Porphyrin−Dithienothiophene π-Conjugated Copolymers: Synthesis and Their Applications in Field-Effect Transistors and Solar Cells

Soluble conjugated alternating porphyrin−dithienothiophene copolymers—single-bond linked ( I ) and triple-bond linked ( IIa and IIb )—were synthesized by palladium(0)-catalyzed Stille and Sonagashira coupling reactions, respectively. The thermal, electrochemical, optical, charge transport, and photovoltaic properties of these copolymers were examined; the effect of the triple bond was studied. I exhibits onset decomposition temperature ( T d ) of 410 °C and glass-transition temperature ( T g ) of 180 °C, higher than those of IIb ( T d, 330 °C; T g, 130 °C). The absorption spectrum of I in thin film exhibits a sharp Soret band at 450 nm and two weak Q-bands at 563−619 nm, while IIb exhibits a sharp Soret band at 491 nm and a strong Q-band at 760 nm. The emission maxima of I and IIb in solution are located at 642 and 722 nm respectively. IIb is electrochemically active in both the oxidation and reduction regions, while I shows only oxidation peak. The field-effect hole mobilities as high as 2.1 × 10 −4 cm 2 V −1 s −1 were obtained for these copolymers. Polymer solar cells (PSCs) were fabricated based on the blend of the polymers and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM). The power conversion efficiency (PCE) of 0.3% was achieved under AM 1.5, 100 mW/cm 2 for the PSC using IIb:PCBM (1:3, w/w) as active layer. The PCE of the PSC based on IIb:PCBM (1:3, w/w) is double that based on I:PCBM (1:2, w/w), consistent with that IIb exhibits stronger Q-band absorption and higher mobility at room temperature.