Flex-Mode Mechatronic Functionality of Lead Iodide Hybrid Perovskite

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Flex-Mode Mechatronic Functionality of Lead Iodide Hybrid Perovskite Systems Aniruddha Basu, Prachi Kour, Swati Parmar, Rounak Naphade, and Satishchandra Ogale J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00192 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018

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The Journal of Physical Chemistry

Flex-Mode Mechatronic Functionality of Lead Iodide Hybrid Perovskite Systems Aniruddha Basu, Prachi Kour, Swati Parmar, Rounak Naphade* and Satishchandra Ogale* Department of Physics and Centre for Energy Science, Indian Institute of Science Education and Research (IISER), Dr. Homi Bhabha Road, Pashan Pune 411 008, India *Corresponding Author: [email protected]; [email protected] ABSTRACT: The mechatronic functionality of lead iodide hybrid perovskite thin films grown on flexible substrate is investigated via the study of current-perpendicular-to-plane (CPP) charge transport modulation under flex-mode compressive and tensile strains for multiple flexing cycles. It is shown that the transport is significantly, reversibly and asymmetrically modulated. Typically, for a strain of 0.088% (0.23%), a remarkable current modulation of +196% (+393%) is achieved for compressive strain (CS), and -49% (-53%) for tensile strain (TS) at an applied potential of 1V. For low levels of bending the response is robust for a large number of bending cycles. The effects of the change of organic cation from methylammonium (MA) to formamidinium (FA) and the grain size on the response are also examined. A comparative study of the structural, morphological and optical properties of the pristine sample and the samples subjected to multiple bending cycles is performed to understand and elucidate the possible mechanisms of the strain induced changes in the transport properties.

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1. INTRODUCTION Hybrid organic inorganic perovskites (HOIPs) have shown remarkable promise in recent years in photo-voltaic1–3 and optoelectronic device applications such as photodetectors.4–6 In addition to the extra-ordinary optical and optoelectronics properties7 these materials also possess intriguing piezoelectric and ferroelectric properties owing to their soft material character rendered by the peculiar integration of the organic and inorganic constituents.8 Hence interest in the mechanical properties of these materials is gaining momentum in the context of novel mechanical/vibration based energy harvesting devices such as nanogenerators9, piezo-field effect transistors (PFETs)10, transducers11, etc. Also, the thin films of hybrid perovskites in flexible device systems can experience small angle mechanical stretching, bending, and twisting strains during the device fabrication and processing steps. Hence, it is of interest to study the influence of mechanical deformation on the electronic properties of these films and the regimes of reversibility of these effects. The results of such study are potentially also of interest to the field of flexible and wearable electronics. Studies of mechanical strain induced charge transport in organic molecular systems and inorganic quantum dots have been investigated previously.12,13 Being endowed with multifunctional properties compounded with low temperature solution processibility, the new class of HOIP materials may also hold great promise for the rapidly developing field of mechatronics or even opto-mechanics which have direct implications for robotics. Indeed, it has been shown that structural deformations in these systems can modify their band gap, and transport characteristics.14 Thus, the response of HOIPs to various strain modes (dynamic, flexural, compressive, tensile etc.) needs more careful and detailed investigations. Recently, by using the nanoindentation technique A.K. Cheetham and co-workers have measured the Young’s modulus and hardness of ABX3 based single crystals (where A= CH3NH3+, CH(NH2)+, B= Pb2+, Sn2+ and X= I, Br) which suggest high ductility and flexibility for this class of materials.15,16 Reyes-Martinez et al. have also shown that the mechanical behaviour (creep deformation and stress relaxation) in HOIP single crystals is a time dependent phenomenon.17 J. Yu et al. have recently reported on the mechanical nature of polycrystalline MAPI thin films and shown their enhanced fracture toughness and nanoductility as compared to their single crystalline counterparts.18 Q. Dong et al. have shown the formation of lateral solar cell on hybrid perovskite single crystals and examined the effect of piezoelectric poling to create fractures in the crystal 2 ACS Paragon Plus Environment

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leading to the formation of grain boundaries which enable the ion migration across the metal contacts.19 More recently, Li et al. have also discussed the strain-doping-ferroelectricity relationship in hybrid perovskite systems.20 These studies emphasize the sensitivity of hybrid perovskite systems to mechanical deformation and strain; and its possible implications for their potential use in flexible and wearable technologies. Methyl Ammonium Lead iodide (MAPI) exhibits a cubic to tetragonal phase transition around 320K. At room temperature, it shows non-centrosymmetric phase which can respond to structural deformations owing to its soft nature. The theoretical Young’s modulus values for the cubic and tetragonal phases are 22.2 GPa and 12.8 GPa, respectively.21 The ratios of the Bulk (B)/Shear(G) moduli [B/G = 1.89 (cubic) and 2.52 (tetragonal)] suggest that this material should have fairly good ductility for larger deformations. In this work, we report the modulation of the current-perpendicular-to-plane (CPP) I-V characteristics of the methylammonium and formamidinium lead iodide (MAPI and FAPI) thin films sandwiched between ITO coated PET substrate and top-deposited gold electrodes under flex-mode multi-cycle deformation. The effect of compressive strain (CS) and tensile strain (TS) on the charge transport is studied and the modulation in the conductivity and the degree of its reversibility are evaluated. We have also studied the effect of grain size on the strain induced charge transport modulation in the case of MAPI thin film. 2. EXPERIMENTAL METHODS MAPI/ FAPI precursor solution: 1mM MAPI/FAPI precursor solution was prepared by dissolving 1 mmole of PbI2 (461mg) and 1 mmole of methylammonium Iodide (MAI) [159 mg] / 1mmole of formamidinium iodide (FAI) [172 mg] in 0.8 mL DMF +0.2 mL DMSO solution. Device Fabrication: The pre-patterned ITO/PET substrate was plasma-cleaned prior to spin coating of MAPI/FAPI solution. The MAPI/FAPI thin films were deposited by spin coating at 2000 rpm for 60 sec followed by annealing at 100°C for 30 min. ~80 nm gold top electrode was deposited through thermal evaporation. The final device architecture was ITO-PET/MAPI/Au. The gold pad area was 3 mm x 1.5 mm and perovskite thin film area is 20 mm X 20 mm. The perovskite films were deposited on the PET substrates as per the aforementioned parameters for characterization. 3 ACS Paragon Plus Environment


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Material Characterization: The powder x-ray diffraction (XRD) measurements on MAPI films were performed on a Bruker D8-Advance X-ray diffractometer (Germany) with Cu Kα radiation (λ = 1.5418 Å). Raman spectrum of MAPI film was recorded on RENISHAW spectrometer using diode laser (532 nm) in the backscattering geometry. The surface morphology of MAPI thin film was analyzed by field emission scanning electron microscopy (FESEM, JEM-2100F, JEOL, Japan). Photoluminescence (PL) spectra were recorded on Horibha Scientific Fluromax 4 Specrtoflurometer. Device characterization: All the measurements were carried out in ambient atmosphere immediately following device fabrication. For all I-V measurements, a voltage range from -1V to +1V was used at a scan rate of 62 mV s-1. For measuring time-dependent dark current modulation by flexing, +1V bias voltage was applied across the electrodes and the device was periodically bent (15 sec bent time; 15 sec relaxed time) to observe the current change. Keithley 2420 A was used for carrying out the electronic measurements. 3. RESULTS AND DISCUSSION Figure 1a shows the device configuration for the current-perpendicular-to-plane (CPP) I-V characteristic measurement. Figure 1b shows the two single axis bent configurations which were employed for applying flex-mode compressive and tensile strains on the film. Since the substrate is much thicker than the film, the neutral plane of strain is expected to lie much below the interface; hence the film is expected to be mostly uniformly strained, albeit with a mild gradient. The magnitude of strain was varied in the experiments by changing the bending angle for either directions, and the strain was calculated by using the formula,22,23

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௟

௭

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Where, εzz is the axial strain, z is the distance from one end of ITO/PET to the middle of the MAPI film, a is the half thickness of the ITO/PET substrate, l is the length of the substrate, and Dmax is the maximum deformation due to bending. Figure 1c shows the I-V characteristics (y-axis on the log scale) for the pristine film in the initial flat state, followed by the application of strain of 0.23% in the compressive and tensile modes (as shown in the inset), and finally followed by the relaxation back to the flat state (relaxed case). 4 ACS Paragon Plus Environment

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The corresponding graph with y-axis on the linear scale is shown in the supporting information (SI) Figure S1a and S1b. As seen, over the voltage scale employed the current varies linearly with the applied voltage for all the cases. For an applied potential of 1V, the initial current of 0.92 x 10-4 A changes to 5.9 x 10-4 A under CS condition and to 0.003 x 10-4 A under the TS condition. Interestingly, in spite of application of this significant strain, the current recovers to 0.81 x 10-4 A which is close to the original value. In order to check the linearity or otherwise of the dependence of transport current on strain value, we subjected the same film to different flexing strains, initially compressive followed by tensile. The corresponding data (with estimated error bars) are presented in terms of the % change of current in Figure 1d. It can be seen that under the application of CS the change is very significant (393 % for strain of 0.23%), while that under the TS condition is relatively less (about -53 % for strain of 0.23%), although still quite significant. Subsequently, we subjected the films to periodic flexing strains (0.23%) for several cycles and noted the changes in the current. Fairly periodic changes were noted in both the cases of CS and TS, as seen from Figure 1e, although notably the base current did show some change, indicating some modifications building in the film over the full mechanical flexing sequence. When subjected to a smaller value of strain of about 0.009%, the cyclic application of strain was seen to render periodic changes over larger number of cycles as shown in Figure 1f. It can be noted from Figure 1e and 1f that upon flexing a jump in the current is noted followed by a slower relaxation component. Thus there is a prompt response as well as a slower response. A prompt response can emanate from direct coupling of the lattice distortions with the transport, as for instance in piezo effects or changes in the electronic states in the band due to strain. The slower components can emanate from ionic movements and related structural or grain boundary effects.24 One could ask whether the changes in current could arise from differences in charge collection efficiency at the contacts due to CS or TS. Piers Barnes and co-workers25 have addressed this possibility in light of the possible reorientation response of the MA+ ion to small applied electric fields and its consequence for ferroelectric/anti-ferroelectric domain wall evolutions. However, addressing this possibility is out of the scope of the present work. Our data reveal that MAPI does have fairly impressive mechatronic functionality and the materials system could be of interest to real applications, especially under small values of 5 ACS Paragon Plus Environment


applied strain fields. Being a material with broadband optical absorbance property, excellent luminescence features, as well as electronic transport capability, coupling of mechanical flexibility with these features could be of interest for flexible electronics and optoelectronics applications. Since tremendous efforts have been currently focused on achieving large grain size in MAPI thin films for enhancement of perovskites solar cell properties,7, 26–28 we decided to study the effect of grain size on the strain induced current modulation in MAPI thin films. We followed the solvent annealing procedure reported by Jiang Liu et. al.29 to synthesize the MAPI thin films with larger grain sizes. Figure 2a and 2b show FE-SEM images of MAPI thin films, with and without the use of solvent annealing process. The grain size can be seen to have increased from about 50-100 nm to 300-400 nm. Figure 2c shows the result of the CPP I-V measurement of device under different types of bending (Bending angle 100o), and Figure 2d shows the cyclic stability data at +1V under compressive and tensile bending (0.009% strain) for the MAPI film with larger grain size. The dark current under flat condition (CPP transport) is seen to have increased in this case of large-grained film as compared to the small-grained MAPI film, which can be attributed to reduced grain boundary scattering. Under CS, the conductivity is seen to increase, while under TS it is seen to decrease, as expected. But in this case the I-V characteristic is not found to restore completely to its original value after the deformation. The current modulation under CS is approximately comparable to the small-grained MAPI film (~2 times in both the samples). However, under TS, the modulation is ~3 x 104 times for films with larger grain sizes, as compared to the ~103 times for MAPI films with smaller grains. These large-grained MAPI films with larger grains also show very good cyclic stability (Figure 2d). One can estimate the gauge factor as gf=(∆R/R)/strain, for the two cases of grain sizes. The gauge factor values for smallgrained (large-grained) films were found to be 17.08 (3.46) and -2.3 (-4.09) for compressive (tensile) strain conditions, for an applied strain of 0.23%. Along the same lines, we also deposited FAPI (FA: formamidinium) thin film on ITO/PET substrate and studied the transport properties under similar flexing condition. The motivation was to examine the effect of the change of the organic cation on the mechatronic functionality. Once again, as seen from Figure 3a, the conductivity of FAPI film is seen to increase under CS and decrease under TS. The transport in FAPI film shows a good modulation under CS and TS 6 ACS Paragon Plus Environment

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bending conditions for lower strain values, however at higher strains (large bending angles) the modulation is comparatively less affected by further increase in the strain. The full lines in the same Figure 3a are for the MAPI case, presented earlier in Figure 1d. This suggests that under compressive strain, the modulation is much less in the FAPI case than the MAPI case. Under tensile strain however the strain modulation of transport is not significantly different in the MAPI and FAPI cases. Subsequently, we applied 1V bias to the FAPI film and the CPP transport data were recorded (Figure 3b) for multiple bending cycles for bending angle 20°. Under CS, the current modulation in bent state and flat state shows good reversible cycling ability. However under TS, the modulation in the conductivity reduces over a few cycles and the film shows irreversible behaviour under mechanical flexing (sloping nature of the flat state current with increasing number of cycles). In order to understand the possible reasons for the observed strain effects, we performed XRD, Raman, PL and SEM characterizations on the pristine film, films subjected to 1000 cycles of bending cycles at a bending angle to 20° and on the strain-cycled films after certain duration of time to compare the corresponding property features. From Figure 4a and 4b which shows the XRD data, it is clear that the nature of changes is distinctly different in the cases of tensile and compressive bends. In the case of application of 1000 tensile bending cycles, the XRD peaks are seen to split indicating structural change; a tetragonal distortion. The new contributions appear on the low 2θ side which suggests higher d-value contribution which is commensurate with the tensile character of the strain. While the peak shapes seems to evolve slightly with time the change seems semi-permanent or relaxing at an extremely slow pace. On the other hand, application of 1000 compressive bending cycles is seen to cause a smaller peak splitting with the secondary peaks appearing on the side of higher 2θ, suggesting decreased lattice parameter, consistent with the compressive nature of strain. Interestingly, the relaxation back to original state is quite rapid in this case as seen from the time dependence. The Raman spectra, which reflects the character of Raman active local modes, for the corresponding cases shown in Figure 4c (Tensile case) and Figure 4d (compressive case) nominally conform to the same picture. The change in the pattern, especially the intensity, for the tensile case is much stronger than that in the compressive case and the sign of change is opposite. In both the cases the relaxed state is not exactly similar to the starting state. Since the Raman intensity is a difficult quality factor to compare due to its dependence on precise focusing and related parameters, we refrain from any 7 ACS Paragon Plus Environment


quantitative analyses at this stage. The SEM micrographs for the pristine film and the two strained cases of interest are shown in Figure 5a, 5b and 5c, and these images reveal that upon 1000 cycle tensile and compressive bending cycles, there are no major changes in the grain structure, neither are there any discernible microcracks in the samples. Finally, we also looked at the PL contributions for the two cases of interest (1000 tensile or compressive cycles) compared to the pristine film case (Figure 5d and 5e). It can be seen that in the tensile case the PL peak is significantly broadened on the longer wavelength side and the same does not relax back to the original shape within an hour or so (thus a case of slow relaxation, as evidenced by other techniques). The broadening on the long wavelength side suggests introduction of trap states below the band edge. Interestingly, in the compressive strain case, the PL is seen to shift fully and significantly (by about 50 nm) to the long wavelength side and the same almost relaxes back to the original state within an hour or so, with some residual trap state contribution left on the high wavelength side. We measured optical absorption of the film before and after 1000 compressive flexing cycles for which the PL of Figure 5e is reported; the corresponding data are shown in the supporting information Figure S2. As seen, there is hardly any change in the optical absorbance, except for a small increase of the spectral weight below the absorption edge at the cost of that above the edge. This reinforces the conclusion that trap states are introduced below the gap due to flexing process and these are transitory and relax back over time. There are no major changes in the material that could cause change in the band gap. The changes in the structural and optical properties and their relaxation times noted and discussed above suggest that ionic movements and related local rearrangements are responsible for the observed effects on the electronic transport via carrier trapping and de-trapping. Since grain boundaries can be the natural locations for strain accommodation, their contributions in influencing the current-perpendicular-to-plane (CPP) transport could be significant. Another possible contribution may emanate from piezoelectric field effect whereby the strain induced polarization field would influence the carrier transport. Evidences have been presented in the literature on the occurrence of piezoelectric effects in the new breed of hybrid perovskite systems.30 One could separately ask whether the strain field could influence the band structure significantly, thereby directly affecting the carrier transport in the bent state. Using DFT 8 ACS Paragon Plus Environment

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calculations C. Grote et al. have shown that at least in the case of inorganic perovskites CsSnI3 and CsPbI3 for which their study was undertaken, the band gaps and band edges are influenced quite substantially by strain.14 In hybrid perovskites the band structure comprises of anti-bonding orbitals primarily contributing to the conduction band (CB) and bonding orbitals contributing to the valence band (VB) formation. Grote et al. have shown that biaxial strains within ±3% of the respective cubic lattice parameters can alter band gaps by several tenths of an electron volt, mainly through the tuning of antibonding interactions in the valence band maximum. Under the tensile bending the band gap is increased while under the compressive strain it decreases; the inorganic matrix (Pb-I bond network) primarily accommodating the deformation induced strain thereby modifying the band edge orbitals in the valence band. The photoluminescence (PL) data of (Figure 5d and 5e) conform to the suggestion that some changes in electronic states do occur in our films as well upon the application of strain, possibly affecting the transport. Finally, a few remarks are in order on the comparison between the observations pertaining to FAPI and MAPI films. Cheetham and co-workers have reported an in-depth study of the factors influencing the mechanical properties of FAPI and other related hybrid perovskites enabling their comparative evaluations.16 They concluded that the bonding in the inorganic framework as well as the hydrogen bonding play key roles in determining the elastic stiffness, and that the influence of organic cation becomes significant for structures at the limit of their perovskite stability. Although a direct comparison of FAPI and MAPI is rendered difficult due to different structures, FAPI has a lower mean Young’s modulus (higher flexibility) than MAPI, though the difference is not very large. Thus the ability to accommodate higher strain without inducing internal irreversible changes is higher in FAPI as compared to the MAPI case. This should reduce the percent current modulation in FAPI case over that in MAPI case. Another factor that may contribute to reduced current modulation in FAPI case over MAPI is the projected higher carrier concentration and therefore much higher conductivity of FAPI. Indeed, Yang Yang and coworkers have studied the structural, optical and electrical properties of FAPI in single crystal form to avoid confusions pertaining to grain boundary effects in the case of thin film samples.31 They found that the conductivity of single crystal FAPI is an order of magnitude higher than that of single crystal MAPI, which they mainly attributed to higher carrier concentration in FAPI that could be related to the lower band gap of this system. The lower resistance in our un-flexed (flat)



FAPI film (than the MAPI film) of the same nominal thickness and preparation method as the MAPI film is then a consequence of its higher intra-grain conductivity. 4. CONCLUSIONS In summary, we have established the interesting mechatronic functionality of lead iodide organic-inorganic hybrid perovskite thin films grown on flexible substrate via the observation of significant and reversible modulation of current-perpendicular-to-plane (CPP) charge transport under flex-mode compressive and tensile strains applied for multiple flexing cycles. Typically, for a strain of 0.088% (0.23%), a remarkable current modulation of +196% (+393%) is achieved for compressive strain (CS), and -49% (-53%) for tensile strain (TS) at an applied potential of 1V. For low levels of bending strain, the response is seen to be quite robust for a large number of bending cycles. These results suggest the potential applicability of the new class of hybrid perovskites for device systems on flexible platforms.

ASSOCIATED CONTENT Supporting Information contains the device I-V characteristics at different bending angles and UV-Visible absorption spectra of MAPI films. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] ACKNOWLEGEMENTS A.B and R.N would like to acknowledge the funding from DST-SERB (File No: PDF/2017/002445 and PDF/2016/003655 respectively). S.O would like to thank DST-APEX-II, DST-Nanomission thematic unit, DST-CERI and UKIERI projects for the funding support.


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Approach to Probe the Charge Carrier Dynamics of the Grain Interior and Grain Boundary of a Perovskite Film for Perovskite Solar Cell Applications. Physical Chemistry Chemical Physics 2017, 19, 9143–9148. (27)

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TOC Graphic:


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Figure 1: a) shows the device configuration for the current-perpendicular-to-plane (CPP) I-V characteristic measurement, b) shows the two single-axis bent configurations which were employed for applying flex-mode compressive and tensile strains on the film, c) shows the current modulation of the MAPI film at +1V bias (under 0.23% strain) under compressive and tensile bending, d) shows the strain dependent dark current modulation of MAPI film at +1V, e) shows the cyclic stability at +1V under compressive and tensile bending (0.23% strain) of MAPI film, and f) shows the cycling stability at +1V under compressive and tensile bending (0.009% strain) of MAPI film.



Figure 2: a) and b) show the FE-SEM images of the MAPI film with and without the use of solvent annealing treatment, c) shows the result of the CPP I-V measurement of device under different types of bending. (Bending angle 100o), and d) shows cyclic stability data at +1V under compressive and tensile bending (0.009% strain) for the MAPI film with larger grain size.


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1E-3

Compression FAPI Tensile FAPI Compression MAPI Tensile MAPI

400

0.01

Current (A) X 10 -4

(∆ Ι /Ι0)x100%

300 200 100 0

-100

a) 0.00

0.05

0.1 1V Bias: Compressive Bending

0.10

0.15

0.20

1E-3 1E-4

1E-4

1E-5 1E-6 1E-7

1E-5

b)

Current (A) X 10 -4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1V Bias: Tensile Bending

1E-8

0.25

0

50

Strain %

100

150

200

250

300

Time (Sec)

Figure 3: a) shows current variation in FAPI film at +1V under different compressive and tensile strain (along with the variation for MAPI case shown with solid lines for comparison), and b) shows the cyclic stability at +1V under compressive and tensile bending (0.009% strain).



a)

Pristine 1000 tensile bent 30 mins relax 60 mins relax 16 hours relax

Pristine 1000 compressive bent 30 mins relax 60 mins relax 16 hours

Intensity

Intensity

b)

13

14

28

29

30

13

14

28

2θ

2θ Pristine 1000 tensile bent 45 mins relax

c)

d)

50

29

30

Pristine 1000 compressive bent 45 mins relax

Intensity

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

150

200

250

-1

300

350

50

Wavenumber (cm )

100

150

200

250

-1

300

350

Wavenumber (cm )

Figure 4: shows the XRD patterns of MAPI film in pristine form along with that under the strain of 0.009% for (a) tensile and (b) compressive bent states, at different times following the bending, (c) and (d) shows the Raman spectra of the pristine MAPI film and that under the strain of 0.009% for tensile and compressive bent states, immediately upon bending and following 45 mins of relaxation.


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Figure 5: (a), (b) and (c) compare the FESEM images of the MAPI film in pristine form and those after application of the strain of 0.009% for 1000 cycles, (d) and (e) shows the photoluminescence (PL) spectra of MAPI films in pristine form and those after the application of a strain of 0.009% for tensile and compressive bent states, for 1000 cycles.


Flex-Mode Mechatronic Functionality of Lead Iodide Hybrid Perovskite

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