  The majority of photoelectrochemical (PEC) water splitting cells cannot drive
the overall water splitting reactions without the assistance of an external
power source. To provide added power, the cells are usually connected to
photovoltaic (PV) devices in a tandem arrangement. This approach suffers from
severe disadvantages since the PEC cell is connected in series to the PV cell
and the overall current is typically limited by the saturation current of the
PEC component. Thus, the operating point of the PV cell is often far from
optimal and the overall system efficiency tends to be low. We propose a
multi-terminal hybrid PV and PEC system (HPEV). As in tandem arrangements, the
PEC cell is optically connected in series with the PV cell. However, a second
back contact is used to extract the PV cell surplus current and allow parallel
production of both electrical power and chemical fuel. Devices consisting of
three-terminal silicon photovoltaic cells coupled to titanium dioxide water
splitting layers are simulated and fabricated. The cells are shown to produce
electricity with little reduction in the water splitting current, surpass the
current mismatch limits, and increase the overall system efficiency.
