Polymer solar cells (PSCs) are fabricated without solvent additives using a low-bandgap polymer, PBDTTT-C-T, as the donor and [6,6]-phenyl-C61-butyric-acid-methyl-ester (PC61BM) as the acceptor. Donor-acceptor blend and layer-by-layer (LL) solution process are used to form active layers. Relative to the blend devices, the LL devices exhibit stronger absorption, better vertical phase separation, higher hole and electron mobilities, and better charge extraction at correct electrodes. As a result, after thermal annealing the LL devices exhibit an average power conversion efficiency (PCE) of 6.86%, which is much higher than that of the blend devices (4.31%). The best PCE of the LL devices is 7.13%, which is the highest reported for LL processed PSCs and among the highest reported for PC61BM-based single-junction PSCs.