Photoelectrochemical (PEC) efficiency of a PEC cell constructed by series connecting two similar to 0.43cm(2) size, 5.95% (AM1.5) efficient CuIn1-xGaxS2 (CIGS2) thin-film photovoltaic (PV) cells having transparent and conducting back contacts, outside the electrolyte, to RuS2 photoanode and platinum cathode, in the electrolyte, for oxygen and hydrogen generation by water splitting was 2.99%. PV electrolysis efficiency of a similar setup prepared using two CIGS2 PV cells having opaque Mo back contacts and highest achieved efficiency of 11.99% (AM1.5) connected to RuS2 and Pt electrodes was 8.78%. This significant result points a way toward attaining higher PEC efficiencies. (c) 2007 Elsevier B.V. All rights reserved.