Multiple wall carbon nanotube (MWNT) networks form Schottky-like heterojunction on n-type silicon substrate. Acid doping down-shifts the Fermi level of the MWNTs, can significantly reduce the internal resistance of the MWNT film. Wetting the MWNT networks with nitric or sulfuric acid can form MWNT-acid-Si photoelectrochemical units. The photoelectrochemical units and the Si-MWNT heterojunctions connected in parallel on the same side of the Si substrate, consequently boosting the power conversion efficiency by more than 10 times. (C) 2014 Elsevier Ltd. All rights reserved.