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Flexible Piezoelectric Energy Harvesting Exploiting Biocompatible AlN Thin Films Grown onto Spin Coated Polyimide Layers Luciana Algieri, Maria Teresa Todaro, Francesco Guido, Vincenzo Mastronardi, Denis Desmaële, Antonio Qualtieri, Cinzia Giannini, Teresa Sibillano, and Massimo De Vittorio ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00820 • Publication Date (Web): 19 Sep 2018 Downloaded from http://pubs.acs.org on September 25, 2018

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ACS Applied Energy Materials

Flexible Piezoelectric Energy Harvesting Exploiting Biocompatible AlN Thin Films Grown onto Spin Coated Polyimide Layers Luciana Algieri *,†,§, Maria Teresa Todaro *,‡,†, Francesco Guido †, Vincenzo Mastronardi †, Denis Desmaële †, Antonio Qualtieri †, Cinzia Giannini ҩ, Teresa Sibillano ҩ, Massimo De Vittorio †,§. †

Istituto Italiano di Tecnologia (IIT), Center for Biomolecular Nanotechnologies, Via Barsanti, 73010 Arnesano, Italy.

§

Dipartimento Ingegneria dell’Innovazione, Università del Salento, via Monteroni, 73100 Lecce, Italy.

‡

Istituto di Nanotecnologia Consiglio Nazionale delle Ricerche NANOTEC, c/o Campus Ecotekne, via Monteroni, 73100 Lecce, Italy. ҩ

Istituto di Cristallografia, Consiglio Nazionale delle Ricerche, v. Amendola 122/O, 70126 Bari, Italy.

e-mail: [email protected]; [email protected] .

Keywords Flexible electronics, piezoelectric energy harvesting, aluminum nitride, thin films, piezoresponse force microscopy (PFM). Abstract The increasing demand of piezoelectric energy harvesters for wearable and implantable applications requires biocompatible materials and careful structural device design, paying special attention to the 1 ACS Paragon Plus Environment

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conformability characteristics, properly tailored to scavenge continuously electrical energy even from the tiniest body movements. The paper provides a comprehensive study on a flexible and biocompatible Aluminum Nitride (AlN) energy harvester based on a new alternative fabrication approach, exploiting a thin polyimide (PI) substrate, prepared by spin coating of precursors solution. This strategy allows manufacturing substrates with adjustable thickness to meet conformability requirements. The device is based on a piezoelectric AlN thin film, sputtered directly onto the soft PI substrate, without poling/annealing processes and patterned by simple and low cost microfabrication technologies. AlN active layer, grown on soft substrate, exhibits good morphological and structural properties with roughness of 6.35 nm, columnar texture and (002) c-axis orientation. Additionally, piezoelectric characterization has been performed and the extracted piezoelectric coefficient d33eff value of AlN thin film resulted to be 4.93 ± 0.09 pm/V. The fabricated flexible AlN energy harvester provides an output peak-to-peak voltage of ~1.4 V and a peak-to-peak current up to 1.6 µA, under periodical deformation, corresponding to a current density of 2.1 µA/cm2, and providing instantaneous power of 1.57 µW under optimal resistive load. Furthermore, the AlN energy harvester exhibits high elasticity and resistance to mechanical fatigue. High quality AlN piezoelectric layers on elastic substrates with tunable thicknesses pave the way for the development of a straightforward technological platform for wearable/implantable energy harvesters and biomechanical sensors.

1.

Introduction

Advances in both low power electronics and energy harvesting are opening new opportunities for the development of power sources able to replace traditional batteries for sustainable, maintenance-free and self-powered systems. Among exploitable environmental energy sources, mechanical energy (from vibrations, wind, sea waves or water currents) has a huge potential. Most importantly, it can be generated by human motions (arm swings, walking, running, breathing) or by biomechanical 2 ACS Paragon Plus Environment

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movements of muscles and organs inside the body (e.g., heartbeat, blood flow, eye blinking or muscle stretching, contraction/relaxation of the diaphragm and lungs, etc…). As a matter of fact, powering portable/wearable personal electronics or implantable biomedical devices (e.g., cardiac-tachometers, pacemakers, prosthetics) requires compact, biocompatible, flexible and compliant transducers. Piezoelectric energy harvesters (PEHs) are a promising class of devices due to their ability to directly convert the applied strain energy into electric energy. Additionally, major advantages are related to their compactness and simple architectures, to the straightforward micromachining fabrication techniques and to the possibility to directly integrate them into monolithic MEMS-scale systems. In this context, the development of flexible piezoelectric devices is particularly attractive, due to their ability to convert the biomechanical energy of extremely tiny body movements, by mechanically straining them by bending/unbending motions, into electricity. In order to achieve flexibility and compliance, all the materials of the piezoelectric stack must be as thin as possible, the lowest possible elastic modulus, in addition to biocompatibility and good piezoelectric response. Typically, piezoelectric thin films for energy harvesting possess perovskite or wurtzite crystal structures. Perovskite films exhibit good piezoelectric properties as a result of annealing

treatments at high temperatures and poling processes by applying high amplitude electric fields for long time

1, 2

. This prevents them from being synthesized directly onto soft substrate, complicating

significantly the device fabrication process. Indeed, complex transfer procedure of the grown layer, from the hard substrate to the soft receiving substrate, is employed. On the other side wurtzite films present moderate piezoelectric properties 3, are commonly use in CMOS technology and show biocompatibility 4. Additionally, being non ferroelectric materials, they can exhibit a permanent polarization along c-axis crystallographic direction depending on the growth conditions 5, without need of poling/annealing processes, thus enabling the direct growth of high quality piezoelectric films on soft substrates

6, 7

and the exploitation of MEMS technologies for device fabrication. Several works 3 ACS Paragon Plus Environment


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reported on flexible piezoelectric energy harvesters exploiting both Pb(Zr,Ti)O3 (PZT)-based layers even for implantable applications

1, 8-15

, and lead-free biocompatible BaTiO3

2

or (Na,K)NbO3

16

perovskite thin films. Other examples of flexible harvesters, based on wurtzite crystal structure piezoelectric materials, such as AlN and ZnO thin films, have been also reported. Typically the piezoelectric energy harvester structure consists of a piezoelectric film embedded into two electrode layers or covered with a patterned electrode film, prepared on a flexible elastic substrate. Despite the number of works published on this topic, the proposed flexible devices are based on commercially available soft foils acting as elastic substrates. We also reported on flexible force sensors 17

for robotics applications with improved performances and efficient energy harvesters operating under

ultralow cut-in wind speed

7

based on Kapton© substrates. However, this approach has some issues:

there are limitations in designing and realizing properly the flexible harvester to maximize the generated voltage/power and to meet conformability requirements for a targeted application. Theoretical models of PEHs and key figure of merit of such devices have been extensively reported in the literature, predicting the dependence of the electrical outputs on the device geometry and on the piezoelectric, dielectric and mechanical properties of the active layer and on the mechanical properties of the elastic substrate 18-21. As an example there is a direct dependence of the generated signals on the quality and crystalline orientation of the piezoelectric layers through their piezoelectric coefficients. It is to noteworthy that several numerical studies focused on the film to substrate ratio and on the elastic mismatch of the piezoelectric /elastic layers

22, 23

. These theoretical studies show the effect of such

parameters on the electric outputs of devices under bending moment giving design rules for high performance flexible and conformal unimorph harvesters. The employment of polymeric commercial foils as elastic layers limits strongly the possibility to validate experimentally these findings and to develop devices with optimal performances. This highlights that the choice of elastic/piezoelectric layers with proper mechanical properties and the possibility to tune both the piezoelectric layer and the 4 ACS Paragon Plus Environment

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elastic layer thicknesses, preserving good piezoelectric properties, are key parameters in engineering high performance flexible and conformable energy harvesters. Another point concerns the morphological properties of commercial soft substrates that often are below the requirements for obtaining high throughput/yield of the flexible devices processed in the same fabrication run. Low roughness surfaces of the elastic layers are demanded to guarantee good adhesion and flexibility of the metallic layers grown on the top 24, to allow the growth of high crystalline piezoelectric films 4, 25and to develop reliable devices having different architectures. In this paper we report a comprehensive experimental study on biocompatible AlN flexible harvester based on an alternative fabrication approach, exploiting a PI thin elastic layer, prepared by spin coated precursors solution. The device is based on a piezoelectric AlN thin film, sputtered directly onto PI substrate and patterned by simple and low cost microfabrication technologies. The PI derived from polyamic acid precursors solution has been chosen because exhibits a desirable combination of film properties such as lightness, deformability, low stress, low coefficient of thermal expansion, appropriate elastic modulus. Additionally PI layers can be easily fabricated of different thicknesses from a few micrometers up to tens of micrometers using spin coating technique and are compatible with the standard microfabrication processes. The work encompasses the study of the employed materials properties, the energy harvester fabrication and the energy generation analysis. We optimized a thin PI layer by controlling deposition and thermal curing processes and obtained a very smooth surface, desirable for a reliable device fabrication process. The piezoelectric AlN thin film integrated into this soft device has been investigated in terms of morphological and structural crystalline properties. Additionally, the piezoelectric behavior has been evaluated by Piezoresponce Force Microscopy (PFM) carrying out the effective piezoelectric coefficient (d33eff ) of the grown layer. The realized flexible harvester based on AlN film has been proven to generate energy under bending/unbending motions. Furthermore, the AlN energy harvester 5 ACS Paragon Plus Environment


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exhibited high elasticity and resistance to mechanical fatigue as evidenced by the long term stability measurements.

2.

Experimental section

1.1

Fabrication of AlN thin film PEH.

The PI 2611 polyimide precursors solution (HD Microsystems) has been deposited onto a carrier Si wafer through sequential spin coating processes at 1000 rpm for 30 s, followed by a soft bake performed on hot plate at 130° C for 180 s, to obtain a soft/flexible layer (thickness of 24 µm). The polyamic acid has been transformed into insoluble PI film by a curing process at high temperature performed into muffle furnace. The AlN interlayer (120 nm) and the Mo bottom electrode (200 nm) have been deposited onto the soft PI layer by sputtering technique in the same deposition run and patterned by optical lithography and chemical etching. AlN interlayer has been deposited using a pure Al target (99.9995%) in a mixture of Ar (20 sccm) and N2 (20 sccm) gases, in DC pulsed power supply at 750 W and with a working pressure of 2.8 × 10-3 mbar. Mo layer has been sputtered using a pure Mo target (99.95%) in pure Ar atmosphere (66 sccm) under DC power supply of 400 W and a working pressure of 5 × 10-3 mbar. The patterning of Mo bottom electrode has been performed by dry etching with ICP-RIE system using a gas mixture of BCl3 (45 sccm) and N2 (25 sccm) with a power applied to the platen and to the coil of 250 W and 600W, respectively. The AlN interlayer has been etched by an ICP-RIE etching system, using a gas mixture of BCl3 (100 sccm) and Ar (25 sccm) with a power applied to the platen and to the coil of 250 W and 600 W, respectively. The piezoelectric AlN thin film (1000 nm) and the Mo top electrode layer (200 nm) have been sputtered in the same run and patterned by optical lithography and dry chemical etching. The AlN film has been sputtered without heating of the substrate, using a pure Al target (99.9995%) in a mixture of Ar (20 sccm) and N2 (20 sccm) gases 6 ACS Paragon Plus Environment

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under direct current (DC) pulsed power supply of 1000 W (frequency of 100 KHz and pulse duration of 1µs) at a working pressure of 2.8 × 10-3 mbar. The Mo top layer has been sputtered in the same condition of the bottom electrode layer. The Mo top layer has been dry etched by an ICP-RIE etching system under the same conditions reported for the Mo bottom layer. The AlN piezoelectric layer has been etched by an ICP-RIE etching system, under the same conditions reported for AlN interlayer etching. At this stage the polymeric layer has been detached from the rigid Si substrate and each flexible device has been cut by a metal blade. The electrodes of each device have been connected through two aluminium clamps by using a crimping tool. 1.2

Characterization

The study of surface morphology of the deposited AlN film has been performed by Atomic Force Microscopy (AFM) measurements (CSI Nano-Observer AFM) in air using oscillating mode and FORT probes by AppNano. The structural properties of AlN piezoelectric film on PI layer have been investigated by a scanning electron microscope (NanoLab 600i SEM/FIB, FEI) for cross-sectional analysis and D8-Discover Bruker diffractometer, equipped with a Cu Roentgen tube for XRD pattern analysis. Piezoelectric properties and effective piezoelectric coefficient (d33eff) have been evaluated by Piezoresponse Force Microscopy (PFM) (CSI Nano-Observer AFM microscope) acquiring imaging and spectroscopy curves in air by selecting a frequency close to the contact resonance to amplify the piezoelectric signal. The platinum coated silicon conductive tips (spring constant of 3 N/m and radius of curvature (ROC) of 30 nm) has been used in contact mode with a frequency of about 280 kHz. An AC bias voltage has been applied between the tip and the sample and the PFM measurements have been performed considering only the vertical displacement. The output signal from the photodiode has

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been calibrated by a Lithium Niobate (LiNbO3) calibration sample consisting of periodically poled structures with known piezoelectric coefficient. Bending/unbending measurements onto flexible devices have been carried out by a built up linear bending system based on a linear micro-actuator characterized by a maximum stroke length of 20 mm and velocity of 28 mm/s with an internal potentiometer that can be used to provide position feedback. The linear micro-actuator has been controlled by ARDUINO© platform using a Linear Actuator Control board interface. The generated output signals as effect of the induced strain on the flexible AlN PEH have been carried out by an oscilloscope (Tektronix MDO 4104-3) for the detection of the open circuit voltage (VOC) and by a SourceMeter (2400 KEITHLEY), managed by the software LabTracer of the Tektronix, for the determination of the short-circuit current (ISC).

2

Results and Discussion

Device configuration consists of a flexible rectangular geometry with the active area covering a part of the elastic layer and positioned close to the hinge. The active area consists of a thin layer of AlN embedded between two Molybdenum (Mo) electrodes deposited by sputtering technique without heating of the substrate to avoid any thermal damage onto a thin PI substrate and patterned by optical lithography and chemical etching. Mo has been chosen as metal material for its interesting properties including the easiness of processing by wet/dry etching, its biocompatibility and great flexibility, making it suitable as electrode on polymeric substrates and the very small lattice mismatch (0.87%) with the AlN hexagonal structure 26, thus promoting the c-axis orientation of the active layer. Despite this, due to the amorphous or semi-amorphous nature of the polymeric substrate, a thin film interlayer has been necessary in order to promote the adhesion and crystalline quality of AlN piezoelectric film. Kamohara et al. reported that AlN crystallographic quality can be improved by using a thin interlayer of few tens of nanometers of AlN grown underneath the bottom electrode 27. Through this strategy it is 8 ACS Paragon Plus Environment

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possible to observe a local hetero-epitaxial growth of the grains of Mo and therefore of the piezoelectric film of AlN, starting from the AlN interlayer grains (AlN-IL). Figure 1a shows a schematic diagram of the fabrication steps of the flexible device. The PI layer has been obtained by sequential spin coating processes of the PI precursors onto a carrier rigid substrate, and optimized thermal curing treatment to obtain a thin layer having a thickness of 24 µm. This thin PI layer exhibited a smooth surface with roughness (Rms) of 1.60 nm and Young’s Modulus of 6.48 ± 0.01 GPa, in good agreement with the values reported in literature extrapolated in the same conditions28 (details in Supporting Information Figure S1 and S2). The AlN-IL/Mo layers have been deposited by sputtering technique onto the PI layer and patterned by optical lithography and dry chemical etching. Then, the AlN piezoelectric layer and the Mo top electrode layer have been sputtered in the same deposition run and subsequently patterned. Devices with active area of 76 mm2 and having size of 14 × 9.6 mm2 have been fabricated by cutting PI substrate and detaching it from the carrier rigid support. An optical photograph of the AlN-based PEH (Figure 1b) reveals that the device is flexible and robust. Scanning Electron Microscopy (SEM) has been conducted to investigate the structural properties of AlN(IL)/Mo/AlN/Mo multilayered structure grown directly on PI layer. Figure 1c illustrates the cross section view of the developed flexible PEH. Four layers can be clearly observed as they are piled up neatly. The Mo bottom layer adheres perfectly to the AlN interlayer and shows columnar texture. The same behavior has been observed for the AlN piezoelectric layer and for the Mo top metal layer in terms of columnar texture. The total thickness of the device resulted to be around 25 µm, comparable with the thinnest and super flexible piezoelectric PEHss reported in the literature 29. To characterize the surface morphology and crystalline quality of the AlN piezoelectric film grown on AlN(IL)/Mo layers, an atomic force microscope (AFM) and an X-Rays diffractometer (XRD) have been used. Figure 1d shows AFM topography image (5µm × 5µm) for AlN based structures grown on PI substrate. AlN piezoelectric layer, grown on AlN(IL)/Mo structure, exhibits a morphology with a pebble-like grain 9 ACS Paragon Plus Environment


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structure and dense surface with a roughness value of 6.35 nm, lower than what has been previously reported for piezoelectric AlN film grown on polymeric substrates 4. Figure 1e shows the XRD curve in θ/2θ configuration of the AlN(IL)/Mo/AlN film structure grown onto flexible PI layer. The XRD pattern shows a pronounced diffraction peak centered at 36°, corresponding to (002) crystal orientation of the AlN wurtzite structure. The full width at halfmaximum (FWHM) of the (002) AlN reflection is 0.24°. The XRD spectrum shows also a peak, centered at 40.55°, attributed to the Mo bottom layer with orientation along the (110) plane. This excellent crystalline properties observed are due to the deposition process, to the choice of the underlying layers and to the semi-crystalline properties of the used PI substrate. PFM has been used to study the polarization (out of plane direction) of AlN film and to determine the effective piezoelectric coefficient d33eff. Figure 2a shows a schematic view of the employed PFM measurement setup. Figure 2b shows the acquired Piezoresponse (PR) amplitude and phase images of the sputtered AlN based structures grown on PI substrate. The PR amplitude image provides the magnitude of the piezoelectric coefficient along the normal direction while the phase image provides information of domains polarization. The PFM phase image reveals a clear piezoelectric contrast associated with the polarization direction, with bright and dark contrasts. The bright contrast indicates that the polarization of the domains is upward and perpendicular to the surface, while the dark contrast corresponds to downward polarization. The upward and downward domains are almost 180° apart. Besides, gray contrast also occurs that indicates weak vertical piezoresponse. The gray contrast can be attributed to the polycrystalline nature of the film where its grains show various orientations with domains where the polarization vector deviates from the direction normal to the film plane. Figure 2c shows a local PFM spectroscopy where amplitude and phase have been recorded in a specific point of the domain region by applying a DC sweep voltage signal. The amplitude signal shows the deformation of the AlN 10 ACS Paragon Plus Environment

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crystalline cell under the effect of the applied electric field. When the applied voltage has positive values the AlN grain locally expands while when the voltage values become negative the material contracts and thus the phase has a switch of around 180°. The phase inversion theoretically should happen at zero voltage value and the amplitude response should be symmetric around this value. However surface charge defects generate local electric fields which translate the inversion at around 2 V. From piezoresponse amplitude curves carried out at different AC voltages ramped from 0 to 5 V (0 V DC bias) at different points of the scanned domain region, it can be extracted the effective piezoresponse coefficient d33eff. Figure 2d presents the Piezoresponse Amplitude versus the applied AC voltage for the AlN thin film grown on the PI layer. The effective piezoelectric coefficient d33eff has been determined from the slope of the piezoresponse amplitude (= vertical deflection × sensitivity) versus modulation voltage amplitude. The d33eff value extrapolated in this case resulted to be 4.93 ± 0.09 pm/V. This value is higher than that reported for AlN films grown on polymeric substrates

30, 31

and in good agreement with our previous founding values of AlN layer on Kapton© foils 6. Representative flexible devices undergoing folding/unfolding states have been characterized to assess the voltage/current generation properties providing a mechanical stimulus by a bending/unbending setup using a linear motor (Figure S3 in the Supporting Information). The thickness of the flexible substrate (~ 24 µm) is much larger than that of the AlN films (~ 1 µm) and therefore a tensile strain is introduced into flexible PEH after bending upward. The external force applied on the AlN based flexible device by linear motor is a compressive stress and the relative bending displacement is defined as:

ߝ=

∆௅ ௅

=

௅ି௅భ ௅

Where L is total length of the device before bending and L1 is the device length after bending.



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The performances of the flexible device are affected by the bending strain and therefore different displacements have been applied to investigate the device electromechanical response (Figure S4 in the Supporting Information). Figure 3a shows the flexible PEH mounted on the bending setup, fixed to its extremities, respectively in the position of unbending and bending, while the Figure 3b represents the forward and reverse connections. Figure 3c shows the open circuit voltage and short circuit current generated by the flexible AlN device from periodic bending/unbending motions with a bending displacement of 0.33 under forward and reverse connection. For both connections the signals have been acquired on a 10 s time scale. The flexible device provides a maximum peak-to-peak voltage of ~1.4 V and a peak-to-peak current up to 1.6 µA from mechanical deformation. In the switching polarity test the inversion of voltage and current signals is observed. In fact the positive and negative signals have been measured by bending and unbending motions, respectively, in forward connection. On the contrary in reverse connection the signal polarities are inverted. It can be noticed that in both connections the current peak is asymmetric during the bending and unbending deformations. Considering the PEH modeled as an equivalent RC circuit, this phenomenon could be attributed to different time constants as effect of charge relaxation processes. 32 To characterize the effective output power of the AlN thin film PEH, the flexible device has been connected to a variable resistive load and cyclically strained. The instantaneous power generated by the device can be calculated approximately by the peak voltage measurements across a resistive load, directly evaluated by the oscilloscope

33

. Figure 4 shows both the instantaneous voltage and power

signals as function of the external loads spanning from 18 kΩ to 1 MΩ under bending displacement of 0.33. The instantaneous voltage signal gradually build up as the resistance increase while the power reaches a maximum value of 1.57 µW corresponding to a power density of 2.07 µW/cm2 at the optimum load resistance close to 56 kΩ. 12 ACS Paragon Plus Environment

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To investigate the mechanical stability of the flexible PEH and its electrical reliability, the durability test is carried out by repeatedly bending and unbending motions for 1800 cycles corresponding to 1500 s. Figure 5 shows the long term-stability characterization of the open circuit voltage. This graph shows stable output voltage signal with slight deviation. This result demonstrates the resistance of the AlN flexible device to mechanical fatigue and therefore its promising employment for energy harvesting.

3

Conclusion

In summary, this manuscript reports a comprehensive study on a flexible and biocompatible energy harvester based on AlN thin film, developed starting from a spin coated PI. The optimized PI layer, as thin as 24 µm, exhibited a smooth surface with roughness of 1.60 nm and a Young’s Modulus of 6.48 ± 0.01 GPa. The AlN active layer grown directly on the PI substrate exhibited good morphological and structural properties with a roughness of 6.35 nm, columnar texture and (002) c-axis orientation. Additionally, from PFM performed on the thin layer, a piezoelectric coefficient d33eff of 4.93 ± 0.09 pm/V has been extracted. The fabricated flexible AlN energy harvester under periodical deformation, provided a peak-to-peak voltage of ~1.4 V, a peak-to-peak current up to 1.6 µA and a maximum generated power of 1.57 µW under optimal resistive load. Furthermore the AlN energy harvester exhibited high elasticity and resistance to mechanical fatigue, as confirmed from the long-term stability measurements. The possibility to adapt the AlN piezoelectric film to elastic substrate thickness ratio could open the way for the development of a straightforward technological platform for conformable high performance wearable/implantable sensors and energy harvesters.



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Associated Content Supporting information AFM topography image of PI substrate; Young’s Modulus extrapolation of PI substrate; schematic view of bending/ unbending setup, and power generation for different bending displacements.

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Graphical Abstract


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Figure 1. Flexible Piezoelectric Energy Harvester (PEH). Schematic illustration of overall fabrication for AlN based flexible PEH (a). Photograph of AlN based device (14 mm × 9.6 mm) bent by fingers (b). SEM image illustrating the cross-section view of the PEH (c). AFM image of AlN thin 19 ACS Paragon Plus Environment


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films on AlN(IL)/Mo layers and PI substrate (d). XRD pattern of AlN thin films on AlN(IL)/Mo layers and PI substrate (e).


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Figure 2. Contact PFM investigation for AlN based thin film. Schematic of the PFM setup where an AC voltage is applied between the tip and the bottom electrode while the deflection signal due to the polarization of thin film is measured using a lock-in amplifier (a). Vertical PFM amplitude image (b) and vertical PFM phase (c) of AlN thin film acquired at 5 V of tip AC voltage and 280 kHz. Local PFM Amplitude and Phase curves in function of bias voltage (-10 to 10 V) with the tip AC voltage is fixed at 21 ACS Paragon Plus Environment


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3.5 V (d). Average Piezoresponse versus the applied tip AC voltage with the linear fit used for piezoelectric coefficient d33eff evaluation (e).

Figure 3. Electrical measurement of the flexible PEH. The captured images of the flexible PEH under bending and unbending states (a). Schematic views of forward and reverse connections (b). Open 22 ACS Paragon Plus Environment

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circuit voltage and short circuit current of the device during periodic motions at bending displacement of 0.33 under forward and reverse connections (c).

Figure 4.The root mean square voltage (left) and the output maximum power (right) of the PEH versus different load resistances.

Figure 5. Long term-stability test of the open circuit voltage carried out by repeatedly bending and unbending motions for 1800 cycles corresponding to 1500 s.



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