Harry Atwater1

1, California Institute of Technology, Pasadena, California, United States

In order to have an impact at scale, next-generation photovoltaics have a formidable challenge: to lower the levelized cost of electricity by increasing efficiency and annual energy yield. Likewise, in order to become a scalable technology, photoelectrochemical generation of solar fuels and chemicals, such hydrogen or reduced products of carbon dioxide, must borrow from the accumulated photovoltaics knowledge base about low-cost large-area electronic device manufacturing. Tandem photovoltaic structures are compelling candidates for the next photovoltaic technology generation; of special interest are tandem structures utilizing existing Si solar cells and manufacturing methods for the bottom cell device in a two-junction tandem photovoltaic structure. Such a “X-on-Si” approach puts great pressure on the top cell device, since it must generate a significant fraction of the power at a very marginal added cost. I will discuss and compare several strategies for tandem photovoltaic module design, including i) epitaxy-free III-V/Si integration, perovskite on Si, transition metal dichalcogenides/Si, and a luminescent solar concentrator/Si. Tandem photoelectrodes are also necessary for integrated photoelectrochemical solar fuel generators to enable the photovoltage to exceed the thermodynamic reaction potential for fuel formation, as well as the kinetic overpotentials for balanced oxidation and reduction reactions. While record solar-to-fuel efficiencies can be achieved in flat-plate, single crystal, monolithic tandem multijunction III-V heterostructure photoelectrodes, such structures are likely too expensive for scalable solar fuel generation. Approaches to photoelectrochemical device architecture based on alternative tandem architectures will be discussed, including the tandem structures noted above as well as oxide-based photoelectrodes, partially-integrated photoelectrochemical structures and separately-wired photovoltaic and electrolyzer structures, with a focus on device efficiency and long-term durability.