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 _g) 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. lib 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 cm² 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/cm² 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 lib exhibits stronger Q-band absorption and higher mobility at room temperature.