The Tandem Cell™ consists of two photo-catalytic cells in series: the front cell absorbs the high energy ultraviolet and blue light in sunlight, using nano-crystalline metal oxide thin films to generate electron-hole pairs. The longer wavelength light in the green to red region passes through the front cell and is absorbed in a Graetzel Cell producing electrical potential under nearly all light conditions The two cells are connected electrically and together provide the potential required to split the water molecules in the electrolyte. The Cell is fabricated from widely-available and cheap materials.   

The key to the Tandem Cell™ is the performance of the metal oxides in reacting to the photons of the incident light. We are developing processes to obtain high efficiency films in a usable form. The metal oxides are expected to be the limiting feature of Tandem Cell™ efficiency, but we also need to optimise all other aspects, including the counter-electrodes, the electrolytes and the mechanical design to maximise light gathering and hydrogen collection. The most expensive component of the Cells is currently the special glass, on which the nano-crystalline films are deposited. Cheaper alternatives will be found to reduce the cost of the Cell. We envisage arrays of Tandem Cells™ incorporating optical collection devices optimised for maximum efficiency.