Low　bandgap　polymer　(LBG):fullerene　mixtures　are　some　of　the　most　promising　organic　photovoltaic　active　layers.　Unfortunately,　there　are　no　post-deposition　treatments　available　to　rationally　improve　the　morphology　and　performance　of　as-cast　LBG:fullerene　OPV　active　layers,　where　thermal　annealing　usually　fails.　Therefore,　there　is　a　glaring　need　to　develop　post-deposition　methods　to　guide　the　morphology　of　LBG:fullerene　bulk　heterojunctions　towards　targeted　structures　and　performance.　In　this　paper,　the　structural　evolution　of　PCPDTBT:PCBM　mixtures　with　solvent　annealing　(SA)　is　examined,　focusing　on　the　effect　of　solvent　quality　of　the　fullerene　and　polymer　in　the　annealing　vapor　on　morphological　evolution　and　device　performance.　The　results　indicate　that　exposure　of　this　active　layer　to　the　solvent　vapor　controls　the　ordering　of　PCPDTBT　and　PCBM　phase　separation　very　effectively,　presumably　by　inducing　component　mobility　as　the　solvent　plasticizes　the　mixture.　These　results　also　unexpectedly　indicate　that　solvent　annealing　in　a　selective　solvent　provides　a　method　to　invert　the　morphology　of　the　LBG:fullerene　mixture　from　a　polymer　aggregate　dispersed　in　a　polymer:fullerene　matrix　to　fullerene　aggregates　dispersed　in　a　polymer:fullerene　matrix.　The　judicious　choice　of　solvent　vapor,　therefore,　provides　a　unique　method　to　exquisitely　control　and　optimize　the　morphology　of　LBG　conjugated　polymer/fullerene　mixtures.　Solvent　annealing　in　a　selective　solvent　provides　a　method　to　invert　the　morphology　of　low　bandgap　polymer　(LBG):fullerene　mixtures　from　a　polymer　aggregate　dispersed　in　a　polymer:fullerene　matrix　to　fullerene　aggregates　dispersed　in　a　polymer:fullerene　matrix.　The　judicious　choice　of　solvent　vapor　provides　a　unique　method　to　exquisitely　control　and　optimize　the　morphology　and　performance　of　LBG:fullerene　mixtures.　©　2014　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim.