Organo-Lead Halide Perovskite Induced Electroactive Î²-Phase in

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Organo-lead Halide Perovskite Induced Electroactive #phase in Porous PVDF Films: An Excellent Material for Photoactive Piezoelectric Energy Harvester and Photodetector Ayesha Sultana, Priyabrata Sadhukhan, Md. Mehebub Alam, Sachindranath Das, Tapas Ranjan Middya, and Dipankar Mandal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17408 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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ACS Applied Materials & Interfaces

Organo-lead Halide Perovskite Induced Electroactive β-phase in Porous PVDF Films: An Excellent Material for Photoactive Piezoelectric Energy Harvester and Photodetector

Ayesha Sultana†, Priyabrata Sadhukhan‡, Md. Mehebub Alam†, Sachindranath Das‡, Tapas Ranjan Middya†, and Dipankar Mandal†,˦* †

Organic Nano-Piezoelectric Device Laboratory, Department of Physics, Jadavpur

University, Kolkata 700032, India ˦

Institute of Nano Science and Technology (INST), Phase-10, Sector-64, Mohali-160062,

India ‡

Department of Instrumentation Science, Jadavpur University, Kolkata 700032, India

*Corresponding Author E-mail: [email protected]; [email protected] Fax: +91-33-2413-8917 Tel.: +91 3324146666×2880

ABSTRACT Methylammonium lead iodide (CH3NH3PbI3) (MAPI) embedded β-phase comprising porous polyvinylidene fluoride (PVDF) composite (MPC) films turns to an excellent material for energy harvester and photodetector. MAPI enables to nucleate upto ~ 92 % of electroactive phase in PVDF to make it suitable for piezoelectric based mechanical energy harvesters (PEHs), sensors and actuators. The piezoelectric energy generation from PEH made with 1 ACS Paragon Plus Environment

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MPC film has been demonstrated under simple human finger touch motion. In addition, the feasibility of photosensitive properties of MPC films are manifested under the illumination of non-monochromatic light that also promises the application as organic photodetectors. Furthermore, fast rising time and instant increase in current under light illumination have been observed in MPC based photodetector (PD) that indicates to work as an efficient photoactive device. Owing to photoresponsive and electroactive nature of MPC films a new class of stand-alone self-powered flexible photoactive piezoelectric energy harvester (PPEH) has been fabricated. The simultaneous mechanical energy harvesting and visible light detection capability of the PPEH is promising in piezo-phototronics technology.

KEYWORDS. PVDF, CH3NH3PbI3, β-phase, porous, perovskite, piezoelectric energy harvester, photoactive, self-powered system

INTRODUCTION The requirements of self-powered portable electronic system eager the development of suitable materials for efficient and hybridized renewable energy harvesting. For example, solar, thermal, mechanical, geothermal, vibration, sonic wave and wind are the various type of renewable energy available in our surrounding, among them solar and mechanical are the most promising and valuable because of its greater accessibility and hybridizing possibilities. However, due to the drastically different physical principles, the existing approaches developed so far are mostly designed to uniquely harvest the mechanical or the solar energy. In general, organo-lead halide perovskites are mostly used materials in light energy harvesting application due to their excellent optical and electronic properties that contribute to high efficiency in solar energy conversion systems.1-4 In particular, photon-absorbing perovskite structured methylammonium lead halides (CH3NH3PbX3, X= Cl, Br, I) have

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attracted much attention in recent years in the arena of photovoltaic related research.5-7 Due to direct band gap semiconducting properties they are also good absorbers of a large spectrum of solar energy and also possess large absorption coefficient and high carrier mobility.6,7 Besides, photovoltaic research, perovskite based photodetectors have also gained enormous interest in recent years.8 However, stability of these materials is of extensive anxiety for the technology of solar cells and photodetectors where moisture is conceived to be the main degradation source on account of their intrinsic hydrophilic property.9 Hence, significant efforts have been devoted toward investigations on stability and encapsulation of perovskite materials for improvement of the lifecycle of the devices.10,11 An interesting functionality may be added if the encapsulation layer which is useful for protecting the perovskite materials can be used for harvesting mechanical energies. Currently, piezoelectric nanogenerators have been developed to convert mechanical energy from irregular mechanical vibrations to electricity. It can be fabricated using flexible polymer materials since inorganic piezoelectric materials are limited due to their fragile nature. Thus a hybrid energy harvester may be fabricated to simultaneously/separately harvest multimode energies, whichever is available if photoactive component is being introduced in piezoelectric polymers. It should also be mentioned that piezoelectric materials are also dielectric thus the output current generated from piezoelectric nanogenerator is generally quite low. Therefore improvement of current from piezoelectric nanogenerator also becomes a challenging task. Generally, ceramic piezoelectric materials show superior mechanical energy harvesting properties but they are limited by their rigid and brittle nature.12 In contrast, piezoelectric polymers are flexible, lightweight non-toxic and easy to process. It has been found that PVDF is one of the best known piezoelectric polymer that has tremendous potential to use in sensors, actuators, biomedical and mechanical energy-harvesting applications owing to its soft, biocompatible and light weight nature. This is a semi-crystalline polymer that exhibit at



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least four distinct crystalline phases viz., α-, β-, γ- and δ-phase.13,14 Among them, the polar β (TTTT conformation) and semi-polar γ (T3G+T3G- conformation) crystalline phases are more attractive due to their superior piezo-, pyro- and ferroelectric properties whereas α (TGTG conformation) phase is electrically nonactive. The alignment of all the –CH2/-CF2 dipoles in the direction normal to the chain axis of PVDF, makes the β-phase highly polar conformation.13 However, from the energy harvesting point of view β-phase is most desirable due to the highest dipolar moment per unit cell compared to γ-phase.13,15 The β-phase can be obtained by preferentially orient the -CH2/-CF2 dipoles of PVDF.16 Among various methods available to achieve β phase (e.g., mechanical stretching, electric poling, annealing, melt under high pressure, addition of hydrated salts and electrospinning) incorporation of external assisting agents in PVDF matrix to prepare a composite structure is most desirable to avoid undesired structural deformations and energy consumption.17,18 Recent study indicates that higher β-phase content, larger charge collecting area and improved strain effect is responsible to achieve enhanced piezoelectric response and sensing in porous PVDF.19,20 Thus several approaches have been undertaken to create porous structure in PVDF films.19,20 Noteworthy to mention that the efficiency enhancement of solar cell is also feasible by incorporation of ferroelectric constituent. For instances, the internal electric field of β-phase of ferroelectric PVDF is thus useful to reduce electron hole recombination by dissociating single electrons holes and charge transfer excitons that improve the device performance.21,22 A single device with multifunctional application has been a new trend in energy technologies.23-26 Thus, induction of polar β-phase in PVDF and optical absorption property of MAPI are both of great importance to make a composite that is suitable for mechanical energy harvester as well as photodetector and their hybridization. The optical absorption property may also be responsible for improvement of the output current of piezoelectric nanogenerator. To the best of our knowledge no attempt has been undertaken for induction of 4 ACS Paragon Plus Environment

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β-phase and porous structure in PVDF by incorporation of MAPI to make it suitable for mechanical energy harvesting, photodetector application and hybridize photoactive piezoelectric energy harvester. In this work, we have synthesized MAPI via simple co-precipitation method and prepared MAPI incorporated β-phase containing PVDF composite film by solution casting method in ambient environment. It seems that MAPI is not only inducing β-phase but also causing porous structure in PVDF. On the other hand, PVDF encapsulation prevents the degradation of MAPI from moisture and oxygen. We report the hybrid photoactive mechanical energy harvester made with MAPI embedded β-phase comprising porous polyvinylidene fluoride (PVDF) composite (MPC) films. It is also capable of detecting visible light which leads to significant change of output voltage (∼42 %) and current (∼39 %) that pave the way to work as self-powered photodetector.

EXPERIMENTAL SECTION

Materials. Methylamine (CH3NH2), γ-butyrolactone (LOBA Chemie), hydroiodic acid (HI) and lead (II) iodide (PbI2) (Otto Chemie). PVDF pellets (Mw ≈ 275 000 gm/mol, SigmaAldrich,

USA),

N,N-dimethylformamide

(DMF),

(Merck

Chemicals,

India)

polydimethylsiloxane (PDMS) (Sylgard184, Dow Corning Corp., USA), indium tin oxide (ITO) coated glass and PET substrates (Sigma-Aldrich).

MAPI Synthesis. Preparation of methylammonium iodide (CH3NH3I) is the first step towards synthesis of methylammonium lead iodide (CH3NH3PbI3). To synthesize CH3NH3I, equimolar solution of methylamine (CH3NH2) and hydroiodic acid (HI) were mixed in ice bath and kept on stirring for 2 hours. Then the resulting solution was heated at 60 ˚C for 1 5 ACS Paragon Plus Environment


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hour in nitrogen atmosphere and then precipitate was recovered. This precipitate was then washed with diethyl ether and filtered to get CH3NH3I powder. After that it was vacuum dried for further use. Equimolar amount of as synthesized CH3NH3I and lead (II) iodide (PbI2) was dissolved in γ-butyrolactone and stirred at 60˚ C in nitrogen atmosphere. Finally, shiny black powder of (CH3NH3PbI3) (digital photograph is shown in supporting information, Figure S1a) was recovered as precipitate which was vacuum dried and stored.

MPC film preparation. 12 wt% (w/v) PVDF was dissolved in DMF to prepare the stock solution. The stock solutions were blended separately with 0.025, 1, 3, 5, 7, 10 and 12 wt% (w/v) of MAPI then vigorously stirred at room temperature for 24 h (resulting solution shown in Figure S1b). PVDF-MAPI solutions were then casted on clean glass slides and subsequently dried at 90 oC for 6 h. Finally as-cast films (thickness ~20 µm) were peeled off from glass slides and named as PVDF (with no additives), and MPC#, # represents concentration of MAPI (Figure S1c).

Characterization. Field emission scanning electron microscopy (FESEM, FEI, INSPECT F50) images were recorded to investigate the detail surface morphological characteristics of the as-cast MPC. MPC were characterized using fourier transform infrared spectroscopy (FTIR, Bruker, Tensor II with ATR attachment (Pike MIRacle)). Thin film of MAPI was deposited on glass substrate in order to perform UV-vis spectroscopy using Optizen POP (South Korea) UV-Vis-NIR spectrophotometer. To investigate the crystallographic structure of the MAPI and MPC films, X-ray diffraction pattern (XRD, Bruker, D8 Advance) was recorded. Dielectric properties at room temperature were examined in the frequency range from 25 Hz to 1 MHz using a precision impedance analyzer (Wayne Kerr, 6500B). The


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ferroelectricity measurements were employed by a FE test system (aixact TF Analyzer 2000). Open-circuit voltage was recorded by connecting the PEH with a national instrument (NI) data acquisition (DAQ) device (NI, USB 6000) using a sampling rate of 1000 samples per second, interfaced with a computer with a standalone program made by using LabVIEW software and current measurements were carried out with Keithley 6485 picoammeter. The current-voltage (I-V) characteristics measurements were carried out using Keithley 2602B dual channel source meter. The photocurrent measurement with different color LEDs were performed with Nvis 105CT digital storage oscilloscope. Light intensities of the LED’s were measured with calibrated optical power meter.

RESULTS AND DISCUSSION

X-ray diffraction pattern (Figure 1a) of synthesized MAPI confirms the formation of pure perovskite phase.27 The UV-vis absorption spectra of MAPI (inset of Figure 1a) shows a broad absorption ranging from 400 to 800 nm. The optical band gap of the MAPI is determined from the Tauc plot as depicted in Figure S2 and associated text. The acquired direct band gap value is found to be 1.51 eV, calculated from the intercept by extrapolating the linear portion of the Tauc plot to the energy axis. This result is in good agreement with previously reported values.28 FE-SEM image and particle size distribution curve shown in Figure 1b illustrates that average diameter of particle around 16±4.2 µm are formed.

FT-IR spectra of neat PVDF and MPC films are shown in Figure 1c. The neat PVDF shows predominantly α-phase as evidenced from the vibrational bands at 1214, 1150, 976, 856, 796, 764 and 613 cm-1. In contrast, MPC films are exhibiting new prominent bands appeared at 1275 and 1234 cm-1 that correspond to the β- and γ phases respectively.29-31 Moreover the



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peak at 841 cm-1 is a dual signature for the presence of β- and γ phases. 29,30 Noteworthy that nucleation of the electroactive β- and γ-phases and reduction of the α-crystalline growth takes place even with a little amount of (0.025 wt%) MAPI.

(a)

(b)

50 µm

(c)

(d)

10 µm

Figure 1. (a) XRD spectra with UV-vis absorption spectra and (b) FE-SEM micrograph of as prepared MAPI with particle size distribution plot in the inset. (c) Static FT-IR spectra of PVDF and MPC films in a range of 1600–550 cm-1. (d) Dependency of electroactive phases of MPC films as a function of MAPI content. Inset presents the FT-IR spectra of PVDF and MPC films in a range of 3070–2935 cm-1.


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The α-phase content decreases with an increase in MAPI concentration and completely disappeared at 1 wt%. The intensity of 1275 and 1234 cm-1 band increases with increment of MAPI content and starts to reduce after 10 wt%. This result clearly revealed that the MAPI is playing a significant role in crystalline phase transformation and improvement of electroative β and γ-phase content in PVDF. The electroactive phase (FEA) content in MPC films as a function of MAPI concentration is illustrated in Figure 1d. It increases with MAPI content, saturates at 10 wt% and then gradually starts to decrease. Due to very close value of electroactive phase content in MPC(3), MPC(5), MPC(7) and MPC(10), mainly we choose two low (MPC(0.025), MPC(1)) and one high MAPI containing film (MPC(10)) for further characterization as well as utilized for device fabrication and its applications. The resultant content of electroactive phase (FEA) and relative proportion of β- (Fβ), and γ(Fγ) phases are individually estimated in selected MPC films and summarized in Table S1. For instance, MPC(10) film contains higher yield of the electroactive phase (~ 92 %) with 41 % and 51 % of β- and γ-phase respectively which is expected to be superior for designing piezoelectric based sensors and energy harvesters. FTIR spectra of the MPC films are also shown in the 3070–2935 cm-1 region (inset of Figure 1d) for studying the electroactive phase formations by interfacial interaction of the -CH2 dipoles with MAPI. This region is extremely useful to check the interfacial interactions between the external filler i.e., MAPI and the CH2 dipoles of PVDF because it is attributed to the -CH2 asymmetric (νas) and symmetric (νs) stretching vibrational bands and they are not coupled with any other vibrational modes.32 Thus, shifting of these bands are expected if any disruption of the vibrations appeared. Likewise, the positions of νas and νs vibrational bands of -CH2 are shifted toward lower energies (ῡ) in MPC films in comparison to PVDF film (ῡo)



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as shown in inset of Figure 1d, manifesting the interfacial interaction between MAPI and PVDF.

(a)

(b)

MAPI

PVDF chain

Figure 2. (a) The crystal structure of MAPI perovskite. (b) A schematic showing the proposed mechanism for the formation of electroactive phase in the MPC film.


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It is due to fact that the interfacial interactions increase the effective mass of the -CH2 dipoles which results a decrease in the vibrational frequency of the CH2 stretching vibration. This interaction act as a damping source and the angular frequency (ωint) of CH2 stretching vibration is related to the damping constant rdc as ω2 = ωo2- rdc2

(1)

where ωo is angular frequency of CH2 stretching vibration for damping free oscillation occurring in neat PVDF. Thus, in terms of wavenumber, rdc = 2πc (̅ o2 ̶ ̅ ) 1/2

(2)

The change of rdc with different MAPI loading is evident from Figure S3. It exhibits that rdc increases with increasing MAPI content. It is also notable that the FEA (shown in Table S1) and rdc as a function of MAPI content are analogous, suggesting a direct relationship between the electroactive phase formation in PVDF due to interfacial interaction of the CH2 dipoles and MAPI.12,13,17,19,32 Therefore, depending on the experimental results and the literature review, a schematic of the interaction between the MAPI and PVDF is presented. In organic-inorganic hybrid lead iodide perovskite compound, the monocationic charge on methylammonium (CH3NH3+) is localized on the three ammonium hydrogen atoms because of much greater electronegative characteristic of the nitrogen atom in the cages constituted by inorganic PbI3− framework (Figure 2a). The electronegative iodine atoms of the PbI3− frame work resulting the negative charge density that in turn electrostatically interact with -CH2 dipoles, which facilitates the nucleation of electroactive phases as shown in the schematic (Figure 2b). It is proposed that the interaction between negative charge (PbI3- anions) and –CH2 dipoles is responsible for the induction of β-phase which is supported by the shifting of -CH2 stretching in the 3070– 2935 cm-1 region. In the interface (where dielectric interface layer was formed) the electrostatic interaction takes place that initiate the preferable nucleation of electroactive 11 ACS Paragon Plus Environment


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phase, successively it leads to the presence of electroactive phase throughout the entire composite film. Thus the resulting film possesses the piezoelectric properties.30 In order to check the stability of β-phase in the MPC composite film, FT-IR measurement were carried out after four months where they kept in ambient condition (Figure S4). It is found that β-phase content is remaining the same that indicates that composite films are quite stable.

XRD pattern illustrated in Figure 3a is also employed to investigate the crystalline phases of PVDF. The diffraction peaks at 17.6° (100), 18.2° (202), 19.7° (110), 27.4° (111), 35.8° (200), and 38.7° (002) appeared in neat PVDF films are the characteristic for α-crystalline phase.33 In contrast, the α-characteristic peaks are decreased in the MPC films, whereas few new peaks (i.e., at 19.2°, 20.8°) are appeared as clearly ascribed from the XRD pattern of MPC(0.025). The diffraction peaks at 19.2° (002) and 20.8° (110/200) are attributing the presence of the γ- and β-phase respectively.32,33 In the MPC(1) film, the β-phase content became more dominant as deduced from the intense diffraction peak at 20.8°. Furthermore, the corresponding diffraction peaks of MAPI are also found in MPC(1) film that ensures the stability of MAPI within the PVDF matrix. In case of MPC(10) film due to the higher concentration of MAPI in PVDF, the β-crystalline peak is suppressed and the MAPI associated diffraction pattern are dominant. In other words, the characteristics diffraction peaks of MAPI are indicating the protection of the perovskite material from degradation that might be mainly due to the interfacial interaction and well encapsulation in PVDF matrix. The surface morphology of the PVDF, MPC(0.025), MPC(1) and MPC(10) films are illustrated in Figure 3b-e respectively.


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(a)

(b)

(c) Tiny spots α-spherulitic growth

5 µm

3 µm 5 µm

(d)

(e)

Inhomogeneous distribution of pores

Pores

5 µm

10 µm



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Figure 3. (a) XRD pattern for PVDF and MPC films. Surface morphology of (b) PVDF (c) MPC(0.025) (d) MPC(1) and (e) MPC(10) film. Pore diameter distribution plot with an average pore diameter of 540 nm in MPC(1) film is shown in the inset of (d).

The PVDF film shows predominant α-phase, evidenced from α-spherulitic fibril growth (Figure 3b), which is consistent with the FT-IR and XRD results. These α-spherulitic features are hindered when the electroactive γ and β-phase start to grow upon inclusion of MAPI filler as shown in Figure 3c-e. The surface morphology of MPC(0.025) film is composed with tiny spots (Figure 3c), however any open pores are not formed. Normally in presence of moisture, MAPI decomposes to CH3NH3I and PbI2 as shown in equation (3) and CH3NH3I further decompose to methylamine and hydroiodic acid (equation (4)).34 CH3NH3PbI3 (s) ↔ CH3NH3I (g) + PbI2 (s)

(3)

CH3NH3I (g) ↔ CH3NH2 (g) + HI (g)

(4)

Here in PVDF solution due to the presence of DMF, dissociation rate of MAPI increases compared to natural degradation. Thus in case of MPC(0.025) the major portion of MAPI may dissociate to form CH3NH2 and HI gases. But due to small amount of MAPI the effective pore formation are not taking place on the film surface. Rather it leads to some tiny spots on the surface of MPC(0.025). In contrast, homogeneous pores are formed on the surface of MPC(1) (Figure 3d). The enlarged cross-sectional view of MPC(1) film (Figure S5) shows that prolate spheroid-shaped voids are separated by insulating dielectric polymer layer which is the characteristic feature of a ferroelectretic material.35 A pore diameter distribution plot is illustrated in the inset of Figure 3d, where the average pore diameter is found to be ~ 540 nm. However, the MPC(10) film exhibits an inhomogeneous distribution of pores (Figure 3e) throughout the surface due to higher content of MAPI filler. These types of electret structure are very common for mechanical energy harvesting applications.36 14 ACS Paragon Plus Environment

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To investigate the piezoelectric response with energy harvesting ability of PVDF, MPC(0.025), MPC(1) and MPC(10) films, PEHs are fabricated (schematics shown in Figure 4a) and named as ref PEH, PEH(0.025), PEH(1), and PEH(10), respectively. Without any electrical poling treatment, mechanical energy harvesting properties of the PEHs were measured, where mechanical energy was applied by repeated human finger imparting (under 2 kPa pressure amplitude).37 Since MPC films are composed of polar β- and γ-phases, they are exhibiting superior piezoelectric properties than PVDF (Figure 4b and c). The open circuit voltage is improved from 0.2 V to 1.5 V when the MAPI loading increases from 0 to 1 wt% in PVDF matrix. As the FEA value of MPC(10) is comparable with MPC(1) (Figure 1d), thus PEH(10) shows comparable output performance to the PEH(1). The maximum open circuit voltage and short circuit current arrived up to 1.8 V (Figure 4b) and 37.5 nA (Figure 4c) respectively in PEH(10). The enhanced piezoelectric response of electroactive phase dominant films are due to the overall charge distributed across the entire crystal structure constituting MAPI and the polar electroactive part of PVDF. Throughout the human finger imparting process PEH’s went through a strain that made a deformation in its crystal structure to a stable one or vice versa, heading to the introduction of a piezoelectric potential across the surface of the film. Then charge induces on upper and lower electrodes due to this piezoelectric potential that contributes to the ultimate electrical output signal. Open circuit voltage and short circuit current with positive (imparting of human finger) and negative (releasing of human finger) amplitudes are produced because of this electrical potential difference appeared across the two electrodes of the PEH’s. Hence, this composite material can be used to harvest biomechanical energy from human body, relevant for touchable sensors such as footpaths, shoes, vehicles etc. In addition, the MPC composite films allow for a new platform for harvesting vibrationally bared energies accessible in surroundings.



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Pressure

(a) Glass Glass

(b)

(d)

Casting of MPC solution MPC

Electrode attachment

Fabricated PEH

Electrode

(c)

(e)

Figure 4. (a) Schematic of PEH fabrication and operation. Generation of (b) open circuit voltage and (c) short circuit current from PEH during human finger touch and release process. (d) Output voltage and instantaneous power density (circuit diagram is shown in the


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inset) as a function of external load resistances under gentle finger touch and (e) Capacitor charging performance of PEH(10).

To analyze the output power of PEH(10), the output voltage signals were recorded as a function of external load resistances (RL) ranging from 30 kΩ to 10 MΩ (Figure 4d) (as shown by circuit diagram in the inset). The output voltage signals (VL) across the load resistances gradually improved with the increasing resistance and getting saturated, equivalent to open circuit voltage at high resistance (10 MΩ). As a result, the instantaneous power density (P =

, where A is the effective area of PEH(10)) is reached up to 2.5

µW/cm2 at the resistance of 750 kΩ (Figure 4d).38 Beside this, to investigate the energy storage capability of PEH(10), different capacitors were charged up by connecting through typical full-wave four probe bridge rectifier circuit. Under continuous dynamic tactile stimuli energy was stored in the capacitors (Figure 4e) that can be further used to drive several low power consumer electronics.39 Furthermore, the good optical absorption ensures the photoresponse behaviour of MPC films as illustrated in Figure S6. It is found that MPC films exhibit the intense broad absorption around 800 nm. It attributes to the fact that MAPI is responsible for this absorption as there is a forbidden absorption in PVDF film. Thus photodetectors with MPC films have been fabricated as described in Figure 5a and recorded the current voltage (I-V) characteristic of the photodetector (PD) under dark and light condition. The I-V characteristic of PD(0.025) and PD(1) show very low current value (Figure S7) under both dark and light condition, is might be due to the insulating behaviour of PVDF that restricts the charge flow by breaking the interconnected network of semiconducting MAPI. However, MPC(10) film made PD(10) shows effective change in I-V characteristic between dark and light condition, thus becomes an effective photodetector. The I–V curves (Figure 5b) both in dark and under light 17 ACS Paragon Plus Environment


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illuminations is almost linear suggesting an Ohmic contact. It is obvious that photocurrent rises almost instantaneously when the device is exposed to the light illumination.

(a) ITO coated Glass

Glass

After etching of ITO coating ITO

Casting of MPC solution on ITO

Fabricated photodetector

MPC

(b)

(c)

(e)

(d)

Figure 5. (a) Schematic of the photodetector fabrication and operation. (b) Current–voltage characteristics of PD(10) under different light illumination. (c) Current vs time curves with


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multiple cycle operation of PD(10) under the irradiance of 42.25 mW/cm2. (d) An enlarge view of photocurrent for rise and decay time calculation. (e) Photocurrent response from PD(10) under illumination of light of different wavelength.

Under the light illumination, MAPI absorbs the photons that generating electron-hole pairs those are separated by the external electric field which conducts to the generation of photocurrent. Figure 5c depicts the photocurrent as a function of time (I-t curves) by illuminating the photodetector (PD(10)) with irradiance of 42.25 mW/cm2, while the bias voltage between two electrodes is kept constant at 10 V. Light illumination was switched on and off periodically with an interval of 20 s by means of shutter to study the photoresponse of the photodetector.

Instantly after turning on the light, the photocurrent quickly arises and settles to a stable and saturated value. Six repeated cycles are displayed in which the photocurrent is noticed to be consistent, repeatable and responds to a similar fashion to light that exhibits good operation stability. The time response speed is a key component for photodetectors for determining the ability of a photodetector to follow fast changing optical signals.40 A quick photoresponse can be assured clearly from the enlarged rising and decaying edges of photocurrent as shown in Figure 5d. The rising time and the decaying time are approximately 44 ms and 153 ms respectively, pointing rapid photoresponse characteristic. Another important parameter of photo detector is its spectral photo response. Being a low band gap (1.51 eV) materials, MAPI is able to absorb full visible spectra and show photo response. Three LEDs of light emission wavelengths 462, 520 and 630 nm were used to check spectral response of the device. Figure 5e shows the photo current produced by the device when illuminated by pulsating LEDs of 10 millisecond pulse width while the device



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was biased at 10 V. The photo current values are normalized to 1 mW/cm2 incident optical light intensity. Here we see that our device produces almost equal photo current for all three wavelengths which confirms it’s capability of detecting light from all over the visible region. Generation of slightly higher photo current when illuminated by lower wavelength (462 nm) may be attributed to the enhanced transition probability of the electron from the valence band to the conduction band for light with higher photon energy.41,42 Light of photon energy higher than the band gap generates electron hole pair which increases conductivity of a photosensitive material. Lower wavelength or higher energy pushes more number of electrons to the conduction band which in turn increases photo current.

So, MPC(10) becomes an excellent material for piezoelectric energy harvester and photodetector application. This also supports the idea that MPC(10) could serve as a reliable self-powered visible light detector.

(a)

(b)

Figure 6. (a) Change of piezoelectric response in terms of output voltage and (b) current on excluding and illuminating of white light with continuous application and removal of stress.


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Thus, a hybrid device, i.e., photoactive piezoelectric energy harvester (PPEH) has been prepared to use it for self-powered visible light detection. The composite solution (which was also used to prepare MPC10 film) is casted on a transparent flexible ITO coated PET substrate that serves as the top electrode. An adhesive conducting carbon tape is attached on the other side that serves as the bottom electrode. Finally, we encapsulate the whole device structure with PDMS, which kept the entire device flexible and also add further protection to perovskite fillers from moisture. Piezoelectric response from PPEH under periodic stress as a function of time is shown in Figure 6a. The average peak value of the open circuit voltage decreases under white light illumination than in dark condition. In contrast, the short circuit current increases in light illumination than dark condition under the similar repeating stress. This might be due to interaction with visible light electron-hole pairs are generated in MAPI that gives rise to a photocurrent. This movement of electrons leading in the improvement of local electromagnetic field, that could interrupts the dipole orientation in the composite film.43 That results in voltage response reduction with respect to the response in the dark condition.

The generated photocurrent created from extra charge carriers within the composite film during photon excitation results in the enhancement of short circuit current (Figure 6b). Thus, under visible light, the internal resistance of the PPEH dropped to ∼294 kΩ from ∼326 kΩ (Figure S8). The voltage and current output values are changed by 42 % and 39 % respectively in comparison to without light illumination. Thus, PPEH could be attractive for futuristic self-powered optoelectronic smart sensors and devices.43 In particular, it indicates that the PPEH can also be used as a self-powered flexible visible light detector/sensor.



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CONCLUSION In summary, organo-lead halide perovskite material methylammonium lead iodide is synthesized via co-precipitation method. The desirable amount of perovskite material incorporation in PVDF can induce the stable electroactive β-phase, thus turns out an attractive material for piezoelectric based energy harvesting applications. Therefore, the traditional stretching and poling method can be avoided to polar phase induction in PVDF. Furthermore, PVDF encapsulation prevents the degradation of MAPI and the resulting composite film shows strong light absorption properties as well. In addition, MPC based photodetectors show considerable performance with fast rising time that suggests that MPC films may have promising applications as a photodetector. Thus MAPI incorporated PVDF composite film promises the possibility of using as active material for multipurpose application (such as mechanical energy harvester, photodetector and hybrid device, i.e., PPEH) which may have potential applications in the next generation of optoelectronic and energy technologies. The coupling of the photoactive component with electroactive polymer permits to detect visible light, which leads to significant change of piezoelectric output voltage (∼42%) and current (∼39%). This result unambiguously suggests that PPEH can also be employed as a self-powered flexible visible light detector.

ASSOCIATED CONTENT Supporting Information Photographic image of synthesized MAPI powder and MPC films, Tauc plot for MAPI, XRD pattern of MPC(1) film, phase content calculation in MPC films, variation of damping coefficient as a function of MAPI loading in PVDF matrix, FT-IR spectra of MPC films when


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the measurement are carried out after four months from the first measurement, UV-vis absorption spectra of MPC films, current voltage characteristic curve.

AUTHOR INFORMATION Corresponding Author Tel.:

+913324146666x2880.

Fax:

+91(0)3324138917.

E-mail

address:

[email protected]; [email protected]

Author Contributions D.M. designs the entire work and motivates to conduct the plan. A.S. wrote the manuscript. All the experiments are carried out by A.S., M.M.A and P.S. P.S. synthesized the MAPI. S. D and T.R.M. shared their fruitful experience and suggestions that help to finalised the manuscript. All authors at last read the manuscript and agreed for the submission.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This work was supported by the Science and Engineering Research Board (SERB/1759/201415, and YSS/2015/000109), Govt. of India. Ayesha Sultana is supported by Maulana Azad National Fellowship (F1-17.1/2015-16/MANF-2015-17-WES-53885/(SA-III/Website)) from UGC. Priyabrata Sadhukhan is supported by INSPIRE fellowship (IF160132). Md. Mehebub Alam is supported by UGC-BSR fellowship (Ref. No.P-1/RS/191/14). Authors are also



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thankful for instrumental facilities developed by DST, Govt. of India under FIST-II programme.

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