) currently attract considerable attention because of their excellent optoelectronic properties and performance in solar cell devices. Despite tremendous research efforts to elucidate their fundamental properties, ion migration with the presence of ionic defects is still not fully understood. Here, various types of ionic defects for specific (100) and (112) lattice facets in single-crystal MAPbI3 have been investigated systematically. Our measurements reveal significant anisotropic properties. Photoluminescence (PL) and electrical transport measurements show that the (100) facet has higher PL intensity and over 1 order lower trap density compared to that of the (112) facet. We find that the facet-dependent variations of contact potential difference measured with Kelvin probe force microscopy under different bias voltages and light illuminations provide insights into different types of ionic defects on the surface of MAPbI3 single crystals. We also observe a completely different ion migration behavior on specific crystal facets through nanoscale scanning probe microscopy investigations. Our results indicate that the (100) facet exhibits an n-type behavior dominated with I− vacancies, whereas the (112) facet exhibits a p-type behavior with MA+ or Pb2+ vacancies. The findings on the facet-dependent configuration of ionic defects provide deeper understanding on facet-dependent optoelectronic properties in single-crystal MAPbI3. dielectric constant,23 field-switching photovoltaic effect,24,25 photoinduced phase separation,26 and photo-generated spontaneous poling.27 It has been theoretically demonstrated that I− ions have a lower activation energy of 0.58 eV compared to MA + ions, 28−30 and it has also been experimentally reported that possible channels for ion migration on the film surface act as the grain boundaries.19,31 The migrated ions at the grain boundaries can lead to hysteretic I−V behavior, thus resulting in instability in solar cells.31 More insight into this phenomenon could possibly be a crucial key to control the grain boundaries or reduce the defect density for suppressing ion migration. Recently, with the successful growth of single-crystal halide perovskites, which have relatively low trap densities,13−15 it is expected that single crystals are increasingly suitable for optoelectronic applications. This makes studies of single crystals of MAPbI3,32 MAPbI3−xClx films,33 and MAPbI334 and MAPbBr335 single crystals important. It has been most recently reported that optoelectronic properties are anisotropic with respect to (112), (100),34,36 and (220) crystal facets.37

1. INTRODUCTION Organic−inorganic metal halide perovskites are currently of intensive research interest owing to their important role in boosting the growth of solar cell power conversion efficiencies from 3.8 to 23.3% in less than just 1 decade1−5 and excellent optoelectronic properties.6−9 Nevertheless, solar cell devices based on polycrystalline thin films are still suffering from stability issues because of ionic defects such as cation and halide vacancies and interstitials at interfaces and grain boundaries, which can have a strong influence on performance in solar cells.10−12 There have been challenging efforts to make high-quality metal halide perovskite crystals,13−15 which have led to higher carrier diffusion lengths15 and longer charge carrier lifetimes16 with better device performances17,18 compared with polycrystalline thin films19 that possess a higher defect concentration. Although improved optoelectronic properties and device performance have been achieved from high-quality crystals, ionic defects still exist and ions migrate, which influences the band gap and absorption properties, thus affecting the operational state of the devices. Because it was initially suggested that ionic defects are at the origin of hysteresis in current−voltage (I−V) measurements,20−22 they have been known to act as a driving force for various phenomena such as the photoinduced giant © 2019 American Chemical Society

Received: January 30, 2019 Revised: April 24, 2019 Published: May 22, 2019 14144

DOI: 10.1021/acs.jpcc.9b00943 J. Phys. Chem. C 2019, 123, 14144−14151

Article

The Journal of Physical Chemistry C

Figure 1. Facet-dependent optical and electronic properties: (a) top view of as-grown MAPbI3 single crystals and (b) polished MAPbI3 single crystals with facets (100) and (112) exposed. (c) PL spectra of (100) and (112) facets, (d) XRD patterns with both (100) and (112) facets exposed simultaneously, and (e) SCLC measurement on facet (100) and (f) on facet (112).

solution growth method called ITC. The MAPbI3 (1 M) precursor solution was prepared by dissolving methylammonium iodide (Dyesol) and PbI2 (Alpha) precursors in γbutyrolactone. The precursor solution was filtered using a 0.2 μm pore size poly(tetrafluoroethylene) syringe filter. The filtered solution was gradually heated up to 90 °C in an oil bath and kept at the same temperature for 1−3 h, forming several hundred micron-sized single-crystal seeds.15 Bulk single crystals in centimeter scale were prepared by the selective growth of a well-shaped crystal seed of the as-prepared singlecrystal seeds with the regular replacement of fresh precursor solution under the same heating condition for 3−5 days. The quality of crystallinity of the bulk single crystals were dependent on the selection of single-crystal seeds, careful control of heating temperature, and regular supply of a fresh precursor solution. 2.2. Characterization. 2.2.1. Steady-State PL Measurements. The steady-state PL measurements upon one-photon excitation were conducted using an Andor iVac CCD detector whose temperature was 30 °C. Luminescence was excited with a 409 nm wavelength light, and exposure time was set to 2 s for MAPbI3 crystals using a PerkinElmer LAMBDA 1050 UV/vis/ NIR spectrophotometer. 2.2.2. SCLC Measurements. The transition in the I−V curves from the Ohmic to Child’s law across the trap-filled limit (TFL) was measured with an electrochemical station (Ivium-n-Stat). The TFL (nt) is defined by three terms, and the two measured values and the equations can be expressed by Child’s law

However, one of the more interesting results in single crystals is that the optoelectronic properties on the surface are highly different compared to those within the bulk.38,39 Thus, differentiated phenomena on the surface compared with bulk should be considered. In addition, different crystal facets can in general lead to distinct characteristics of optoelectronic properties because of the difference of defect densities and types, which could also lead to different ion migration behaviours.37 In this sense, the investigation of ion migration behavior, dominating defect type, and trap densities at different facets on the surface of crystals is indispensable for understanding the origin of facet-dependent optoelectronic properties with respect to defect chemistry. In this work, centimeter-scale MAPbI3 single crystals with defined (100) and (112) facets are successfully grown by an inverse temperature crystallization (ITC) process. Photoluminescence (PL) is used to quantify the surface trap states and measure the difference of band gap on the (100) and (112) facets. X-ray diffraction (XRD) is performed to check the crystallinity of the crystals. Electric transport measurements confirm the space-charge-limited conduction (SCLC) as the dominant conduction mechanism in the crystals and associated facet-dependent trap densities. We report on variations of average surface potentials in light and dark conditions, and under variable bias conditions using non-contact mode Kelvin probe force microscopy (KPFM). We identified that the (100) and (112) facets have different doping types, which is responsible for different types of dominating defects at the surface. Contact mode atomic force microscopy (AFM) shows the effect of morphological variation, which is also attributed to the different types of defects and their migration behavior on individual facets. Our results provide new insightful understanding of the ionic movement related to different types of defects on different facets of single-crystal MAPbI3.

VTFL =

entL2 2εε0

(1)

where VTFL is the onset voltage, e is the electronic charge, L is the thickness of the crystal, ε is the dielectric constant of the material (32, MAPbI3),6 and ε0 is the vacuum permittivity. The trap densities were calculated from the Child’s region. 2.2.3. XRD Measurement. The sharp XRD peaks in MAPbI3 single crystals were obtained using XRD (a Bruker D8

2. EXPERIMENTAL SECTION 2.1. Single-Crystal Growth. Centimeter-scale bulk MAPbI3 single crystals were grown with a bottom-seeded 14145

DOI: 10.1021/acs.jpcc.9b00943 J. Phys. Chem. C 2019, 123, 14144−14151

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The Journal of Physical Chemistry C ADVANCE powder X-ray diffractometer). The used X-ray diffractometer uses Cu Kα radiation, a tube voltage of 40 kV and 30 mA, and temperatures in the range of 10−60°. 2.2.4. Scanning Probe Microscopy Measurements. Individual facets in the growth of MAPbI3 crystals were polished using commercial diamond polishing papers (Allied High Tech Products Inc., diamond lapping film) downsizing from 35 μm particle size to 0.1 μm to clean the contaminated surface and make the surface flat. In this process, any chemical and water was not used while polishing to prevent surface damage. The prepared samples were measured with an atomic force microscopy (AIST-NT Smart SPM 1000) under ambient conditions at room temperature. Nitrogen gas was used for cleaning the sample surfaces before measurements. The AFM and KPFM measurements were performed using diamondcoated conductive probes (DCP20, force constant, k = 48 N/ m) which possess a resonance frequency of 420 kHz for both measurements. The applied dc biases were supplied to the tip in all measurements. The contact potential difference (CPD) spatial maps were gained in non-contact mode. Light sources were blocked for dark conditions. Tunable white light sources were used for illumination at an intensity of 0.01 mW/m2.

excitation condition at a wavelength of 409 nm. To check the crystallinity of the single crystals, XRD analysis is also carried out with both facets simultaneously exposed to the X-ray beam. The presented peaks correspond to the {100} and {112} facets, as can be seen in Figure 1d. These peaks exhibit excellent crystallinity along with [100] and [112] crystallographic orientations. The SCLC measurement is commonly used as a steady-state method to measure the hole and electron mobilities in organic semiconductors. This measurement has been used to calculate trap states. Figure 1e,f shows the I−V curves obtained on the (100) and (112) lattice facets. They confirm SCLC at a higher bias including an Ohmic contact region at lower bias. We calculated the TFL for different facets by the equation detailed in the Experimental Section; the values are shown in Table 1.

3. RESULTS AND DISCUSSION Figure 1a,b displays images of as-grown crystals of MAPbI3 with different pronounced facets. The detailed procedure for crystal growth is described in the Experimental Section. Note that the (100) and (112) facets are naturally exposed. Similar d-spacing lattice parameters (d100 and d112 are 4.4195 and 4.4535 Å, respectively) give rise to their growth rates being similar in solution.37 The PL spectra of the MAPbI3 single crystals were obtained on the (100) and (112) surfaces and are shown in Figure 1c. Note that the spot size of PL measurements is 0.5 cm, which covers the whole crystal surfaces on individual facets. The peak for the (100) facet is very sharp and high in intensity with a full width at half-maximum (FWHM) of 41.4 nm, whereas the peak for the (112) facet is broader and lower with an FWHM of 53.2 nm. Also, the PL peak positions of individual facets in MAPbI3 single crystals are placed at 776 and 784 nm, respectively, although these values are not matched completely with previous reported values for the same crystals because of differences of crystallinity.13,14,37 This implies that individual facets have different band gaps and their values are 1.60 and 1.58 eV, respectively. In fact, the peak position difference for each facet in the PL spectra could result from various effects. The peak variation in PL measurement can, for example, occur because of the interplay between the electron or electron−hole pair and trap states.40,41 When the defect densities are higher, it causes changes in the recombination rate of the excited carriers, which could result in band gap shift and change in the PL intensity and peak position. Besides, facet-dependent efficiencies have been reported in polycrystalline films, which are strongly associated with the recombination sites caused by different defect states.33 In our measurement, we confirmed different band gaps on each facet with PL spectra, observed at different PL peak positions.42 Nevertheless, other effects cannot be completely excluded, for example, photon reabsorption.43 The PL shift driven by the reabsorption effect can be observed under a two-photon (2P) excitation condition with a longer wavelength of 800 nm, rather than a one-photon (1P) excitation condition. Thus, in our case, it is difficult to observe the reabsorption effect in PL measurement with 1P

The listed values show the TFL voltage (VTFL) from the I−V curves. The calculated trap densities on the (100) and (112) facets are 2.80 × 1013 and 1.05 × 1014 cm−3, respectively. The trap-state density on the (112) lattice facet is around 1 order higher than that on the (100) facet. This confirms that the (100) facet has a lower trap density compared with the (112) facet. We also calculate the work function from the CPD via KPFM measurement at ambient dark conditions. The obtained work function values on the (100) and (112) facets are 4.7 and 4.6 eV, respectively, resulting from the CPD between a highly orientated pyrolytic graphite reference and a Pt-coated tip. The difference of traps densities and work functions between two lattice facets could contribute to facet-dependent optoelectronic properties such as photo detectivities, photocurrent, responsivity, and external quantum efficiency in published reports.34,36,37 KPFM measurements were employed to further investigate the facet-dependent trap sites that limit the charge carrier collection. All experiments are carried out under ambient conditions and low humidity conditions below 20%. Our results are consistent over several repetitive scans, so we expect that there is no degradation during the measurement. However, we do not completely exclude the effect of the atmosphere that can affect the charge carrier dynamics, photoemission properties, surface-state density, and even ion migration, leading to variations of the photophysics in perovskites.44−46 It has been demonstrated that the atmosphere influences perovskite performance as non-coordinated atoms in the film surfaces are partially passivated with H2O and O2 molecules.47,48 To avoid contamination of the used tip, it was always maintained in fresh state. KPFM allows simultaneous mapping of the topography and the local CPD between the tip and the sample by tracking the reflection of a laser off the cantilever onto a photodiode. Surface photovoltage is obtained from the CPD measured in dark subtracted from the CPD measured in light, that is, surface photovoltage = CPDlight − CPDdark. The surface photovoltage measurements help to understand the p−n junction properties in a semiconductor and local charge properties.49 Figure 2a,d shows the CPD measured in dark and light on (100) and

Table 1. Trap Densities and Work Functions for Different Crystal Facets

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samples

VTFL/V

nt/cm−3

Φ/eV

MAPbI3(100) MAPbI3(112)

0.56 2.41

2.80 × 1013 1.05 × 1014

4.7 4.6

DOI: 10.1021/acs.jpcc.9b00943 J. Phys. Chem. C 2019, 123, 14144−14151

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Figure 2. Variation of average CPD and proposed photoinduced effects in single-crystal MAPbI3 on individual (100) and (112) facets. (a) Lightinduced (on: 0.01 mW/m2) average CPD as a function of time on (100) and (d) on (112). (b) N-type behavior: the surface state of holedominated charge on (100) facet under light illumination, and (e) P-type behavior: the surface state of electron-dominated charge on (112) facet under light illumination, (c) energy band diagram at the MAPbI3 crystal surfaces on (100) and (f) on (112) facets. ECB is the conduction band energy, EVB is the valence band energy, and EF is the Fermi level.

Figure 3. Ion movement on different facets investigated by KPFM measurements. Average CPD on (100) and (112) facets as a function of (a) positive voltage biases (c) and negative voltage biases. A series of KPFM images on each facet (b) after applying the positive bias from +1 to +3 V and (d) after applying the negative bias from −1 to −3 V.

(112) facets, respectively. The full series of the CPD maps is shown in Figure S1. The initial average CPD values (dark) are approximately 35 and 96 mV on the (100) and (112) facets, respectively. On the basis of our previous reports, a higher CPD value implies a more positively charged surface.19 Such a

difference indicates that each facet has different charge states which could arise from different types of ionic defects that are commonly reported in halide perovskites, that is, vacancy (Iv, Pbv, and MAv), interstitial (II, MAI, and PbI), and antisite defect (IMA, MAI, PbI, IPb, PbMA, and MAPb). These defects can 14147

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charged tip on the (100) facet. This change is relatively higher compared with that of the (112) facet. However, the changes with negative biases reveal that one goes down on the (100) facet from approximately 76 to −1 mV, whereas the other one moves up on the (112) facet from approximately 56 up to 75 mV. Different starting points of CPD values shown in Figure 3c compared with those shown in Figure 3a indicate that the sample does not return to the completely pristine state once positive biases are applied, although several minutes passed by without bias voltages under dark conditions. Surface degradation is also not observed during this measurement because of the nature of the non-contact mode measurement that prevents ions moving through the tip from the surface. On the basis of the above results, it is possible to deduce the type of specific ionic defects in each facet. In materials with the ABX3 perovskite structure, vacancy-mediated diffusion is the most common process. Interstitial migration has not been commonly observed in inorganic perovskite oxides or halides because of the lack of interstitial space in such closely packed structures.29 Also, a single crystal does not comprise grain boundaries which act as ion migration channels. For the (100) facet, we suspect that iodine vacancy is the dominant defect type. An iodide vacancy creates energy states near the conduction band; thus, the surface should be n-type, which is supported by the above surface photovoltage measurements (Figure 2). Also, the change in CPD upon biasing the tip (Figure 3a,c) is largely varying with the bias voltages. This indicates that the ions are vigorously migrating, and many theoretical and empirical studies have shown that an iodide vacancy has the lowest migration activation energy.29 Table S1 summarizes the migration activation energy of vacancies in the MAPbI3 perovskite (I−, Pb2+, and MA+).28,29,52 In this scenario, it is likely that the positively charged iodide vacancy is moving away from the surface when positive voltages are applied to the tip, thus changing the surface to less n-type, which is also confirmed by an increase in CPD, and the same analogy can be applied when negative voltages are applied to the tip. The change in CPD upon biasing the tip for the (112) facet is only 19 and 16 mV for positive and negative voltages, respectively. Also, CPD increases for both cases, which implies that the surface is almost not changing and preserving its ptype characteristics regardless of the AFM tip bias polarity. Because this facet exhibited a p-type behavior (Figure 2), we suspect that the dominating defects are MA+ and Pb2+ vacancies which create energy levels near the valence band. In fact, the migration activation energy of Pb2+ is almost 4 times higher compared to the iodide vacancy.29 It is also suspected that the interstitial MA+ and Pb2+ do not actually migrate but could exist at this facet. These defects do not only create energy levels near the valence band (p-type) but also near the mid-gap. Mid-gap states are Shockley−Read−Hall recombination centers, which coincide with higher trap density and lower PL intensity of the (112) facet observed in Figure 1.9,50 To investigate the impact of ion migration behavior on the morphology of MAPbI3 single crystals with different facets, bias-dependent contact mode AFM measurements were carried out in dark conditions. The measurements were taken in dark in order to rule out photo-generated carriers and ionic movement. Because the tip is now directly contacting and surveying the surface, the effect of bias voltage applied to the tip is enormously larger compared to the above non-

create a trap state within the band gap and provide unintentional doping.9,50 For instance, the trap state existing near the valence band and conduction band results in p-type and n-type doping, respectively. As soon as lights are turned on after the first scan, the average CPD value is increased on the (112) facet by 148 mV, whereas the value on the (100) facet is decreased until around 7 mV. The decrease in CPD observed in the (100) facet implies that excess electrons dominate the charge on the surface of the crystal and excess holes are separated toward the bulk, that is, the n-type behavior at the surface, which is described in Figure 2b. Conversely, the increase in CPD indicates that excess holes dominate the charge on the surface and excess electrons are separated toward the bulk as in a ptype semiconductor for the (112) facet as shown in Figure 2e. Our relevant band diagrams of each facet according to our results are shown in Figure 2c,f.51 Our results clearly indicate that the types of majority of the carriers at each surface of the facets are different, which could be resulting from different types of defects dominating for each surface. Different surface orientations have different surface energy, so each facet has its own nucleation and growth conditions. It is also noticeable that error bars for the (112) facet are larger compared with the (100) facet. CPD distribution plots for both facets provided in Figure S2 show that the excited charge carriers behave differently under light illumination. Note that the plots are obtained from individual KPFM images shown in Figure S1. We have further studied the migration behavior of ionic defects on each facet using KPFM by applying the bias to the tip in dark. Figure 3 shows variations of the average CPD extracted from KPFM images of a 2 μm2 area taken on the crystal surfaces of (100) and (112) orientation under dark conditions as a function of various positive and negative biases. The full series of KPFM images after applying the positive bias is shown in Figure S3. To elucidate whether the observed phenomenon is a cross-talk or artifact, the line profiles from the selected data of both facets are carried out as shown in Figure S4. As a result, there is no direct correlation between the magnitude of the measured CPD and the height. When applied negative bias, the full series of KPFM images are also presented in Figure S5. Furthermore, the KPFM images after applying sequence-positive and -negative biases are presented in Figures S3 and S5 to check for varying values of CPD as a function of time. A change of topography on the crystal surfaces is not observed during the measurement. All measurements are performed consecutively from 0 to +1, +2, +3, 0, −1, −2, and −3 V. Note that individual scans take 8 min 40 s to complete with scan direction from bottom to top. The main phenomena we address here is different ionic migration behavior in the differentiated facets of MAPbI3 crystal surfaces. We have previously demonstrated that ion migration on the film surface could be measured through the variation of CPD under dark condition as a function of bias voltage.19 The measurement of CPD from KPFM is capable of observing local potential differences induced by specific ionic charge state.19 On the basis of this knowledge, the variation of CPD over biases is measured in this study with the same method as can be seen in Figure 3. An increased trend is seen on both facets when positive biases are applied up to +3 V, indicating a relatively abrupt increase on the (100) facet from around 34 to 86 mV and a moderate increase on the (112) facet from around 76 to 92 mV. This result shows that the attracted negative ions move toward the top surfaces by a positively 14148

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Figure 4. Ion migration behavior on different crystal facets in contact with charged AFM tips. (a) AFM topographic images on the (100) facet in pristine and after scan at the selected area (green box) with the positively charged AFM tip (upper lines) and the collected images at the same area after scan with the negatively charged tip (lower lines). (b) AFM topographic images on the (112) facet in pristine after applying positive biases at the marked boxes of green lines (upper lines) and the images at the relocated place after applying negative biases (lower lines). (c) Corresponding line profile for topography with a dc bias voltage of −2 V (red line) and (d) line profile of topography with a dc bias voltage of −3 V (blue line).

Also, there is a chance that MA+ ions dissociate because of the strong attraction to the surface.19 It is noticeable that the surface damage still occurs upon applying higher negative bias (−3 V) for the (112) facet. On the basis of this result, it is expected that I− vacancies still exist, possibly, in the bulk or just below the top surface where they can readily migrate to the surface as the migration of I− vacancies is the most favorable vacancy-assisted diffusion in the MAPbI3 perovskite.29 To further check how much the surfaces are damaged on individual facets, the corresponding line profiles from the height images at a dc bias voltage of −2 V (red line) on the (100) facet and of −3 V (blue line) on the (112) facet shown in Figure 4a,b are presented in Figure 2c,d. As a result, destructed surface areas caused by negative bias voltages on both facets are almost the same. These results clearly imply that the observed anisotropic surface properties of MAPbI3 single crystals at the nanoscale foster the formation of different ionic defects on the surface. Our finding leads to the conclusion that the specific configuration of ionic defects is determined by atomic densities, and the surface properties caused by different facets may have an influence on the device performance in solar cells. Therefore, the characteristics of ion migration in MAPbI3based materials should be considered with crystallographic orientations in mind as well as chemical composition and crystal structures. In fact, ion migration in metal halide perovskites is an important key to understand the underlying mechanisms for operating conditions in photovoltaic solar cells. Possible channels for ion migration in polycrystalline films act as grain boundaries.19,31 However, ionic defects in MAPbI3 perovskite single crystals are still in existence, and such a phenomenon has an influence on operating solar cells.

contact mode measurements. In this experiment, the positively or negatively charged tip is in contact with the crystal surfaces during this measurement and morphological change is confirmed by subsequent measurement without any bias voltage applied to the tip. Figure 4a,b shows the AFM topographic images on the polished crystal surfaces of (100) and (112) facets after applying positive and negative bias voltage at the marked boxes. The full series of the biasdependent AFM images on (100) and (112) facets are shown in Figures S6 and S7, respectively (Supporting Information). Note that all measurements are performed consecutively from +1 to +10 V and −1 to −10 V on both facets to determine a structural change caused by ion migration on the surface. A positive bias will attract negative ions such as MA+ or Pb2+ vacancies to the surface and repel positive ions such as I− vacancies and vice versa for negative bias.19 As a result, nothing is likely to change on the (100) facet, whereas surface damage starts to occur on the (112) facet when positive bias larger than +7 V is applied to the tip. This result reaffirms the types of different dominating defects for (100) and (112) facets. For the (100) facet, the dominant defects are I− vacancies, which are repelled from the surface. On the other hand, the (112) facet that has dominant MA+ and Pb2+ vacancies will attract these defects at higher voltages because of their high migration activation energy. Removing these vacancies through the surface will eventually collapse the structure; therefore, damage on the surface is clearly visible. When applying negative biases, the surface damage occurs at −2 V for the (100) facet as shown in Figure 4a, whereas no apparent damage on the surface is observed at −2 V for the (112) facet and the damage is observed at −3 V instead as shown in Figure 4b. This is resulting from the dominant I− vacancies for the (100) facet. 14149

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and technical assistance of the Electron Microscope Unit (EMU) at the University of New South Wales (UNSW) and the Queensland node of the Australian National Fabrication Facility (ANFF) at the University of Queensland (UQ).

In this regard, it is crucial to understand precise mechanisms related to this phenomenon even in perovskite-based single crystals. For future work, the systematic investigation of anisotropic ion migration behaviors on (100) and (112) lattice facets of MAPbI3 single crystals may inspire further improvement of perovskite solar cells.

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(1) Green, M. A.; Hishikawa, Y.; Dunlop, E. D.; Levi, D. H.; HohlEbinger, J.; Ho-Baillie, A. W. Y. Solar cell efficiency tables (version 51). Prog. Photovoltaics Res. Appl. 2018, 26, 3−12. (2) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for high-performance inorganic−organic hybrid perovskite solar cells. Nat. Mater. 2014, 13, 897. (3) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional engineering of perovskite materials for highperformance solar cells. Nature 2015, 517, 476. (4) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (5) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015, 348, 1234. (6) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013, 342, 341. (7) Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M. High charge carrier mobilities and lifetimes in organolead trihalide perovskites. Adv. Mater. 2013, 26, 1584−1589. (8) Xiao, Z.; Yuan, Y.; Wang, Q.; Shao, Y.; Bai, Y.; Deng, Y.; Dong, Q.; Hu, M.; Bi, C.; Huang, J. Thin-film semiconductor perspective of organometal trihalide perovskite materials for high-efficiency solar cells. Mater. Sci. Eng., R 2016, 101, 1−38. (9) Yin, W.-J.; Shi, T.; Yan, Y. Unusual defect physics in CH3NH3PbI3 perovskite solar cell absorber. Appl. Phys. Lett. 2014, 104, 063903. (10) Ahn, N.; Kwak, K.; Jang, M. S.; Yoon, H.; Lee, B. Y.; Lee, J.-K.; Pikhitsa, P. V.; Byun, J.; Choi, M. Trapped charge-driven degradation of perovskite solar cells. Nat. Commun. 2016, 7, 13422. (11) Soufiani, A. M.; Hameiri, Z.; Meyer, S.; Lim, S.; Tayebjee, M. J. Y.; Yun, J. S.; Ho-Baillie, A.; Conibeer, G. J.; Spiccia, L.; Green, M. A. Lessons learnt from spatially resolved electro- and photoluminescence imaging: interfacial delamination in CH3NH3PbI3 planar perovskite solar cells upon illumination. Adv. Energy Mater. 2017, 7, 1602111. (12) Yun, J. S.; Kim, J.; Young, T.; Patterson, R. J.; Kim, D.; Seidel, J.; Lim, S.; Green, M. A.; Huang, S.; Ho-Baillie, A. Humidity-induced degradation via grain boundaries of HC(NH2)2PbI3 Planar perovskite solar cells. Adv. Funct. Mater. 2018, 28, 1705363. (13) Saidaminov, M. I.; Abdelhady, A. L.; Murali, B.; Alarousu, E.; Burlakov, V. M.; Peng, W.; Dursun, I.; Wang, L.; He, Y.; Maculan, G.; et al. High-quality bulk hybrid perovskite single crystals within minutes by inverse temperature crystallization. Nat. Commun. 2015, 6, 7586. (14) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; et al. Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 2015, 347, 519. (15) Zhumekenov, A. A.; Saidaminov, M. I.; Haque, M. A.; Alarousu, E.; Sarmah, S. P.; Murali, B.; Dursun, I.; Miao, X.-H.; Abdelhady, A. L.; Wu, T.; et al. Formamidinium lead halide perovskite crystals with unprecedented long carrier dynamics and diffusion length. ACS Energy Lett. 2016, 1, 32−37. (16) Tian, W.; Zhao, C.; Leng, J.; Cui, R.; Jin, S. Visualizing carrier diffusion in individual single-crystal organolead halide perovskite nanowires and nanoplates. J. Am. Chem. Soc. 2015, 137, 12458− 12461. (17) Chen, Z.; Dong, Q.; Liu, Y.; Bao, C.; Fang, Y.; Lin, Y.; Tang, S.; Wang, Q.; Xiao, X.; Bai, Y.; et al. Thin single crystal perovskite solar

4. CONCLUSIONS We successfully grew MAPbI3 single crystals with specific (100) and (112) facets. We confirmed that the grown MAPbI3 crystals have excellent crystallinity as a single crystal by XRD patterns. Facet-dependent surface properties are apparent in PL and electrical transport measurements. Anisotropic ionic migration behavior has been investigated with KPFM and AFM. The effect of light in KPFM measurements clearly shows different formation of ionic defects on the surface of MAPbI3 crystals. Here, light illumination-induced potential changes, resulting in band bending with opposite directions, are observed. This effect is more pronounced on the (112) facet. Bias-dependent KPFM measurements on both (100) and (112) facets show different ionic movements under dark condition. The results indicate that iodine vacancy is the dominant defects for the (100) surface while dominating defects for the (112) surface are MA+ and Pb2+ vacancies. Finally, bias-dependent AFM measurements in contact mode show that the damage was not observed when positive bias is applied on the (100) facet where iodide vacancies are dominant, whereas the damage occurs on the MA+ and Pb2+ vacancies dominating the (112) surface. The severe damages induced by MA+ ions on both facets emerge under negative biases. These results offer further understanding of the anisotropic surface properties in solar cell operation and for enhancing the design of MAPbI3-based perovskite materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b00943. KPFM images under varying illumination and bias conditions and their analysis, investigation of migration energy of vacancies in MAPbI3 perovskite, and AFM images with different bias conditions (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.S.Y.). *E-mail: [email protected] (L.W.). *E-mail: [email protected] (J.S.). ORCID

Dohyung Kim: 0000-0002-1586-1466 Lianzhou Wang: 0000-0002-5947-306X Jan Seidel: 0000-0003-2814-3241 Author Contributions ⊥

D.K. and J.-H.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been partially financially supported by the Australian Research Council (ARC) through Discovery grants and a DECRA grant. The authors acknowledge the scientific 14150

DOI: 10.1021/acs.jpcc.9b00943 J. Phys. Chem. C 2019, 123, 14144−14151

Article

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