Here we demonstrate a more effective use of III–V photoconversion material to achieve an ultrahigh power-per-weight ratio from a solar cell utilizing an axial p-i-n junction GaAs/AlGaAs nanowire (NW) array grown by molecular beam epitaxy on a Si substrate. By analyzing single NW multicontact devices, we first show that an n-GaAs shell is self-formed radially outside the axial p- and i-core of the GaAs NW during n-core growth, which significantly deteriorates the rectification property of the NWs in the axial direction. When employing a selective-area ex situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such a controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ∼1.3% areal coverage of the NWs shows a photoconversion efficiency of ∼7.7% under 1 Sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on a Si substrate so far. This corresponds to a power-per-weight ratio of the active III–V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and low-cost III–V NW solar cells and III–V NW/Si tandem solar cells.