Single-junction Ge solar cells have been epitaxially grown on GaAs (001) substrates by a low pressure metalorganic chemical vapor deposition using isobutylgermane. Various growth conditions have been studied to reduce the high doping level of over 1x10(19) cm(-3) of the p-type Ge epitaxial layer, which is detrimental to the minority carrier collection efficiency. By increasing the growth rate and employing a discontinuous doping technique, the p-type doping level of the epitaxial Ge layer has been lowered to 2x10(18) cm(-3), and the hole mobility increased from 44 to 162 cm(2)/V s. A high power conversion efficiency of 6.72% can be achieved under AM1.5 G illumination by the epitaxial Ge solar cell with the lowered doping level in the p-type Ge base region. The spectral response of the epitaxial Ge solar cell with a 5 mu m thick base layer is good enough to use for InGaP/GaAs/Ge triple-junction solar cells.