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Electroactive and High Dielectric Folic Acid/PVDF Composite Film Rooted Simplistic Organic Photovoltaic Self-Charging Energy Storage Cell with Superior Energy Density and Storage Capability Swagata Roy, PRADIP THAKUR, Nur Amin Hoque, Biswajoy Bagchi, Nayim Sepay, Farha Khatun, Arpan Kool, and Sukhen Das ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05540 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017

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ACS Applied Materials & Interfaces

Electroactive and High Dielectric Folic Acid/PVDF Composite Film Rooted Simplistic Organic Photovoltaic Self-Charging Energy Storage Cell with Superior Energy Density and Storage Capability Swagata Roy1, Pradip Thakur1, 2*, Nur Amin Hoque1, Biswajoy Bagchi1+, Nayim Sepay3, Farha Khatun1, Arpan Kool1, Sukhen Das1* 1

Department of Physics, Jadavpur University, Kolkata- 700032, India

2

Department of Physics, Netaji Nagar College for Women, Kolkata-700092, India

3

Department of Chemistry, Jadavpur University, Kolkata-700032, India

+

Present address: Fuel Cell and Battery Division, Central Glass and Ceramic Research Institute,

Kolkata-700032, India *Corresponding authors: Email address: [email protected]; Mobile: +919433091337 Email address: [email protected]; Mobile: +919830366215

Keywords: Electroactive PVDF, MWS Interfacial Polarization, Dielectric Relaxation, Self-charging Photovoltaics, Energy Storage.

Abstract: Herein we report, a simplistic prototype approach to develop an “Organic Photovoltaic Selfcharging Energy Storage Cell (OPSESC)” rooted with bio-polymer folic acid (FA) modified high dielectric and electroactive β crystal enriched poly(vinylidenefluoride) (PVDF) composite (PFA) thin film. Comprehensive and exhaustive characterizations of the synthesized PFA composite films validate the proper formation of β-polymorphs in PVDF. Significant improvement of both β-phase crystallization (F (β) ~ 71.4%) and dielectric constant (ε ~ 218 at 20 Hz for PFA 7.5

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mass %) are the twosome realizations of our current study. Enhancement of β-phase nucleation in the composites can be thought as a contribution of the strong interaction of the FA particles with the PVDF chains. Maxwell-Wagner-Sillars (MWS) interfacial polarization approves the establishment of thermally stable high dielectric values measured over a wide temperature spectrum. The optimized high dielectric and electroactive films are further employed as an active energy storage material in designing our device named as OPSESC. Self-charging under visible light irradiation without an external biasing electrical field and simultaneous remarkable selfstorage of photogenerated electrical energy are the two foremost aptitudes and the spotlight of our present investigation. Our as fabricated device delivers an impressively high energy density of 7.84 mWh/g and an excellent specific capacitance of 61 F/g which is superior relative to the other photon induced two electrode organic self-charging energy storage devices reported so far. Our device also proves the realistic utility with good recycling capability by facilitating commercially available light emitting diode.

1. Introduction: Polyvinylidene fluoride (PVDF), a partially crystalline, mechanically flexible and sparsely available fluoropolymer having the molecular constitution (-CH2-CF2-)n, is one of the most fascinating electroactive polymer which has intrigued the precious attention of the present-day researchers for its versatile and unique inherent properties and has been in the limelight of polymer science research, especially in the burgeoning arenas of energy harvesting, piezo- and pyro-sensing and memory devices, batteries, thin film transistors, piezoelectric nano generators, self-charging storage devices as well as in the biomaterials and bio-medical horizons.1-6 Classified as smart materials, PVDF is bestowed with five diverse crystalline polymorphic


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phases viz. α , β, γ, δ and ε. Generated from melt crystallizations, α (alpha) polymorph is the most thermodynamically stable and the principal phase of PVDF. Self-nullification of dipole moments amidst the two antiparallel chains in the monoclinic unit cell of the lattice structure introduces it with the non-polar groups and designs its TGTGʹ (trans-gauche+-trans-gauche -) dihedral polymeric chain conformation. A huge spontaneous electrical polarization in PVDF in the presence of an electric field initiates from the disparity in electronegativity between the carbon and fluorine atoms of the monomer hydrocarbon units resulting in an extremely polar C-F bond of strong dipole moment of 6.4 x 10-30 C m. Electroactive β polymorph of PVDF possessing an orthorhombic unit cell matrix and all trans TTTT planar zigzag conformation, presents the highest dipolar moment per unit cell (8 x 10-30 C m ) when compared with the other two polar γ (T3GT3Gʹ) and δ (TGTGʹ) phases.1,7 Hereafter, it is contemplated as the inimitably striking and imperative polymorph owing to its indubitable piezo-, pyro-, ferro- and dielectric performances with reference to its all other existing phases. Henceforth, promotion and enhancement of the electroactive β-phase in PVDF matrix by various strategies is gaining impetus in the modern research community. Apart from the standard methods of augmenting the β-phase and the dielectric properties of PVDF like mechanical stretching and quenching of α-PVDF,8 electro-spinning,9 application of strong fields,10 impregnation of the PVDF matrix with inorganically and organically modified clays,11-14 metal and their oxide nanoparticles,15-18 inorganic nano/micro fillers like carbon black and carbon nanotubes,19-20 ferrites,21,22 graphene,23 inorganic salts24 and ceramics25 are only a few research approaches out of the extensive studies executed in the progression of the polar phase transformation and dielectric enhancement in PVDF for its application in numerous disciplines.



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Nevertheless, no such reports henceforth have been noticed to investigate the role of an organic bio-polymer as a modifier assisting in the β-phase transformation of PVDF. Folic acid, also known as pteroylglutamic acid (C19H19N7O6), is a thermodynamically stable, cost-effective yellowish crystalline powder and has earned its significance as a targeting ligand to the cell membranes exploited for therapeutic purposes. Researchers throughout have been utilizing this multi bio-functional water-soluble vitamin in the biological extents as an ideal vector for anticancer drug delivery,26,27 antimicrobial agents,28 as an ideal vector for fluorescent endoscopic detection29 and in the fields of photoelectrochemistry and optoelectronics.30 This article presents FA particles as a successful phase modifier of PVDF. Impregnation of αPVDF with various mass percentages of FA particles not only succeeded in its polar and electroactive β-phase transformation but also has enhanced its dielectric constant considerably. Our work further implements the PFA film composites in the fabrication of visible light induced self-charging energy storage devices assisted by an organic dye phenosafranine. Adequate and sustainable energy harvesting from renewable sources has become a solemn and indispensable disquiet in the contemporary civilization owing to the ecological demolition and depletion of fossil fuels at a swifter pace. Amidst all the renewable sources, solar energy has profoundly mesmerized the researchers worldwide since it is a remarkably inexhaustible silent energy source, environmental benign and does not yield any detrimental components which can disrupt the ecological systems. Hence, a sincere pursuit to harness this reliable and affordable light energy source by photoelectric conversion and to develop simultaneous energy storage systems in order to facilitate the world power consumption is today’s sole concern.31,32 Electrochemical reactions occurring between two electrodes and electrolyte interfaces using different inorganic combinations rule the vastly studied energy storage systems like Li-ion


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batteries33-37 and supercapacitors.38-42 However, majority of these devices require external voltage source for their charging. Though, various two electrode photo-self charging cells (PSCs) based on dye-sensitized solar cells (DSSCs)43-46 and quantum dot solar cells47 are also in the current chaos, but energy harvesting implementing a dye source as a photon absorber and biopolymer FA doped PVDF based high dielectric organic material as an energy storage system is not reported to the best of our extensive studies. Our fabricated device is a cost-effective simplistic endeavor for light-weight, biocompatible and two-electrode self-charging energy storage systems which involves no external electrical biasing. Henceforward, this work connotes a novel approach for a combined harvest and storage of electric energy directly from visible light.

2. Experimental: 2.1. Materials: Commercially available glass coated FTO substrates and aluminium foil are purchased from Sigma Aldrich. The subsequent precursors used in this study are Poly(vinylidene fluoride) (PVDF) pellets (Aldrich, Germany, Mw: 275000 GPC; Mn: 110000), Folic Acid (FA) [C19H19N7O6] (Lobachemie), Dimethyl Sulfoxide (DMSO) (Merck India), Polyvinyl Alcohol (PVA) [-C2H4O)n] (Lobachemie) and Phenosafranine (PSF) [C18H15N4Cl] (Sigma, USA). Exclusive of additional refinement, all these supplies are used as received. 2.2. Synthesis of PFA Composite Films: FA (1.0, 2.5, 5.0 and 7.5 mass % of PVDF; cf. Table 1) is added to the previously prepared 5 % PVDF solution (prepared by mixing 250 mg of PVDF in 5 ml DMSO stirred at 60 ºC). The mixture is vigorously stirred overnight and ultrasonicated for 30 min to obtain homogeneity.



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Hereafter, it is casted in dirt free glass petri plates and placed in a hot air oven at 80 ºC for 12 h. Pristine PVDF (for control system) and PFA thin films of thickness 30-50 µm are harvested at similar experimental conditions by the entire evaporation of the solvent DMSO from the mixture. The as prepared samples are carefully preserved in a vacuum desiccator for subsequent experimental studies.

Table1. Sample designations and calculated amount of Folic Acid (FA) incorporated in PVDF during the synthesis of pure PVDF and PFA films

Sample Name

PVDF PFA 1.0 % PFA 2.5 % PFA 5.0 % PFA 7.5 %

Amount of PVDF mg 250 250 250 250 250

Mass % of FA % 0.0 1.0 2.5 5.0 7.5

Amount of FA mg 0.00 2.50 6.25 12.50 18.75

2.3. OPSESC Fabrication: Meticulous cleansing of the purchased FTO coated glass with acetone and distilled water is executed prior to the fabrication of the device. First, PSF-PVA dye solution (0.1 mg/ml PSF in 10 mass% PVA solution), formerly synthesized in distilled water, is deposited on the conducting side of the FTO and dried out in air at room temperature till a jelly-like layer is obtained. Next, the previously synthesized pure PVDF and PFA 7.5 mass % thin films are placed on the jelly layer and dried at 80 oC. The Al counter electrode is inserted on the top and two wires are connected from either side of the two electrodes for photovoltaic studies. The thickness of the active material and PSF-PVA films are ~ 30 µm and ~ 20 µm approximately.


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2.4. Material Characterization Techniques: Simplistic solution casting technique obtained PVDF and PFA thin films, are all investigated by X-ray diffractometer (Model-D8, Bruker AXS Inc., Madison, WI) for electro active β−phase evaluation at an atmospheric pressure and room temperature from 10º to 45º 2θ values at a scan speed of 0.3 s stepping time by means of nickel filtered Cu-Kα radiation. A voltage of 40 kV is applied in the experiment. Secondly, the IR spectra of the identical samples are studied by Fourier transform infrared spectrometer (FTIR-8400S, Shimadzu) to inspect the influence of the doped organic particle on the phase nucleation of the polymer. The wavenumber range set for this study is from 400 cm-1 to 1000 cm-1. The absorbance spectra of each sample, scanned for 50 times, are obtained under a resolution of 4 cm-1. Lambert-Beer law which is given by the equation, =

(1)

is used to compute the fraction of β–phase, F (β) (%) of the pure and doped composite thin films where Aβ = the absorbance at 840 cm-1, Aα = the absorbance at 764 cm-1. Kβ = 7.7 x 104 cm2 mol1

and Kα = 6.1 x 104 cm2 mol-1 are the absorption coefficients at 840 cm-1 and 764 cm-1

respectively.12,22 Third characterization involves differential scanning calorimetry (DSC-60, Shimadzu (Asia Pacific) Pte. Ltd., Singapore. This apparatus is used to observe the phase nucleation and melting character of the pure and doped PVDF films. All the films weighing ~ 5 mg are placed in ceramic crucibles of capacity 70 µl and heated from 30 oC to 600 oC at an increasing rate of 10 o

C/min under N2 gas atmosphere. DSC thermographs are obtained and simultaneous assessment



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of enthalpies (∆Hm) of fusion and the degree of crystallinity (Xc) of the same samples are evaluated using the equation written below: Xc= ∆Hm / ∆H100%

(2)

where ∆Hm = heat of melting or enthalpy of fusion and ∆H100% = melting enthalpy of 100% crystalline PVDF with value 104.6 J/g.11,12 Further characterization to obtain the absorption spectra of the doped and un-doped PVDF films in the wavelength range 200-800 nm is carried out using UV-Visible Spectrophotometer (Lambda 25, Perkin Elmer, USA and UV-3101PC, Shimadzu). Micro structural and morphological alterations of the pristine PVDF film upon addition of the FA into it are captured via FESEM (INSPECT, F50, Netherlands). The samples are positioned under gold coating unit through plasma spraying at 0.1 mbar pressure. Each of the samples is kept within a vacuum chamber at a pressure of about 5x10-3 Pa and at a remoteness of 12 mm away from the detector with spot size of 3 mm. Emission current of 170 µA and an operating voltage of 20 kV are applied whilst taking the FESEM images. Dielectric behavior of the pure and FA incorporated PVDF thin films in the frequency range of 100 Hz to 2 MHz under an 1 V ac voltage signal, are noted at room temperature and also by variation of temperature at a step of 30 ºC till 150 ºC. Digital LCR meter (Agilent, E4980A), a PID temperature controller and Ag electrodes are used for this measurement. Evaluation of the dielectric constant (ε) and the ac conductivity (σac) of the thin films involves the following equations numbered (3) and (4) respectively, ε = C.d / ε0A

(3)

σac= 2πfε0 ε tanδ

(4)


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where, C, A, d and tan δ are the capacitance, area, thickness and tangent loss of the film samples correspondingly, ε0 (8.854 x 10-12 Fm-1 ) is the permittivity of free space and f is the applied frequency in Hz.11,17 Photovoltage studies are carried out using a 40 W tungsten filament lamp, a multimeter (Agilent U1252A) and commercial blue LEDs (light emitting diodes). Energy density, power density, charge density and specific capacitance are all calculated from the discharge graph using the well-known equations given below respectively. E = (i x area under the discharge curve) / m

(5)

Pmax = (Vmax x i) / m

(6)

Q = idt / m

(7)

Cmax = Q / Vmax

(8)

Where, E, Pmax i, Vmax, Q, Cmax and m are the energy density, maximum power density, discharge current , maximum voltage attained by the single device, charge stored by the device, maximum specific capacitance of the device and mass in grams of the pure PVDF and PFA film.

3. Results and Discussions: 3.1. X-ray Diffraction (XRD): Well exhibited diffraction patterns of pure PVDF and PFA thin films are portrayed in Figure 1 (a). The characteristic crystalline α-phase reflections in pure PVDF film are marked by red dotted lines at 2θ = 17.5° (100), 18.2° (020), 19.9° (021), and 26.5° ((201), (310)). Similar marking is made at 2θ = 38.5° (211) which denotes a petite characteristic peak of γ phase crystals.



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Spectra of the PFA composite films confirms the gradual disappearance of all bigger spherulites of non-polar α and γ-phase (2θ = 17.5°, 18.2°, 19.9°, 26.5° and 38.5°) and simultaneous increase of a prominent electroactive β-phase peaks at 2θ = 20.6° and 36.5° which are relative to the sum of diffraction intensities of (110), (200) and (020), (100) planes respectively with the increase in doping concentration of FA in the polymer matrix.1,17 Emergence and enhancement of β-phase with the increase in doping confirms the formation of the electro active beta phase crystallization in the synthesized composite thin films. The significant phase transformation may be a consequence of the well intercalation and fine distribution of the FA particles all over the polymeric matrix. Longer polymer chain TTTT i.e all trans conformation and re-orientation of molecular order in the polymeric structure are the major outcomes leading to this phase transformation. PFA 7.5 % offers the optimal β- phase transformation amongst all the doped concentrations. Amount of α and β phase in the pure and doped PVDF is determined by the ratio of the intensities (I20.6 / I18.2) of 18.2° (corresponding to α-phase) and 20.6° (corresponding to β-phase) in Figure 1 (b). Computation of the ratios yields a value of 0.92 in case of PVDF while it gradually raises to 8.14 for PFA 7.5 % suggesting improved nucleation of β-phase. FTIR and DSC values studied shortly show excellent consistency with the XRD spectra obtained signifying the authenticity of the phase transformation in the PVDF films upon addition of FA.


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Figure1. X-Ray diffractograms of (a) Pure PVDF and PFA thin films (b) Ratio of I20.6 and I18.2 of the pure PVDF and PFA thin films. (c) FTIR spectra of pure PVDF and PFA thin films (d) Evaluation of the F (β) (%) of pure PVDF and PFA thin films.

3.2. Fourier Transform Infrared (FTIR) Spectroscopy: Fourier transform infrared spectral analysis has proved to be a crucial tool for the determination of the β-phase transformation from the non-polar α-polymorphs. Figure 1 (c) provides a clear spectrum which assigns the diverse and distinguished phases of un-modified and doped PVDF composite films.



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Absorbance peaks at 488 cm-1 (CF2 waging), 532 cm-1 (CF2 bending), 614 and 764 cm-1 (CF2 bending and skeletal bending), 796 cm-1 and 975 cm-1(CH2 rocking) which relates to α-phase polymorphs and a little absorbance peak at 840 cm-1 (CH2 rocking, CF2 stretching and skeletal CC stretching) which designates the β-phase are observed in the spectra of the untreated pristine PVDF. The α-phase absorbance peaks are found to fade away upon the addition of FA to the pure polymer matrix while the absorbance peak at 840 cm-1 prominences. Distinguished and new absorption bands of polar β-polymorphs around 479 cm-1 (CF2 deformation), 510 cm-1 (CF2 stretching) and 600 cm-1 (CF2 wagging) are also observed to emerge in the spectra of the modified thin films which adds to the previous corroboration.1,12,17. Gradual improvement in the intensities of the main β-phase characteristic absorbance peak at 840 cm-1 and emergence of other surrounding β-phase peaks with the increase in percentages of the additives in the pure polymer specifies the strong and successful β-phase transformation from non-polar α-polymorphs. The highest absorption at 840 cm-1 is recorded for PFA 7.5 mass %. The FA particles finely distributed and environed by the polymer thin layers are held responsible for the maximal β-phase nucleation in the modified composite films for that particular concentration. Histograms illustrated in the Figure 1 (d) elucidate the variation of the relative fraction of the βphase content F (β) (%) with respect to the different doping concentrations of FA in the pure αphase PVDF. Lambert-Beer law [equation (1)] is applied to compute all the F (β) (%) values. Untreated PVDF showed an F (β) (%) value of only 38. Escalation of F (β) (%) is denoted from the histograms with the increase of doping concentration (mass %) and an utmost value of ~71.4 % is achieved for the highest doping concentration. Subsequently, FTIR spectroscopy technique


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proved prolific in establishing the stabilization and enhancement of the electro active β-phase nucleation in the FA modified PVDF composite films.

3.3. Differential Scanning Calorimetry (DSC): DSC thermographs are studied in the Figure 2 (a) as an effective complementary tool of X-ray and FTIR analysis for the identification and establishment of various crystalline phases of the pure and composite polymer samples. α-phase crystallization in untreated PVDF gives rise to a strong melting characteristic peak at 163.5 oC12,17 while the peak is seen to shift to higher temperatures by 2-3 oC in the FA modified polymer films. The nucleating process gives rise to a prominent transformation of non-polar α-phase to its electro-active and polar β-form resulting in this upshift in temperature upon impregnating the polymer with the various mass percentages of FA. Graphical illustrations of the calculated enthalpy of fusion and degree of crystallinity of all the samples calculated from DSC thermographs are represented by the Figure 2 (b) and 2 (c). Lowest crystallinity of neat PVDF implied its amorphic nature and slight homogeneity in the crystals. The melting enthalpies as well as the degree of crystallinity of the FA loaded PVDF film exhibits a prominent upsurge with the increasing concentration of the additives compared to the pristine PVDF. This upsurge in the enthalpies and crystallinity in the doped films may be accredited to the strong interaction of the FA particles with the electrophilic F groups of PVDF chains and the simultaneous nucleation of electroactive β-polymorphs from non-polar α-domains.12,20,48,49 A maximum value of 58 % of melting enthalpy is obtained for PFA 7.5 % while degree of crystallinity of 55 % is calculated for the same doping concentration.



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Figure 2. (a) DSC thermographs of pristine PVDF and PFA thin films; Evaluation of (b) Melting enthalpies and (c) Degree of crystallinity of pure PVDF and PFA thin films; UV-visible absorption spectra of (d) Folic acid and (e) Pristine PVDF and PFA thin films.

3.4. UV-Vis Spectroscopy: The underlying fact that FA is responsive to ultraviolet radiation is certified by the spectrophotometric analysis in Figure 2 (d). The distinctive absorption maxima at 314 nm and a broader band around 382 nm which are attributed to п- п* and n-п* transitions in enone moiety of folate respectively, are all the significant characteristic peaks of FA lying in the UV region. The minor deviation of the obtained absorbance peaks from the reported data may be due to the DMSO solvent medium in which FA is dispersed.50-52 Nevertheless, the spectra of the composite (cf. Figure 2 (e)) depict a different picture. Pure PVDF exhibits no such absorbance within the recorded wave band while upon doping the polymer with FA17 absorption bands at 318 nm and


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393 nm are observed which are located nearby the plausible characteristic peaks of FA. A clear and effective conjugation of FA with that of PVDF chains is anticipated from the slight shift in the absorption peaks in the spectra of the composite thin films. A reformation in size and morphology of the FA particles in PVDF matrix may be responsible for the red shift in the absorbance spectrum of the FA-PVDF film composites. 3.5. Surface Morphological Analysis: Illustrations of microstructural and morphological features of the surface of FA, pure PVDF and PFA thin films are displayed in the Figure 3. The captured images demonstrate plate like microstructures of FA of wide size variations and its finely demarcated uniform distribution in the PVDF matrix. Due to the fine intercalation of FA in the polymer, their plate like structures measuring in microns crumbles down to structures with size in the nano domain. Excellent and uniform distribution of particles in the polymer matrix is observed and this contributes to the nucleation of electro active β-phase. Extents stretching in diameter from 20-40 µm are noted in pure PVDF matrix (cf. Figure 3 (f)) while smaller and even surfaced spherulites extending in diameter from 3-5 µm are formed with absolute clarity when FA embedded PVDF film composites are taken into account (cf. Figure 3 (g, h, i and j)). Literature review shows that α and γ phase dominant PVDF regions have diameter larger than 10 µm.53,54 Henceforth, formation of smaller spherulites (~ 3 µm to 5 µm) attained from FESEM study can also be a substantial conclusion which fit in good agreement with the previous XRD, FTIR and DSC deductions authorizing the enhanced β-phase nucleation in the FA altered PVDF thin films.55



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Figure 3. FESEM micrographs showing (a) FA microstructures and (b, c, d and e) Various mass % of PFA thin films showing uniformly intercalated FA particles in PVDF matrix (f) α-spherulites of Pristine PVDF film and (g, h, i and j) Formation of β-spherulites in various mass % of PFA thin films.

3.6. Dielectric Properties: 3.6.1. FA Content (mass %) Dependence: To study the variation in the dielectric constant of PVDF upon impregnating it with FA, detailed dielectric analysis is carried out at room temperature. Figure 4 (a) and 4 (b) explicates the disparity of dielectric constant (ε) and ac conductivity (σac) between the pristine PVDF and PFA thin films with reference to FA content (mass %) assessed at frequencies of 20 Hz, 100 Hz, 2 kHz and 1 MHz at room temperature and atmospheric pressure using equation (3) and (4). Pure PVDF has a dielectric constant of only ~ 9 at 20 Hz. A noticeable escalation in the dielectric values is realized with the increase in doping concentration of FA in the polymer matrix. It is interesting to note that PFA 7.5 % attains a maximum dielectric value of 218 at 20 Hz which is ~ 24 times that of pure PVDF. The values of dielectric constants of the PFA 7.5 % at other frequencies viz. 100 Hz, 2 KHz and 1 MHz are 100, 44 and 21 respectively as displayed in


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Figure 4 (a). Additional doping of more than 7.5 mass % of FA in PVDF vitiates the integrity of the structure of the material generating pores in it which restricts us from achieving higher dielectric constant by increasing the loading percentage of FA in PVDF. Enrichment of dielectric constant with increase in doping concentration can be elucidated by the Maxwell–Wagner–Sillars (MWS) interfacial polarization phenomenon which arises on account of the difference in conductivity of the dopants and insulating polymer chains. Owing to the fine homogeneous mixing of the FA in PVDF matrix and strong conjugation of FA particles with the polymer chains, inter-molecular distance shrinks and interfacial area per unit volume linked with the interfaces of FA and PVDF matrix crop up. These act as an impetus for the huge deposition of charges and substantial augmentation of average localized polarization. Again, improved βphase nucleation with the increase of the additive percentage induces longer planar zigzag conformations into the matrix resulting enhanced dipole density in the film samples resulting in the escalation of the dielectric strength.1,11,17,48,49,56,57 Since dipolar polarization is the dictating parameter accountable for the dielectric constant values at higher frequencies the dielectric values shows a feeble and alike increment with the increase of doping percentage at higher frequencies of 2 KHz and 1 MHz. A linear increase of the ac conductivity with the doping concentration is yet another noteworthy observation of this study.11,56 3.6.2. Frequency Dependence: Frequency dependent dielectric constant, tangent losses and ac conductivity of all the film samples are graphically presented in Figure 4 (c), 4 (d) and 4 (e) respectively. The vivid representations denote the gradual decrease of dielectric constant values of PFA 7.5 % and PFA



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5.0 % composite films while the dielectric values of lower percentage doped and un-doped polymer samples exhibits insignificant variation of dielectric constant relative to the frequency. Also the prominent increase of dielectric constant values with the increase of additive percentage is a major observation. At lower frequencies the dielectric constant is primarily influenced by the MWS interfacial polarization. Formation of abundant small range mobile dipoles due to good interaction and excellent dispersion of FA in the polymer matrix and a colossal deposition of the same in the interfaces created between the two heterogeneous medias of FA and PVDF matrix during the application of alternating electric field to the samples, results in the large interfacial polarization and a considerable upswing in dielectric constant in the doped PVDF film samples.11,12,56-58 At higher frequency, the charge carriers or dipoles fail to switch rapidly enough to keep pace and suffers a lag relative to the frequency of the electric field applied and the material loses its polarizing capability. Effective depletion and confinement of the dipole movement at the higher frequencies add to the reason for the exhibition of a complex behavior of dielectric values with respect to the supplied electrical energy. MWS interfacial polarization is constrained due to poor accumulation of charges and the dielectric constant values of synthesized composites saturate regardless of increase in the frequency.12,48,49,56-58 Tangent loss of the highest FA doped composite film is miniscule i.e. only 0.6 with respect to its high dielectric value which is fairly promising. Dipolar polarization, long range intermolecular hopping of electrons and damped oscillations of electrical dipoles contribute to the dielectric loss variation with frequency.59


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Inter-particle distance being quite small amidst the conductive fillers, dielectric constant is always accompanied by ac conductivity. A sharp and linear rise in the ac conductivity of the doped films composites with increase in frequency as well as with the increase in doping concentration when compared with pure polymer film is an interesting observation (cf. 4 (e)). The dc conductivity chiefly dominates the lower frequency range while higher frequency range is ruled by the ac conductivity. The enormous space charge growth in the interfaces due to MWS polarization and dipolar relaxation modes are the duo confirming the rise of the ac conductivity with both the frequency and concentration of dopants. Swift and linear intensification in the conductivity at low frequencies can be a reason for the nonappearance of lower frequency plateau in the diagrammatic presentation.11,12,56,58



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Figure 4. FA content (mass %) dependency of (a) dielectric constant and (b) ac conductivity at 20 Hz, 100 Hz, 2 kHz and 1 MHz of Pure PVDF and PFA thin films. Frequency dependency of (c) dielectric constant (d) tangent loss (e) ac conductivities of pure PVDF and PFA thin films.

3.6.3. Temperature Dependence: Further extended study to comprehend the dielectric stability with temperature is performed and results achieved are unveiled in Figure 5. It is observed that pure PVDF shows minimal deviation in dielectric value with the increase in temperature at low temperature and low frequency region but achieves a dielectric value of 63 at 150 °C owing to its total polarization.60 On the other hand, FA embedded PVDF films exhibits a monotonic rise in dielectric constant with the increase in temperature and doping concentration at the low frequency region. PFA 7.5 mass % attains a dielectric constant of ~1609 at 150 ºC; 20 Hz which is noticeably higher with respect to pure as well as all other percentage doped composite films. But at high frequencies, regardless of increase in temperature, dielectric values of both the pure and composite films show a decrease and subsequent saturation which is quite fascinating. Both these observations can be rightly justified by MWS phenomenon and dipolar polarization which strictly dominates the lower and higher frequency range respectively.11,56 The polymer chains remain entangled and intimate and the hence the segmental movement freezes at low temperatures. However, with the increase in the temperature, the volume of the bulk material expanses, entanglement releases and number of molecules per effective length diminishes; hence dipolar orientation comes at ease.11,58,61,62 All these factors added with the


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supply of thermal energy feasibly boost up the net polarization and simultaneous increase in dielectric constant at low frequency across the temperature spectrum can be achieved. Thermal energy supplied to the FA doped PVDF composite film samples releases the various layers of intensely bound water molecules entrapped in the remote periphery of the FA matrix to its free state.63 Huge expanse of free dipoles thus created travel along to occupy the interfaces sandwiched between the FA and PVDF boundaries. Strong crosslinking between –CF2 polymeric chains and FA particles along with a large space charge polarization give rise to MWS effect which further contributes to the extensive enhancement of the dielectric constant values with the increase in doping percentages of FA in PVDF over the wide temperature spectrum. Significant improvement of dielectric constant also comprises of αa - relaxation supremacy and the chain scissoring process as the couple of factors attributing to the rapid movement of the functional groups at the lower frequency range with the rise in temperature.11,56,60,64,65 However, at higher frequency, there is a swift alteration in the ac field and dipoles are unable to respond to the oscillations of the field due to αa - relaxation and hence dipolar polarization predominates. Though the thermal energy tries to randomize and create Brownian motion amongst the dipoles and increases the mobility of the dipoles to a large extent, still they cannot contribute to the net space charge polarization and it results in the net decrease in dielectric constant even with the rise in temperature.60-62,64 Dielectric losses and ac conductivities portray a peculiar nature with the rise in temperature at low and high frequency area and this is clearly implied in the Figure 5. The quick internal bond rotation of the polymer chains and consequential intra and inter ionic hopping in the polymer chain impairs its rigidity, creates numerous vacant states and a considerably massive deposition



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of free dipoles in the interfaces (MWS phenomenon) is announced due to improved mobility when high temperatures intervene. This enhances the dielectric constant and αa - relaxation which turns out to be a reason for the increases of ac conductivity of the doped composites with temperature.58 Loading PVDF with 7.5 mass % of FA not only reaps the best dielectric constant value and low tangent loss but also attains a satisfactory conductivity over a wide range of temperature which makes it a fairly reliable and a prospective candidate for our further device fabrication amongst all the other loaded percentages of FA.

Figure 5. Variation of dielectric constant with temperature in the pure PVDF and PFA thin films at (a) 20 Hz (d) 1 MHz; Variation of tangent loss with temperature in the pure PVDF and PFA thin films at (b) 20 Hz (e) 1 MHz; Variation of ac conductivity with temperature in pure PVDF and PFA thin films at (c) 20 Hz (f) 1 MHz


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3.7. Performances and Self-Charging Mechanism of the OPSESC Detailed characterizations and dielectric studies establish the PFA 7.5 mass % sample as the best suited candidate for fabrication of our OPSESC. The diagrammatic and digital illustrations of the fabricated device along with its cross-sectional FESEM micrograph are displayed in the Figure 6.

Figure 6. (a) Diagrammatic illustration of the OPSESC architecture, (b) FESEM micrograph of the crosssection of the device, (c) and (d) The digital images of both sides of the OPSESC.

FTO coated glasses and Aluminum foils are acting as the working and counter electrodes respectively. PSF-PVA film adjoined with working electrode is the photon absorber and generator of photo-electrons. PFA 7.5 mass % film (dimensions ~ 0.65 cm x 0.8 cm) of thickness ~ 30 µm (cf. Figure 6 (b)) is the active material in our device acting as the storage medium of



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photogenerated electrons. We emphasize on the fact that our device is activated by capturing the visible spectrum of light only, emitted from a tungsten filament lamp of intensity 110 mW/cm2 and no external bias voltage is supplied. Figure 7 (a) and (b) represents the self-charging and discharging characteristics (V-t and I-t graphs) of the synthesized devices made by using pristine PVDF (OPSESC1; control set-up) and PFA 7.5 mass % film (OPSESC2). Under an irradiation of visible light (Intensity = 110 mW/cm2), the device OPSESC2 has been charged up to 1.0 V in 23 minutes while it is only 686 mV in OPSESC1. The devices are exposed to darkness for examining the discharge characteristics. Though the charging ceases, the voltage is still maintained by the devices and a very gradual drop is noted for OPSESC2 with very long discharge time of ~127 minutes compared to OPSESC1. Moreover, the consecutive reversible photo-charging and discharging cycles upholds the excellent constancy of the device (cf. Figure 7 (c)). Energy density bars in Figure 7 (d) calculated from the discharge graphs using equation (5) also signifies that OPSESC2 has the remarkable energy density of 7.84 mWh/g which is much better relative to the OPSESC1 i.e. 0.56 mWh/g. Maximum power density is obtained ~ 8 mW/g for the OPSESC2 while it is only 1.4 mW/g for OPSESC1 device (calculated using equation 6). Evaluations from equation (8) authorize that OPSESC2 is an energy storage device having a maximum specific capacitance of 61 F/g which is strikingly noteworthy for a self-charging device. The energy density, power density and storage capability of our device is also very much larger than the previously reported prototype photovoltaic cells integrated with in situ charging ability.46, 66-68


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Figure 7. Charging-discharging (V-t and I-t) graphs of (a) OPSESC1 (b) OPSESC2; (c) Charge-discharge repeat cycle of OPSESC2 (d) Histograms representing the comparative values of energy density, maximum power density, maximum specific capacitance and charge density between OPSESC1 and OPSESC2 .

It is challenging at times to achieve the power requirements to light up small electronic gadgets using a single device. Thereby, three analogous OPSESC2 devices are connected in series to produce enough voltages and current. The digital photograph of the series assembly of three OPSESC2 is shown in Figure 8 (a). The three devices connected in series are self-charged to maximum voltage of 2.8 V under the same illumination intensity and then connected to a commercially available blue light emitting diode (LED). The charged OPSESC2, is able to light up a LED which stays so for 7 s (See supporting Video S1). The digital snapshot of the lighted



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LED (in dark) is presented in the inset of Figure 8 (a). Thus, our device OPSESC2, successfully demonstrates its high potential in realistic utility in our daily life. The schematic illustration of the self-charging and storage mechanism of our device is depicted in Figure 8. It can be explained that when the tungsten filament bulb is switched on, the PSF molecule absorbs the photon energy in visible spectrum (~ 519 nm, cf. Figure 8 (b)) and actively generates the photo induced electrons. The reaction mechanism of the activation of the PSF molecule with visible light photons and its subsequent electron liberation can be designated as: PSF + hʋ ↔ PSF* + e-

; PSF*+ PVA ↔ PVA* + PSF

The HOMO and LUMO energy states of PSF are clearly visible in the proposed schematic. Electrons generated due to the excitation of PSF molecules reach the FTO electrode as it occupies an energy state much lower in work function ~ - 4.8 eV with reference to the LUMO of PSF and travel along the direction of Al electrode in the external circuit. PVA supplies electrons to the oxidized form of photosensitizer and the dye returns to its original form. PSF again absorbs a photon and the process is repeated. A large deficit of electrons accounts for the positive charge on the FTO and the adjoining PSF-PVA film. Huge surplus of electrons bestow a negative charge accumulation on the periphery of Al electrode. As the illumination lengthens, the voltage escalates reaching a maximal value of 1.0 V. The device hence gets charged up due to the created potential difference across the two electrodes. The maximum charge density achieved in the OPSESC2 is ~ 61 C/g. When the light source is switched off, photovoltaic effect terminates and discharging occurs at a very slow rate indicating charge storage in the device. The electrons on the Al side cannot reach the PSF-PVA layer at once due to the barrier created by our synthesized high dielectric PFA 7.5 % thin film. Hence, polarization in the PFA film composites


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and charge separation occurring on the either side of the PFA 7.5 % film contributes to significant charge storage.46,69 A satisfactory amount of conductivity obtained in the PFA 7.5 % film, allows slow transport of electrons. A constant discharge current of 8 µA is formed in the circuit. When a blue LED is connected across a series assembly of three self-charged similar OPSESC2 devices, it gets lighted up. It is obvious from our experiments that our device gets charged up only under the action of visible light and simultaneously has the capability of memorizing the acquired voltage for a considerable expanse of time in dark as we switch off the illumination contributing to the energy storage. OPSESC2 is a simplistically fabricated economic organic self-charging energy storage device and is a promising approach in the energy harvesting fields. This study provides a simple approach in the energy harvesting and storage arena and contributes significantly though studied in a miniature scale. This is a simplistic prototype attempt to fabricate a bio-compatible and cost effective self-charging energy storage device though further investigations are on the way and much progress in the development of the performance of the device by focusing on the improvement and optimization of the active material is yet to arrive.



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Figure 8. (a) Digital photograph of three similar OPSESC2 devices in series with lighted LED (inset shows glowing LED in dark) (b) UV-visible spectrum of PSF (c) Schematic of the mechanism of charging and storage in the OPSESC2.

4. Conclusion: In summary, our study revolves around the development of PFA thin film composites implementing a facile and low-cost technique and its efficacy in energy harvesting fields. Thorough characterizations of the synthesized films not only inveterate the effectual


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enhancement of the β-phase of PVDF (~ 71.4 %) but also confirmed the augmentation of its dielectric constant from 9 to 218 at 20 Hz. The bio-polymeric ligand FA, incorporated in various concentrations in PVDF, played the role of a successful modifying agent. Detailed dielectric study over a wide temperature range confirmed its thermal stability. An honest attempt to harness the renewable solar energy and designing a light-weight, flexible, low-cost and selfcharging OPSESC2 device using our synthesized high dielectric PFA film as the energy storage medium is reflected in our investigation. After being photo-irradiated, an open circuit output voltage ~1.0 V is obtained. Good repeatability of self-charge discharge cycles is also achieved. Superior energy density ~ 7.84 mWh/g and high specific capacitance of 61 F/g are attained in our fabricated device. Furthermore, our device is capable to light up commercially available LED representing its realistic applicability in our daily modern life. Supporting Information: A supplementary video of the glowing commercial Light Emitting Diode is provided as the Supporting Information. Acknowledgements: The authors wish to express gratitude to the University Grants Commission (UGC), Govt. of India and Department of Science and Technology, Govt. of India for providing fellowship and financial assistance devoid of which it would have been certainly difficult for us to execute our present investigation. S. Roy is highly obliged to her colleague Ms. Somtirtha Banerjee for her assistance during this work. References:



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GRAPHICAL ABSTRACT


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PVDF ... - ACS Publications

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