Bee Keen Gan

Bio: Bee Keen Gan is an academic researcher from Agency for Science, Technology and Research. The author has contributed to research in topics: Ferroelectricity & Thin film. The author has an hindex of 10, co-authored 11 publications receiving 374 citations.

Phase transition and properties of a ferroelectric poly(vinylidene fluoride-hexafluoropropylene) copolymer

Abstract: The phase transition, ferroelectric, and piezoelectric properties of a poly(vinylidene fluoride-hexafluoropropylene) [PVDF∕HFP (10 mol %)] copolymer have been investigated. A rapid quenching process led to the formation of the ferroelectric β phase in the PVDF∕HFP (10 mol %). An irreversible crystal phase transition from the β phase to the α and γ phases at a temperature from 120 to 130 °C, which is below its melting point 159 °C, was observed through x-ray diffraction (XRD) experiments. Stretching the melt-quenched material or annealing the material at a temperature below the crystal phase transition temperature resulted in a significant increase in the β phase. Based on the XRD results, the stretched samples had oriented β phase, but the amount of β phase is less than that in the annealed samples; however, our ferroelectric property measurements showed that the stretched samples had a higher polarization than the annealed samples. A theoretical analysis was conducted to clarify the effects of the polyme...

Large photo-induced voltage in a ferroelectric thin film with in-plane polarization

Abstract: A large photoinduced voltage of 7 V was obtained with an in-plane poled ferroelectric thin film having a composition of WO3 modified Pb0.97La0.03(Zr0.52Ti0.48)O3 (PLWZT), under ultraviolet (UV) illumination for about 80 s. By poling the ferroelectric film along the surface plane through pairs of interdigital electrodes, the interelectrode distance constraint on the voltage magnitude arising from the small film thickness was broken. Our experimental results showed that both the direction and magnitude of the photovoltaic output could be tuned with the poling electric field. The dependence of the photoinduced voltage on light wavelength, light intensity, and the gap of the interdigital electrode were investigated. The advantages of the photovoltaic configuration on the basis of an in-plane poled ferroelectric thin film were analyzed.

Stability of photovoltage and trap of light-induced charges in ferroelectric WO3-doped (Pb0.97La0.03)(Zr0.52Ti0.48)O3 thin films

Abstract: The stability of photovoltage in WO3-doped (Pb0.97La0.03)(Zr0.52Ti0.48)O3 (PLWZT) ferroelectric thin films was investigated. For in-plane polarized configuration, with a greatly enhanced electrode gap, the reduction ratio of photovoltage during multicycle UV illumination was significantly smaller and stability of photovoltage was greatly improved over the sandwich capacitor configuration. The ferroelectric-metal interfacial effects including Schottky barriers and polarization screening due to the trap of photoinduced charges at interfaces were found to determine the magnitude, stability, and even the polarity of the photovoltage, particularly for the electrode-sandwiched PLWZT thin films.

An Ultraviolet (UV) Detector Using a Ferroelectric Thin Film With In-Plane Polarization

Abstract: Different from the semiconductor photovoltaic device on the basis of an interfacial effect, UV detection was realized in this letter by using bulk photovoltaic mechanism involving the depolarization field in a W-doped Pb0.97La0.03(Zr0.52Ti0.48)O3 (PLWZT) ferroelectric thin film. A prototype of UV detector was demonstrated in the laboratory as well as under sunlight, in which the ferroelectric PLWZT thin film was polarized in the plane of the film surface. Characteristics of the photovoltaic response of the ferroelectric thin films and their application values for UV detection were analyzed.

Ferroelectric poly(vinylidene fluoride-hexafluoropropylene) thin films on silicon substrates

Abstract: Ferroelectric poly(vinylidene fluoride-hexafluoropropylene) (PVDF/HFP) thin films were prepared through a solution coating approach on aluminum-coated silicon substrates. The crystalline phase and morphology of the obtained PVDF/HFP thin films strongly depended on the drying temperature for the spin-coated films and chemical additives. When magnesium nitrate hexahydrate (Mg(NO 3 ) 2 ·6H 2 O) was introduced into the solution precursor, dominant β phase, the desired ferroelectric phase, was formed in the thin films. Their ferroelectric and piezoelectric properties were investigated. The results indicate that our methods have overcome a critical barrier for integrating ferroelectric PVDF/HFP thin films for device applications.

Mutual Insight on Ferroelectrics and Hybrid Halide Perovskites: A Platform for Future Multifunctional Energy Conversion.

Abstract: An insight into the analogies, state-of-the-art technologies, concepts, and prospects under the umbrella of perovskite materials (both inorganic-organic hybrid halide perovskites and ferroelectric perovskites) for future multifunctional energy conversion and storage devices is provided. Often, these are considered entirely different branches of research; however, considering them simultaneously and holistically can provide several new opportunities. Recent advancements have highlighted the potential of hybrid perovskites for high-efficiency solar cells. The intrinsic polar properties of these materials, including the potential for ferroelectricity, provide additional possibilities for simultaneously exploiting several energy conversion mechanisms such as the piezoelectric, pyroelectric, and thermoelectric effect and electrical energy storage. The presence of these phenomena can support the performance of perovskite solar cells. The energy conversion using these effects (piezo-, pyro-, and thermoelectric effect) can also be enhanced by a change in the light intensity. Thus, there lies a range of possibilities for tuning the structural, electronic, optical, and magnetic properties of perovskites to simultaneously harvest energy using more than one mechanism to realize an improved efficiency. This requires a basic understanding of concepts, mechanisms, corresponding material properties, and the underlying physics involved with these effects.

High energy density nanocomposites based on surface-modified BaTiO(3) and a ferroelectric polymer.

Abstract: The dielectric permittivity and electric breakdown strength of nanocomposites comprising poly(vinylidene fluoride-co-hexafluoro propylene) and phosphonic acid surface-modified BaTiO3 nanoparticles have been investigated as a function of the volume fraction of nanoparticles. The mode of binding of pentafluorobenzylphosphonic acid on the BaTiO3 particles was investigated using infrared and 31P solid-state nuclear magnetic resonance spectroscopy, and the phosphonic acid was found to form well ordered, tightly bound monolayers. The effective permittivity of nanocomposites with low volume fractions (<50%) was in good agreement with standard theoretical models, with a maximum relative permittivity of 35. However, for nanoparticle volume fractions of greater than 50%, the effective permittivity was observed to decrease with increasing nanoparticle volume fraction, and this was correlated with an increase in porosity of the spin-coated nanocomposite films. The dielectric breakdown strength was also found to decre...

Photovoltaic Switching Mechanism in Lateral Structure Hybrid Perovskite Solar Cells

Abstract: In this study, long range electromigration of methylammonium ions (MA+) in methyl ammonium lead tri-iodide (MAPbI3) film is observed directly using the photo­thermal induced resonance technique. The electromigration of MA+ leads to the formation of a lateral p-i-n structure, which is the origin of the switchable photovoltaic effect in MAPbI3 perovskite devices.

Bulk Photovoltaic Effect at Visible Wavelength in Epitaxial Ferroelectric BiFeO3 Thin Films

Abstract: Adv. Mater. 2010, 22, 1763–1766 2010 WILEY-VCH Verlag G T IO N While silicon-based diodes have been the dominant solar cell type, novel photovoltaic mechanisms are being explored in pursuit of lower cost or improved efficiency. In a semiconductor photodiode, such as a Si solar cell, photons with energy higher than the band gap are absorbed to produce electron-hole pairs, which are separated by the internal field in the p–n junction and collected with the electrodes. However, a p–n junction is not a prerequisite for the photovoltaic effect. For exitonic solar cells, photon absorption creates excitons, which dissociate at a heterojunction. In materials without a center of symmetry, such as ferroelectric materials, steady-state photocurrent can exist in a homogeneous medium under uniform illumination, a phenomenon called bulk photovoltaic effect (BPVE). BPVE is a fascinating mechanism with many unique features such as extremely large photovoltage, a photocurrent proportional to the polarization magnitude, and charge-carrier separation in homogeneous media. Observed in bulk ferroelectrics in as early as 1950s, BPVE has seen a resurgent interest recently, especially in ferroelectric thin films. It has been proposed that remarkably higher photovoltaic efficiency can be achieved in thin films. On the other hand, open-circuit voltage much larger than the band gap has also been achieved with ferroelectric thin films with in-plane interdigital electrodes, which has led to the development of UV sensors and dosimeters. The ferroelectric thin-film materials under the previous study, such as BaTiO3 and Pb(ZrTi)O3, have wide band gaps (typically larger than 3.3 eV) corresponding to the UV region. BPVE in visible wavelength could lead to the development of new photovoltaic cells or other novel optoelectronic devices. BiFeO3 (BFO), a multiferroic material at room temperature with a band gap near 2.74 eV and a very large remnant ferroelectric polarization, offers a unique opportunity for such an investigation. Appreciable photoconductivity in visible light has been reported in BFO. Optical studies by absorption spectroscopy and spectroscopic ellipsometry have shown that BFO has a direct band gap with high absorption coefficient. Recently, a switchable-diode effect and a visible-light photovoltaic effect has been observed in BFO bulk crystals. However, no value of photovoltage has been reported for BFO single crystals and significant bulk photovoltaic response has not been demonstrated in BFO thin film. It is also unclear if the photovoltaic response in BFO is due to the diode effect. Here, we studied the photovoltaic effect in epitaxial BFO thin films and obtained an open-circuit voltage Voc of 0.3 V. We further demonstrated that photocurrent direction can be switched by the polarization direction of the BFO film and that the ferroelectric polarization is the main driving force of the observed photovoltaic effect. Moreover, the as-deposited BFO films were self-polarized and they could readily function as a photovoltaic cell without any poling. Epitaxial BFO thin films of 170 nm were grown by radiofrequency (RF) magnetron sputter deposition on a (001)c SrTiO3 (STO) substrate, with a 60-nm layer of SrRuO3 (SRO) as the bottom electrode. The resulting films show good epitaxy as determined by high-resolution X-ray diffraction (HRXRD; Supporting Information, Fig. S1). The polarization–electric field (P–E) hysteresis measurement shows a remnant polarization (Pr) of more than 65mCcm 2 with a Au top electrode (Fig. S2). Devices with an indium tin oxide (ITO) top electrode have a slightly smaller Pr. Figure 1a shows the spectral response of the short-circuit current (Jsc) of the BFO film. Highest current density is detected at 460 nm, closely corresponding to the measured BFO band gap of 2.72 eV (Fig. S3). Incident light at 435 nm, slightly above band gap, was used for the current-density–voltage (J–V) measurement (Fig. 1b). The as-deposited samples were electrically poled before measurement. The poling direction is termed positive if a positive bias voltage is applied to the top electrode with the bottom electrode grounded. In the J–Vmeasurement, the applied voltage is positive if a positive bias voltage is applied to the bottom electrode. Fig. 1b shows that for the positively poled samples the photocurrent is positive (i.e., it flows out of the top electrode). In contrast, after the negative poling, the photocurrent direction is reversed. The magnitudes of both the photocurrent and photovoltage are smaller in positively poled samples than in negatively poled ones. Jsc is observed to increase almost linearly with the illumination intensity (Fig. 1c), whileVoc saturates at high illumination intensity (Fig. 1d). At the highest illumination intensitymeasured,Voc in the negatively poled film of 170-nm thickness is 0.286V. The substantialVoc obtainedhere is probably a result of the low conductivity of our samples, which is on the order of 10V 1 cm , six orders of magnitude smaller than that reported by Basu et al. and also much smaller than that reported by Choi. The photovoltaic response for the as-deposited films without any poling was also measured. The results are surprisingly