Conjugated polymers with polar side chains in bulk heterojunction solar cell devices

Two polymers with polar side chains, namely poly[2,7‐(9,9‐dioctylfluorene)‐alt‐5,5‐(5',8'‐di‐2‐thienyl‐(2',3'‐bis(3''‐(2‐(2‐methoxyethoxy)ethoxy)phenyl)quinoxaline))] (P1) and poly[2,7‐(9,9‐bis(2‐(2‐methoxyethoxy)ethyl)fluorene)‐alt‐5,5‐(5',8'‐di‐2‐thienyl‐(2',3'‐bis(3''‐(2‐(2‐methoxyethoxy)‐ethoxy)phenyl)quinoxaline))] (P2), were synthesized for solar cell application. A series of bulk heterojunction solar cells were systematically fabricated and characterized by varying the electron‐acceptor materials, processing solvents and thickness of the active layer. The results show that P1, with a higher molecular weight and good film‐forming properties, performed better. The best device showed an open circuit voltage of 0.87 V, a short circuit current of 6.81 mA cm−2 and a power conversion efficiency of 2.74% with 1:4 polymer:[6,6]‐phenyl‐C71‐butyric acid methyl ester (PCBM[70]) mixture using o‐dichlorobenzene (o‐DCB) as processing solvent. P2 on the other hand showed a poorer performance with chlorobenzene as processing solvent, but a much improved performance was obtained using o‐DCB instead. Thus, an open circuit voltage of 0.80 V, short circuit current of 6.21 mA cm−2 and an overall power conversion efficiency of 2.22% were recorded for a polymer:PCBM[70] mixing ratio of 1:4. This is presumably due to the improvement of the morphology of the active layer using o‐DCB as processing solvent.