Photovoltaic Effect in Poly(vinylidene fluoride)

Abstract

ABSTRACT:The generation of photocarriers and their transport processes in poled poly(vinylidene fluoride) films under light illumination are discussed from the viewpoint of a ferroelectric property of {3 form crystals. We observed not only a pyroelectric current but an additional current due to anomalous photovoltaic effect. The open circuit photovoltage was estimated as 4 x 10 4 V, which exceeds the band gap potential of PVDF more than three decades. Because of the high resistivity of PVDF, the conversion efficiency of light to electric energy is still quite low (ca. 0.25%), but further improvements are suggested.KEY WORDS Poly(vinylidene fluoride) I Ferroelectric Polymer Photovoltaic Effect I Pyroelectricity I Photocarrier Generation Photoelectric Current I Industrial applications of poly(vinylidene fluoride) (PVDF) have been extensive in recent years, resulting in remarkable product, such as electroacoustic transducers used in audio-speakers and microphones/ ultrasonic diagnostic equipment in the medical electronics/ ultrasonic hydrophones for the ocean technology, 3 and the photo-electric (or pyro-electric) transducers 4 for vidicon and IR sensors. These applications are based essentially on the ferroelectric property of PVDF, and its success in industrial application has made PVDF the most promising polymer as an "active material." Here we use the term "ferroelectric" for a polymer which undergoes a spontaneous (remanence) polarization and obeys Curie-Weiss' rule. A detailed morphological study of ferroelectric PVDF has also been carried out, 5 and it was clarified that the ferroelectricity of PVDF is associated with the crystal structure of the f3 form prepared, for example, by drawing or poling of the film.According to the works of Fridkin 6 and Glass et a/., 7 the anomalous photovoltaic effect has been observed in inorganic ferroelectric crystals such as BaTi0 3 and LiNb0 3 , in which open circuit photovoltage exceeds the band gap potential of the crystals by several orders of magnitude. This effect seems applicable for the conversion of solar energy into electric power. In this paper, we investigate the possibility of anomalous photovoltaic effect in a poled PVDF film, and discuss the generation of photocarriers and their transport in relation to photoconductive properties of PVDF. EXPERIMENTALUnder light excitation, electronic carriers (electrons or holes) are generated in a polymer film. If an electric field is applied across a sample, a current, observed in the external circuit, corresponds to a photocurrent !Ph' whereas if an electric field is absent, then the current corresponds to a shortcircuit photovoltaic current Ipv· We carried out measurements of !ph and Ipv on both unpoled and poled PVDF samples qS functions of temperature (T) and wavelength (A.) of the exciting light in the ranges of 20-l20°C and 300-700 nm. In the case of an !ph measurement, the applied voltage across the sample ranged from 0 to ± 600 V. Figure I shows a schematic diagram of the measuring system used in this study. A fil...

Photoelectric Properties of the Ferroelectric Polymer Poly(vinylidene fluoride)

“…The sign and magnitude of the current depend on the orientation of the light polarization vector with projections Ei and Ek. Taking into account the point group of the /3-phase of PVDF (mm 2) and the difference from zero components of the photovoltaic tensor 'Y.iik• we obtain from eq I the expression for Jz (the direction of the linear polarized light propagation coincides with z-axis) (2) where I is the light intensity, 8 is the angle between the light polarization plane and stretching axis and the z-axis is perpendicular to the film surface. The expression for Jz under illumination with natural light has the Figure 4 shows the kinetics of the shortcircuit photocurrent generated in the poled sample (type a) when the light is switched on and off.…”

“…The experimental results reported in these works are in discrepancy. The open-circuit saturation voltage for a 30-µm-thick film was estimated by Sasabe et al 2 to be 4 x 10 4 V, which exceeds the energy gap Eg of PVDF more than by three orders of magnitude (Eg""9eV). An analogous result for copolymer VDF-TrFE was obtained.…”

Polymers for Electronic Applications

“…the time evolution of short-circuited photocurrent lse) observed in the ferroelectric polymer PVDF. 12 The sharp rise of I se follo.wed by slow decay corresponds to the pyroelectric effect. At the …”

“…The time dependent current corresponds to the pyroelectric current, whereas the steady state current under photoexcitation corresponds to the anomalous photovoltaic current (after Sasabe et ale 12) thermal equilibrium state of the film under steady illumination, I sc reaches a steady state current corresponding to I apv • The reversal current observed after light off shows the same shape and absolute value (but opposite polarity) as I pyro' and hence corresponds to the pyroelectric current. Figure 8.…”

“…Figure 3 shows a typical measuring system for photocurrent/photovoltaics. 12 ._. In general the thickness of a depletion layer formed at the metal/insulator interface is less than 10 nm, hence it is necessary to prepare ultrathin films of polymers as insulating layers, preferably of thickness less than 20 nm.…”