Kui Yao

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

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

Gate-controlled nonvolatile graphene-ferroelectric memory

In this letter, we demonstrate a nonvolatile memory device in a graphene field-effect-transistor structure using ferroelectric gating. The binary information, i.e., “1” and “0”, is represented by the high and low resistance states of the graphene working channels and is switched by controlling the polarization of the ferroelectric thin film using gate voltage sweep. A nonvolatile resistance change exceeding 200% is achieved in our graphene-ferroelectric hybrid devices. The experimental observations are explained by the electrostatic doping of graphene by electric dipoles at the ferroelectric/graphene interface.

High efficient photovoltaics in nanoscaled ferroelectric thin films

Photovoltaic effect in ferroelectric thin films with thickness below 100nm was investigated through both theoretical and experimental approaches. Unprecedented high photovoltaic power conversion efficiency around ∼0.28% was achieved with epitaxial (Pb0.97La0.03)(Zr0.52Ti0.48)O3 ferroelectric thin films, which is about 2 orders of magnitude higher than the reported in literature for ferroelectrics. Theoretical analysis indicated that efficiency can be further significantly improved by reducing the thickness in nanoscale. Extremely high efficient bulk photovoltaic effect is predicted in high quality ferroelectric ultrathin films.

Graphene field-effect transistors with ferroelectric gating.

Recent experiments on ferroelectric gating have introduced a novel functionality, i.e., nonvolatility, in graphene field-effect transistors. A comprehensive understanding in the nonlinear, hysteretic ferroelectric gating and an effective way to control it are still absent. In this Letter, we quantitatively characterize the hysteretic ferroelectric gating using the reference of an independent background doping (n(BG)) provided by normal dielectric gating. More importantly, we prove that n(BG) can be used to control the ferroelectric gating by unidirectionally shifting the hysteretic ferroelectric doping in graphene. Utilizing this electrostatic effect, we demonstrate symmetrical bit writing in graphene-ferroelectric field-effect transistors with resistance change over 500% and reproducible no-volatile switching over 10⁵ cycles.

Graphene-ferroelectric hybrid structure for flexible transparent electrodes.

Graphene has exceptional optical, mechanical, and electrical properties, making it an emerging material for novel optoelectronics, photonics, and flexible transparent electrode applications. However, the relatively high sheet resistance of graphene is a major constraint for many of these applications. Here we propose a new approach to achieve low sheet resistance in large-scale CVD monolayer graphene using nonvolatile ferroelectric polymer gating. In this hybrid structure, large-scale graphene is heavily doped up to 3 × 10(13) cm(-2) by nonvolatile ferroelectric dipoles, yielding a low sheet resistance of 120 Ω/□ at ambient conditions. The graphene-ferroelectric transparent conductors (GFeTCs) exhibit more than 95% transmittance from the visible to the near-infrared range owing to the highly transparent nature of the ferroelectric polymer. Together with its excellent mechanical flexibility, chemical inertness, and the simple fabrication process of ferroelectric polymers, the proposed GFeTCs represent a new route toward large-scale graphene-based transparent electrodes and optoelectronics.

Large strain and high energy storage density in orthorhombic perovskite (Pb0.97La0.02)(Zr1−x−ySnxTiy)O3 antiferroelectric thin films

Antiferroelectric (Pb0.97La0.02)(Zr1−x−ySnxTiy)O3 (PLZST) thin films with orthorhombic perovskite structure were prepared on Si substrates by a chemical solution deposition process. A secondary pyrochlore phase, which was not detectable with x-ray diffraction, was revealed with transmission electron microscopy. The pyrochlore phase was effectively suppressed by the introduction of polyethylene glycol (PEG) in the precursor solution and applying PbO capping layer on the surface of the films. With the persistent and detrimental pyrochlore phase removed completely, our PLZST antiferroelectric thin films exhibited excellent electrical and electromechanical properties. A large energy storage density up to 13.7 J/cm3 was exhibited from the polarization measurement, and a strain of 0.49% under the clamping of the substrate was also achieved in the thin film with high Zr content.

Crystallization mechanism and piezoelectric properties of solution-derived ferroelectric poly(vinylidene fluoride) thin films

β-phase dominant poly(vinylidene fluoride) (PVDF) ferroelectric thin films were obtained on silicon substrates by spin coating the precursor solutions with addition of hydrate salt and drying at an elevated temperature. The remnant polarization of the dense β-phase dominant PVDF thin film was 69.5mC∕m2. The apparent longitudinal piezoelectric coefficient d33 was −17.4pm∕V without taking into account the substrate clamping effect. It was suggested that the hydrate salt functioned as nucleation sites for the crystallization of PVDF molecules, and the intermolecular hydrogen bonding together with dipolar interactions promoted all-trans conformation and β-phase formation.

Nonlinear dielectric thin films for high-power electric storage with energy density comparable with electrochemical supercapacitors

Although batteries possess high energy storage density, their output power is limited by the slow movement of charge carriers, and thus capacitors are often required to deliver high power output. Dielectric capacitors have high power density with fast discharge rate, but their energy density is typically much lower than electrochemical supercapacitors. Increasing the energy density of dielectric materials is highly desired to extend their applications in many emerging power system applications. In this paper, we review the mechanisms and major characteristics of electric energy storage with electrochemical supercapacitors and dielectric capacitors. Three types of in-house-produced ferroic nonlinear dielectric thin film materials with high energy density are described, including (Pb_0.97La_0.02)(Zr_0.90Sn_0.05Ti_0.05)O₃ (PLZST) antiferroelectric ceramic thin films, Pb(Zn_1/3Nb_2/3)O_3-Pb(Mg_1/3Nb_2/3) O_3-PbTiO₃ (PZN-PMN-PT) relaxor ferroelectric ceramic thin films, and poly(vinylidene fluoride) (PVDF)-based polymer blend thin films. The results showed that these thin film materials are promising for electric storage with outstandingly high power density and fairly high energy density, comparable with electrochemical supercapacitors.

Measurement of longitudinal piezoelectric coefficient of thin films by a laser-scanning vibrometer

A laser scanning vibrometer (LSV) was used for the first time to measure the piezoelectric coefficient of ferroelectric thin films based on the converse piezoelectric effect. The significant advantages of the use of the LSV or this purpose were demonstrated. Several key points were discussed in order to achieve reliable and accurate results.

Piezoelectric K0.5Na0.5NbO3 thick films derived from polyvinylpyrrolidone-modified chemical solution deposition

Lead-free K0.5Na0.5NbO3 (KNN) ferroelectric films with enhanced thickness of 3.5 μm were prepared by a polyvinylpyrrolidone-modified chemical solution deposition method. A single perovskite phase with a dense morphology and (100) orientation was obtained at relatively low annealing temperature of 600 °C. A large effective piezoelectric coefficient d33, of 61 pm/V was demonstrated at 100 kHz without considering the substrate clamping effect. A well-saturated polarization hysteresis loop was obtained with a high remnant polarization Pr of 16.4 μC/cm2. These results showed that KNN is a promising lead-free piezoelectric film candidate, and that crystallizing the film at low processing temperature to obtain (100) orientation and dense morphology is critical to achieving excellent ferroelectric and piezoelectric properties.