We report that sequential deposition of a highly crystalline polymer donor and a soluble fullerene acceptor leads to a well-defined interpenetrating network and enhanced power conversion efficiencies in bilayer polymer solar cells. Even without the use of solvent additives, layered thin films of poly[(5,6-difluoro-2,1,3-benzothiadiazol4,7-diyl)-alt-(3,3.-di(2-octyldodecyl)-2,2'; 5',2''; 5.,2.-quaterthiophen-5,5.-diyl)] (PffBT4T-2OD) and [6,6]phenyl C71-butyric acid methyl ester (PC71BM), as electron donor and acceptor materials, respectively, showed bicontinuous networks similar to those of a PffBT4T-2OD:PC71BM bulk-heterojunction (BHJ) thin film processed with 1,8-diiodooctane (DIO) as a solvent additive. Transmission electron microscopy results confirmed the BHJlike morphology of the bilayered PffBT4T-2OD/PC71BM thin films. Bilayer solar cells fabricated without the DIO additive produced a power conversion efficiency of. 7.65%, which is even higher than that of a BHJ solar cell fabricated with the DIO additive (. 7.04%). These results demonstrate that a highly crystalline polymer donor and an electron-accepting small molecule can be a good combination for efficient bilayer polymer solar cells.