Voltage enhancement in multijunction solar cells and the application to water splitting [electronic resource]

Over the past 50 years, cost has been the primary obstacle to the generation of electricity by solar cells. Two major obstacles have now emerged that limit further scaling and installation of photovoltaics (PVs): first, the reduction of material cost of solar devices is approaching its limit while "soft cost" is becoming the limiting factor; second, the intermittency of solar power puts an increasing burden on the power grid and increases the cost of solar electricity. Therefore, further advances of PV technology must focus on improving the PV module efficiency as well as finding efficient energy storage solutions. Among the different methods of solar energy storage, solar hydrogen generation through water splitting is one of the most promising approaches and has attracted significant research interest in the past three decades. Meanwhile, hydrogen is also considered as an ideal fuel due its zero emission, elemental abundance and ultrahigh mass energy density. However, high-efficiency solar-to-hydrogen conversion is very challenging due to the absolute voltage requirement for water splitting. Although various solar hydrogen generation technologies have been demonstrated in the past two decades, the solar-to-hydrogen efficiencies in all previous works are much lower than the typical efficiency of PV solar cells. Therefore, voltage enhancement becomes key to not only high PV efficiency, but also high solar-to-hydrogen efficiency for energy storage. On the other hand, multijunction solar cells have become increasingly interesting for water splitting applications due to not only the intrinsically high voltage, but also the potential for further voltage enhancement as well. Among the different voltage enhancement techniques, luminescent coupling has recently drawn a lot of research attention since it allows cell performance improvement by simply changing the design, incurring trivial additional cost. In addition, luminescent coupling effects are also demonstrated to mitigated efficiency loss due to non-ideal illumination spectrum, improving PV module efficiency in real operation conditions. Therefore, my research focuses on the design techniques in voltage enhancement with luminescent coupling effects, and the application of voltage-enhanced multijunction solar cells to high-efficiency water splitting. The first half of this dissertation focuses on solar cell design, modeling and characterization with the presence of luminescent coupling effects. We demonstrated a 3D distributed equivalent-circuit model that allows the modeling of luminescent coupling effects in 3D, as well as novel methods to model and measure luminescent coupling efficiency more accurately then traditional methods. The second half of this dissertation demonstrates a PV-electrolysis system that couples two electrolyzers with a triple-junction solar cell at the optimal operation point, and achieves a solar-to-hydrogen efficiency of 30%, which is significantly higher than that previously reported by any water-splitting system.

Keywords: Electrical Engineering.
Advisor: Harris J. S. (James Stewart)
Fan Shanhui
Jaramillo Thomas Francisco advisor. Thesis advisor
Kamins Theodore I.