We report on the fabrication and photovoltaic characterization of In_0.12Ga_0.88N/GaN multi-quantum-well (MQW) solar cells grown by metal–organic vapor phase epitaxy on (0001) sapphire substrates. Increasing the number of MQWs in the active region from 5 to 30 improves a factor of 10 the peak external quantum efficiency of the device at the price of a slight reduction and increase of the shunt and series resistance, respectively. Solar cells with 30 MQWs exhibit an external quantum efficiency of 38% at 380 nm, an open circuit voltage of 2.0 V, a short circuit current density of 0.23 mA/cm² and a fill factor of 59% under 1 sun of AM1.5G-equivalent solar illumination. Solar cells with the grid spacing of the top p-contact varying from 100 to 200 µm present the same device performance in terms of spectral response and conversion efficiency.