The field of photovoltaic cells has been dominated so far by solid state p-n junction devices made of silicon, CdTe and copper indium gallium diselenide profiting from the experience and material availability of the semiconductor industry. However there is an increasing awareness of the possible advantages of devices based on mesoscopic inorganic or organic semiconductors commonly referred to as “bulk“ junctions due to their interconnected three-dimensional structure. It is now possible to depart completely from the classical solid-state cells, replacing them by devices based on interpenetrating network junctions. These cells are formed, for example, from nanocrystalline inorganic oxides, ionic liquids and organic hole conductor devices, which offer the prospect of very low cost fabrication without expensive and energy intensive high temperature and high vacuum processes. They can feasibly be produced employing flexible substrates and are compatible with a variety of embodiments and appearances to facilitate market entry, both for use in domestic devices as well as in architectural applications. The prototype of this new PV family is the dye-sensitized solar cell (DSC), which accomplishes the separation of the optical absorption and the charge separation processes by the association of a sensitizer as light-absorbing material with a wide band gap semiconductor of nanocrystalline morphology. The DSC reaches today over 11 percent conversion efficiency and excellent stability rendering it a credible alternative to conventional p-n junction photovoltaic devices. Mesoscopic oxide semiconductors such as hematite show also great promise for use as photo-anodes in tandem cells for water splitting by visible light.