Since the high efficiency tandem solar cell structure has been reported, the conversion efficiency of triple-junction solar cells could achieve 40% with concentrator . To further enhance the conversion efficiency of solar cells, the fourth-junction is used to decrease the wasting of energy transformation caused by current limiting in tandem solar cells, so that the conversion elficiency of four junction tandem solar cell is expected to achieve up to 45%. Thus, InGaP/GaAs/ InGaAsN/Ge (1.8/1.42/ ∼/ 0.7eV) hetero-structures have attracted much attention [2,3] recently. In this study, we tried to optimize the device structure by reducing the N content to 2.8% in the InGaAsN absorption layer. However, we decreased the nitrogen composition from 3.3% to 2.8% by reducing the DMHy/VT ratio, and the 600 nm thick InGaAsN absorption layer is lattice-match to GaAs substrate. Thus, the short-circuit current density could also be considerably increased to 14.8 m A/cm2 without antireflection coating layer. Therefore, the conversion efficiency could be improved to 2.94%.