Ferroelectric Thin Film Photovoltaic Conversion Mechanism And Improve The Efficiency

The photovoltaic effect in ferroelectric films （FEFs） has recently drawn attentions.However, the photoelectric conversion efficiency in traditional electrode/FEFs/electrodedevices is as low as nA/cm². Both the mechanism and the efficiency behind thephotovoltaic effect of ferroelectric films need to be studied further. In this thesis, weselected the PZT and BNT films as examples to do the following two aspects of theresearch work:A. The mechanism of the photoelectric conversion in FEFs.（1） The interface effect. We investigated and compared the photoelectric behavior ofthe Pt sandwiched BNT and PZT films where the Pt upper electrodes were prepared bysputtering under room temperature. The two films exhibit nearly the same remnantpolarization, and thus the depolarization electric fields and the correspondingphotocurrents are nearly the same. However, the surface states of the two films aredifferent. PZT has a flat surface while BNT surface is rough. Based on the analysis of thedark I-V characteristics, the top and bottom Pt/PZT interface Schottky barriers aredetermined to be0.29eV and0.76eV, respectively. The top and bottom Pt/BNT barriers aremore symmetric （0.64eV vs.0.72eV）. Therefore, the photocurrent arised from theinterface barriers is larger for Pt/PZT/Pt. Furthermore, we investigated the experimentalevidence of top Pt/PZT interface layer thickness effect on the photocurrent. It was wellestablished before that the photocurrent of metal/ferroelectric film is attributed to theheight of Schottky contact barrier. However, our results suggest that the photocurrent ofPt/PZT interface contact is determined not only by the barrier height but also by theinterface layer thickness, namely, by the built-in electrical field at the interface layer. Themechanism behind such photocurrent phenomenon is proposed.（2） Based on the analysis of the photocurrent behavior of the Pt sandwiched BNTfilms deposited by sol-gel method, the mechanism of the polarization effect on the photocurrent of Pt sandwiched multi-crystalline ferroelectric films was clarified that, inferroelectric films irradiated by the extra light, the depolarization field directly gives morecontribution to the photocurrent when the polarization aligned under the external polingvoltage, while the variation of the top or bottom interface Schottky barriers, because of thepresence of the polarization charge near the top or bottom interface, have a indirect andsubordinate influence on the photocurrent.（3） It is widely accepted that ultraviolet （UV） light illumination of ferroelectric filmscan result in polarization imprint because of the accumulation of photoinduced carriers onthe domain walls and/or on the electrode–film interfaces, and then the decrease of thereversible remnant polarization. In this paper, however, the enhancement of remnantpolarization was exhibited in PZT films when irradiated by UV light. The time-dependentphotocurrent and hysteresis loop of PZT films indicated that the transient behavior ofphotocurrent and coercive voltage offset were closely related to the polarization states,moveable defect charge （mainly oxygen vacancy） density, and aging time. Based on theobservation of piezoresponse force microscopy, the mechanism behind the observedphotoelectric and ferroelectric phenomena was proposed.B. The improvement of photoelectric conversion efficiency of the FEFs.（1） In traditional metal/ferroelectric film/metal structure, it is widely accepted that thephotocurrent is originated from two factors: one is the remnant polarization, whichproduces a depolarization electric field extending over the whole film volume; the other isthe top or bottom film/metal interface Schottky barrier. However, the high reflection ofopaque or translucent metal electrodes, as well as almost symmetric Schottky barriers inthe top and bottom metal/film interfaces result in a low photovoltaic output. In this paper, atransparent indium tin oxide （ITO） electrode was introduced in ITO/PZT/Pt structure, inorder to not only make more incident light absorbed by PZT film but also artificiallyenlarge the Schottky barrier difference between bottom ITO/PZT interface and top PZT/Pt one. The results show that the photocurrent of ITO/PZT/Pt structure is enhanced one orderthan that of Pt/PZT/Pt structure under the same irradiation of a simulative sunlight （AM1.5G）.（2） Becasue of the interface barriers in metal/EFFs/metal are usually opposite, makingthe separation of the photocarriers not efficient. Here we demonstrate that the insertion ofan n-type cuprous oxide （Cu₂O） layer between the PZT film and the cathode Pt contact in aITO/PZT/Pt cell leads to the short-circuit photocurrent increasing120-fold to4.80mA/cm2and power conversion efficiency increasing of72-fold to0.57%under AM1.5G （100mW/cm2） illumination. Ultraviolet photoemission spectroscopy and dark J Vcharacteristic show an ohmic contact on Pt/Cu₂O, an n⁺n heterojunction on Cu₂O/PZTand a Schottky barrier on PZT/ITO, which provide a favorable energy level alignment forefficient electron-extraction on the cathode. Our work opens up a promising new methodthat has the potential for fulfilling cost-effective ferroelectric-film photovoltaic.