Highly efficient core-shell TCO/a-Si/Si nanowires (SiNWs) heterojunction solar cells were fabricated on SiNW arrays prepared by metal assisted wet chemical etching of an n-type silicon wafer. The silver catalyst was carefully removed after the etching by a three-step procedure. A stack of intrinsic and p-type amorphous-Si (a-Si) was deposited as a shell onto the SiNW arrays by plasma enhanced chemical vapour deposition (PECVD), and finally, a ~200 nm TCO layer was deposited on top of the a-Si layer by atomic layer deposition (ALD). No shunt was detected in our cells, which was a big problem in the cells prepared on similar substrates published in the literature. The core-shell heterojunction solar cells on nanowire arrays show also great improvement of the performance in comparison with those published previously. In a mesa-structured solar cell with contact area of 7 mm2, an open circuit voltage of 476 mV, short circuit current density of 27 mA/cm2, filling factor of 56.2% and conversion efficiency of 7.29% was determined at AM 1.5. Electron beam induced current (EBIC) measurements were performed on the solar cells, which demonstrate unambiguously that the nanowire arrays work as active photovoltaic components.