Organometal Halide Perovskites and Gold Nanoclusters for Solar Energy Conversion

It is essential and urgent to develop an efficient yet cost-effective approach to convert solar energy into useful energy sources to sustain our society in the near future. TiO₂ supported nanostructures have the potential to tackle this challenge due to its low-cost nature and flexibility in device design. The light absorbers that are anchored onto TiO₂ surface play important roles on determining the device performance, however, the current systems provide low efficiencies even after decades of studies. In this dissertation, I have investigated two emerging nanomaterials, glutathione-capped gold nanoclusters and organometal halide perovskites as light absorbers, respectively. Their performances in TiO₂ supported systems for solar to electricity and solar to fuel conversion applications are examined and discussed.

Glutathione-capped gold nanoclusters which exhibit molecule-like properties are first investigated for solar to electricity conversion applications. The excited state behavior of the gold nanoclusters elucidated from time-resolved spectroscopy shows favorite long-lived excited state lifetime. These long-lived excited states are found to inject energetic electrons into the conduction band of TiO₂ upon light excitation, confirming the role of gold nanoclusters as the new class of photosensitizer alternative to dye molecules and quantum dots. We fabricated a metal cluster-sensitized solar cell (MCSCs) using these clusters as the photosensitizers and demonstrated its ability of delivering stable power conversion efficiency of 2 %. This new class of photosensitizer is further explored as a co-sensitizer in a dye-sensitized solar cell (DSSC). The metallic core of the gold nanoclusters is found to accumulate electrons and raise the quasi-fermi level of TiO₂ under illumination when two of them are in contact. The resulting co-sensitized solar cells show greater energy conversion efficiency compared to the solar cells composed of single photosensitizer.

The promising gold nanoclusters-sensitized TiO₂ nanostructures are further studied for light-driven hydrogen production application in two electrode photoelectrochemical cells (PECs) and photocatalytic slurry systems. The glutathione-capped gold nanoclusters characterized using cyclic voltammogram show suitable energy levels for hydrogen and oxygen evolution reaction at neutral pH. The gold nanoclusters-sensitized TiO₂ nanostructures are capable of producing hydrogen gas under visible light illumination at neutral pH without applying external bias or sacrificial reagents. These exceptional performances are superior to the pre-existing systems using dye molecules and quantum dots as the photosensitizers.

Lastly, organometal halide perovskite is explored for light-driven hydrogen production. This emerging light absorber composed of inexpensive, earth-abundant elements and can be prepared using low-cost solution processes, has already demonstrated its ability of delivering power conversion efficiency of 20 % in TiO₂ supported nanostructures. The uniqueness of perovskite solar cells in delivering high open circuit voltage is designed to supply additional photovoltage to a n-type semiconductor photoanode to drive the overall water splitting reaction in a photoanode-photovoltaic tandem device. The solar to hydrogen conversion efficiency of this device is 2.5 % under simulated sunlight illumination. Further improvement on the device efficiency and stability are expected via optimizing the electrochemical activity of the front photoelectrode.