Sung Sik Won

Antiferroelectric Thin-Film Capacitors with High Energy-Storage Densities, Low Energy Losses, and Fast Discharge Times

We demonstrate a capacitor with high energy densities, low energy losses, fast discharge times, and high temperature stabilities, based on Pb(0.97)Y(0.02)[(Zr(0.6)Sn(0.4))(0.925)Ti(0.075)]O3 (PYZST) antiferroelectric thin-films. PYZST thin-films exhibited a high recoverable energy density of U(reco) = 21.0 J/cm(3) with a high energy-storage efficiency of η = 91.9% under an electric field of 1300 kV/cm, providing faster microsecond discharge times than those of commercial polypropylene capacitors. Moreover, PYZST thin-films exhibited high temperature stabilities with regard to their energy-storage properties over temperatures ranging from room temperature to 100 °C and also exhibited strong charge-discharge fatigue endurance up to 1 × 10(7) cycles.

Lead-free Mn-doped (K0.5,Na0.5)NbO3 piezoelectric thin films for MEMS-based vibrational energy harvester applications

Lead-free Mn-doped (K0.5, Na0.5)NbO3 (KNN) thin films were fabricated by the chemical solution deposition method. The addition of small concentration of Mn dopant effectively reduced the leakage current density and enhanced the piezoelectric properties of the films. The leakage current density of 0.5 mol. % Mn-doped KNN film showed the lowest value of ∼10-7 A/cm2 at 10 V compared to the films with other doping concentrations and the piezoelectric d33 and e31 coefficients of this film were ∼90 pm/V and −8.5 C/m2, respectively. The maximum power and power density of the lead-free thin film-based vibrational energy harvesting device were 3.62 μW and 1800 μW/cm3 at the resonance frequency of 132 Hz and the acceleration of 1.0 G. The results prove that the 0.5 mol. % Mn-doped KNN film is an attractive candidate transducer layer for the piezoelectric MEMS energy harvesting device applications with a small volume and a long-lasting power source.

Flexible Pb(Zr0.52Ti0.48)O3 Films for a Hybrid Piezoelectric-Pyroelectric Nanogenerator under Harsh Environments

In spite of extremely high piezoelectric and pyroelectric coefficients, there are few reports on flexible ferroelectric perovskite film based nanogenerators (NGs). Here, we report the successful growth of a flexible Pb(Zr0.52Ti0.48)O3 (PZT) film and its application to hybrid piezoelectric-pyroelectric NG. A highly flexible Ni-Cr metal foil substrate with a conductive LaNiO3 bottom electrode enables the growth of flexible PZT film having high piezoelectric (140 pC/N) and pyroelectric (50 nC/cm(2)K) coefficients at room temperature. The flexible PZT-based NG effectively scavenges mechanical vibration and thermal fluctuation from sources ranging from the human body to the surroundings such as wind. Furthermore, it stably generates electric current even at elevated temperatures of 100 °C, relative humidity of 70%, and pH of 13 by virtue of its high Curie temperature and strong resistance for water and base. As proof of power generation under harsh environments, we demonstrate the generation of extremely high current at the exhaust pipe of a car, where hot CO and CO2 gases are rapidly expelled to air. This work expands the application of flexible PZT film-based NG for the scavenging mechanical vibration and thermal fluctuation energies even at extreme conditions.

Piezoelectric poly(vinylidene fluoride trifluoroethylene) thin film-based power generators using paper substrates for wearable device applications

Solution-derived poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) piezoelectric thin films on cellulose paper substrates were prepared as flexible power generators for wearable device applications. Optimization of appropriate annealing and cooling sequences of the co-polymer films resulted in the formation of dense and uniform microstructures exhibiting a well-developed β-phase. A maximum open-circuit voltage of 1.5 V was generated from the periodic bending and releasing of the paper power generator at approximately 1 Hz. To demonstrate the wearable applications, P(VDF-TrFE) piezoelectric film-based paper power generators were directly attached on the back of a human hand, and they generated a maximum output open-circuit voltage of 0.4 V at low bending frequencies of 0.25 Hz. Good open-circuit voltage performance at low frequencies makes P(VDF-TrFE) piezoelectric thin films on paper substrates a strong candidate for future self-powered wearable devices.

BiFeO3-doped (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 ferroelectric thin film capacitors for high energy density storage applications

Environmentally benign lead-free ferroelectric (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 (KNMN) thin film capacitors with a small concentration of a BiFeO3 (BF) dopant were prepared by a cost effective chemical solution deposition method for high energy density storage device applications. 6 mol. % BF-doped KNMN thin films showed very slim hysteresis loops with high maximum and near-zero remanent polarization values due to a phase transition from the orthorhombic structure to the pseudo-cubic structure. Increasing the electric field up to 2 MV/cm, the total energy storage density (Jtotal), the effective recoverable energy density (Jeff), and the energy conversion efficiency (η) of lead-free KNMN-BF thin film capacitors were 31.0 J/cm3, 28.0 J/cm3, and 90.3%, respectively. In addition, these thin film capacitors exhibited a fast discharge time of a few μs and a high temperature stability up to 200 °C, proving their strong potential for high energy density storage and conversion applications.

Photovoltaic effect of lead-free (Na0.82K0.18)0.5Bi4.5Ti4O15 ferroelectric thin film using Pt and indium tin oxide top electrodes

We have grown a Bi-layer structure (Na0.82K0.18)0.5Bi4.5Ti4O15 (NKBiT) ferroelectric thin film on Pt(111)/TiO2/SiO2/Si(100) substrate by using the chemical solution deposition method and deposited two kinds of thin Pt and indium tin oxide (ITO) top electrodes. The photovoltaic behaviors of Pt/NKBiT/Pt and ITO/NKBit/Pt capacitors were investigated over the wavelength range of 300–500 nm. When NKBiT thin film is illuminated by the corresponding wavelength of the films energy band gap (Eg), a photocurrent is generated due to the Schottky barrier between electrode and film, and an internal electric field is originated by the depolarization field. The maximum photocurrent density and power conversion efficiency of the ITO/NKBiT/Pt capacitor in the poled-up state are obtained as 45.75 nA/cm2 and 0.035%, respectively, at 352 nm. The photocurrent density and power conversion efficiency of the ITO/NKBiT/Pt capacitor increased to 3.5 times higher than that of the Pt/NKBiT/Pt capacitor.

Flexible high energy density capacitors using La-doped PbZrO3 anti-ferroelectric thin films

Flexible high energy density capacitors were fabricated by depositing 6 mol. % La-doped anti-ferroelectric PbZrO3 thin films using chemical solution deposition on ultra-thin metal foil substrates. The integration of a LaNiO3 buffer layer on the flexible austenitic metal foil substrate resulted in substantial improvements in microstructure uniformity and density of the La-doped PbZrO3 thin film capacitors. The recoverable energy density and the energy loss of the flexible thin film capacitors were 15.2 ± 0.2 J/cm3 and 4.8 ± 0.2 J/cm3 at 1 MV/cm, respectively. The La-doped PbZrO3 flexible thin film capacitors showed no noticeable degradation in ferroelectric hysteresis behavior and the energy storage density even after 1000 bending cycles.