Smaller Voc of 1.0eV/0.75 eV InGaAsP/InGaAs double-junction solar cell（DJSC） than the Voc sum of individual subcells has been observed， and there is little information of the origin of such Voc 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 Voc 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 Voc for further four-junction solar cells.