Mechanism and Impact of Cation Polarization in Methylammonium

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Mechanism and Impact of Cation Polarization in Methylammonium Lead Iodide Susanne T. Birkhold, Hao Hu, Pius T. Höger, Ka Kan Wong, Philipp Rieder, Andreas Baumann, and Lukas Schmidt-Mende J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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The Journal of Physical Chemistry

Mechanism and Impact of Cation Polarization in Methylammonium Lead Iodide

AUTHOR NAMES Susanne T. Birkhold,1 Hao Hu,1 Pius T. Höger,1 Ka Kan Wong,1 Philipp Rieder, 2 Andreas Baumann, 3 Lukas Schmidt-Mende1*

AUTHOR ADDRESS 1

2

Department of Physics, University of Konstanz, 78457 Konstanz, Germany Experimental Physics VI, Julius-Maximilian University of Würzburg, 97074 Würzburg,

Germany 3

Bavarian Center for Applied Energy Research, 97074 Würzburg, Germany

* E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT Despite the intense research effort on metal halide perovskites, the fundamental correlation between the crystal structure and optoelectronic properties remains unclear. As many intriguing phenomena are expected to base on the dipolar character of the rotating organic cations, an improved understanding of the material’s polarizability is of high relevance. Here, we study the orthorhombic-tetragonal phase transition of methylammonium lead iodide to gain insight into the polarization mechanism at low temperatures and the resulting effects on solar cell performances. Using thermally stimulated current measurements, we detect polarization currents when cooling or heating across the phase transition. These (de-)polarization currents are found to correlate with a sudden change in rotational freedom of the organic cations, with a temperature range of 20 K separating polarizing and depolarizing processes. The nature of this cation polarization within the orthorhombic phase is investigated with respect to its intrinsic and extrinsic polarizability. Although the amount of unamplified polarization is determined to be only in the order of a few nC/cm2, our results show significant increases in solar cell performances upon polarization, highlighting the intimate relationship between the alignment of organic cations and optoelectronic properties in metal halide perovskites.

INTRODUCTION Since their first application as sensitizer in 2009,1 metal halide perovskites, with methylammonium lead iodide (CH3NH3PbI3, hereafter abbreviated MAPbI3) being the most studied representative, have emerged as a highly promising absorber material for thin film photovoltaics. However, despite the skyrocketing power conversion efficiencies of perovskite


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solar cells, now reaching above 20 %,2 a full understanding of the material’s crystal structure and its impact on optoelectronic processes is still missing. With decreasing temperature, the high symmetry cubic crystal structure of MAPbI3 (T > 330 K) transforms into lower symmetry tetragonal (330 K > T > 160 K) and orthorhombic (T < 160 K) structures by tilting of the PbI6 octahedron around the C-axis.3-4 While the cubic crystal structure has been refined by the space group Pm3m,3, 5 the crystallographic classification of tetragonal MAPbI3 is still under debate, as the non-polar I4/mcm and the polar I4cm space group are very similar with regard to the PbI6 octahedra. Although the majority of reports proposes the existence of the non-polar I4/mcm space group,3, 5-8 significant disagreement persists in literature about the polar nature of MAPbI3 at room temperature.4, 7, 9-10 Yet, also the crystal structure of the orthorhombic phase and the exact mechanism of the tetragonal-orthorhombic phase transition still lack a thorough understanding. Upon cooling below 160 K, the transition of MAPbI3 into the orthorhombic phase is assumed to be a first order phase transition.5 Baikie et al. have assigned the space group Pnma to the orthorhombic phase based on powder X-ray diffraction measurements at 100 K, which has been generally accepted to be without dynamic disorder of the organic cations.3, 11-12 However, as already pointed out by Baikie et al.,3 a missing group-subgroup relationship between the space groups I4/mcm and Pnma makes a continuous transformation between these tetragonal and orthorhombic phases impossible, suggesting the existence of a transient intermediate phase. Experimental observations of a dielectric dispersion in orthorhombic MAPbI3 around 60 – 100 K, 11, 13 as well as unexpected cation dynamics between 60 – 160 K indicated by inelastic neutron scattering,14 photoluminescence measurements,15 and theoretical calculations,16 support the notion of


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significant structural transformations with increasing cation rotation within the orthorhombic phase of MAPbI3. An improved understanding of the temperature dependent ordering of organic cations is of significant relevance, as their collective polarization is expected to play a critical role for ferroelectric properties in MAPbI3,17-20 as well as for other perovskite compounds containing organic cations.21-22 Enhanced polarization retention at low temperatures has been proposed to be necessary for (anti-)ferroelectric behavior.18-20, 23 Furthermore, amplified spontaneous emission during the orthorhombic-tetragonal phase transition,24-26 as well as improved photoluminescence quantum yield and charge carrier mobility after thermal cycling highlight the necessity to better understand the structural changes happening both to the inorganic and organic framework of MAPbI3 when cooling below the orthorhombic-tetragonal phase transition.25, 27 In this work, we study the structural evolution of MAPbI3 across the orthorhombic-tetragonal phase transition by thermally stimulated current (TSC) measurements. We correlate observed (de-)polarization currents with a transformation in rotational motion of organic cations across the phase transition and analyze the reversible polarization and depolarization mechanisms, that are separated by a temperature range of around 20 K. While the polarization direction can be controlled by relatively low electric fields during cooling, we find that application of electric fields up to ~ 1 V/µm cannot switch the polarization once the material is fully polarized at low temperature. Our results show that the collective polarization of organic cations causes an immediate enhancement in solar cell performances, highlighting the close relationship between the dynamic crystal structure of metal halide perovskites and their optoelectronic properties. METHODS


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Solar Cell Fabrication. Unless otherwise noted, measurements were done on device A. The device area was 0.125 cm2. Device A: ITO/PEDOT:PSS/MAPbI3/C60/LiF/Ag. Details of preparation and performance of the devices have been reported elsewhere.28 A 30 nm PEDOT: PSS film (purchased from Clevios™ VP AI 4083) was spin-coated on cleaned ITO substrates and annealed at 180 °C for 5 min. Then, substrates were transferred into a nitrogen-filled glove box. For the mixed-halide perovskite precursor solutions, PbI2 (purchased from Sigma-Aldrich), PbCl2 (purchased from SigmaAldrich) and MAI (purchased from Dyenamo) were mixed in anhydrous DMF with a molar ration of 1:1:4 and a solution concentration of 40 wt %. After the precursor solution was spincoated on the substrates at 3000 rpm, films were transferred immediately to a vacuum chamber on a hotplate at 90 °C. After the films turned red-brown, they were annealed at ambient pressure at 100 °C. Finally, a C60 layer (20 nm), LiF layer (1nm) and Ag back contact (100 nm) were thermally evaporated. Device B: FTO/c-TiO2/PCBA/MAPbI3/P3HT/WO3/Ag. Details of preparation and performance of the devices have been reported elsewhere.29 TiO2 sol gel (purchased from Solaronix) was spin-coated on cleaned ITO substrates at 5000 rpm and sintered at 450 °C for 30 min. After surface treatment of TiO2 by immersing samples in 0.2 mM PCBA solution in chloroform for 1 hour, samples were spin-coated with 1 M PbI2 (purchased from Sigma Aldrich) solved in DMF and dried at 70 °C for 1 hour in a nitrogen-filled glovebox. Afterwards, 0.58 mM MAI (purchased from Solaronix) solved in anhydrous isopropanol was drop-casted, spun, and annealed at 100 °C for 10 min. 80 mg/ml Spiro-MeOTAD (purchased from Merck) solved in chlorobenzene (doped with 28.8 µl 4-tert-butylpyridine and 17.5 µl bis(trifluoromethane)


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sulfonimide lithium salt solution) was spun at 4000 rpm for 120 s. Finally, back electrodes with WO3 (3 nm) and Ag (100 nm) were thermally evaporated. Temperature Dependent Measurements. All measurements were performed in a continuous flow cryostat (Janis Research Company). Samples were attached to a copper plate, whose temperature was controlled by liquid nitrogen or helium and a temperature controller (Lake Shore Cryotronics Inc.). Before starting to heat or cool, samples was kept for 10 min at the start temperature to reach thermal equilibrium. If not otherwise noted, measurements were performed after an initial thermal cycling from the orthorhombic phase into the tetragonal phase. Thermally Stimulated Current (TSC) Measurements. All TSC measurements were performed in complete darkness at short circuit to avoid overlap of thermally stimulated currents with injection or photo-induced currents. Solar cells were continuously heated or cooled at constant rate and thermally stimulated currents were detected with a subfemtoamp remote source measurement unit (Keithley 2636A). Unless otherwise noted, temperature rates of 3 K/min were applied. Several variations of TSC measurements have been applied. For illuminated TSC measurements, the solar cell was illuminated with a blue LED for 15 min at 78 K. After a dwell time of 15 min in darkness, the TSC signal was measured during continuous heating of the device. For biased TSC measurements, the solar cell was cooled down under an external bias. After a dwell time of 15 min with no external bias, the TSC signal was measured during continuous heating of the device. For current vs. time measurements, the solar cell was cooled down in dark either with or without an external bias from 295 K to a target temperature at a rate of 12 K/min. When the target temperature was reached, the TSC signal was immediately measured against time at the constant target temperature at short circuit.


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Dielectric Measurements. Dielectric measurements were done on ITO/MAPbI3/Au devices (perovskite film deposited as in device A) using a Metrohm Autolab PG-STAT302N and an integrated FRA2 module. Impedance spectra were acquired in dark at 1 kHz and 0 V bias. At each temperature, the device was rested for 10 min to reach thermal equilibrium. Solar Cell Characterization. Solar cells were illuminated with a white light LED of reduced light intensity (5 mW/cm2) to prevent heating. Slow heating and cooling rates of 1.5 K/min were applied, while continuously measuring the short circuit current or the open circuit voltage with a source measurement unit (Keithley 2400). RESULTS AND DISCUSSION TSC Measurements Across Phase Transition. In the following, thermally stimulated currents (TSC) upon heating or cooling of a MAPbI3 based solar cell across the orthorhombic-tetragonal phase transition are investigated. In general, TSC measurements are a well-established technique to characterize the defect nature of semiconductors and insulators,30 with recent applications to perovskite solar cells.31-32 To study the energetic distribution of trap states, the device is photoexcited at low temperatures to fill trap states with electronic carriers, followed by a continuous temperature increase in dark that results in a measurable current caused by the thermal release of the trapped carriers. Figure 1 displays a typical TSC measurement of a MAPbI3 based solar cell, using a ITO/PEDOT:PSS/MAPbI3/C60/LiF/Ag device architecture. While two TSC peaks around 150 K and 190 K appear when traps are filled by photo-excitation at 78 K, only one peak at 145

Figure 1. TSC measurements of a MAPbI3 based solar ACS Paragon Plus Environment (ITO/PEDOT:PSS/MAPbI3/C60/LiF/Ag) with and without previous illumination at 78 K.

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K appears in case of no trap filling. The high temperature TSC peak has been assigned to deep trap states in MAPbI3 with a trap state energy of around 0.5 eV.31 However, the origin of the TSC peak at 145 K remains under debate and has been recently proposed to be a displacement current in the outer circuit caused by the orthorhombic-tetragonal phase transition of MAPbI3.31 In fact, thermally-induced polarization of a material can cause additional TSC signals, because the (de-)polarization of a material can attract (release) surface charges at the electrodes, causing a current flow in the outer circuit. In case of thermally-induced spontaneous polarization in the absence of an electric field, the resulting currents are called pyroelectric currents. These polarization currents can superimpose on de-trapping currents, making it necessary to distinguish between the two processes when analyzing TSC signals.33-34 In this respect, the origin of the TSC peak at 145 K will be discussed in the following. In Figure 2a, TSC measurements without trap filling show similar current peaks for heating and cooling, yet in reverse direction. The opposite direction of the otherwise similar current peaks indicates a reversible process upon thermal cycling across this temperature range. Furthermore, we observe that these TSC peaks are similar for different device architectures of ITO/PEDOT:PSS/MAPbI3/C60/LiF/Ag

and

FTO/c-TiO2/PCBA/CH3NH3PbI3/P3HT/WO3/Ag,

indicating that their height and direction are independent from selective layers (see Fig. S1).


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The thermal position of these TSC peaks depends on the applied temperature rate, as displayed in


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detail in Fig. S2. Figure 2b summarizes the TSC peak positions for different temperature rates,


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that approach temperatures of around 125 K and 145 K for cooling and heating, respectively, if


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temperature rates are reduced below 1.5 K/min. We assign the shifted TSC peak positions for


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higher temperature rates to a delayed temperature transfer from the temperature-controlled


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Figure 2. (a) TSC measurements of a ITO/PEDOT:PSS/MAPbI3/C60/LiF/Ag solar cell without illumination during heating (red line) and cooling (blue line) at a temperature rate of 1.5 K/min. (b) Peak positions of TSC measurements without illumination during heating (red line) and cooling (blue line) at different temperature rates. (c) Dielectric constant Ɛr measured on a ITO/MAPbI3/Au device at 1 kHz. Shaded area highlights the position and thermal separation of TSC peaks. copper plate to the perovskite layer. However, the consistent TSC peak position at 145 K for


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heating with temperature rates slower than 1.5 K/min verifies that a delayed temperature transfer is eliminated for slower rates. Interestingly, Figure 2b displays that a temperature range of around 20 K persist between TSC peaks for heating and cooling, even if slow temperature rates are applied. In fact, we find that temperatures have to be decreased below a certain temperature of around 120 K (small temperature variations exist between samples) to induce the reversed TSC peak (see Fig. S3). It is therefore assumed that significantly lower temperatures (T < 125 K) are required to reverse the transformation process that is observed for increasing temperatures (T > 145 K). For decreasing temperatures, thermally activated de-trapping processes of charge carriers or migration of ions cannot explain the reversed TSC peak, as trapped carriers and mobile ions are thermally “freezed” at low temperatures. Indeed, threshold temperatures to activate the motion of ions in metal halide perovskites have been reported to be above 250 K.35 Furthermore, a reversed and sudden motion of ions at 125 K and 145 K for heating and cooling would require a symmetric inversion of an internal electric field by a spontaneous change in the contacts’ work functions, which is unlikely. Therefore, the TSC peak is best described by polarization currents due to a reversible polarization and depolarization of the perovskite film around the orthorhombic-tetragonal phase transition. The here observed phase transition temperature around 145 K deviates from the typically observed phase transition at 160 K, which we assign to variations in film processing conditions and grain size.26,

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The thermal separation of the

polarization currents of around 20 K between heating and cooling indicates an extended structural transformation that takes place between the polarized and unpolarized states of MAPbI3. Furthermore, the hypothesis of the TSC peak being a displacement current in the outer


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circuit due to a change in the material’s polarization is supported by the observed independence from the diode direction (Fig. S1).31 The correlation of the TSC peak and the material’s polarization is further studied by dielectric measurements on ITO/MAPbI3/Au devices (Figure 2c). Generally, the dielectric constant Ɛr of MAPbI3, and hence its polarizability, decreases from the tetragonal to the orthorhombic phase,11, 13

which has been associated with a freezing of organic cation rotations around the C-N axis.37-38

Figure 2c displays the transformation of the dielectric constant across the orthorhombictetragonal phase transition measured for our MAPbI3 films. We find that the dielectric constant Ɛr peaks at 125 K during cooling and at 145 K during heating. The continuous rise in Ɛr over a temperature range of 25 K prior to the phase transition is in good accordance with previous reports of increasing cation dynamics within the orthorhombic phase.14-15 The discontinuity in the dielectric constant, and hence in the polarizability of the perovskite, coincides with the occurrence of the TSC peaks at 125 K and 145 K. We therefore propose that organic cations gain increasing rotational freedom when the temperature is increased towards the phase transition, resulting in a collective depolarization of organic cations at 145 K that gives rise to the observed current peak in the TSC measurement. The reversed process of cation polarization upon cooling below 125 K results in a TSC peak of opposite current direction, which we find to be thermally delayed by around 20 K. Repeatability and Retention of Polarization. The repeatability of the polarization and the impact of repeated thermal cycling are displayed in Figure 3a. Here, the device is quickly cooled across the phase transition from 295 K to 78 K with a rate of 12 K/min. Due to a delayed temperature transfer from the copper plate to the perovskite layer, the polarization current is stimulated with a certain time delay when measuring current against time at a set temperature of


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Figure 3. (a) Current vs. time measurements at 78 K, measured directly after cooling from 295 K to 78 K without external bias. All measurements were performed on the same device on consecutive days. For first measurements of each day, the TSC peak appears later in time (bright blue) and for all following measurements, the TSC peak appears earlier (dark blue). (b) TSC measurement during heating in dark at 3 K/min for two separate solar cells, showing polarization either along one or two opposite directions. 78 K. The similarity of TSC peaks for repeated cooling cycles demonstrates a highly reproducible process, both concerning the dynamics and the degree of polarization. Figure 3a shows that only the first cooling cycle of each day results in a delayed polarization, indicating that the first polarization cycle enhances the material’s polarizability. Recently, Senanayak et al. reported a preserved ordering of the organic cation at room temperature after thermal cycling across the orthorhombic-tetragonal phase transition.27 We therefore suggest that some polarized domains persist after heating across the orthorhombic-tetragonal phase transition and facilitate consecutive polarization processes. Yet, the similarity of first measurements on consecutive days displays that this remnant polarization dissipates after overnight storage at room temperature. Polarization Direction. Although the majority of TSC measurements displayed identical and repeatable polarization directions, a few measured devices featured polarization peaks that comprised both positive and negative current contributions (see Figure 3b). This indicates that


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opposite polarization directions can be present within a polycrystalline perovskite film. The observed high repeatability of the polarization process reveals that organic cations repeatedly align with the same orientation relative to the solar cell stacking direction. Using X-ray diffraction (XRD) to characterize the crystallographic orientation of our films, we find that our perovskite is predominantly orientated with the (110) crystal plane parallel to the surface of the film (Fig. S4). We therefore conclude that the observed TSC signal, that is measured perpendicular to the surface, originates from a polarization perpendicular to the (110) plane. Please note that calculations by Stroppa et al. predict that a coupling of organic cations with the inorganic framework by hydrogen bonding causes a distortion of the PbI6 octahedra upon cation polarization, with additional inorganic polarization components both parallel and perpendicular to the cation alignment.39 It is further expected that variations in film preparation can lead to different polarization directions of organic cations within the polycrystalline perovskite film, e.g. by varying grain orientations on the substrate, interface dipoles, surface charges, etc. In order to control the alignment of organic cations, external electric biases are applied to the solar cell. As TSC signals are in the order of a few picoamperes, overlapping high injection currents prevent a sensitive TSC analysis while simultaneously applying an electric field. Hence, the sample is biased during cooling at a fast rate of 12 K/min and currents are measured at short circuit as soon as a temperature of 78 K has been reached. Due to the delayed heat transfer from the temperature-controlled cooper plate to the perovskite layer, thermally-induced processes can then be displayed against a time axis. Figure 4a demonstrates that the application of an electric bias during cooling from 295 K to 78 K can enhance or reverse the TSC peak and likewise the polarization at low temperatures. To reverse the polarization direction, applied biases above + 65 mV are necessary. Heating the poled device results in a similar amplification of the


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depolarization current in the respective direction (Figure 4b). Higher polarization currents for negative biases demonstrate a non-symmetric polarization enhancement for opposite electric


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biases. Such an increased polarizability along the direction of spontaneous polarization is in good agreement with recent simulations by Filippetti et al.23 Additionally, electric poling during


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cooling causes a filling of trap states due to charge injection at low temperatures, giving rise to a trap state peak at around 190 K when temperatures are increased again (Figure 4b). Interestingly,


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its direction is reversed by positive biasing. Although the mechanism of the reversal of the trap state peak by electrical poling is not fully understood yet, we propose that a bias during cooling


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causes additional ion migration within the perovskite. At low temperatures this ionic distribution “freezes” and the resulting internal electric field could cause a reversed current flow within the

Figure 4. (a) Current vs. time measurements at 78 K after cooling from 295 K to 78 K at different external biases. (b) TSC measurement during heating at 3 K/min after the device has been cooled from 295 K to 78 K at different external biases. (c) TSC measurement during heating at 3 K/min after the device was poled for 10 min at 78 K with different external biases.


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perovskite layer. Besides application of an electric field, illumination at low temperatures also amplifies the peak height of the lower temperature TSC peak (see Figure 1). The origin of this photo-induced peak amplification may base on various mechanisms: It has been suggested that either an overlay of a shallower trap state peak around 150 K or an early release of deeply trapped charges, that is precipitated by the phase transition, might cause an increase in peak height when the device is illuminated at low temperatures.31 Figure S5 shows that a selective filling of trap states by illumination at different temperatures amplifies the TSC peak at 150 K even if only deep traps are selectively filled. This indicates that an overlay of a shallower trap state peak is not responsible for the observed current amplification at 150 K. Furthermore, local variations in polarization direction that are present at polar domain walls, have been proposed to cause heterogeneities in the electrostatic potential of MAPbI3,17,

40

which could potentially act as

electronic trapping centers that release trapped carriers upon depolarization of the material. Besides a release of trapped charges, an enhanced polarizability of organic cations in metal halide perovskites under illumination could also lead to increases in the observed polarization current.38, 41-42 In fact, it was found that either light-driven molecular reordering due to weakened hydrogen bonds between organic cations and the inorganic cage or the presence of photoinduced charge carriers increase piezo – and ferroelectric properties in MAPbI3.41, 43

Discussion of Ferroelectric and Pyroelectric Properties. As the reversibility of polarized domains is a requirement for ferroelectricity, we further investigated if the polarization direction can be switched once the perovskite has completely polarized at low temperatures. However, Figure 4c shows that in contrast to electric poling during cooling, application of reasonably


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strong biases of up to + 6 V at a fixed temperature of 78 K does not allow to switch the polarization direction. Instead, we observe a slight increase in the peak height of the polarization peak, which may base on an effect related to charge-injected trap filling, as discussed above. Unfortunately, device stability was not sufficient to apply higher electrical biases to the solar cell. With the electric fields probed herein (~ 1V/µm), we do not observe evidence of electrical polarization switching and therefore cannot confirm the existence of ferroelectricity within the orthorhombic phase of MAPbI3. Nevertheless, our data is not sufficient to definitely exclude ferroelectricity, as the applied electric fields may not be effective enough in switching the polarization direction. A distinct identification of pyroelectric properties as the origin of the polarization current is not possible either, as the asymmetric electrodes of the device cause a build-in field within the perovskite film. Hence, the here presented polarization is not observed in the absence of an electric field, as required to prove pyroelectricity. However, our results show that the polarization direction is not exclusively determined by the build-in field of the device, but can depend on film properties (Figure 3b). Furthermore, the low magnitude of the external bias of 65 mV, that is necessary to reverse the polarization direction, is significantly smaller than work function differences within the device, which are in the order of several hundreds of meV. These results provide evidence that the polarization direction is not determined by the build-in field, but rather by properties of the perovskite film itself, like the common orientation of crystal grains on the substrate and a preferred polarization direction within each unit cell. Amount of Polarization. Without amplification by external electric fields, the amount of the observed polarization is relatively low. Integration of the polarization current over time without external biasing gives a lower limit of around 4 nC/cm2 (see Figure S6). In contrast, theoretical


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predictions have suggested a polarization of several µC/cm2 in MAPbI3, if all organic cations polarize in parallel.17, 19, 22-23, 39 This nearly zero polarization indicates that only around 0.1 % of all dipole moments are aligned perpendicular to the surface of the perovskite film. Several ordering mechanisms of organic cations could be responsible for this low macroscopic polarization, including a random distribution of polar domains over the entire film, that cancel out the overall polarization, significant polarization parallel to the surface, a predominantly antipolar ordering, or a mixture of polar and antipolar domains. Experimentally, the coexistence of both polar and antipolar domains of organic cations has been observed in orthorhombic MAPbBr3 single crystals by scanning tunneling microscopy, while theoretical simulations suggest an antiparallel alignment of organic cations as a stable polarization state in orthorhombic MAPbI3.12, 18, 40 Nevertheless, amplification of the polarization current by external electric fields, as shown in Figure 4a,b, proves the possibility to enhance the alignment of organic cations perpendicular to the surface across the entire film. Solar Cell Performance Across Phase Transition. Finally, we investigate the impact of the transformation of organic cation rotation on optoelectronic properties in our MAPbI3 films. As displayed in Fig. S7, we find that the photoluminescence (PL) emission spectrum of our MAPbI3 films undergoes a significant redshift between 90 – 130 K. Similar PL redshifts have already been reported for other MAPbI3 films,15, 25-26, 44 and have been explained by Dar et al. to originate from an increasing disorder of organic cations within the orthorhombic phase,15 which is in good accordance with the continuous rise in dielectric constant that we observe between 110 – 145 K (see Fig. 2c). The effect of the orthorhombic-tetragonal phase transition on the performance of perovskite solar cells is presented in Figure 5 and Fig. S8. Figure 5 displays changes in the short circuit current


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Figure 5. (a) Short circuit current Jsc and (b) open circuit voltage Voc measured during heating (red line) and cooling (blue line) of an ITO/PEDOT:PSS/MAPbI3/C60/LiF/Ag solar cell at 1.5 K/min. Shaded areas highlight characteristic features for heating (red) and cooling (blue). (Jsc) and open circuit voltage (Voc) of an ITO/PEDOT:PSS/MAPbI3/C60/LiF/Ag solar cell that is measured continuously while increasing or decreasing the temperature at a rate of 1.5 K/min. As reduced illumination intensities are used to ensure minimal heating of the device, absolute values differ from standardized measurements at 100 mW/cm2. The shaded areas in Figure 5 highlight distinctive features in the solar cell performance upon heating and cooling. Interestingly, we observe that both Jsc and Voc are affected by abrupt increases at around 115 K and decreases at around 145 K upon cooling and heating, respectively. These significant changes in Jsc and Voc at 115 K and 145 K are intrinsic properties of the perovskite, as they are also observed in MAPbI3 solar cells based on other selective layers (see Fig. S8). Again, we note that minor differences in


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characteristic temperatures might arise due to variations in film properties, as well as a comparison of measurements done in dark and under illumination. Nevertheless, these temperature-dependent measurements of the solar cell performance highlight a correlation between increased device efficiencies and organic cation polarization, as improvements in Jsc and Voc upon cooling coincide reasonably well with the polarization TSC peak and decreases in Jsc and Voc upon heating coincide with the depolarization TSC peak at 145 K. Such a positive impact of cation polarization on solar cell performance is supported by several theoretical studies: Calculations by Quarti et. al. show that a net orientation of organic cations with parallel alignment can give rise to a strong band bending in the conduction and valence band of MAPbI3, which may assist charge carrier separation and reduce charge carrier recombination.45 Likewise, Frost et al. suggest that domain walls between polar domains can act as charge carrier “highways” by forming peaks and troughs in the electrostatic potential via the local dipole order.17 The resulting internal junctions are assumed to aid charge separation and reduce recombination through a segregation of opposite charge carriers. Furthermore, an increased charge carrier mobility has been calculated to occur within polarized domain walls.46 These theoretical considerations of improved charge carrier separation and transport, as well as a reduced charge carrier recombination upon cation polarization in the perovskite are in good accordance with our experimental observations of sudden increases in Jsc and Voc upon cooling, respectively. An additional jump in the short circuit current is detected at 177 K for heating and at 145 K for cooling, though being of smaller magnitude (see Figure 5a and Figure S8). Although we cannot determine the exact origin of this high temperature jump in short circuit current, we interpret this current feature as indication of another structural transformation within the perovskite film, e.g.


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caused by a separated phase transition of the inorganic sublattice. Similar characteristic temperatures have also been observed by acoustic phonon anomalies that indicate a broad, multistep structural transition in MAPbI3 between 130 – 200 K.47 These results reveal that the performance of perovskite solar cells is significantly affected by a multi-stage structural transition of organic and inorganic sub-lattices and subsequent polarization processes. However, we want to note that multiple processes happen across the orthorhombictetragonal phase transition, including changes in bandgap,48 exciton binding energy,49 as well as the potential formation of ferroelectric domains,17 that might have additional effects on the device characteristics. CONCLUSION In conclusion, we have studied thermally stimulated currents across the orthorhombic-tetragonal phase transition of MAPbI3 and observed currents due to thermally-induced (de-)polarization of the organic cations. The mechanism of polarization and depolarization organic cations is found to be highly reversible and to be separated over a temperature range of around 20 K. While the polarization can be controlled by external electric biases during cooling, application of electric fields of up to ~ 1 V/µm is not sufficient to switch the polarization direction once the material is fully polarized at low temperatures. Despite the low magnitude of the unamplified polarization of only a few nC/cm2, our results demonstrate significant increases in solar cell performances upon the polarization of organic cations and highlight the crucial impact cation orientation on optoelectronic properties in metal halide perovskites.

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Supporting Information Available: XRD measurements, additional TSC measurements, analysis of the amount of polarization, photoluminescence measurements, additional temperature dependent measurements of the short circuit current and open circuit voltage. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Carl Zeiss Foundation, Federal Ministry of Education and Research (BMBF)

ACKNOWLEDGMENT We would like to thank Vladimir Dyakonov for fruitful discussions, as well as Sabrina Ruess and Susanne Koch for assistance with TSC measurements. S.T.B. acknowledges funding by the Carl Zeiss Foundation and also from the Federal Ministry of Education and Research (BMBF) for the Project “MesoPIN”.


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Mechanism and Impact of Cation Polarization in Methylammonium

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