We explore the interrelation between density of states, recombination kinetics, and device performance in efficient poly[4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one)thieno[3,4-b]thiophene-2,6-diyl]:[6,6]-phenyl-C71-butyric acid methyl ester (PBDTTT-C:PC71BM) bulk-heterojunction organic solar cells. We modulate the active-layer density of states by varying the polymer:fullerene composition over a small range around the ratio that leads to the maximum solar cell efficiency (50–67 wt% PC71BM). Using transient and steady-state techniques, we find that nongeminate recombination limits the device efficiency and, moreover, that increasing the PC71BM content simultaneously increases the carrier lifetime and drift mobility in contrast to the behavior expected for Langevin recombination. Changes in electronic properties with fullerene content are accompanied by a significant change in the magnitude or energetic separation of the density of localized states. Our comprehensive approach to understanding device performance represents significant progress in understanding what limits these high-efficiency polymer:fullerene systems.