Probing Facet Dependent Surface Defects in MAPbI3 Perovskite

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C: Energy Conversion and Storage; Energy and Charge Transport

Probing Facet Dependent Surface Defects in MAPbI Perovskite Single Crystals 3

Dohyung Kim, Jung-Ho Yun, Miaoqiang Lyu, Jincheol Kim, Sean Lim, Jae Sung Yun, Lianzhou Wang, and Jan Seidel J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 27, 2019

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The Journal of Physical Chemistry

Probing Facet Dependent Surface Defects in MAPbI3 Perovskite Single Crystals

Dohyung Kim1†, Jung-Ho Yun2†, Miaoqiang Lyu2, Jincheol Kim3, Sean Lim4, Jae Sung Yun3*, Lianzhou Wang2*, and Jan Seidel1* 1

School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052,

Australia 2

Nanomaterials Centre, School of Chemical Engineering and Australian Institute for Bioengineering

and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia 3Australian

Centre for Advanced Photovoltaics (ACAP), School of Photovoltaic and Renewable and

Engineering, University of New South Wales, Sydney, NSW2052, Australia 4Electron † These

Microscope Unit, University of New South Wales, Sydney, NSW 2052, Australia

authors contributed equally to this work.

* Corresponding

author E-mail:

[email protected]; [email protected]; [email protected]

1

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ABSTRACT Halide perovskites such as methylammonium lead iodide (MAPbI3) currently attract considerable attention due to 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, 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 and electrical transport measurements show that the (100) facet has higher PL intensity and over one order lower trap density compare to that of the (112) facet. We find that facet-dependent variations of contact potential difference (CPD) measured with Kelvin probe force microscopy (KPFM) under different bias voltages and light illuminations provide insight into different types of ionic defects on the surface of MAPbI3 single crystals. We also observe completely different ion migration behaviour on specific crystal facets through nanoscale scanning probe microscopy investigations. Our results indicate that (100) facet exhibits n-type behaviour dominated with I- vacancies while (112) exhibits p-type behaviour with MA+ or Pb2+ vacancies. The findings on facet-dependent configuration of ionic defects provide deeper understanding on facet-dependent optoelectronic properties in single crystal MAPbI3.

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1. Introduction Organic-inorganic metal halide perovskites are currently seeing 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 one decade1-5 and excellent optoelectronic properties.6-9 Nevertheless, solar cell devices based on polycrystalline thin films are still suffering from stability issues due to 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 crystals13-15, which have led to higher carrier diffusion lengths,15 and longer charge carrier lifetimes16 with better device performances17-18 than polycrystalline thin films19 that possess a higher defect concentration. Although improved optoelectronic properties and device performance have been gained from high quality crystals, ionic defects still exist and ions migrate, which influences bandgap and absorption properties, and thus affect the operational state of devices. Since it was initially suggested that ionic defects are at the origin of hysteresis in current-voltage (I-V) measurements20-22, they have been known to act as a driving force for various phenomena such as the photo-induced giant dielectric constant23, field switching photovoltaic effect24-25, photo-induced phase separation26, and photo-generated spontaneous poling27. 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 are the grain boundaries19, 31. Migrated ions at grain boundaries can lead to hysteretic I-V behaviour, thus resulting in instability in solar cells.31 More insight into this phenomenon could be a possibly crucial key to control 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 densities13-15, it is expected that single crystals are increasingly suitable for optoelectronic applications. This makes studies of single crystals of MAPbI332, MAPbI3-xClx films33, 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 facets37. However, one of the more interesting results in single crystals is that optoelectronic properties on the surface are highly different compared to 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 due to difference of defect densities and types, which could also lead to different ion migration behaviour.37 In this sense, the investigation of ion migration behaviour, dominating defect type and trap densities at different facets on the surface of crystals is 3



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indispensable for understanding the origin of facet-dependent optoelectronic properties with respect to defect chemistry. In this work, centimetre 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 (100) and (112) facets. X-ray diffraction (XRD) is performed to check 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 noncontact mode Kelvin probe force microscopy (KPFM). We identified that (100) and (112) facets have different doping types which is responsible from different types of dominating defects at the surface. Contact mode atomic force microscopy shows the effect of morphological variation which is also attributed to the different types of defects and their migration behaviour 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.

2. Experimental Section 2.1. Single crystal growth. Centimetre scale bulk MAPbI3 single crystals were grown with a bottom-seeded solution growth method called inverse temperature crystallization. 1 M MAPbI3 precursor solution was prepared by dissolving MAI (Dyesol) and PbI2 (Alpha) precursors in butyrolactone (GBL). The precursor solution was filtered using 0.2 μm pore size PTFE syringe filter. The filtered solution was gradually heated up to 90 ℃ in an oil bath and remained at the same temperature for 1 – 3 h, forming several hundred micron sized single crystal seeds.15 Bulk single crystals in centimetre scale were prepared by the selective growth of a well-shaped crystal seed of the asprepared single crystal seeds with the regular replacement of fresh precursor solution under the same heating condition for 3 to 5 days. The quality of crystallinity of the bulk single crystals were dependent on the selection of single crystal seeds, the careful control of heating temperature, and the regular supply of fresh precursor solution. 2.2. Characterization. 2.2.1.

Steady-state

photoluminescence

(PL)

measurements.

The

steady-state

photoluminescence (PL) measurements upon one-photon excitation were conducted using an Andor iVac CCD detector whose temperature was 30 °C. Luminescence was excited with 409 nm wavelength 4


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light, and exposure time was set to 2 seconds for MAPbI3 crystals using a Perkin Elmer Lambda1050 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 trap filled limit (TFL, nt) is defined by three terms and the two measured values and the equations can be expressed by Child’s law:

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 crystal were obtained using X-ray diffraction (a Bruker D8 Advance powder XRD). 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. SPM measurements. Individual facets in the growth MAPbI3 crystals were polished using commercial diamond polishing papers (Allied High Tech Products Inc, Diamond lapping film) downsizing from 35㎛ particle size to 0.1㎛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 AFM (AIST-NT Smart SPM 1000) under ambient conditions at room temperature. Nitrogen gas was used for cleaning sample surfaces before measurements. The AFM and KPFM measurements were performed using diamond-coated 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 CPD spatial maps were gained in non-contact mode. Light sources were blocked for dark conditions. Tuneable white light sources were used for illumination at an intensity of 0.01 mW/m2.

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 (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 5



The PL spectra of the MAPbI3 single crystals were obtained on (100) and (112) surfaces and are shown in Figure. 1c. Note that the spot size of PL measurements is 0.5 cm which covers 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 while the peak for the (112) facet is broader and lower with a FWHM of 53.2 nm. Also, the photoluminescence peak positions of individual facets in MAPbI3 single crystals are placed at 776 nm and 784 nm, respectively, although these values are not matched completely with previous reported values for the same crystals due to differences of crystallinity.13-14, 37 This implies that individual facets have different band gap and their values are 1.60 eV and 1.58 eV, respectively. In fact, the peak position difference for each facet in the PL spectra could result from various effects. Peak variation in PL measurement can e.g. occur due to interplay between electron or electron-hole par and trap states.40-41 When the defect densities are higher, it causes change in the recombination rate of the excited carriers, which could result in bandgap shift and change in PL intensity and peak position. Besides, facet-dependent efficiencies have been reported in polycrystalline films, which are strongly associated with recombination sites caused by different defect states.33 In our measurement, we confirmed different bandgaps on each facet with PL spectra, seen as different PL peak positions.42 Nevertheless, other effects cannot be completely excluded, e.g. photon-reabsorption43. PL shift driven by the reabsorption effect can be observed under 2-photon (2P) excitation condition with longer wavelength of 800 nm, rather than 1-photon (1P) excitation condition. Thus, in our case it is difficult to observe the reabsorption effect in PL measurement with 1P excitation condition at a wavelength of 409 nm. To check 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 {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 (Space charge limited current) measurement is commonly used as a steady-state method to measure 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 (100) and (112) lattice facets. They confirm space charge limited currents at higher bias including an Ohmic contact region at lower bias. We calculated the trap filled limit for different facets by the equation detailed in the experimental section, the values are shown in Table 1. The listed values show the trap filled limit voltage (VTFL) from the I-V curves. The calculated trap densities on (100) and (112) facet are 2.80 × 1013, and 1.05 × 1014 cm-3, respectively. The trap state density on (112) lattice facets is around one order higher than on the (100) facet. This confirms that the (100) facet has a lower trap density compared with the (112) facet. We also calculate work function from the contact potential difference (CPD) via KPFM measurement at ambient dark conditions. The obtained work function values on (100) and (112) facets are 4.7 and 4.6 eV respectively resulting from CPD between a highly orientated pyrolytic graphite reference 6


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(HOPG) 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 (EQE) in published reports.34, 36-37

Figure 1. Facet-dependent optical and electronic properties (a) Top view of as-grown MAPbI3 single crystals (b) polished MAPbI3 single crystal 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 (e) space charge limited current measurement on (100) and (f) on (112) Table 1. Trap densities and work functions for different crystal facets Samples

VTFL / V

nt / cm-3

Φ / eV

MAPbI3 (100)

0.56

2.80 x 1013

4.7

MAPbI3 (112)

2.41

1.05 x 1014

4.6

7



Figure 2. Variation of average contact potential difference and proposed photo-induced effects in single crystal MAPbI3 on individual (100) and (112) facets. (a) light-induced (On: 0.01 mW/m2) average contact potential difference as a function of time on (100) and (d) on (112) (b) N-type behaviour: the surface state of hole-dominated charge on (100) facet under light illumination and (e) P-type behaviour: 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). ECB is the conduction band energy, EVB is the valence band energy, and EF is the Fermi level. KPFM measurements were employed to further investigate 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 perovskites44-46. 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 contaminate the used tip, it was always kept in fresh state. KPFM allows simultaneous mapping of the topography and the local contact potential difference (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 CPD measured in dark subtracted from CPD measured in light, i.e. surface photovoltage=CPDlight-CPDdark. The surface photovoltage measurements help to understand p-n junction properties in semiconductor and local charge properties.49 Figure. 2a and d show CPD measured in dark and light on (100) and (112) facets 8


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respectively. The full series of the CPD maps are shown in figure S1. The initial average CPD values (dark) are approximately 35 and 96 mV on (100) and (112) facets, respectively. Based on our previous reports, higher CPD value implies 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, i.e., vacancy (Iv, Pbv, MAv), interstitial (II, MAI, PbI), and antisite defect ((IMA, MAI, PbI, IPb, PbMA, MAPb)). These defects can create a trap state within the bandgap and provide unintentional doping.9, 50 For instance, 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 (112) facets by 148 mV while the value on (100) facets is decreased until around 7 mV. The decrease in CPD observed in (100) facets implies that excess electrons dominate charge on the surface of the crystal and excess holes are separated towards the bulk, i.e. n-type behaviour at the surface, which is described in Figure 2b. Conversely, the increase in CPD indicates that excess holes dominate charge on the surface and excess electrons are separated toward the bulk as in a p-type semiconductor for the (112) facets as shown in Figure 2e. We relevant band diagrams of each facet according to our results in Figure 2c and f.51 Our results clearly indicate that the types of majority 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) facets are larger compare to the (100) facets. CPD distribution plots for both facets in Figure S2 show that excited charge carriers behave differently under light illumination. Note that the plots are obtained from individual KPFM images in Figure S1.

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Figure 3. Ion movement on different facets investigated by KPFM measurements. Average contact potential difference (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 applied positive bias from + 1 V to + 3 V and (d) applied negative bias from – 1 V to – 3V. We have further studied migration behaviour of ionic defects on each facet using KPFM with bias applied to the tip in dark. Figure 3 shows variations of average contact potential difference (CPD) extracted from KPFM images of a 2 µm2 area taken on 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 when applied positive bias are shown in Figure S3. To elucidate whether the observed phenomenon is cross-talk or artefact, 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 also are presented in Figure S5. Furthermore, KPFM images after applying sequence positive and negative biases are presented in Figure S3and 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 V to +1 V, +2 V, +3 V, 0 V, -1 V, -2V, and -3V. Note that individual scans take 8 mins 40 seconds to complete with scan direction from bottom to top. The main phenomena 10


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we address here is different ionic migration behaviour 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 Based on this knowledge, the variation of CPD over biases is measured in this study with the same method as can be seen in Figure 3. A trend is seen on both facets when positive biases are applied up to + 3 V increases indicating relatively abrupt increase on (100) facets from around 34 to 86 mV and moderate increase on (112) facets from around 76 to 92 mV. This result shows that the attracted negative ions move towards the top surfaces by positively charged tip on (100) facet. This change is relatively higher than for the (112) facet. However, the changes with negative biases reveal that one goes down on (100) facets from approximately 76 to -1 mV whereas the other one moves up on (112) facets from approximately 56 up to 75 mV. Different starting points of CPD values in figure 3c compared with figure 3a mean that sample do not return to completely pristine states 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 due to the nature of the non-contact mode measurement that prevents ions moving through the tip from the surface.

Based on above results, it is possible to deduce 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 due to the lack of interstitial space in such close-packed structures.29 Also, a single crystal does not comprise grain boundaries which act as ion migration channels. For the (100) facet, we suspect 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 above surface photovoltage measurements (Figure 2). Also, the change in CPD upon biasing the tip (Figure 3 a and 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 summarises migration activation energy of vacancies in 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 mV and 16 mV for positive and negative voltages respectively. Also, CPD increases for both cases, which implies that the 11



surface is almost not changing and preserving its p-type characteristics regardless of the AFM tip bias polarity. Since this facet exhibited p-type behaviour (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 interstitial MA+ and Pb2+do not actually migrate but could be existing 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 SRH recombination centres, this coincides with higher trap density and lower PL intensity of (112) facets observed in Figure 1.9, 50

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Figure 4. Ion migration behaviour on different crystal facets in contact with charged AFM tips. (a) AFM topographic images on (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 (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) (d) Line profile of topography with a DC bias voltage of - 3 V (blue line). To investigate the impact of ion migration behaviour 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. Since the tip is now directly contacting and surveying the surface, the effect of bias voltage applied to the tip is enormously larger compare to the above non-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 AFM topographic images on the polished crystal surfaces of (100) and (112) surfaces after applying positive and negative bias voltage at the marked boxes. The 13



full series of the bias-dependent AFM images on (100) and (112) are shown in Figure S6 and S7, respectively (supplementary material). Note that all measurements are performed consecutively from + 1 V to + 10 V and -1 V 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 (100) facets whereas surface damage starts to occur on (112) facets 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) facets that have dominant MA+ and Pb2+ vacancies will attract these defects at higher voltages due to 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 -2V for the (100) facet as shown in Figure 4a, whereas no apparent damage on the surface is observed at -2V for the (112) facet and the damage is observed at -3V instead as shown in Figure 4b. This is resulting from dominant Ivacancies for the (100) facet. 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. Based on this result, it is expected that I- vacancies still exists, possibly, in the bulk or just below the top surface where they can readily migrate to the surface as the migration of Ivacancies is the most favourable vacancy-assisted diffusion in 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 in figure 4a and b are presented in figure 2c and 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 crystal 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 MAPbI3 based materials should be considered with crystallographic orientations in mind as well as chemical composition and crystal structures. In fact, Ion migration in metal halide perovskite is an important key to understand underlying mechanisms for operating conditions in photovoltaic solar cells. Possible channels for ion migration in polycrystalline films are grain boundaries19, 31 However, ionic defects in MAPbI3 perovskite single 14


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crystals are still in existence, and such phenomenon has an influence on operating solar cells. In this regard, it is crucial to understand precise mechanisms related to this phenomenon even in perovskitebased single crystals. For future work, the systematic investigation of anisotropic ion migration behaviours on (100) and (112) lattice facets of MAPbI3 single crystal may inspire further improvement of perovskite solar cells. 4. Conclusions We successfully grew MAPbI3 single crystals with specific (100) and (112) facets. We confirmed that grown MAPbI3 crystal have excellent crystallinity as a single crystal by X-ray diffraction patterns. Facet-dependent surface properties are apparent in photoluminescence and electrical transport measurements. Anisotropic ionic migration behaviour has been investigated with Kelvin probe force microscopy and atomic force microscopy. 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 is observed. This effect is more pronounced on (112) facets. Bias-dependent KPFM measurements on both (100) and (112) facets show different ionic movement 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 damage was not observed when positive bias is applied on (100) where iodide vacancies are dominant, whereas the damage occurs on the MA+ and Pb2+ dominating (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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Supporting Information Supporting Information is available on the ACS Publications or from the author. Full series of KPFM images under varying illumination and bias condition and their analysis, investigation of migration energy of vacancies in MAPbI3 perovskite, and full series of AFM images with different bias condition. Acknowledgements This research has been partially financially supported by the Australian Research Council (ARC) through Discovery grants and a DECRA grant. We acknowledge scientific 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).

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Probing Facet Dependent Surface Defects in MAPbI3 Perovskite

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