Total water splitting into oxygen and hydrogen using solar light is one of the most sustainable ways to produce energy. Unfortunately, this is not an easy process, as it is a thermodynamically unfavorable reaction. Moreover, the required band gap to perform both the oxidation and the reduction is large, so only the high energy fraction of the solar light can be used. This can be overcome by a Z-scheme photoelectrochemical (PEC) cell setup. In this case, a tandem cell is created, in which the one excitation in the UV light is replaced by two tandem excitations in the visible light. Current cells suffer however from unstable, costly and often toxic electrodes and a low efficiency. Covalent Organic Frameworks (COF) are a relatively recent discovery (2005). They are crystalline, highly porous and light organic frameworks, consisting typically only of C, H and H. They are extremely stable in all kinds of environments, including acid and basic aqueous environments. Their high porosity allows an intimate contact between the photo-anode and the photo-cathode. We will design a Z-Scheme PEC cell in which the cathode and the anode consist of COFs. By designing the functionalities in the COFs, these materials can either act as a p-type or an n-type semiconductor. The cells will be compared to the current state of the art and should show a solar efficiency higher than 10%.