Tandem-junction microwire array photoelectrodes have been fabricated by coating np(+)-Si radial homojunction microwire arrays sequentially with fluorine-doped tin oxide (FTO) and titanium dioxide (TiO2). These photoelectrodes effected unassisted water splitting under simulated 1 Sun conditions with an open-circuit potential (E-oc) of -1.5 V vs the formal potential for oxygen evolution, E-0' (OH-/O-2), a current density at E = E-0' (OH-/O-2) of 0.78 mA cm(-2), a fill factor (ff) = 0.51, and a photovoltaic-biased photoelectrochemical ideal regenerative cell efficiency of 0.6%. (C) 2016 The Electrochemical Society. All rights reserved.