Compared with fullerene-based bulk heterojunction (BHJ) organic solar cells (OPV), all-polymer solar cells provide advantages such as flexibility of HOMO-LUMO tuning, versatility of chemical design and potentially low cost. Yet the power convention efficiency (PCE) of all-polymer solar cells is still much lower than that of fullerene-based BHJs. Fundamental understanding of all-polymer solar cell device structure-function relationships is indispensible in order to enhance PCE of all-polymer solar cells in an efficient way. During previous research, domain details have been intensively studied and correlated with device performance[1]. For example, domain size is often correlated with short circuit current density (Jsc) and domain purity is correlated with fill factor (FF). Among these key parameters, molecular orientation has recently been found critical to determine the device performance in some fullerene-based OPV[2]. However, morphology study on all-polymer solar cells is difficult due to the intrinsically low contrast between components in all-polymer solar cells. Resonant x-ray scattering (R-SoXS) has emerged as a powerful tool of morphology study for all-polymer solar cells because of the relative strong material contrast it provides. Here we use PBDTBDD:PNDI and PBDTBDD-T:PNDI as a model system to elucidate the correlation between molecular orientation and device performance. By utilizing R-SoXS, the molecular orientation relative to donor/acceptor interface in the blend is characterized and correlated with device performance. We found that by replacing the donor polymer PBDTBDD with its 2-D conjugated counterpart PBDTBDD-T, the PCE was boosted from 2.4% for PBDTBDD:PNDI to 5.8% for PBDTBDD-T:PNDI and significant differences in homo-polymer and heteropolymer pi-pi interactions are observed. The molecular orientation variation relative to donor/acceptor interface can be correlated with the charge separation and recombination processes. Improved charge transport for PBDTBDD-T is related to improved π-stacking and face-on orientation relative to the electrodes. The results indicate that molecular design can be used to control molecular orientations correlations in a device for improved performance. [1] BA Collins, Z Li, JR Tumbleston, E Gann, CR McNeill… - Advanced Energy Materials, 2013, 3, 65-74 [2] JR Tumbleston, BA Collins, L Yang, AC Stuart, E Gann… - Nature Photonics, 2014, 8, 385-391