This paper reports the study on p-n GaInP2/GaAs tandem cells. The cell samples were produced by metal-organic chemical vapor deposition at a low gas pressure. In order to optimize the device configuration, numerical modeling has been performed for the impacts of a fieldaided collection on the performances of the top cells. On the basis of modeling results, a modified configuration of top cells is introduced, using p+-p—n—n+ structure instead of p+n structure. This modification has brought about much improved photovoltaic performance of the top and tandem cells, with the conversion efficiency Eff=14.26% and 23.82% (AMO, 25℃, 2×2cm2), respectively.