Hybrid metal oxide nanostructures for solar water splitting and inorganic perovskite photovoltaics

The conversion and storage of solar energy is already a strategic pillar of global strategy to reduce emissions and fight climate change. Despite showing major promise in application to solar water splitting and photovoltaic cells, hybrid metal oxide nanostructures have not been widely adopted as commercial energy materials. This is largely due to their infancy, requiring extensive research for further optimisation and performance enhancement. The aim of this thesis is to explore the beneficial properties of these materials such as high n-type doping and stability in water, and improve them using various strategies. Ultimately, to test their validity by first measuring water splitting photocurrent, and second incorporating the materials into inorganic perovskite solar cells and measuring power conversion efficiency. The first strategy implemented in this thesis was the novel synthesis of vertically aligned zinc oxide nanorods. By utilising microwave heating, growth duration of the high surface area material was cut down from the conventional method’s 16 hours to under three hours. The resistive heating of the conductive glass substrate led to defect rich structures with higher n- type doping as measured by electrical impedance spectroscopy. The quantity of these defects was then reduced in order to restrict surface recombination by thermal annealing in air. The second strategy went a step further, adding extrinsic yttrium atom dopants to the nanorods. This finely tuned defect population with greater control, resulting in increased electron mobility due to the neutralisation of transport restrictive traps. These strategies led to a fourfold and 23% enhancement in solar water splitting photocurrent for microwave synthesis and yttrium doping respectively. Furthermore, such dopants led to resistance to photo-corrosion, a critical problem for zinc oxide photoanodes. Despite these improvements, the critical issue of large band gap energy renders zinc oxide unable to generate photoelectrons when exposed to most visible light. In order to overcome this problem, a-Fe2O3 (hematite) was applied as a surface coating on the nanorods. This material has a low band gap of 2.1 eV, capable of absorbing visible light. Electrochemical deposition was chosen for this application due to high control of layer thickness along with various electrolytes compatible with ZnO. Initially cathodic deposition was applied, wherein Fe3+ ions were reduced at the nanorod surface from a low concentration solution. Following annealing, the presence of both a-Fe2O3 phase and ZnFe2O4 interphase layers were confirmed. External quantum efficiency measurement indicated the success of the photoanode; achieving 8.96% at 400 nm irradiation, in comparison to zero activity for uncoated ZnO. The critical limitation of a-Fe2O3, sluggish charge transportation, is overcome using the novel three dimensional structure, forming nanoparticles on the surface of conductive yttrium doped ZnO. Utilising the cores as electron highways in conjunction with visible light sensitisation led to the highest water split- ting photocurrent achieved in this thesis, 1.59 mA cm-2 at 1.23 VRHE. Furthermore, an anodic current deposition led to the growth of hematite nanosheets on the surface of the wires. This too, led to greater water splitting performance with added surface area for increased charge transfer. Photolysis of water may have the advantage of storing solar energy in chemical bonds, but thus far has achieved limited commercialisation and wide application. This is due to the com- plex material requirements resulting in either low power conversion efficiency or expensive device construction. Therefore the previously employed strategies were applied to photovoltaic devices for high efficiency solar energy generation. Inorganic CsPbBr3 perovskite allows ambient synthesis and long term stability, it was selected as light absorber due to remarkable opto- electronic properties and visible light sensitive band gap of 2.4 eV. Devices were constructed by applying a layer of the material upon ZnO nanorod arrays, followed by drop casting of liquid phase exfoliated graphite ink produced by sonication. The suitable work function of the final material formed a Schottky junction yielding a hole selective contact, completing the photo- voltaic device. Optimisation and enhancement was achieved by applying yttrium doping, NR length control and finally TiO2 coating for surface passivation and recombination reduction. The success of the enhanced device yielded quantum efficiency of 77.8% at 400 nm illumination, and a champion power conversion efficiency of 5.8%. In nearly all cases the performance of said cells improved as time passed under dark ambient storage conditions. Therefore this thesis is concluded with a final investigation into the potential causes after storage in various conditions.