We report the fabrication of heterojunction solar cells via the thermal chemical vapor deposition (CVD) of gallium nitride (GaN) nanostructures on clean Si substrates. GaN epitaxial layers were synthesized via the direct reaction of Ga vapor and NH3 solution at 1050 degrees C. The structural and optical characteristics of the as-grown GaN layers were investigated. The effects of Si orientation (100 vs 111) and doping type (n- vs p-) on the structural and optical properties of the deposited GaN nanostructures and solar cell performance were explored. The fabricated GaN nanostructures exhibited p-type behavior at the GaN/Si interface as revealed from the Hall-effect measurements. The J V characteristics showed rectifying behavior for the GaN/n-Si junction and Ohmic behavior for the GaN/p-Si junction. Upon illumination (30 mW/cm(2)), the as-deposited heterojunction solar cell devices showed conversion efficiencies of 6.18% and 3.69% for GaN/n-Si (1 1 1) and GaN/n-Si (1 0 0) heterojunctions, respectively. The growth of GaN on Si substrates in the presence of NH3 solution has strong effect on the morphological, optical and electrical properties and consequently on the efficiency of the solar cell devices made of such substrates. (C) 2013 Elsevier Ltd. All rights reserved.