Study On Resistive Switching Behavior And Photoelectric Devices Consisting Of Oxide Thin Films

Since the wide applications of conventional oxides materials in the fields ofsemiconductors, magnets and sensing devices, the investigations into nano effectsand heterojunction performances of various functional oxides, such assuperconductors, giant magnets and multiferroics, have attracted a lot of attention,because of potential unique phenomenon as well as the emerge devices consisting ofthese oxide materials. Moreover, electrochemical devices based on oxide films,especially such as dye sensitized solar cell （DSSC）, have been widely studied due toits advantages of high thermal stability, low-cost, flexibility, and friendly toenvironment. Michael Graztel et al, who discovered and improved the DSSC，haveachieved the photo-electric conversion efficiency of12%for such anelectrochemical solar cell, demonstrating a very promising future. It is believed thatthe DSSC is applicable for novel photo-electronic devices, just as Silicon-based p-njunctions available for various transistors and relevant photo-electric devices.In the first part of this dissertation, we study on the transport performance ofDSSC photoanode at heterostructrual interfaces, and then explore the novelphototransistor device based on the dye sensitized solar cell, On the other hand, wenote the iodide based electrolyte may serve as a special electrode, allowing anemerged electrochemical devices with distinct transport phenomenon, such asresistive switching behavior. In the second part, we analyze and study on the originof resistive switching and negative capacitance existing in the heterostructure unitconsisting of Cerium oxides/electrolyte.To realize the researches mentioned above, a number of preliminary work isdone for the sample preparation and characterization, such as （a） building up twomeasurement systems, for characterizations of the low temperature electric andmagnetism，as well as of the photovoltaic behaviors. With the software of ANSYS,the temperature distributions of sample plates are analyzed. As well, the measurement control programs are compiled for a transport measurement system byusing LabVIEW;（b） improving the preparation technology of DSSC and oxide thinfilms, ensuring the quality of the studied samples;（c） creating an etching methodwith electrical burning to separate the hard conducting layer of F-doped SnO2（FTO）into two parts. This approach employs one electrode connecting the side of FTOplate, and the other one, shaped as a needle, scratching along the middle of FTOpiece. With sufficient voltages, Joule heating occurring at the contact spot, willaccumulate and increase the local temperature, due to low specific heat and weakthermal conductivity of the conducting oxides. This makes the crystal structure meltand the stechiometry shift, resulting in an insulating groove of approximate100m.With the techniques mentioned above, we establish some interestingelectrochemical devices and study their electric transport properties, as well aspropose relevant models for explanation of the observed phenomenon. Main researchcontents and associated results are summarized as follows:（1） A novel DSSC-based phototransistor is developed. Such a phototransistormay work at the operation voltage as low as1V. Regardless of the light sensitivityof0.1A/W, its on/off ratio approximately reach a ratio of1000at100mW/cm2, and100even at3.5mW/cm2. The dynamical measurements confirm the stability ofrepeatable operation and indicate the cutoff frequency of such a device,demonstrating a law relation of load resistances. By using the energy level diagramtogether with the structure analysis, an equivalent electric circuit is modeled tocharacterize the DC transport, providing a theoretical basis for further improvementnot only on the technology but also on the performance.（2） A distinct photovoltage decay measurement is established using the methodof constant current discharge to study the electronic transport behavior in thephotoanode heterojunction for a typical dye-sensitized solar cell. With such atechnique, a clear mechanism is revealed with respect to the electron transport. Theexperimental results are well simulated by the theoretical prediction, indicating not only the charge transfer dynamics in such an n+-n heterojunction, but also thefeasibility of the present decay measurement. It is believed that the proposedmeasurement is a promising and effective method to characterize variousphotoanodes, giving significant information which is hard to realize for thoseconventional transport measurements.（3） A special electrochemical device consisting of FTO/CeO₂/electrolyte/FTO isdesigned. The resistive switching （RS） behavior is observed, being induced throughapplying a sufficient negative pulse, as well as through the light illumination.Moreover, the endurance measurements are done, confirming its stability in therepeatable operation. It is revealed as well that as the depth of CeO₂thin filmincreases, the observed RS phenomenon becomes significant, implying that thesurface deficiencies, e.g. Ce³⁺substituting Ce⁴⁺, is probably the cause of thisbehaviors. It is assumed that the diiodide radical arising from the reaction with surfacedeficiencies, i.e., storing amounts of electrons at CeO₂surface, results in the observedswitching behavior.（4） A negative capacitance behavior is observed in the CeO₂electrochemicaldevice discussed above. Under a negative bias as large as-2V, a lower semicircle ispresent in the electrical impedance spectrum （EIS）. It is believed that the cation, i.e.Li⁺, can be driven by the applied voltage, and be accumulated at the interface, leadingto an intermediate state. This state allows the electrons hopping through the junctioneasier, and results in junction current larger than the virginal current. With the abovedynamical mechanism, the negative capacitances observed in various measurementsof impedance spectrum, LCR measurement and current response are reasonable andunderstandable.