Nonfullerene　acceptors　are　being　investigated　for　use　in　polymer　solar　cells　(PSCs),　with　their　advantages　of　extending　the　absorption　range,　reducing　the　energy　loss　and　therefore　enhancing　the　power　conversion　efficiency　(PCE).　However,　to　further　boost　the　PCE,　mobilities　of　these　nonfullerene　acceptors　should　be　improved.　For　nonfullerene　acceptors,　the　pi-pi　stacking　distance　between　cofacially　stacked　molecules　significantly　affects　their　mobility.　Here,　we　demonstrate　a　strategy　to　increase　the　mobility　of　heteroheptacene-based　nonfullerene　acceptors　by　reducing　their　pi-pi　stacking　distances　via　control　over　the　bulkiness　of　lateral　side　chains.　Incorporation　of　2-butyloctyl　substituents　into　the　nonfullerene　acceptor　(M36)　leads　to　an　increased　mobility　with　a　reduced　pi-pi　stacking　distance　of　3.45　angstrom.　Consequently,　M36　affords　an　enhanced　PCE　of　16%,　which　is　the　highest　among　all　acceptor-donor-acceptor-type　nonfullerene　acceptors　to　date.　This　strategy　of　control　over　the　bulkiness　of　side　chains　on　nonfullerene　acceptors　should　aid　the　development　of　more　efficient　PSCs.