Microscopic Origin of Piezoelectricity in Lead-free halide Perovskite

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Letter

Microscopic Origin of Piezoelectricity in Lead-free halide Perovskite: Application in Nanogenerator Design Richa Pandey, Gangadhar S. B., Shivani Grover, Sachin Kumar Singh, Ankur Kadam, Satishchandra Ogale, Umesh V. Waghmare, V. Ramgopal Rao, and Dinesh Kabra ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00323 • Publication Date (Web): 29 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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ACS Energy Letters

Microscopic Origin of Piezoelectricity in Lead-free halide Perovskite: Application in Nanogenerator Design

Richa Pandey1, Gangadhar SB#2 Shivani Grover#3, Sachin Kumar Singh4, Ankur Kadam5, Satishchandra Ogale4, Umesh V. Waghmare3, V. Ramgopal Rao*6 and Dinesh Kabra2* 1

Centre for Research in Nanotechnology and Science, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India. 2

Department of Physics, 6Department of Electrical Engineering Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India. 3

Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India. 4

Physics and Centre for Energy Science, Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pashan, Pune 411008, India. 5

Applied Materials, India Pvt. Ltd. Mumbai 400076, India.

AUTHOR INFORMATION Corresponding Author [email protected], [email protected] / [email protected] #Authors contributed equally

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ABSTRACT In this work, we report a lead-free hybrid halide perovskite system with a very high piezoelectric charge density for applications in nanogenerator. We use materials engineering by incorporation of Formamidinium Tin Iodide, FASnI3, in soft polymer (polyvinylidene fluoride, PVDF) matrix, and demonstrate high performance large-area flexible piezoelectric nanogenerators. This is achieved by using self-poled thin films of FASnI3:PVDF nano-composite. The fabricated devices show the output voltage up to ~23 V and power density of 35.05 mWcm-2 across a 1MΩ resistor, under a periodic vertical compression, with a release pressure of ~ 0.1 MPa. Measured values of the local piezoelectric coefficient (d33) of these films reach up to 73 pm/V. We provide the microscopic mechanism using first-principles calculations, which suggest soft elastic nature and soft polar optic phonons are responsible for the high piezoelectric response of FASnI3. Our studies open up a route to high performance nanogenerators using lead-free organic-inorganic halide perovskite family of materials.


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TOC GRAPHIC


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Organic-Inorganic Hybrid Perovskites (OIHPs) have been the focus of research for quite some time in view of the immense success achieved in realizing high energy conversion efficiency in the new breed of solution processible solar cell architectures1. With the growing understanding of their physical and chemical properties, gained through intense research, their applicability in other areas such as electronics2, light sources3, 4, 5 , photodetectors6, and energy storage devices7 like capacitors and batteries, is also beginning to be examined. Ferroelectricity and piezoelectricity

8,9,10

exhibited by these materials could have interesting applications in the field

of mechanical energy harvesting and mechatronics, which are yet to be explored in depth. Towards this end, high performance, eco-friendly lead-free energy harvesters continue to attract immense research activity aimed at further improving their present power conversion efficiencies. In the domain of such devices, piezoelectric nanogenerator (PENG) are quite interesting and have the potential for applications because of their promising capability to generate electric power locally through impact stress, vibrations, or flexing

11,12

using relatively

simple device structures, low cost and ease of large-scale production. After the first report by Z. L. Wang on Zinc Oxide (ZnO) nanowire based piezoelectric nanogenerators in 2006 14,15

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NaNbO3 16, ZnSnO3

, several other kinds of non-centrosymmetric materials (e.g. BaTiO3

17

were studied to fabricate

, PZT nanofibers

18

and PMN-PT nanowires

19 20

,

and many more)

piezoelectric nanogenerators. However, many of these materials

contain lead, and require high processing temperature and pressure, and involve irregular stress induced mechanical deformations owing to their brittle ceramic nature21,22. As a result, these materials are ineffective for further advances in nanogenerators, and especially in the domain of flexible devices.


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The performance of piezoelectric devices directly depends on its piezoelectric coefficients (d33 α εPr)

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, where ε and Pr are dielectric permittivity and remnant (spontaneous) polarization,

respectively. A large remnant polarization is a parameter relevant to enhanced performance of the device. Spontaneous polarization is quite pronounced in OIHPs due to coupled dynamics of molecular ion and inorganic octahedral cage, which results in broken centro-symmetry in the crystal

24

. Initial theoretical studies by Walsh, et. al, using density functional theory (DFT)

reported a spontaneous polarization of 38 μC/cm2 in methyl ammonium lead iodide (MAPbI3), which is comparable to that of ferroelectric oxide perovskites (30 μC/cm2 for KNbO3)25 wellexplored for applications in piezoelectric devices. The first hybrid organic-inorganic perovskite employed in fabrication of a piezoelectric nanogenerator is methyl ammonium lead iodide (MAPbI3), which gave the voltage output and current density of 2.7 V and 140 nA cm-2

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,

respectively, at a pressure of 0.5 MPa. Later, a system based on formamidinium lead bromide (FAPbBr3)27 was developed, achieving the output voltage and current density of 8.5 V and 3.8 µAcm-2, respectively, at a pressure of 0.5 MPa. In this study, we focus on Formamidinium (FA) based hybrid perovskites. In Comparison to methyl ammonium cation based hybrid halide (MASnI3) perovskite, formamidinium (FA) cation based perovskite possesses higher temperature stability as HC(NH2)2SnI3 ,i.e., FASnI3 has shown single phase formation till 200 ͦ C28. The higher stability of the FA+ analogue could arise due to a more rigid perovskite structure from the enhanced hydrogen bonding between FA

+

cations and the inorganic cage29. FASnI3 (Fig. 1a), which occurs in a cubic structure at room temperature, unlike the tetragonal MAPbI3 30 having a weak piezoresponse (d33 = 5 pm/V) 31. We demonstrate that FASnI3 exhibits almost eight times of magnitude higher piezoresponse [d33 = 38pm/V] than that of MAPbI3. We note that the theoretical estimates of spontaneous polarization


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of MAPbI3 (6.8µC/cm2)32 and FASnI3 (6.1µC/cm2 ) are quite comparable (without incorporating phonon modes). To understand this, we use first-principles calculations to identify the specific phonon modes that cause such significantly stronger piezoelectric response of FASnI3. We then demonstrate that the piezoresponse of FASnI3 can be further enhanced to achieve d33 = 73 pm/V by blending it in a soft matrix polymer system (Polyvinylidene fluoride (PVDF), Fig. S2a) which has its own inherent ferroelectric property. Along with giving environmental, thermal, electrical and flexible nature to the nanocomposite films, it also removes shunting effect due to the semiconducting nature of hybrid-halide perovskite which is very essential requirement for piezoelectric nanogenerator. PVDF with its repeating molecular unit (–CH2–/–CF2–) exhibits five different polymorphs viz. α-phase, β-phase, γ-phase, δ-phase and ε-phase. To achieve more desirable property for energy harvesting applications, the nucleation of the polar phase; α and β phase of PVDF in nano-composite films needs to be enhanced33.

To further check the

compatibility of FASnI3: PVDF nanocomposite film as a flexible device, we fabricated the metal-semiconductor-metal type piezoelectric nanogenerator. Our most optimal piezoelectric nanogenerator showed a repeatable and stable voltage output of ~23 volts and power density of ~ 35.05 mW cm-2, respectively under the vertical compression and release of pressure at 0.1 MPa. The proposed approach demonstrates the viability of using lead-free organic-inorganic hybrid perovskite systems in obtaining a large piezoelectric charge density for high-performance energy harvesting devices. The different volume percentage of FASnI3:PVDF films were prepared by chemical solution route and the X-ray diffraction (XRD) patterns at room temperatures are shown in Fig. 1(b). The XRD pattern can be indexed to a cubic phase of FASnI3 (schematic of FASnI3 crystal structure shown in Fig. 1a) with space group of Pm-3m and lattice constants of a= 6.321 Aͦ34. As is


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known, PVDF is a semi crystalline polymer with four different lattice and chain configurations resulting in four crystalline phases, α, β, γ and δ. Among these phases, only α and β phase (dipole moment per monomer unit converges to a nearly constant 5 × 10−3 C.m for α-PVDF and 8 × 10−3 C.m for β-PVDF 33) is the one which is relatively more polar as compared to other phase35,36. In Fig. 1(b), PVDF shows a prominent peak at 20.2° reflecting the characteristic peak of β-phase of PVDF. PVDF polar vs non-polar phase as a function of composition is sudden within the range/step measured by us, which is derived from structural studies carried out on 11 samples with different compositional ratio (Fig. S10 and Fig. S11). We do not observe co-existence of polar (i.e., α and/or β) and non-polar (i.e., γ) phases of PVDF. A particular phase in PVDF system is result of solid-state packing or microstructure of thin-film prepared, which can be influenced by many known parameters like; annealing conditions, molecular weights, polydispersity and compositional engineering. In this case where we observed beta-phase is going through different transitions is result of compositional ration of FASnI3 vs PVDF. PVDF being in polar phase for α-phase and β-phase as matrix for FASnI3 will further boost the spontaneous charge formation33,37. We note that the increasing concentration of FASnI3 in PVDF amplifies the local distortion of PVDF chains in the nanocomposite resulting in diminishing of characteristic polar peak of PVDF. Further, less concentration of FASnI3 in PVDF results in less stabilization of cubic phase of FASnI3 in nanocomposite. Also, it leads to the formation of non-polar γ phase (2θ = 19.3 ͦ) of PVDF (see Fig. 1c). Hence, we identified that FASnI3:PVDF in 0.5:0.5 volume ratio results in stabilization of PVDF chains in polar α-phase as well as cubic phase of FASnI3, as evidenced by XRD in Fig. 1(b,c). Annealing more than 70ºC results in distortion of FASnI3 crystal structure, hence nano-composite films were prepared at 70ºC, which allowed us to keep FASnI3 unaffected


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and also get the polar phase of PVDF matrix in 0.5:0.5::FASnI3:PVDF composition. In order to fabricate flexible piezoelectric nanogenerator from these composite materials, the first prerequisite is compact pin-hole free smooth and continuous films. Hence, we investigated the surface morphological properties with scanning electron micrographs of pristine FASnI3 and FASnI3:PVDF nanocomposite films (Fig. S1a-e). We note that pristine FASnI3 has poor coverage (Fig. S1b) which is not favourable for device fabrication. A significant morphological improvement (uniform dense coverage, no voids, more flexibility Fig. S1c-g) can be seen when pristine FASnI3 is mixed with PVDF matrix in different volume percentage in Fig. S1 (b-f). The key role in piezoelectric device performance is their ability to convert mechanical deformation into electricity, which can be represented by the magnitude of piezoelectric coefficient (d33) of the material system. To determine the piezoelectric coefficient of the FASnI3:PVDF nanocomposite

and pristine FASnI3 films, we performed Piezoresponse Force Microscopy

(PFM), details about this technique is given in supporting information. For both composite and pristine films lateral PFM was performed where the conductive tip act as one of the electrode and the applied electrical field is parallel to the measured deformation38. The obtained images of topography, amplitude (A), and phase angle (φ) are shown in Fig. S3 with an area of 3µm × 3µm. In Piezoresponse Force Spectroscopy study (i.e. where the PFM was operated in spectroscopy mode), a butterfly-shaped amplitude loop [A (E)] are observed at a bias voltage of ± 8 V for FASnI3:PVDF (Fig. 2 a) and FASnI3 (Fig. 2 b) films. Amplitude goes upto 772 pm for the voltage range of ± 8 V in the case of nano-composite film, whereas it is upto 398pm only in the case of pristine FASnI3 film. The piezoelectric coefficient can be obtained using the formula, Adeflection = d33 Eac. The absolute value of the d33 value for nanocomposite and pristine FASnI3 films are, 73 pm/V and 38 pm/V, respectively. We note that these numbers are significantly


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higher than reports on other lead-free perovskites like BaTiO3 (28 pm/V)39, NaNbO3 nanowires (4 pm/V)16 and other halide based hybrid perovskite systems like MAPbI3 (5pm/V)31 and FAPbBr3(25pm/V)40. The phase response with respect to applied bias is shown in Fig. 2c,d for FASnI3:PVDF nanocomposite and pristine FASnI3. The noticeable rectangular shape hysteresis loop in nanocomposite clearly shows a 180° switching, and this indicates that by the applied external electric field the polarization can be switched to upward or downward directions. The marked area in Fig. S3e shows the location of PFS study. In order to reduce the effect of electrostatic interactions, the measurements are done at “OFF” state. The piezoresponse hysteresis loop can be derived and plotted using the equation PR(E) = A(E)cos[ φ(E)]38. The obtained PR(E) hysteresis loops are asymmetric, which shows that the shift along the electric field axis is towards

positive bias. This inferred that there is already a built-in field in

nanocomposite, which favour the polarization in particular orientation. In Fig. 2e.f, the magnitude of shift along the both axis is more prominent and clearly visible in FASnI3:PVDF than pristine FASnI3, which further confirms that the ferroelectric like behaviour of the nanocomposite FASnI3 is enhanced, as compared to pristine FASnI3. In addition, as seen (Fig. 1b,c and Fig. S2b,c) that nanocomposite film has same orientation but relatively higher crystallinity in comparison to pristine FASnI3 film, which can lead to higher piezoresponse 41,42. Considering that FASnI3 has a cubic structure (Pm-3m), its ferroelectricity and piezoelectric properties are intriguing. We used first-principles DFT calculations to optimize the structure to a minimum energy configuration maintaining the unit cell of cubic structure (Pm-3m) of FASnI3, as observed by us (Fig. 1) and others34. Methodology and parameters used for these computational studies are given in the supporting information. The primitive unit cell of cubic structure of FASnI3 contains 12 atoms. We considered various orientations of FA+ in the initial


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structure used during structural optimization, corresponding to orientation of the dipole moment of formamidinium (FA+) cation along , and directions(Fig. S6). Structural relaxations of these configurations reveal that the polarization of FASnI3 is roughly along direction in its ground state as shown in Fig. 3a (energies of the three optimized structures are given in Table S1). While our estimate of polarization (∼ 6.1 μC/cm2) of FASnI3 is lower than that of a typical inorganic ferroelectric perovskite (e.g. 27 μC/cm2 for BaTiO3)43, it is comparable to the polarization of a related hybrid perovskite, MAPbI3 (6.8 μC/cm2 )32. Further, to estimate the piezoelectric response of FASnI3, we applied hydrostatic strain ranging from -5% to 5%, and optimized its internal structure at each strain, estimating the polarization of the relaxed structure using Berry phase method44. At small strains, polarization change arising from the strain ε is linear, ΔP = e33E, where e33 is a piezoelectric compliance. At strains from -1 % to +1 % (Fig. 3b), P is along direction, analogous to the rhombohedral state of BaTiO3. From the slope of this graph (Fig. 3b), our estimate of e33 is 1.53 C/m2, which is lower, but of the same order of magnitude as the piezoelectric response (e33 = –3.52 C/m2) of rhombohedral BaTiO345. The piezoelectric response measured in our experiments is d33T = ε/E, which gives strain ε induced by applied electric field E, and can be obtained using e33 = d33 ς, where ς is the relevant elastic modulus, defined as the second derivative of energy density with respect to strain ς = 𝟏𝟏/𝑽𝑽 (𝝏𝝏^𝟐𝟐 𝑬𝑬)/(𝝏𝝏𝝏𝝏^𝟐𝟐 )), where V is the equilibrium cell volume. From energy of the

structure as a function of strain ε, our estimate of elastic modulus ς of FASnI3 is 120 GPa, and of the piezoelectric constant d33 is 12.8 pm/V. This is comparable to d33= -14.7 pm/V of rhombohedral BaTiO345, although e33 response of FASnI3 is weaker. This is because FASnI3 is a relatively softer crystal. Our estimate of d33 is smaller than its observed value, and a good part of the difference is probably due to its temperature dependence (first-principles calculations gives


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d33 at T= 0 K, while experiments are at T = 298 K). Since this value of d33 is relatively smaller than the one measured experimentally using PFM of the pristine film, we now determine the mechanism of its ferroelectricity through analysis of phonons. Using DFT linear response, we obtained the zone center (Γ-point) phonon spectrum of cubic FASnI3 (at the experimental volume, see Fig. 4b). Among its 36 phonon modes (Fig.S7a-c), we find one unstable mode with an imaginary frequency (ω=i57 cm–1). Visualization of its eigenvector reveals that it is a non-polar mode, and involves rotation of the FA+ cations(see Fig. 4a). While it does not contribute to piezoelectric response, it entropically stabilizes the ferroelectric state. To uncover the phonon dominating the piezoelectric response, we examine the overlap between the phonon eigenvectors (eν) and the strain-induced changes in the structure in terms of atomic displacements (ΔS) with respect to the equilibrium structure. The variation of |ΔS.eν| with strain (see Fig. 4b) shows that the low frequency modes (at frequencies of 38 cm–1 and 75 cm–1) show remarkable variation in |ΔS.eν| with strain. As these are polar phonons contributing Z∗ (|ΔS.eν|) to e33, Z* being the dynamical charges, they dominate and cause a large piezoelectric response observed in FASnI3, as depicted in Fig. 4a. Thus, soft polar optic phonons and a soft elastic modulus are responsible for the high value of d33 observed in pristine FASnI3 films. Considering significant ferroelectric character in pristine and nanocomposite film, we planned to demonstrate the application in flexible piezoelectric nanogenerator having different compositions of FASnI3: PVDF nanocomposite films (Fig. S1a-e). The different composition of air-stable, free standing nanocomposite films (Fig. S1 g) are sandwiched in between Cr/Au-coated PET sheets which act as top and bottom electrodes. In the experimental section, the fabrication process and characterization scheme of nanogenerator (schematic and digital image of energy harvesting


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setup shown in Fig. S4a and S4b, respectively) are presented in details. The FASnI3:PVDF nanocomposite films generate electric potential when external stress is applied and also serve as an active layer generating the voltage output. The piezoelectric output from the nanogenerator is assessed under periodic vertical compression (impact) and release of 0.1MPa at different frequency ranging from 5Hz to 25 Hz. The active area under the testing of FASnI3:PVDF nanocomposite film is 2 cm × 2 cm. The output voltage values measured at the frequencies of 5Hz, 10 Hz, 15 Hz, 20 Hz and 25 Hz are 7.28 V, 10.56 V, 19.6 V, 22.8 V and 10.8V, respectively (Fig. 5a). The output performance of the nanocomposite gradually increases (Fig. 5 b) with increase of frequency and it reach up to ≈23 V at 20 Hz but beyond a certain force the performance of nanocomposite decreases (see Fig.5a). Devices started to get deformed permanently at around 25Hz impact and lower frequency responses could not be reproduced from those PENGs (Fig. S17). The highest power density achieved by 0.5:0.5::FASnI3:PVDF is ≈35 mWcm-2. Similarly, the piezoelectric response from the devices prepared from different volume percentage composition of FASnI3 and PVDF was also measured. The output voltage values from (20 FASnI3:80 PVDF) and (0 FASnI3:100 PVDF) at different frequencies are (2.84 V and 1.64 V)5Hz, (4.96 V and 3.2 V)10Hz, (9.2 V and 4.56 V)15Hz, (12.2 V and 3.52 V)20Hz respectively, as shown in Fig. S5a,b and Table S2. The piezoelectric domains of nanocomposite films are not along any particular orientation, as FASnI3 is randomly distributed in the PVDF matrix and the net dipole moment is zero. Hence, the voltage output was zero from the devices when they are not exposed to any external stress. While applying vertical compression (force at different frequency can be seen in Fig. 5b inset) the dipoles of FASnI3:PVDF films gets strongly aligned


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along one direction due to ‘stress-induced poling effect’46,47. resulting in the generation of a significant amount of piezoelectric potential across the electrodes. When only PVDF films are subjected to vertical compression we observe that there is no significant piezoelectric voltage output (Fig.S5a). However, after addition of FASnI3 in PVDF with different volume concentration, there is a significant enhancement in voltage output and current output as shown in Fig. S5a (see in Table S2). We note that the piezoelectric output achieved from the FASnI3:PVDF composite nanogenerator is high as compared to previously reported nanogenerators based on organic-inorganic hybrid metal halide perovskite materials (see in Table S3). Since, the nanocomposite films are self-poled hence without undergoing any external poling treatment, here the maximum voltage obtained is 23 V at 0.1 MPa compression is three times higher than that of 6 V at 0.5MPa compression for the FAPbBr3:PVDF composite nanogenerator, hence it suggest that nanocomposite based nanogenerator is at least 15 times better than FAPbBr3:PVDF composite nanogenerator10. We note that the previously reported nanogenerators are lead-based hence using toxic-materials would cause pollution to the environment during the synthesis, fabrication and disposal process, which also restricted their real time application for body implantable devices48. However, this composition being lead-free should overcome most of these drawbacks of the earlier reported lead-based hybrid organicinorganic perovskite based nanogenerator. The FASnI3:PVDF composite nanogenerator can be used to charge capacitor and light up a commercial light-emitting diode (LED) for a practical application(schematic and digital image of capacitor charging and LED driving shown in Fig. S8a and Fig. S8c, respectively). Fig. S8d shows the captured picture when the LED is turned on from Video S9.


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Based on above results we understand that despite having a cubic structure (Pm-3m, nonpolar point group) the FASnI3 based hybrid halide perovskite can exhibit a ferroelectric character. This is shown for pristine film experimentally and further; the results could be explained using computational studies. Based on our limited literature survey, we find that this is the first report where 3D hybrid halide lead-free perovskite with cubic structure showing such a high magnitude of piezoelectric coefficient

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without any extrinsic poling treatment. Our computational study

not only provided insight about the possible mechanism for ferroelectricity in these system due to distorted octahedral based rhombohedral ground state by presence of FA+ being aligned in direction, but a remarkable significant contribution coming from the soft-phonon modes. Identification of those soft phonon modes is very important for future materials design. In addition, this knowledge can be further used to explain many photo-physical studies being carried out on these systems 49. Moreover, we understand that utilization of PVDF to enhance the piezoelectric properties can be seen as an addition due to intrinsic ferroelectric character of PVDF (α and β phase), however, we confirmed that it is more than just addition by analysing the nanogenerator devices (Fig. 5 and Fig. S5a,b) based on pristine PVDF. Enhancement of ferroelectric character in 0.5:0.5::FASnI3:PVDF nanocomposites can be explained by simple electrostatics, where a cascaded effect of dipole alignment can be observed when two ferroelectric materials are kept in vicinity50,51,52. Moreover, as mentioned improved crystallinity without compromising on the orientation of FASnI3 can support the observed higher piezoresponse (PFM and nanogenerators) in nanocomposite films41. We also note that in pristine FASnI3 films there are grain boundaries and pin-holes (Fig. S1b), which might be responsible for observed relatively lower d33 values (Fig. 2f). Addition of PVDF not only enhanced the ferroelectric properties of nano-composite, but also provided an essential environmental and


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ACS Energy Letters

mechanical stability to FASnI3. These nano-composite based devices found to be more compact and suitable for flexible nano-generator devices.

(a) Sn I HC(NH2)2+ (b) (b)

(c)

Figure 1| Crystal structure and XRD pattern of FASnI3, FASnI3 : PVDF nanocomposite (a) Schematic crystal structure of Formamidinium tin iodide, (b) XRD pattern of (FASnI3 : PVDF)


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nanocomposite mixed in the volume ratio of (0.8 : 0.2), (0.5 : 0.5), (0.2 : 0.8), (1 : 0 ), (0 : 1) (c) focus on the graph of the XRD pattern between 18 ͦ to 22 ͦ.

(a)

FASnI3 : PVDF

(b)

(c)

(d)

(e)

(f)

FASnI3

Figure 2| Local piezoelectric behaviour of 0.5 FASnI3 : 0.5 PVDF nanocomposite film and pristine FASnI3 : Piezoelectric ‘butterfly’ displacement curve at 298 K in the ‘OFF’ state of (a) (0.5 FASnI3 : 0.5 PVDF) nanocomposite film and (b) pristine FASnI3, elucidated by using DART-PFM mode. Phase change corresponding to the hysteresis loop of (c) (0.5 FASnI3 : 0.5 PVDF) nanocomposite film and (d) pristine FASnI3. Piezoresponse loop of (e) (0.5 FASnI3 : 0.5


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PVDF) nanocomposite film and (f) pristine FASnI3. The arrow shows the sweep direction of the bias voltage.

(a)

b

[100] c

[010]

[111]

(b)

(c)


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Figure 3| Structural and piezoelectric response of FASnI3. (a) The crystal structure corresponds to the alignment of FA+ dipole along [100], [110] and [111] respectively. Color scheme: I, blue; Sn, grey; C, brown; N, green; H, pink. (b) Change in the polarization vector components |P_x|, |P_y|, |P_z| with hydrostatic strain, (c) Energy as a function of hydrostatic strain.

(a)

c b

(b)


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Figure 4| Phonon mode and Eigen vector visualization: (a) Atomic displacement of low energy mode (calculated frequency of 38 cm-1) of FASnI3 structure with cubic structure along [111] direction, and (b) Variation in overlap matrix |ΔS.eν| elements with strain.

(a)

(b)

Figure 5| Signal generation from 0.5 FASnI3 : 0.5 PVDF nanocomposite based flexible piezoelectric nanogenerator device at different frequencies. (a) Frequency dependent open circuit voltage output from nanocomposite device prepared by mixing FASnI3 and PVDF in 0.5:0.5 volume ratio, (b) Comparison of voltage output and corresponding power density from 0.5 FASnI3 : 0.5PVDF nanocomposite film at different forces. This change in force occurs due to change in frequency from 5Hz to 20 Hz, see in inset.


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ASSOCIATED CONTENT Supporting Information file The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. This file contains experimental methods, Figure S1 to S17, Supplementary VideoS9 and Table S1 to S4

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] [email protected] / [email protected]

Notes: The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the Applied Materials US., (Grant Number 16AMAT004) and the National Centre for Photovoltaic Research and Education (NCPRE). This work was also partially supported by the UKRI Global Challenge Research Fund project, SUNRISE (EP/P032591/1). We acknowledge support from Prof. Viswanadham B V S and Nissar Khan for providing the magnetic shaker at Civil Department, IIT Bombay. UVW acknowledges support from a JC Bose National Fellowship, Sheikh Saqr fellowship and IKST, Bangalore.


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Microscopic Origin of Piezoelectricity in Lead-free halide Perovskite

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