Most piezoelectric energy harvesters utilize cantilever-type crystalline ferroelectric oxide thin films on rigid substrates, relying on vibrational energy sources. High-temperature processing (above 700 °C) is typically required for crystallization, limiting flexibility and compatibility with polymer substrates. Flexible harvesters offer advantages by utilizing various mechanical inputs like bending, stretching, and warping. Current methods for creating flexible harvesters involve complex and size-limited processes to transfer pre-prepared thin films onto flexible substrates, including exfoliation from mica, wet-etching of Si, laser lift-off, and stress-driven exfoliation from Ni. This research aims to overcome these limitations by developing a flexible piezoelectric energy harvester using perovskite CaCu3Ti4O12 (CCTO) thin films deposited directly onto a plastic substrate without post-annealing. While polycrystalline CCTO isn't typically considered a piezoelectric material, it exhibits an ultrahigh dielectric constant (>105 at low frequencies) and stable temperature dependence. This colossal dielectric permittivity is attributed to Maxwell-Wagner polarization at low frequencies, involving charge build-up at interfaces and grain boundaries. The amorphous state of CCTO, sputtered at room temperature, offers a promising alternative, with a high dielectric constant (~192), successfully exploited in amorphous thin-film transistors. The unusual permittivity in amorphous CCTO is suggested to be due to defect-dependent chemical states and short-range grain boundaries. Previous studies have indicated dipolar polarization in amorphous perovskite oxide films (e.g., amorphous BaTiO3 and SrTiO3), with higher Ti off-centered displacement than in crystalline counterparts due to weak orientational ordering of TiO6 octahedra. This study investigates the potential for high-performance power generation using amorphous CCTO thin films in a flexible device architecture, leveraging bending-driven electromechanical energy conversion. This approach avoids the complexities of transferring films to plastic substrates, potentially providing a significant advance in flexible energy harvesting.

The literature review highlights the existing limitations of traditional piezoelectric energy harvesters based on crystalline perovskite oxides such as PZT, (K,Na)NbO3, and (Bi,Na)TiO3. These materials require high-temperature processing for crystallization, typically on rigid Si substrates, making them incompatible with flexible applications. Several methods have been explored to integrate these films into flexible systems, including complex transfer processes such as exfoliation, wet etching, laser lift-off, and stress-driven exfoliation. These techniques often involve limitations in scale and complexity. The literature also discusses the unique properties of CaCu3Ti4O12 (CCTO), specifically its ultrahigh dielectric constant and the underlying mechanisms responsible for this property, which are primarily related to Maxwell-Wagner polarization and internal barrier layer capacitor effects. The review also delves into the existing knowledge on dipolar polarization in amorphous perovskite oxide films and the limited reports on effective piezoelectric coefficients measured by piezoresponse force microscopy (PFM) in such films. The relatively low piezoelectric coefficient values reported for amorphous perovskite oxide films in prior studies make this research into the use of amorphous CCTO particularly noteworthy.

Amorphous CCTO thin films were deposited onto polyethylene naphthalate (PEN) substrates coated with platinum using room-temperature RF magnetron sputtering from a stoichiometric CaCu3Ti4O12 target. Film thickness and oxygen partial pressure (pO2) were varied to optimize energy harvesting performance. Cross-sectional transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed the amorphous nature of the films. High-resolution X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical states of Cu, Ti, and O, revealing the coexistence of Cu+ and Cu2+, and Ti3+ and Ti4+, with their ratios dependent on pO2. The flexible piezoelectric energy harvester structure comprised PEN/indium tin oxide (ITO)/polydimethylsiloxane (PDMS)/CCTO/Pt/PEN. The top thin PEN layer helped to position the neutral plane, optimizing tensile strain in the CCTO film during bending. Piezoelectric energy harvesting performance was evaluated under various bending strains and frequencies. Piezoresponse force microscopy (PFM) was employed to characterize the piezoelectricity of the CCTO films, measuring piezoresponse amplitude and phase images at different pO2 and DC biases. The effective piezoelectric coefficient (d33,eff) was calculated from the PFM data. Electric displacement versus electric field measurements were performed using single-beam laser interferometry, and the effective transverse piezoelectric coefficient (e31,eff) was measured using a four-point bender unit. Dielectric constant (εr) was measured using an impedance analyzer over a wide frequency range to investigate polarization mechanisms. Complex impedance measurements were also conducted to analyze grain boundary and interfacial resistances. Finally, the energy harvesting performance was tested with and without poling under optimal bending conditions, and the results were compared with those of previously reported piezoelectric thin-film harvesters and polymer-matrix composite harvesters.

The study found that the amorphous CCTO thin films exhibited excellent piezoelectric energy harvesting performance, exceeding previously reported values for polycrystalline oxide thin-film cantilevers. The optimal oxygen partial pressure (pO2) significantly impacted harvesting performance; higher pO2 led to higher output voltage and current. Thicker films also resulted in stronger energy harvesting, likely due to reduced substrate interface influence. The optimal bending conditions were identified as a bending strain of 0.77% and a frequency of 3.10 Hz. Long-term stability tests demonstrated consistent output over 11,000 bending cycles. PFM analysis revealed a pO2-dependent piezoresponse, with the average effective piezoelectric coefficient (d33,eff) increasing from 1.73 ± 0.3 pm V-1 to 27.6 ± 4.4 pm V-1 with increasing pO2. The nearly stoichiometric film processed at the highest pO2 (4.0 mTorr) showed evidence of ferroelectricity. Laser interferometry measurements confirmed piezoelectricity, yielding effective d33 values of -28.5 pm V-1 (4.0 mTorr) and -18.1 pm V-1 (3.0 mTorr). A four-point bending measurement yielded an e31,eff of -1.1 C m-2 for the 4.0 mTorr sample. Dielectric measurements showed a strong pO2 dependence of the dielectric constant, with two polarization mechanisms identified: interfacial polarization at frequencies below ~105 Hz and dipolar polarization in the frequency range of ~104 to ~105 Hz. Applying an electric field of up to 120 kV cm-1 further enhanced the output, reaching peak values of -38.7 V and -1.24 μA. The maximum power output was 413 μW at a load resistance of ~106 Ω, and the power density was 2.8 × 106 μW cm-3. This performance significantly surpasses those of existing piezoelectric thin-film energy harvesters and even many piezoelectric composite harvesters, representing a record high.

The superior performance of the amorphous CCTO-based energy harvester is attributed to the combined effects of localized permanent dipoles of TiO6 octahedra and defect dipoles (Ti3+VO and Cu+Cu2+VO) within the amorphous structure. The higher oxygen partial pressure resulted in near-stoichiometric ratios and reduced oxygen vacancies, enhancing both dipolar and interfacial polarization. The bending operation potentially contributes to higher electromechanical conversion compared to vibrational modes by providing a larger input force. While a higher dielectric constant generally reduces the piezoelectric voltage coefficient, the significantly enhanced piezoelectric coefficient in this case outweighs this effect, resulting in a higher figure of merit (FOM). The observed superior performance compared to other piezoelectric thin-film and composite harvesters highlights the potential of this approach for practical applications. The direct deposition of amorphous CCTO films onto a flexible substrate simplifies the fabrication process, making it more scalable and cost-effective.

This study demonstrated a high-performance, flexible piezoelectric energy harvester based on amorphous CCTO thin films deposited directly on a plastic substrate. The achieved output voltage (-38.7 V), power (413 μW), and power density (2.8 × 106 μW cm-3) significantly exceed previously reported values for piezoelectric thin-film harvesters. The success is attributed to the unique properties of amorphous CCTO, including localized dipoles and a balance between enhanced piezoelectricity and dielectric constant. Future research could explore other amorphous perovskite materials, optimize film deposition parameters, and investigate device designs to further improve the energy harvesting performance and explore various applications in flexible electronics.

While the study demonstrates exceptional performance, several limitations should be considered. The PFM measurements, while providing valuable insights, might not fully reflect intrinsic piezoelectric coefficients due to measurement limitations. The influence of long-term environmental factors on the stability of the amorphous CCTO films under various operating conditions requires further investigation. The generalizability of these findings to other amorphous perovskite materials needs further exploration.