Smaller V_oc of 1.0 eV/0.75 eV InGaAsP/InGaAs double-junction solar cell(DJSC) than the V_oc sum of individual subcells has been observed, and there is little information of the origin of such V_oc loss and how to minimize it. In this paper, it is disclosed that the dominant mechanism of minority-carrier transport at back-surface-field(BSF)/base interface of the bottom subcell is thermionic emission, instead of defect-induced recombination, which is in contrast to previous reports. It also shows that both InP and InAlAs cannot prevent the zinc diffusion effectively. In addition, intermixing of major III-V element occurs as a result of increasing thermal treatment. To suppress the above negative effects, an initial novel InP/InAlAs superlattice(SL) BSF layer is then proposed and employed in bottom InGaAs subcell. The V_oc of fabricated cells reach 997.5 mV, and a reduction of 30 mV in Voc loss without lost of Jsc, compared with the results of conventional InP BSF configuration, is achieved. It would benefit the overall V_oc for further four-junction solar cells.