Dawei Cao

High-Efficiency Ferroelectric-Film Solar Cells with an n-type Cu2O Cathode Buffer Layer

Because of the existence of interface Schottky barriers and depolarization electric field, ferroelectric films sandwiched between top and bottom electrodes are strongly expected to be used as a new kind of solar cells. However, the photocurrent with a typical order of μA/cm(2) is too low to be practical. Here we demonstrate that the insertion of an n-type cuprous oxide (Cu(2)O) layer between the Pb(Zr,Ti)O(3) (PZT) film and the cathode Pt contact in a ITO/PZT/Pt cell leads to the short-circuit photocurrent increasing 120-fold to 4.80 mA/cm(2) and power conversion efficiency increasing of 72-fold to 0.57% under AM1.5G (100 mW/cm(2)) illumination. Ultraviolet photoemission spectroscopy and dark J-V characteristic show an ohmic contact on Pt/Cu(2)O, an n(+)-n heterojunction on Cu(2)O/PZT and a Schottky barrier on PZT/ITO, which provide a favorable energy level alignment for efficient electron-extraction on the cathode. Our work opens up a promising new method that has the potential for fulfilling cost-effective ferroelectric-film photovoltaic.

Interface effect on the photocurrent: A comparative study on Pt sandwiched (Bi3.7Nd0.3)Ti3O12 and Pb(Zr0.2Ti0.8)O3 films

We investigated and compared the photoelectric behavior of the Pt sandwiched (Bi3.7Nd0.3)Ti3O12 (BNT) and Pb(Zr0.2Ti0.8)O3 (PZT) films deposited by sol-gel method. Based on the analysis of the photocurrent and I-V characteristics, the top and bottom Pt/film interface Schottky barriers are found to be more symmetric in BNT, compared to that in PZT. The photocurrents originated from the depolarization field due to the alignment of ferroelectric polarization are different, although the polarizations of the two films are about the same. The mechanism behind the origin of the different photocurrent behaviors between Pt/BNT/Pt and Pt/PZT/Pt capacitors was discussed.

Space charge effect on the photocurrent of Pt-sandwiched Pb(Zr0.20Ti0.80)O3 film capacitors

The space charge effect on the photocurrent of the Pt-sandwiched Pb(Zr0.20Ti0.80)O3 (PZT) films was investigated. The films were prepared under different annealing temperatures and atmospheres to control the densities of space charges in the PZT films. By testing the short-circuit photocurrent after the films were poled under different voltages and correlating the loops of the static photocurrent-poling voltage with the dynamic polarization-switching voltage, it was manifested that the space charges, especially the time-dependent trapped charges, affected significantly the photoelectric properties of the PZT films through an indirect way of influencing the Schottky barrier in the Pt/PZT/Pt capacitors.

Interface layer thickness effect on the photocurrent of Pt sandwiched polycrystalline ferroelectric Pb(Zr,Ti)O3 films

Based on the analysis of the photocurrent behavior of Pt sandwiched Pb(Zr0.2Ti0.8)O3 (PZT) films, the experimental evidence of top Pt/PZT interface layer thickness effect on the photocurrent is reported. It was well established before that the photocurrent of metal/ferroelectric film is attributed to the height of Schottky contact barrier. However, our results suggest that the photocurrent of Pt/PZT interface contact is determined not only by the barrier height but also by the interface layer thickness, namely, by the built-in electrical field at the interface layer. The mechanism behind such photocurrent phenomenon is proposed.

Understanding the nature of remnant polarization enhancement, coercive voltage offset and time-dependent photocurrent in ferroelectric films irradiated by ultraviolet light

It is widely accepted that ultraviolet (UV) light illumination of ferroelectric films can result in polarization imprint because of the accumulation of photoinduced carriers on the domain walls and/or on the electrode–film interfaces, and then the decrease of the reversible remnant polarization. In this paper, however, the enhancement of remnant polarization was exhibited in Pb(Zr0.2Ti0.8)O3 (PZT) films when irradiated by UV light. The time-dependent photocurrent and hysteresis loop of PZT films indicated that the transient behavior of photocurrent and coercive voltage offset were closely related to the polarization states, moveable defect charge (mainly oxygen vacancy) density, and aging time. Based on the observation of piezoresponse force microscopy, the mechanism behind the observed photoelectric and ferroelectric phenomena was proposed.