Ferroelectric Polarization in CH3NH3PbI3 Perovskite - NESEL - Skku

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Ferroelectric Polarization in CHNHPbI Perovskite Hui Seon Kim, Sung Kyun Kim, Byeong Jo Kim, Kyung-Sik Shin, Manoj Gupta Kumar, Hyun Suk Jung, Sang-Woo Kim, and Nam-Gyu Park J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 21 Apr 2015 Downloaded from http://pubs.acs.org on April 22, 2015

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The Journal of Physical Chemistry Letters

Ferroelectric Polarization in CH3NH3PbI3 Perovskite Hui-Seon Kim,§1 Sung Kyun Kim,§2 Byung-Jo Kim,§2 Kyung-Sik Shin,2 Manoj Gupta Kumar,2 Hyun Suk Jung,2 Sang-Woo Kim,*2,3 and Nam-Gyu Park*1 1

School of Chemical Engineering and Department of Energy Science, Sungkyunkwan

University, Suwon 440-746, Korea 2

School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon

440-746, Korea 3

SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746,

Korea §

These authors contributed equally to this work

*Corresponding authors S.-W.K.: E-mail: [email protected] N.-G.P.: E-mail: [email protected]

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Abstract We report on ferroelectric polarization behavior in CH3NH3PbI3 perovskite in dark and under illumination. Perovskite crystals with three different sizes of 700 nm, 400 nm and 100 nm were prepared for piezoresponse force microscopy (PFM) measurements. PFM results confirmed the formation of spontaneous polarization in CH3NH3PbI3 in the absence of electric field, where the size dependency to polarization was not significant. Whereas the photo-induced stimulation was not significant without an external electric field, the stimulated polarization by poling was further enhanced under illumination. The retention of ferroelectric polarization was also observed after removal of electric field, in which larger crystal showed longer retention behavior compared to the smaller sized one. Additionally we suggest the effect of perovskite crystal size (morphology) on charge collection at the interface of ferroelectric material even though insignificant size dependency in electric polarization was observed.


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Since the reports on perovskite solar cells based on mesoscopic sensitization concept1 and meso-superstructured extremely thin layer concept2 in 2012, organometal halide perovskite has been considered as promising light harvester for high efficiency solar cell. Power conversion efficiency (PCE) of perovskite solar cell surged to over 15% in 2013 by modifying a perovskite deposition method from a conventional one-step method to two-step sequential deposition3 and vapor evaporation process4. So far, the highest PEC of 19.3% was reported using planar structure5 and 20.1% was certified by the National Renewable Energy Laboratory6. The outstanding photovoltaic performance from perovskite solar cells has urged researchers to investigate the fundamental properties of perovskite. Consequently, balanced charge transport7,8 and charge accumulation9 properties of organolead iodide perovskite were unraveled. Besides light harvesting and balanced charge transport properties, unusual properties were observed with CH3NH3PbI3 (MAPbI3), such as ambipolar self-doping property,10 high dielectric constant,11 current-voltage hysteresis12 and slow dynamics13. Polarization was suggested as one of bases for the origin of these phenomena. In ABO3 oxide perovskite materials, ferroelectricity along with high dielectric constants was observed.14 In addition, phase transition was accompanied in ferroelectric oxide perovskite materials. Thus, anomalous properties observed for MAPbI3 are likely to be related to the underlying ferroelectric property because of its high dielectric constant and cubic-tetragonal phase transition. Regarding the issue on ferroelectricity in MAPbI3, theoretical calculations suggested the possibility of ferroelectricity of MAPbI3.15-17 Ferroelectric domains were directly observed from the solution-processed MAPbI3.18 Interestingly, no ferroelectricity was measured from MAPbI3 but the ion migration was suggested to interpret the phenomena.10,19 This means that the argument on ferroelectricity in MAPbI3 has not been terminated with clear conclusion, which motivates us to investigate ferroelectric behavior of MAPbI3. 4 ACS Paragon Plus Environment

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We report here on ferroelectric polarization behavior in MAPbI3 by piezoresponse force microscopy (PFM) investigation. We measured the electric polarization change of the samples with different crystal size with and without electric field in the dark and under illumination. In addition, polarization retention characteristics after removal of external electric field were investigated. All experimental was designed similar to the practical solar cell operation including morphology of MAPbI3 and range of external electric field in order to closely correlate with the solar cell characteristics and give insight into working principle of perovskite solar cell. Photovoltaic performance was previously found to be significantly altered by crystal size of MAPbI3,20 where large crystal was beneficial to light harvesting and charge extraction compared to small one. Moreover, large crystal was less sensitive to current-voltage (I-V) hysteresis than small one.12 Thus perovskite layers with different crystal size were selected as samples for PFM study in this work in order to correlate the PFM results with the studied photovoltaic properties. The crystal size was controlled by varying the concentration of CH3NH3I (MAI) solution in the two-step spin-coating deposition method as described elsewhere,20 where 0.038 M, 0.044 M, and 0.063 M MAI produces average MAPbI3 crystal size of about 700 nm, 400 nm, and 100 nm, respectively, as shown in Figure 1(a)-(c). Compared to the compact nature of the 100 nm-sized cuboids, the number of gaps appears and increases upon increasing size, which is consistent with the previous results.20 No structural change was observed depending on crystal size as confirmed by X-ray diffraction (XRD) patterns in Figure 1(d)-(f), which underlies that the chemical composition is not modified by crystal size. Because there is no significant difference in XRD peaks among three samples with different morphologies, we expect the grain sizes in the samples with poly-crystalline nature are comparable irrespective of different crystal size. In addition, the prepared MAPbI3 shows phase purity without unreacted PbI2. 5 ACS Paragon Plus Environment


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The piezoresponse measurements were conducted using a PFM set-up as illustrated in Figure 2. The detailed measurement condition is described in the experimental section. To confirm the ferroelectric characteristics of the MAPbI3, we measured the topography and the piezoresponse of the MAPbI3 on the fluorine-doped tin oxide (FTO)/glass substrate using atomic force microscopy (AFM) and PFM. It is well known that PFM is a powerful tool which detects polarization states based on the detection of the local piezoelectric deformation induced by an external electric field in a ferroelectric sample. The topography image of the MAPbI3 was built by measuring the z-axis movement of the AFM tip in a contact mode. For poling the sample, we applied a DC bias voltage between the MAPbI3 cuboid and the conductive tip. During the PFM measurement, an AC voltage was applied from the conductive tip, as top electrode, to the sample’s surface. The amplitude and the phase signals of the response of the MAPbI3 material to the applied electrical signal were detected using the vertical displacement of the AFM tip.

Figure 1. Scanning electron microscopy (SEM) images for the MAPbI3 crystals with average cuboid size of (a) 700 nm, (b) 400 nm and (c) 100 nm prepared using 0.038 M, 0.044 M and 0.063 M MAI, respectively. Scale bars represent 500 nm. X-ray diffraction patterns of (d) 700 6 ACS Paragon Plus Environment

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nm-, (e) 400 nm- and (f) 100 nm-sized MAPbI3 cuboids directly formed on fluorine-doped tin oxide (FTO) substrate.

Figure 2. Schematic illustration of the experimental setup for PFM study of the MAPbI3.

To determine the ferroelectric polarization of the MAPbI3 perovskite material, positive and negative poling processes were sequentially applied and PFM phase was measured both in the dark and under illumination. Figure 3 shows PFM phase along with topography observed in the dark, where no significant deformation of the perovskite sample is assumed throughout the successive poling process due to the sequential distinctive image of PFM phase. The topographic image of 700 nm-sized perovskite sample and the corresponding PFM phase images obtained without bias voltage (unpoled) and with bias voltages of +3 V (positively poled) and -3 V (negatively poled) are depicted in Figures 3(a)(d), respectively. Figure 3(a) shows some extent of PFM phase contrast even in the absence of electric field (unpoled), indicative of spontaneous polarization. Unlike the change in 7 ACS Paragon Plus Environment


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displacement of B-site cation in oxide perovskites,21,22 the polarization behavior in organolead iodide perovskite is likely due to large freedom of rotation of the polar CH3NH3+ cation.15 When positive poling (+ 3V) is applied, stimulated polarization in the positive direction is distinctively observed as shown in Figure 3(c). Figure 3(d) shows the PFM image under the negative poling process. Interestingly the opposite electric field (- 3V) is incapable of rotating dipoles 180o, which implies that the dipoles cannot easily rotate by the applied electric field due to the interaction with defects in crystals.23 The spontaneous and stimulated polarizations are also observed for the 400 nm- and the 100 nm-sized cuboids. As can be seen in Figures 3 (f)-(h) and (j)-(l), PFM phase contrast is enhanced upon positive poling and relieved with negative poling but the dependence of polarization on the crystal size seems to be not significant. It is suggested that the negligible size dependency originates from poly-crystalline nature of MAPbI3 composed of multiple grains. Thus, we conclude that grain size is a critical factor to realize effective manipulation of ferroelectric polarization rather than crystal morphology. However, careful interpretation is still needed considering the fact that the perovskite crystals with different size of 700 nm, 400 nm, and 100 nm simultaneously generate different thickness of 360 nm, 290 nm, and 170 nm, respectively. Because few studies of MAPbI3 ferroelectric behavior on thickness effect have conducted, little information was given to explain the impact of thickness around hundreds of nanometers.


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Figure 3. AFM topographic images and PFM phase images measured in the dark for the MAPbI3 cuboids with size of (a)-(d) 700 nm, (e)-(h) 400 nm and (i)-(l) 100 nm. Scale bars in (a)-(h) represent 1 µm and those in (i)-(l) represent 500 nm. Measured areas were 5 µm × 5 µm for 700 nm- and 400 nm-sized samples, 2 µm × 2 µm for 100 nm-sized sample.

Light-induced polarization of the perovskite cuboids was also investigated with respect to the crystal size and poling process using the PFM measurement system equipped with a white light source (50,000 lux). Figure 4 shows topographic images and the corresponding PFM phase images under illumination depending on the perovskite polycrystal size. For the case of unpoled samples (Figure 4(b),(f),(j)), it is found that the spontaneous polarization is not enhanced by light irradiation compared to the unpoled samples in dark (Figure 3(b),(f),(j)). The 700 nm-sized large sample shows notably attenuated polarization even under illumination (Figure 4(b)) compared to that in dark (Figure 3(b)), which indicates that the ferroelectric polarization is efficiently screened by photo-generated conducting electrons.22 However, photo-induced stimulated polarization is clearly distinct in the presence of electric field, especially positive poling (Figure 9 ACS Paragon Plus Environment


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4(c),(g),(k)) compared to those in dark (Figure 3(c),(g),(k)). Upon negative poling, similar tendency is observed showing weakened polarization (Figure 4(d),(h),(i)) compared to the positively poled samples (Figure 4(c),(g),(k)).

Figure 4. AFM topographic images and PFM phase images measured under illumination (50k lux) for the MAPbI3 cuboids with size of (a)-(d) 700 nm, (e)-(h) 400 nm and (i)-(l) 100 nm. Scale bars in (a)-(h) represent 1 µm and those in (i)-(l) represent 500 nm. Measured areas were 5 µm × 5 µm for 700 nm- and 400 nm-sized samples, 2 µm × 2 µm for 100 nm-sized sample.

Retention of ferroelectric polarization is investigated as a function of time after a removal of the external electric field. PFM phase of positively poled-crystals are measured at an interval of 30 min under illumination (Figure 5). Highly aligned dipoles in 700 nm-sized crystals are maintained for 60 min as shown in Figure 5(a), while dipoles in the 400 nm- and 100 nm-sized MAPbI3 are retained only for 30 min as shown in Figure 5(b) and (c), respectively. It is obvious from our observation that the retention of the light-induced 10 ACS Paragon Plus Environment

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polarization is dependent on MAPbI3 crystal size, in which 700 nm-sized large crystal shows better retention behavior than smaller ones. Generally large crystal is assumed to have lower defect density compared to the smaller crystal where significant defects hinder the retention of polarization.24

Figure 5. PFM phase images under illumination for (a) 700 nm, (b) 400 nm, and (c) 100 nm MAPbI3 cuboids, measured at positive poling (0 min) and at 30 min and 60 min after a removal of poling (+ 3 V). Scale bars represent 1 µm for 700 nm- and 400 nm-sized MAPbI3 and 500 nm for 100 nm-sized MAPbI3. Measured areas were 5 µm × 5 µm for 700 nm- and 400 nm-sized samples, 2 µm × 2 µm for 100 nm-sized sample.

In this study our results reveal ferroelectric characteristics of MAPbI3 and its dependence on light. Lastly we correlate photo-ferroic behavior with photovoltaic performance and suggest carrier collection mechanism in the ferroelectric MAPbI3 material. Forward bias voltage more than 1 V is applied to the real device while measuring photocurrent-voltage curve. Poling at 3 V in this study is acceptable for correlation with real 11 ACS Paragon Plus Environment


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device performance because the actual extent of applied external electric field between tip and plane in PFM measurement is two- or three-times weaker than the case between plane and plane in solar cell measurement.18 Even though ferroelectric property is comparable regardless of crystal size, excluding retention behavior, photovoltaic performance is highly dependent on crystal size.12,20 Here we suggest charge collection mechanism with related to the effect of morphology of the ferroelectric MAPbI3 material. Figure 6 illustrates carrier pathway depending on the perovskite crystal morphology. Photo-induced stimulated polarization is evident in MAPbI3 irrespective of crystal size. It is theoretically found that the well aligned dipoles in MAPbI3 have a substantial effect on PbI3 inorganic cage and lead an enhanced current response.25 In addition, at positive poling (forward bias direction in actual I-V measurement) the external electric field will be essentially weakened by generated internal electric field due to strong polarization, which is good for separation and collection of photogenerated electrons toward an interface of perovskite and selective contact. However, transported electrons are easy to be trapped at the interface and thus hardly injected into the selective contact.26 Whereas densely populated small crystals cause inefficient electron injection due to the higher density of trapped electrons (Figure 6(b)), sparsely populated large crystals sufficiently provide an efficient pathway for electrons along the edge (Figure 6(a)). Then, the sparse morphology with large crystals is beneficial for charge collection leading high photocurrent density. This result is well accordance with the previous report, where the perovskite solar cells incorporating large crystals of about 700 nm and 400 nm exhibit higher amounts of the extracted charges compared to the smaller cuboids of around 100 nm.20 Furthermore, a similar interpretation can be applied to I-V hysteresis or perovskite solar cell depending on the crystal size. It is reported that I-V hysteresis problem is intensified with decreasing crystal size.12 The higher density of trapped electrons in small crystals can afford to induce severe hysteresis. It was 12 ACS Paragon Plus Environment

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observed that there was a strong correlation between transient ferroelectric polarization induced by negative poling in the dark and I-V hysteresis enhancement in photovoltaic characteristics.27 Our results support this because internal electric field formed by negative poling is adverse to carrier collection.

Figure 6. Expected charge collection mechanism with carrier pathway depending on MAPbI3 crystal morphology under illumination and forward bias condition.

Another important aspect is the retention of polarization as observed in Figure 5. Large crystal of 700 nm-sized MAPbI3 showed longer retention of polarization than smaller ones. Actually we expected the decay (relaxation) of the polarization would be around a few second according to our previous result,12 where the current response time of capacitive current leading I-V hysteresis was observed in the range of seconds (~5 s) and its corresponding capacitance also responded in low frequency range (~0.28 Hz). However, the observed retention time of the induced polarization by poling was much longer than expected. Therefore, we suggest that the transient current response is related to the traps for charge carriers at the interface. On the other hand, Coll et al. observed ferroelectric retention 13 ACS Paragon Plus Environment


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behavior in the one second regime from the similar morphology of MAPbI3.28 This definitely different retention behavior might result from the presence of mesoporous TiO2 layer where the ferroelectric domain could be limited by pore size. In terms of light effect, they also found increased polarization upon illumination which was confirmed by PFM amplitude. Xiao et al. recently reported that ion drift occurs during a poling process and they could not find any trace of ferroelectric characteristics from their PFM measurements even though they used a sequential two-step deposition method (same method used in this study) to prepare MAPbI3 perovskite film.10 The difference might come from the crystal quality or/and significant crystal size difference since much smaller crystal size would be expected due to the high concentration of 45 mg/mL MAI solution.10 It was mentioned that the ion drift by poling was negligible in the large grain (600-1000 nm) owing to the reduced vacancy concentration based on the fewer grain boundaries. In conclusion, MAPbI3 was found to exhibit ferroelectric characteristics according to PFM study. Spontaneous polarization behavior was commonly found in MAPbI3 where the size dependency was not significant. The electric polarization was further stimulated by positive poling and released by negative poling, in which 180o dipole rotation by opposite electric field is inhibited by defects in crystals. Photo-induced polarization is negligible in the absence of electric field but evident in the presence of electric field showing strong polarization. Photo-induced polarization with positive poling is basically beneficial to charge transport and collection especially for the large crystals by providing efficient electron pathway. Polarization was remained after removal of electric field and retention of polarization was longer in large crystal than smaller ones. The observed ferroelectric behavior in organolead iodide perovskite is expected to give crucial insight into design and working mechanism of perovskite solar cells.


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Methods Synthesis of MAI. The MAI was synthesized according to a previous report.12,20 28 mL of methylamine (40% in methanol, TCI) was added to 30 mL of hydroiodic acid (57 wt % in H2O, Aldrich) and the mixture was stirred in the ice bath for 2 h. The product was obtained by evaporating the solvent at 70 oC for 1 h and washed three times with diethyl ether and filtered. For higher purity, the product was dissolved in a small amount of ethanol (~150 mL) and the solution containing the product was dropped into a large amount of diethyl ether (~1000 mL) with stirring. The white precipitate was filtered and dried under vacuum overnight. Preparation of MAPbI3 films for PFM measurements. FTO glasses (Pilkington, TEC-8, 8Ω/sq) were cleaned with detergent, which was followed by sonication in an ethanol bath for 30 min and UV-ozone treatment in an UVO cleaner for 30 min. MAPbI3 was prepared by a sequential deposition method.20 0.5 M PbI2 (99%, Aldrich) solution dissolved in N,N-dimethylformamide (99.8%, Aldrich) was spin-coated on the cleaned FTO substrates. The spin-coated films were heated at 40 oC for 5 min and 100 oC for 5 min to form PbI2 layer. 200 µL of 2-propanol solution of MAI (0.038 M, 0.044 M, and 0.063 M were used in this experiment) was dropped on the PbI2 deposited substrate with dimension of 2.4×2.4 cm2, which was spun after a given loading time (30 s, 20 s, and 10 s for 0.038 M, 0.044 M, and 0.063 M, respectively). The spin-coated film was heated at 100 oC for 5 min to form MAPbI3. Surface perovskite cuboids morphologies were obtained using a field emission scanning electron microscope (FE-SEM, JSM-7600F, JEOL). X-ray diffraction patterns were measured by a HP-Thin Film XRD system (D8 Advance for high power, Bruker Corporation) using a turbo X-ray source of 18 kW with a scan rate of 2° min−1. PFM measurement. The AFM-based investigations were carried out using a XE-100 15 ACS Paragon Plus Environment


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AFM (Park Systems). The ferroelectric polarization of the halide perovskite samples was confirmed by PFM. A lock-in amplifier (Stanford Research Systems SR830) was used to detect the piezoresponse signal equipped with conductive Pt/Cr coated silicon tips (spring constant 3 N/m), operating in contact mode for imaging of topography and relative polarization by PFM. Samples were basically kept in dark before measuring without any pretreatment. For ferroelectric induced dipole orientation measurement, a poling voltage of +3 V or -3 V was applied to the tip to align the dipoles in a single direction. Afterwards, the PFM signal was measured. The deflection of the cantilever was set to 30 nN and the scanning rate was 0.5 Hz. PFM measurement was conducted at room temperature in ambient air with 20% of humidity. The sample scanning area was 5×5 µm2 for 700 nm and 400 nm-sized perovskite cuboids and 2×2 µm2 for 100 nm-sized perovskite cuboids sample. A white-light point source (50000 lux) with a light guide GS6-1000F integrated with AFM system was used to detect light induced polarization.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contracts No. NRF-2010-0014992,

NRF-2012M1A2A2671721,

NRF-2012M3A7B4049986

(Nano

Material Technology Development Program), NRF-2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System). H.-S.K is grateful to Global Ph.D. Fellow Grant funded by NRF. S.-W.K. acknowledges financial support by Basic Science Research Program (2009-0083540) through the National Research Foundation (NRF) of Korea Grant funded by the Ministry of Science, ICT & Future Planning.


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References (1) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E. et al. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. (3) Burschka, J; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316-319. (4) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. (5) Zhou. H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells, Science 2014, 345, 542-546. (6)

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