In a future energy system, chemical energy carriers that can easily be stored, like hydrogen, are of vital importance. One possible way of producing carbonneutral hydrogen is by direct solar to hydrogen conversion in a photoelectrochemical cell (PEC). Silicon based multijunction solar cells are a possible candidate for the photovoltaic stack of these PEC devices. To make high efficiency PEC devices, the photovoltaic stack has to be optimised, especially with respect to reaching high current densities in the middle (ncSi) cell. In this work it was attempted to increase the current density of a triple junction device based on cSi/ncSi/aSi absorber layers by varying the ncSi absorber thicknesses and implementing various intermediate reflecting layers (IRLs) between the middle and bottom cell. These layers were based on silicon oxide, transparent conducting oxides (TCOs) and thin silver films. For each method it was attempted to give a quantitative comparison of how much the electrical performance is affected per unit of current gained in the middle cell. Both increasing the ncSi absorber thickness and the silicon oxide reflector thickness were found to be feasible methods of increasing the middle cell current density. The ncSi absorber thickness leads to an initial rapid current gain, of about 2.6mA/cm2 between 2.5 and 3.75 μm, reducing to just 0.8mA/cm2 between 3.75 and 5 μm. The electrical performance cost between 2.5 and 3.75 μm was calculated as a 2% reduction in 푉 oc*퐹퐹 product per mA/cm2 current gain. This cost increased to about 5% per mA/cm2 between 3.75 and 5 μm. Increasing the absorber thickness beyond 5 μm is not considered feasible due to reducing current gains and mounting electrical performance losses. Increasing the silicon oxide thickness can result in a current gain of about 1mA/cm2. This comes at an electrical loss of 3% 푉 oc*퐹퐹 product per mA/cm2. TCO based IRLs were found to quickly result in shunting, and in the case of indium doped tin oxide (ITO) based IRLs also damage to the surrounding cell structure resulting in poorly performing cells. Aluminium doped zinc oxide (AZO) based reflectors with proper electrical isolation from the edges gave only slightly reduced electrical performance, but failed to lead to a current gain in the middle cell. Thin silver films were found to quickly rearrange into nanoparticles, effectively forming a plasmonic IRL. This IRL however suffered from high parasitic absorption, and as a result also did not lead to a current gain. A very thin silver film was however shown to slightly improve the electrical performance, at the cost of bottom cell current density. Finally, a device was fabricated with both a thick ncSi absorber as well as a silicon oxide IRL. The resulting device achieved a current density of 9.5mA/cm2, a 푉 oc of 1.947V and a FF of 0.789, giving an efficiency of 14.6%. To the knowledge of the author this is the highest efficiency reported for this device configuration to date. Furthermore, attempts were made to incorporate these stacks into PEC, by fabricating an microstructured anode, cathode and ionconducting pores, to form a porous membrane PEC (PMP). The cathode and anode were successfully demonstrated, but the pore etching is yet to be optimised.