  Strong interference in ultrathin film semiconductor absorbers on metallic
back reflectors has been shown to enhance the light harvesting efficiency of
solar cell materials. However, metallic back reflectors are not suitable for
tandem cell configurations because photons cannot be transmitted through the
device. Here, we introduce a method to implement strong interference in
ultrathin film top absorbers in a tandem cell configuration through use of
distributed Bragg reflectors (DBRs). We showcase this by designing and
fabricating a photoelectrochemical-photovoltaic (PEC-PV) stacked tandem cell in
a V-shaped configuration where short wavelength photons are reflected back to
the photoanode material (hematite, Fe2O3), whereas long wavelength photons are
transmitted to the bottom silicon PV cell. We employ optical simulations to
determine the optimal thicknesses of the DBR layers and the V-shape angle to
maximize light absorption in the ultrathin (10 nm thick) hematite film. The DBR
spectral response can be tailored to allow for a more than threefold
enhancement in absorbed photons compared to a layer of the same thickness on
transparent current collectors. Using a DBR to couple a bottom silicon PV cell
with an ultrathin hematite top PEC cell, we demonstrate unassisted solar water
splitting and show that DBRs can be designed to enhance strong interference in
ultrathin films while enabling stacked tandem cell configuration.
