Metal oxide Graphene oxide nanocomposite thin film for optoelectronic applications

Abstract

This thesis deals with the study of the solution-processed wide band gap metal oxide (TiO2) - graphene oxide (GO) nanocomposite materials in thin film form for their optoelectronic applications, such as UV-photodetector and dye-sensitized solar cells (DSSCs). Here, we demonstarte the fabrication of individual metal oxide (TiO2) - graphene oxide (GO) nanocomposites, as well as hybrid nanostructures (ZnO NW/TiO2), with GO incorporation using an easy, cost-effective and simple sol-gel spin coating technique. The formation of GO-composited highly transparent nanocomposite thin films, comprised of the rutile phase of TiO2 nanoparticles, as well as hybrid nanostructures (ZnO NW/TiO2), has been confirmed through microstructural, morphological, optical, and electrical characterizations. Modification of optical and electrical characteristics with a small amount of GO reinforcement into the host TiO2, as well as hybrid nanostructures (ZnO NW/TiO2), is also examined. Due to the incorporation of a small amount of GO into metal-oxide films, as well as hybrid nanostructures, the optical band gap values of those nanostructures are slightly reduced. At room temperature, DC bias dependent impedance spectroscopic analysis of TiO2 as well as hybrid nanocomposites (ZnO NW/TiO2) with GO, was performed for various external bias voltages in the frequency range of 4 Hz to 5 MHz. To evaluate and analyze the various contributions originating from the core grains and grain boundaries, the experimental Nyquist plot derived from the bias-dependent impedance spectra was fitted with an appropriate model electrical circuit consisting of two parallel RC circuits combined with a series resistance. The modification of grain boundary and its consequential effect on charge transport in individual metal oxide semiconductors, as well as hybrid nanocomposites, were confirmed by the variation of relaxation times (τ = RC) with an external bias and its modification after graphene oxide (GO) reinforcement. This demonstrates that a conducting graphene oxide (GO) network is capable of modifying the grain boundaries of individual metal oxides, as well as hybrid nanocomposites, and facilitates better charge transport through it, which may be beneficial for numerous optoelectronic applications. The fabricated nanocomposite thin films are used as efficient photoanodes for dyesensitized solar cells (DSSCs). In this study, the ruthenium-based N3 dye, widely employed as a prominent photosensitizer for the absorption of solar energy, is utilized along with the iodinebased redox pair (I- /I3-) mediator serving as the electrolyte. A transparent and conductive FTOcoated glass substrate is used as the counter-electrode. Under the irradiation of a 1.5 AM solar spectrum and with a power density of about 100 mW/cm2 , the current density-voltage (J–V) characteristic curves of TiO2, TiO2-GO, ZnO NW, ZnO NW/TiO2, and ZnO NW/TiO2-GO–based DSSCs are evaluated and analyzed. In the TiO2-GO (10 %) based DSSC structure, the maximum value of power conversion efficiency (η ~ 0.48 %) is obtained. The inclusion of GO in the TiO2 photoanode leads to a notable increase in the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and power conversion efficiency (η). This improvement is ascribed to the less recombination of photogenerated charge carriers and improved charge transport enabled by the conductive GO network. The hybrid nanostructure that comprises ZnO NW/TiO2-GO (10 %) exhibits the highest level of performance, with an optimized power conversion efficiency value of η ~ 0.72 %. This observed enhancement in performance is due to the combined effects resulting from the improvement in charge transport along the axis of ZnO nanowires (NWs), an increase in contact area, less recombination of photogenerated charge carriers, and the modification of the conductivity of TiO2 by the incorporation of a conductive GO network. Therefore, high transparency with enhanced photocurrent density and large power conversion efficiency make TiO2-GO, ZnO NW-GO, and hybrid ZnO NW/TiO2-GO nanocomposite thin films more efficient photoanodes for dye-sensitized solar cells (DSSCs). Enhanced UV photoresponse properties of fabricated nanocomposite thin films were studied through UV photoconductivity measurements. Here, we present an investigation into the modification of UV photo-response properties of transparent TiO2-GO nanocomposite thin films, as well as hybrid ZnO NW/TiO2-GO nanocomposites. Schottky-enabled enhanced UV-light detection with high photoresponsivity, rapid response, and recovery time were observed by the incorporation of small amounts of GO into TiO2, as well as hybrid nanostructures (ZnO NW/TiO2). Under UV irradiation of wavelength ~ 365 nm with a power of ~ 8.07 µW, better photoresponsivity, sensitivity, specific detectivity, and external quantum efficiency were achieved by the inclusion of a small amount of GO into TiO2 thin films and hybrid ZnO NW/TiO2 nanostructures. Under the same circumstances, a maximum photocurrent of 1.78 mA/cm2 and a maximum UV photo-responsivity of 6.93 A/W, as well as a shorter rise time (tON ~ 0.28 sec) and recovery time (tOFF ~ 0.41 sec), were observed for TiO2–GO (15 %) thin film. These observed phenomena may be related to the modification of the space charge region produced by the incorporation of GO. This modification results in the passivation of interface defect states, which enhances the transport of charge carriers while simultaneously reducing the recombination of photo-generated charge carriers. When subjected to UV irradiation at a wavelength of 365 nm and an incident light power of 8.07 μW, the potential barrier height dropped from 0.54 eV to 0.49 eV, and the ideality factor decreased from 9.40 to 8.71 in ZnO NW/TiO2 films with 15 % GO incorporation. These results are beneficial for the development of optoelectronics applications and are advantageous for the film's overall performance. Also, for ZnO NW/TiO2-GO (15 %) nanostructures, a notable increase in photocurrent density (3.47 mA/cm2 ) and an impressive photoresponsivity of 13.52 A/W at a low bias (0.5 V), along with a speedy rise time of 0.89 sec and a quick recovery time of 1.66 sec, were achieved. Impedance spectroscopic analysis has been adapted to describe the transport mechanism in those prepared nanocomposite films in dark and UV-irradiated conditions. Therefore, the construction of a good Schottky junction with an Ag electrode, effective UV-stimulated charge separation, and improved transportation of charge via a conducting GO network all contribute to the fact that TiO2-GO thin films, as well as hybrid nanostructures (ZnO NW/TiO2-GO), are excellent UV response devices even when the external bias voltage is relatively low.