The Preparation And Photovoltaic Properties Of Bismuth-based Ferroelectric Oxide Films

Bismuth-based ferroelectric oxides has been applied in medicine,catalyst, electric field for its unique properties, including ferroelectric,nonlinear optical effect, photoelectric and photocatalyst properties.Recently, BiFeO3(BFO) and BiVO4(BVO) has drawn increasing attentionin conversion light to electric for its relatively low band gap (2.2～2.8eV)in the visible range. BFO and BVO has gained increasing attention inferroelectric photovoltaic research. The shortage in fossil energy and thegreenhouse effect has further driven the photovoltaic research.This paper is aimed to synthesize BFO and BVO thin films depositedon fluorine doped tin oxides (FTO) glass substrates. The crystallinestructure and surface morphology of the films were measured. Thephotovoltaic effect of the BFO and BVO based thin film solar cells wereinvestigated. Based on the previous work, BFO thin film with nano-scalethickness was inserted between the BVO and FTO interface, the effect ofthe as-inserted BFO layer on the photovoltaic properties of BVO wasinvestigated.Main work and results were summarized as follows:（1） Polycrystalline BFO thin films were deposited on FTO glasssubstrates successfully by a modified chemical solution depositionmethods based on using N,N-Dimethylformamide (DMF) assolvent. Structural analyses show that DMF is beneficial to reducethe structural strain, and hence rhombohedral BFO films rather thanthe conventional pseudocubic ones were obtained. Absorptionspectrum indicates that the modified process derived rhombohedral BFO has a narrower band gap (2.54eV) than that of theconventional process derived pseudocubic BFO (2.76eV).Different electrodes, including Au, Indium tin oxides (ITO), andAl-doped tin oxide (AZO), were used to investigate thephotovoltaic properties of BFO thin films. Different top electrodescan have much effect on photovoltaic output of the BFO basedsolar cells. The BFO shows good photovoltaic properties mainlydue to its narrow band gap compared with conventionalpolycrystalline BFO thin films previously reported. The shortcircuit current density JSCand open circuit voltage VOCof theheterojunctions are measured to be130μA/cm2and0.65V,respectively, much higher than the reported values for theconventional process derived pseudocubic BFO basedheterojunction. AZO, as a low cost and promising transparentelectrode materials, can provide three times larger photovoltaiceffect (JSC=130μA/cm2, VOC=0.63V) than that with Au electrode(JSC=43μA/cm2, VOC=0.4V). The solar cell with AZO topelectrode can provide comparable photovoltaic output with that ofthe ITO ones, which indicates that AZO can be a promisingelectrode materials in ferroelectric film based solar cells.（2） BVO films deposited on FTO glass substrates were prepared bya green and economic chemical solution deposition processsuccessfully. The deposited BVO films consist entirely ofmonoclinic scheelite structure. The BVO films exhibit uniformsurface structure. Optical spectrum indicates that the depositedBVO film has a narrow band gap2.66eV. Photovoltaic propertiesof the BVO films were investigated by an Au/BVO/FTO structurefor the first time. The open circuit voltage and short circuit currentdensity are measured to be0.2V and63μA/cm2, respectively. Thestructure shows an obvious diode behavior due to an idealmetal-semiconductor Schottky barrier. The photovoltaic effect is mainly attributed to the Schottky barrier froming at the Au/BVOinterface.（3） Photovoltaic effect of BVO semiconductor was investigated byincorporation an ultrathin inorganic ferroelectric layer. This bilayerstructure shows unique properties with a reversed diode polarityand greatly enhanced efficiency due to an n-n heterojunction at theBVO/BFO interface and a polarization-induced Schottky-likebarrier on BFO/FTO. The photovoltage increases by5-fold to1Vand the photocurrent density increases by2.5-fold to140μA/cm2,in which the photovoltage is the highest compared with thereported values in polycrystalline and epitaxial ferroelectric thinfilm capacitors. Our work provides good guidance for thefabrication cost-effective semiconductor photovoltaic devices withhigh performance. And this kind of ultrathin ferroelectric film mayalso have promising applications in copper indium gallium selenidesolar cell, dye-sensitized TiO2solar cell, lighting emitting diode,and other photo-electron related devices.