New Two-Dimensional Thiophene−Acceptor Conjugated Copolymers for Field Effect Transistor and Photovoltaic Cell Applications

We report the synthesis, properties, and optoelectronic device applications of two-dimensional (2D) like conjugated copolymers, P4TBT, P4TDTBT, P4TDTQ, and P4TDPP, consisting of 2′,5′′-bis(trimethystannyl)-5,5′′′-di-(2-ethylhexyl)[2,3′;5′,2′′;4′′,2′′′]quarterthiophene ( 4T ) with the following four acceptors of 4,7-dibromo-2,1,3-benzothiodiazole ( BT ), 4,7-di-2-thienyl-2,1,3-benzothiadiazole ( DTBT ), 2,3-bis(4-(2-ethylhexyloxy)phenyl)-5,8-bis[5′-bromodithien-2-ylquinoxalines] ( DTQ ), and 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4- c ]pyrrole-1,4-dione ( DPP ). The optical band gaps (eV) of the studied conjugated copolymers are in the order of P4TDPP (1.29) < P4TDTBT (1.60) < P4TDTQ (1.83) < P4TBT (1.88). The 2D-like conjugated copolymers exhibited high field effect (FET) hole mobilities in the range of 10 −1 −10 −4 cm 2 V −1 s −1 . On the other hand, the FET electron mobilities were observed for P4TDTBT and P4TDPP because of their relatively low-lying LUMO level suitable for electron injection. In particular, P4TDPP showed the ambipolar characteristics with the hole and electronic mobilities of 0.115 cm 2 V −1 s −1 (on/off ratio: 2.49 × 10 4 ) and 3.08 × 10 −3 cm 2 V −1 s −1 (on/off ratio: 7.34 × 10 2 ), respectively, which was strongly related to its ordered intermolecular chain packing based on the DSC and XRD results. The power conversion efficiencies (PCE) of the prepared polymer/PC 71 BM (1:3) based photovoltaic cells were in the range 1.28−1.67% under the illumination of AM 1.5G (100 mW/cm 2 ). The PCE could be enhanced up to 2.43% of the P4TDPP /PC 71 BM (1:2) based device because of the balanced hole/electron mobility. The above results indicate that these two-dimensional 4T −acceptor conjugated copolymers could enhance the charge-transport characteristics and are promising materials for organic optoelectronic devices.