Increased Exciton Dipole Moment Translates into Charge-transfer Excitons in Thiophene-fluorinated Low-bandgap Polymers for Organic Photovoltaic Applications - Collado, Fregoso E; Boufflet, P; Fei, Z; Gann, E; Ashraf, S; Li, Z; McNeill, C; Durrant, J; Heeney, M (Chemistry of Materials, 2015)