A major challenge in GaN based solar cell design is the lack of holes compared with electrons in the multiple quantum wells (MQWs). We find that GaN based MQW photovoltaic devices with five different Mg-doping concentrations of 0 cm^-3, 5×10¹⁷ cm^-3, 2×10¹⁸ cm^-3, 4×10¹⁸ cm^-3and 7×10¹⁸ cm^-3in GaN barriers can lead to different hole concentrations in quantum wells (QWs). However, when the Mg-doping concentration is over 1×10¹⁸ cm^-3, the crystal quality degrades, which results in the reduction of the external quantum efficiency (EQE), short circuit current density and open circuit voltage. As a result, the sample with a slight Mg-doping concentration of 5×10¹⁷ cm^-3exhibits the highest conversion efficiency.