A Photoelectron Spectroscopy Study of Ethylenedioxythiophene Adsorption on Polycrystalline Gold Surfaces

Enhanced photovoltaic effect in BiVO4 semiconductor by incorporation with an ultrathin BiFeO3 ferroelectric layer

The photovoltaic effect of BiVO4 semiconductor was investigated by incorporating an ultrathin BiFeO3 ferroelectric layer. It is found that the ultrathin ferroelectric layer with strong self-polarization and high carrier density is desirable to enhance the photovoltaic effect and to manipulate the photovoltaic polarity of the semiconductors. The photovoltage increases by 5-fold to 1 V, and the photocurrent density increases by 2-fold to 140 μA/cm(2), in which the photovoltage is the highest compared with the reported values in polycrystalline and epitaxial ferroelectric thin film solar cells. The mechanism for the observed effect is discussed on the basis of a polarization-induced Schottky-like barrier at the BiFeO3/fluorine doped tin oxide interface. Our work provides good guidance for fabrication of cost-effective semiconductor photovoltaic devices with high performance, and this kind of ultrathin ferroelectric film may also have promising applications in copper indium gallium selenide solar cell, dye-sensitized TiO2 solar cell, lighting emitting diode, and other photoelectron related devices.

Evidence for oxygen vacancy or ferroelectric polarization induced switchable diode and photovoltaic effects in BiFeO3 based thin films

The diode and photovoltaic effects of BiFeO3 and Bi0.9Sr0.1FeO(3-δ) polycrystalline thin films were investigated by poling the films with increased magnitude and alternating direction. It was found that both electromigration of oxygen vacancies and polarization flipping are able to induce switchable diode and photovoltaic effects. For the Bi0.9Sr0.1FeO(3-δ) thin films with high oxygen vacancy concentration, reversibly switchable diode and photovoltaic effects can be observed due to the electromigration of oxygen vacancies under an electric field much lower than its coercive field. However, for the pure BiFeO3 thin films with lower oxygen vacancy concentration, the reversibly switchable diode and photovoltaic effect is hard to detect until the occurrence of polarization flipping. The switchable diode and photovoltaic effects can be explained well using the concepts of Schottky-like barrier-to-Ohmic contacts resulting from the combination of oxygen vacancies and polarization. The sign of photocurrent could be independent of the direction of polarization when the modulation of the energy band induced by oxygen vacancies is large enough to offset that induced by polarization. The photovoltaic effect induced by the electromigration of oxygen vacancies is unstable due to the diffusion of oxygen vacancies or the recombination of oxygen vacancies with hopping electrons. Our work provides deep insights into the nature of diode and photovoltaic effects in ferroelectric films, and will facilitate the advanced design of switchable devices combining spintronic, electronic, and optical functionalities.