TiO₂ protected III-V Multijunction Solar Cell for PhotoelectrochemicalWater-splitting

Despite a growing international awareness of environmental problems from carbon-rich energy carriers, the rising trend in global fossil fuel consumption has increased consistently since its inception. In an effort to reverse the situation, technological advances have been made to implement carbon-free energy carriers.

One of the most promising candidates for the future energy carrier is hydrogen, and vibrant research on replacing fossil fuels with hydrogen is ongoing. However, the most cost-effective process to produce (in other words, convert energy into) hydrogen is steam methane reforming and gasification, which are already releasing more than 2 % of global CO2 emission to meet industrial uses of hydrogen.

Therefore, extended use of hydrogen as an energy carrier would not be possible unless an emission-free way for producing hydrogen is developed. Water electrolysis would be an attractive way to produce hydrogen without emission, especially when the electricity is supplied from renewable energy sources.

In this regard, solar hydrogen production by combining a solar cell and water electrolyzer has been under investigation. A technically viable way to combine them at this moment would be electrically wiring a separate solar cell and water electrolyzer. Another interesting but immature approach is photoelectrochemical (PEC) water splitting, which is immersing a solar cell into an electrolyte without any wiring to derive the same electrochemical reaction.

Because PEC carries out the reaction directly over a large solar cell surface with a low current density, it can reduce efficiency losses from activation energy and ohmic drop. Furthermore, a monolithic device structure can save the system cost. However, PEC water splitting faces critical challenges to be employed for practical hydrogen production.

Because semiconductors are usually unstable in an electrolyte, PEC devices cannot operate sufficiently long time until it finds economic feasibility. In addition, the optical properties of PEC devices are not optimized yet, and it results in a lower activity compared to a dry solar cell. This thesis focuses on the major bottlenecks of PEC water splitting and it, I believe, has taken a small but important new step toward making it feasible for hydrogen production.

The optical properties of the PEC devices depending on its structure are systematically investigated and suggests a way forward to minimize optical losses. Furthermore, the PEC devices in this study reach record-high stability by improving its protection layer.