Photoelectric Conversion Properties Of Pmn-pt Single Crystals And Thin Films

Ferroelectric-semiconductors possess both ferroelectricity and semiconductor.Under light, they absorb photons, and then produce photon-induced carriers separated by their own polarization electric field, and directional move to the electrode layers,resulting in a photovoltaic signal output. The photovoltaic voltage is 2 to 4 orders higher than that of traditional p-n junctions because the effective field of ferroelectric materials is an order of magnitude higher than that of conventional semiconductor p-n junctions, which make them favorable for light sensor, light actuator, and the solar cell applications.The Ferroelectric photovoltaic(FE-PV) effect, in which polarization electric field is the driving force for the photocurrent, is distinctly di FFerent from the typical photovoltaic(PV) effect in semiconductor p-n junction’s devices.(1-x)Pb(Mg1/3Nb2/3)O3-x Pb Ti O3(PMN-PT) ralaxor-based ferroelectric single crystals and thin films exhibit high value of spontaneous polarization, which can be regulated and controlled by composition, crystal orientation, phase structure, domain structure etc. Previous results demonstrated that rhombohedral PMN-PT single crystals and thin films with compositions close to the morphotropic phase boundary showed large spontaneous polarization as well as excellent temperature stability. Therefore, the aim of this thesis is to investigate the microstructures and photoelectric conversion properties for <111>-oriented PMN-0.28 PT, Mn-doped PMN-0.28 PT single crystals and PMN-0.30 PT thin films.0.96 Zn O:0.04Al(AZO) transparent conducting electrode, Au metal electrode and Cu2 O semiconductor buffered layer were fabricated using Pulsed Laser Deposition(PLD) and R-F sputtering deposition methods. And then, the effect of growing conditions on the orientation, surface morphology and electrical properties ofelectrode and buffered layer were investigated. Finally, AZO/PMN-PT/Cu2O/Au heterojunctions were prepared under the optimized conditions.The absorption spectrum, secondary emission cuto FF and Fermi cutoff were characterized by ultraviolet-visible spectrophotometry(UV-VIS), Ultraviolet Photoelectron Spectroscopy(UPS). And the work function and optical gap of Cu2O、PMN-PT、Mn-PMN-PT as well as interface energy band structures were obtained by fitting UV-VIS and UPS spectrums. The results showed that the work function were3.21 e V, 3.66 e V, 4.21 e V, and the energy gap were 2.63 e V, 3.05 e V, 3.65 e V for Cu2 O, PMN-PT, Mn-PMN-PT, respectively, which are all n-type semiconductors.The current-voltage(J-V) characteristics of AZO/PMN-PT/Cu2O/Au heterojunctions were measured by Solar Simulator using sunlight(AM1.5G), U-V light 365 nm and 254 nm as light source. The influence of the composition, crystal orientation, domain structure, defect structure, buffered layer, polarization direction,built-in electric field, and electrode on the performance of photoelectric conversion was investigated. The results were as following. 1) The interface barrier potential was reduced by inserting n-type Cu2 O semiconductor buffered layer between PMN-PT single crystals and Au, and an ohmic contact was formed for an n-n+heterojunction,which was favorable for electronic transport from ferroelectric material to electrode and result in an increased photocurrent by more than three orders of magnitude compared to traditional ferroelectric/metal interface. 2) The short-circuit photocurrent density(JSC) and power conversion efficiency(η) of poled PMN-PT single crystals improved dramatically about three times compared with the unpoled one. The value of JSC、open-circuit voltage(VOC) and η for negative poled PMN-PT is higher than that of positive poled ones, which demonstrated the direction of built-in electric field consistent with the Schottky barrier and the built-in electric field in ferroelectric materials played an important role for separating photon-induced carriers. Mn-doped PMN-0.28 PT single crystals heterojunction exhibited JSC and η(7.35 μAcm-2, 6.63%)higher than those of pure PMN-0.28 PT ones(4.20 μAcm-2, 4.07%) under negative poling, which may be caused by the defects resulted from B atoms replaced by Mnchanging the electronic band structure of the heterojunctions. 3)The value of η under254 nm U-V light exhibited a 15% increase in comparison to 365 nm U-V light for the same testing conditions and samples. Since the photon energy for 365 nm U-V light is enough to stimulate electrons from valence band to the conduction band, so we suppose that the increment is caused by using low work function metals as electrode, with combining classic photoelectric and ferroelectric photovoltaic effects together. 4) The fill factors(FF) changed with polarization direction of the ferroelectric-semiconductors. The value of FF for negative polarization is always higher than that of positive polarization under the same testing conditions and samples.The PMN-0.30 PT ferroelectric thin films with <110> preferred orientation were fabricated successfully under an oxygen pressure of 30 Pa and a substrate temperature of 750℃ by Pulsed laser deposition(PLD). The ferroelectric, dielectric, leakage,photoelectric conversion properties of the thin films were studied with different characterization techniques. The crystalline quality and electric properties of the thin films were improved by inserting La0.5Sr0.5Co O3(LSCO) buffered layer between the ferroelectric and electrode layers. The PMN-0.30 PT ferroelectric thin film exhibited a large remnant polarization of 31.97 μC/cm2 and relative dielectric constant of 2800 which were close to the value of its bulk counterpart. The PMN-PT/LSCO interface exhibited P-F conductive mechanism while PMN-PT/Pt interface exhibited Schottky conductive mechanism. The value of JSC, VOC and η for PMN-0.30 PT heterojunction under AM1.5G(100 m W/cm2) light as light source were 83.28 μAcm-2, 1.89 V and0.049%, respectively.The short circuit current density of 7.35 μAcm-2, open circuit voltage of 36.58 V and the photoelectric conversion efficiency of 6.63% were obtained under 254 nm UV light by regulating the polarization direction, domain structure, defect structure of PMN-PT single crystals and thin films, as well as ferroelectric and schottky barrier caused by contact between ferroelectric materials and electrode. These studies can help deepen the understanding of the perovskite ferroelectric-semiconductors, as well as paved the way for the practical applications of ferroelectric-semiconductors.