Efficient solar water splitting by enhanced charge separation in a bismuth vanadate silicon tandem photoelectrode

Metal oxides are generally very stable in aqueous solutions and cheap, but their photochemical activity is usually limited by poor charge carrier separation. Here we show that this problem can be solved by introducing a gradient dopant concentration in the metal oxide film, thereby creating a distributed n n homojunction. This concept is demonstrated with a low cost, spray deposited and non porous tungsten doped bismuth vanadate photoanode in which carrier separation efficiencies of up to 80 are achieved. By combining this state of the art photoanode with an earth abundant cobalt phosphate water oxidation catalyst and a double or single junction amorphous Si solar cell in a tandem configuration, stable shortcircuit water splitting photocurrents of 4 and 3 mAcm 2, respectively, are achieved under 1 sun illumination. The 4 mAcm 2 photocurrent corresponds to a solar to hydrogen efficiency of 4.9 , which is the highest efficiency yet reported for a stand alone water splitting device based on a metal oxide photoanod