The influence of the number-average molecular weight (M-n) on the blend film morphology and photovoltaic performance of all-polymer solar cells (APSCs) fabricated with the donor polymer poly[5-(2-hexyldodecyl)-1,3-thieno[3,4-c]pyrrole-4,6-dione-alt-5,5-(2,5-bis (3-dodecylthiophen-2-y1)-thiophene)] (PTPD3T) and acceptor polymer poly{[N,N'-bis ( 2-octyldodecyl) naphthalene-1,4,5,8-bis (dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2); N2200) is systematically investigated. The M-n, effect analysis of both PTPD3T and N2200 is enabled by implementing a polymerization strategy which produces conjugated polymers with tunable M(n)s. Experimental and coarse-grain modeling results reveal that systematic M-n, variation greatly influences both intrachain and interchain interactions and ultimately the degree of, phase separation and morphology evolution. Specifically, increasing M-n for both polymers shrinks blend film domain sizes and enhances donor-acceptor polymer-polymer interfacial areas, affording increased short-circuit current densities (J(sc)). However, the greater disorder and intermixed feature proliferation accompanying increasing M-n promotes charge carrier recombination, reducing cell fill factors (FF). The optimized photoactive layers exhibit well-balanced exciton dissociation and charge transport characteristics, ultimately providing solar cells with a 2-fold PCE enhancement versus devices with nonoptimal M(n)s. Overall, it is shown that proper and precise tuning of both donor and acceptor polymer M(n)s is critical for optimizing APSC performance. In contrast to reports where maximum power conversion efficiencies' (PCEs) are achieved for the highest M(n)s, the present two-dimensional M-n optimization matrix strategy locates a PCE "sweet spot" at intermediate M(n)s of both donor and acceptor polymers. This study provides synthetic methodologies to predictably access conjugated polymers with desired M-n and highlights the importance of optimizing M-n for both polymer components to realize the full potential of APSC performance.