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Polarization Switching and Light-enhanced Piezoelectricity in Lead Halide Perovskites Mariona Coll, Andres Gomez, Elena Mas Marzá, Osbel Almora, Germà Garcia-Belmonte, Mariano Campoy-Quiles, and Juan Bisquert J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 30 Mar 2015 Downloaded from http://pubs.acs.org on March 31, 2015
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Polarization Switching and Light-Enhanced Piezoelectricity in Lead Halide Perovskites Mariona Coll*l, Andrés Gomez1, Elena Mas-Marza2, Osbel Almora2, Germà GarciaBelmonte2, Mariano Campoy-Quiles,1 Juan Bisquert*2,3 1
Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus UAB, 08193, Bellaterra, Catalonia, Spain 2
Photovoltaics and Optoelectronic Devices Group, Departament de Física, Universitat Jaume I, 12071, Castelló, Spain 3
Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia
Abstract We investigate the ferroelectric properties of photovoltaic methylammonium lead halide CH3NH3PbI3 perovskite using Piezoelectric Force Microscopy (PFM) and macroscopic polarization methods. The electric polarization is clearly observed by amplitude and phase hysteresis loops. However the polarization loop decreases as the frequency is lowered, persisting for a short time only, in the one second regime, indicating that CH3NH3PbI3 does not exhibit permanent polarization at room temperature. This result is confirmed by macroscopic polarization measurement based on a standard capacitive method. We have observed a strong increase of piezoelectric response under illumination, consistent with the previously reported giant photoinduced dielectric constant at low frequencies. We speculate that an intrinsic charge transfer photoinduced dipole in the perovskite cage may lie at the origin of this effect. TOC GRAPHICS
KEYWORDS: perovskite solar cells, CH3NH3PbI3, piezoelectric force microscopy, dipole, ferroelectric, dipole moment, hysteresis, loops, polarization
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Very high power conversion efficiencies of sunlight to electricity have been obtained for photovoltaic devices based on hybrid inorganic-organic lead halide perovskites in a short time of research.1-4 Methylammonium lead halide materials as CH3NH3PbI3 (MAPbI3) and the associated substitutional variants display excellent photovoltaic properties in terms of light absorption, electronic charge separation, transport and recombination. These materials provide a very promising novel route to massive solution-based facile fabrication of low cost solar cells. However, the chemical and structural properties of the inorganic-organic perovskite, that are unprecedented in the previously known high efficiency photovoltaic materials, produce a number of physical features that are not understood and are intensely investigated at present. In the hybrid perovskite structure ABX3, the organic cation A+ is in a cage formed by four BX6 octahedra. It has become evident that under photovoltaic operation (light or voltage biasing) the “soft” structure of the hybrid perovskite, associated to the combination of organic and inorganic ionic components, produces peculiar and significant phenomena in the ultraslow time regime as current-voltage hysteresis5, 6 and persistent photovoltage decay.7-9 These properties have been assigned to either ferroelectric or ionic polarization, or else to electronic traps.10, 11 Another connection between structure and electronic response is established by the giant dielectric constant observed in MAPbI3, that is strongly enhanced by illumination. This property has been observed by capacitance spectroscopy12 and suggests the formation of a large photoinduced dipole.13 This paper investigates the ferroelectric polarization that may have a substantial impact on device operation and give rise to novel applications of the inorganic-organic lead halide perovskites in multiferroic materials.14 In general, many oxide perovskites are highly polar and show a ferroelectric response that provides a static dielectric constant in the order of 103. MAPbI3 with tetragonal symmetry belongs to the 4mm point group and I4/mcm space group and as such could be ferroelectric. The polarization P in hybrid perovskites may arise from three major mechanisms: the ionic offcentering, the atomic BX6 cage rotations, and the rotation of dipolar MA+. The latter mechanism is facile15 and it has been favored in recent simulation studies of polarization in MAPbI3,16-18 as dipole rotation is simpler to treat than cooperative ionic displacements. Former studies19, 20, both for CH3NH3 PbI3 and CH3NH3PbBr3, showed that at room temperature the PbX6 octahedra rotate alternatively around the c-axis conforming the SrTiO3-type tetragonal perovskite structure, and the rotation angle of the PbI6 octahedra increases with decreasing temperature. The transition from tetragonal to orthorhombic phase is accompanied by a peak of the dielectric constant, suggesting that the orthorhombic phase is ferroelectric. More recently, the local dipole structure property has been invoked to explain a giant dielectric constant,12 slow dynamic response10 and current-voltage hysteresis.21, 22 However, local polarization distortion may be combined with long range ionic drift that establishes macroscopic polarization at the contacts of the sample.6
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Importantly, there are only a few direct observations of polarization features of inorganic-organic lead halide perovskites and the results reported so far are not conclusive. One study23 claimed the direct observation of ferroelectric domains while another one reported the absence of such effects.11 Piezoelectric force microscopy (PFM) is a variant of AFM that is widely used to image polarization structure and local switching. However, the observation of contrast region is not sufficient evidence for the existence of ferroic domains, since such contrast may be due to several factors, namely: existence of ferroelectric domains, electrochemical phenomena, ion migration, electrostatic effects and it can also have a contribution from surface topography.24-26 Nevertheless, the states of opposite polarization under applied bias are clearly revealed by a change of phase of ac voltage, since the piezoelectric response has different sign in either state of polarization, and this method is shown below. Another important point is whether voltage induced polarization remains permanent, as in ferroelectric material, or disappears when the external voltage is removed. In order to obtain a clear picture of polarization in MAPbI3, we have investigated polarization switching, the light dependence and the relaxation time in MAPbI3, using PFM and macroscopic polarization measurement. The structure of the samples prepared as indicated in Experimental methods consists of FTO/TiO2 buffer layer/TiO2 mesoporous layer (200 nm)/MAPbI3. The samples were measured without upper contact, but their related solar cells provided a power conversion efficiency of about 9% with a photovoltage of ca. 1 V (see SI). Samples with different crystal size have been prepared27 with the aim to investigate a possible interaction between morphology and polarizability. Topographic images were acquired simultaneously with the PFM-phase images. From the topographic image, the samples with small MAPbI3 crystals display a surface roughness (rms) of 25 nm and an average crystal size diameter of 200 nm, figure 1(a), whereas samples with large MAPbI3 crystals show a rms of 38 nm and a measured crystal size diameter of 500 nm, Figure 1(c). This morphological analysis is in good agreement with the SEM study as shown in the corresponding inset images. High humidity conditions (80%-90%) can promote the existence of electrochemical phenomena between the tip and the sample surface, which can lead to surface damage and misinterpretation of PFM data.28 In order to minimize the amount of water layers between the tip and the sample surface and therefore minimize the existence of electrochemical reactions, we performed the experiments under low humidity conditions (
