Photovoltaic Properties Of Ferroelectric And Semiconductor Oxide Thin Films

Recent researches have witnessed another potential value of ferroelectrics in new solar cells,photodetectors and novel optoelectronic memory device,which can be attributed to the unique characteristic of ferroelectrics containing the reversal of spontaneous polarization with the external electric field and the open circuit voltage is much larger than the optical band gap.However,because of most ferroelectrics share wide bandgaps and poor conductivity,the photoelectric conversion efficiency in traditional electrode/ferroelectric film/electrode devices is as low as in the order of?A/cm².Considering about the key scientific questions emerged as the low photoelectric conversion efficiency and uncleared physical mechanism,we prepated ferroelectric film,semiconductor film and ferroelectric semiconductor composited film and tried to explore the physical mechamism of photovoltaic effect of these films.An effective method to enhance the photoelectric conversion efficiency of ferroelectric thin films is proposed.The main contents are as follows:1.Polycrystalline Bi₄Ti₃O₁₂?BTO?film was prepared on commercially fluorine-doped tin oxide?FTO?conductive glass substrate by a sol-gel method.The thickness,the root mean square surface roughness and the grain size of the film are approximately 400 nm,8.54 nm and 120 nm,respectively.It is found that the BTO thin films have good local ferroelectric and piezoelectric properties by the piezoresponse force microscope?PFM?.Moreover,the as-grown BTO film have upward self-polarization.The direct optical bandgap?E_g?is calculated from modified square law based bandgap calculations using the?????² vs??plot,by extrapolating the linear portion of the absorption to the X-axis,where absorption coefficient nears zero.The band gap of the BTO film is estimated to be about 3.45 eV.Ag and Pt metal electodes were deposited on the same BTO films at room temperature.Through the comaprasion of absorption curves for three kinds of strucrures of BTO/FTO,Pt/BTO/FTO and Ag/BTO/FTO,it is obvious that the absorption intensity of Ag/BTO/FTO structure is clearly larger than that of Pt/BTO/FTO structure.The light absorption enhancement phenomenon may be attributed to the easily formation of surface plasma for Ag nanoparticles combied to BTO film with natural rough surface.It was found that the photovoltaic responses of the devices strongly depended on the excitation light sources and top electrodes.The devices showed large ON/OFF photocurrent ratio with good stability and repeatability.Under the illumination of purple light,the photocurrent of the Ag/BTO/FTO device is about 0.4 mA/cm²,and the photoelectric conversion efficiency is about 0.45 V.We investigated the influence of metal top electrode and light source on the photoelectric conversion efficiency of BTO film and the mechanism of enhancement of photoelectric conversion efficiency were briefly discussed combined with the band gap,schottky barrier and surface plasmon effect.2.P-type Cu₂O epitaxial film has been fabricated on n-type?001?Nb-SrTiO₃?NSTO?single crystale by pulsed laser deposition?PLD?technique.The high resolution X-ray diffraction?XRD?and scanning electron microscopy?TEM?studies of the heterostructure revealed a cube-on-cube epitaxial relationship between?001?Cu₂O and?001?NSTO.The influence of deposition temperature,deposition time and Nb doping concentration on the photovoltaic properties of Cu₂O/NSTO heterojunction solar cells were investigated.A maximum photovoltaic output was observed in ITO/Cu₂O/NSTO?550 deg.,60 min,0.7wt%?devices under simulated sun light,in which V_oc,J_sc,fill factor and photoelectric conversion efficiency were about 0.45 V,1.1 mA/cm²,46%and 0.23%,respectively.When the film thickness is less than 76 nm,the photoelectric conversion efficiency increases with the thickness of Cu₂O films,when the film thickness is larger than 76 nm,the photoelectric conversion efficiency decreases with the increase of the thickness of Cu₂O films.The former can be attributed to the decrease of the resistivity and the improvement of crystallinity with the increase of the deposition time of Cu₂O thin films,while the latter is mainly attributed to the shorter lifetime of the current with the increase of the Cu₂O film thickness.It is found that the photovoltage,the current density,the fill factor and the photoelectric conversion efficiency amplitude increases with the content of Nb from 0.028 wt%to 0.7 wt%.Thus,it is suggested that the Nb doping concentration in single crystal substrates Nb-SrTiO₃ plays an important part in determining the photovoltaic output of Cu₂O films.3.The epitaxial BFTO thin films and heteroepitaxial BFTO/ZnO junction thin films were successfully prepared on?001?Nb-SrTiO₃?NSTO?single crystal substrate by PLD method.The crystal structure and microsture of these films have been characterized by high resolution XRD and TEM.These experimental results substantiated the highly crystalline quality,clearly layered structure and highly oriented epitaxy of these films.In addition,we also observed the domain structure of BFTO thin films with layered regular arrangement by high resolution TEM images.Based on structure of electrode/ferroelectric film/electrode,two back-to-back Schottky barriers are formed at the ferroelectric film/electrodes interface owing to the discrepancy between the workfunction of the ferroelectric film and the bottom or topelectrodes.By inserting a ZnO semiconductor layer with spontaneous polarization into the structure of Ag/BFTO/NSTO ferroelectric thin films,we formed Ag/ZnO/BFTO/NSTO heterojunction devices.The short-circuit photocurrent increased by 50-fold to 2.2 mA/cm²,and the power density increased by 15-fold to 0.09mW/cm² under purple light.We tested the performance of the Ag/BFTO/NSTO devices and the Ag/ZnO/BFTO/NSTO heterojunction devices by pulsed voltage respectively,and found that the two devices have photovoltaic performance polarization tunability.In addition,the photovoltaic properties of ferroelectric thin films were improved by adding ZnO layer without changing the polarization tunable properties of ferroelectric thin films.We combined with the experimental results,the band theory of semiconductor and the characteristics of ferroelectric thin films to establish the schematic interpretation,in order to understand the physical mechanism of improved photoelectric conversion efficiency for Ag/ZnO/BFTO/NSTO heterojunction.This work lay a foundation for the pratical appliations of new ferroelectric/semiconductor heterojunction film devices.