  We present a concept for an ultra-thin solar fuel device with a
nanostructured back contact. Using rigorous simulations we show that the
nanostructuring significantly increases the absorption in the semiconductor,
CuBi$_2$O$_4$ in this case, by 47\% (5.2~mAcm$^{-2}$) through the excitation of
plasmonic modes. We are able to attribute the resonances in the device to
metal-insulator-metal plasmons coupled to either localised surface plasmon
resonances or surface plasmon polaritons. Rounding applied to the metallic
corners leads to a blueshift in the resonance wavelength while maintaining
absorption enhancement, thus supporting the possibility for a successful
realization of the device. For a 2D array, the tolerance of the
polarization-dependent absorption enhancement is investigated and compared to a
planar structure. The device maintains an absorption enhancement up to incident
angles of 75$^{\circ}$. The study highlights the high potential for plasmonics
in ultra-thin opto-electronic devices such as in solar fuel generation.
