Aerosol Processes Enabling Solar Energy Applications.

다국어 초록 (Multilingual Abstract)

Development of alternative energy sources is essential to satisfy future energy demands. While remaining fossil fuel resources may substantially meet these demands, anthropogenic carbon dioxide (CO2) emissions from combustion processes will continue accumulating in the atmosphere and thus exacerbate climate change. There are a variety of carbon-free energy sources, including hydroelectricity, tidal energy, wind energy, biomass, and solar energy. Among those, solar energy would allow for a natural resolution to global energy problems if harvested effectively and used efficiently. Although solar energy can produce clean power in a renewable and sustainable manner, its expensive unit cost, which is mostly attributed to its high production cost, makes people hesitate to switch their energy sources. Artificial photosynthesis, which borrows partial steps from natural photosynthesis, suggests promising ways to produce clean energy: electron generation by light-harvesting molecules, hydrogen (H2) formation by water photolysis, and transformation of CO2 into hydrocarbon fuels. Photocatalytic metal oxides are commoly used for those energy production processes.
Photocatalytic metal oxides, such as titanium dioxide (TiO2), are attractive materials for solar energy applications, since they are low-cost materials and are a plentiful resource. The applicability of photocatalytic metal oxides will be enhanced if an economically viable process for synthesis of highly efficient metal oxide thin films is developed. In the work reported here, Aerosol processes, which are simple operations that can be easily scaled up, were used to fabricate metal oxide-based solar cell devices. Aerosol-chemical vapor deposition (ACVD), a simple, one-step process operating at atmospheric pressure, was developed to deposit nanostructured metal oxide films with controlled morphologies. The as-synthesized nanostructured thin films were used as photoanodes for both dye-sensitized solar cells and water photolysis. One problem is that the fast recombination of photogenerated electron-hole pairs suppresses the electrochemical reaction. It was found that noble metal nanoparticles with specific sizes can delay the recombination of electron-holes by forming junctions with metal oxides. Long lived photogenerated electrons improved device performance in both water photolysis and photocatalytic CO2 reduction. Along with the fast recombination of electron-hole pairs, another critical problem is limited light absorption of photocatalytic metal oxide, especially TiO 2. Quantum dots (QDs) are promising solid-state photosensitizers, whose optical properties can be tuned by controlling their sizes. An electrospray system was employed to deposit QDs onto the nanostructured metal oxide films to enhance light absorption in the visible regime. Unlike existing methods such as chemical linking and chemical bath deposition, the electrospray method rapidly deposits QDs in a controlled manner. Solar irradiance mostly lies in the visible as well as the near infrared (NIR) regime. Natural light-harvesting complexes inspired us to develop a concept of a bio-hybrid solar device. The device could harvest photons over a wide range of visible and NIR light by combining chlorosomes as a light antenna system and lead sulfide (PbS) QDs as artificial reaction centers for charge separation.
In conclusion, ACVD and an electrospray system was used to fabricate highly efficient photoelectrodes for solar energy applications. These aerosol processes are expected to reduce the production cost of solar energy devices, and eventually will accelerate the wide utilization of solar energy.