Investigations　on　the　relationships　among　the　chemical　structures,　morphology　and　photovoltaic　properties　of　conjugated　polymers　are　crucial　in　designing　high-efficiency　semiconducting　polymers.　Here,　two　novel　copolymers,　PIBTO-T　and　PIBTO-TT,　were　designed　and　synthesized　to　demonstrate　the　improvement　in　photovoltaic　performance　of　conjugated　polymers　by　a　small　change　in　their　chemical　structures.　PIBTO-TT　with　thieno[3,2-b]thiophene　π-bridges　has　a　more　linear　backbone　conformation,　thereby　resulting　in　enhanced　intermolecular　π-π　interactions　compared　to　PIBTO-T　with　thiophene　π-bridges.　Benefiting　from　the　closer　intermolecular　π-π　stacking,　PIBTO-TT:PC71BM　exhibits　a　higher　hole　mobility　than　PIBTO-T:PC71BM.　Morphological　studies　reveal　that　the　miscibility　of　PC71BM　in　PIBTO-TT　is　better　than　that　in　PIBTO-T.　This　enhanced　miscibility　could　shorten　the　distances　between　adjacent　fullerenes　and　improve　electron　transportation　in　the　miscible　region.　Meanwhile,　PIBTO-TT:PC71BM　blends　have　larger　donor/acceptor　interfacial　areas　than　PIBTO-T:PC71BM　samples.　All　these　factors　contribute　to　the　better　photovoltaic　performance　of　PIBTO-TT-based　devices.　This　study　clearly　shows　that　the　morphological　characteristics　and　photovoltaic　properties　of　conjugated　polymers　are　closely　related　to　their　molecular　structures,　and　the　manipulation　of　backbone　conformation　through　π-bridge　modulation　is　a　promising　molecular　engineering　approach　to　improve　the　photovoltaic　properties　of　conjugated　polymers.　©　The　Royal　Society　of　Chemistry　2018.