A pentacyclic nitrogen-bridged thienyl-phenylene-thienyl arene for donor-acceptor copolymers: Synthesis, characterization, and applications in field-effect transistors and polymer solar cells

A pentacyclic benzodipyrrolothiophene (BDPT) unit, in which two outer thiophene rings are covalently fastened with the central phenylene ring by nitrogen bridges, was synthesized. The two pyrrole units embedded in BDPT were constructed by using one-pot palladium-catalyzed amination. The coplanar stannylated Sn-BDPT building block was copolymerized with electron-deficient thieno[3,4-c]pyrrole-4,6-dione (TPD), benzothiadiazole (BT), and dithienyl-diketopyrrolopyrrole (DPP) acceptors by Stille polymerization. The bridging nitrogen atoms make the BDPT motif highly electron-abundant and structurally coplanar, which allows for tailoring the optical and electronic properties of the resultant polymers. Strong photoinduced charge-transfer with significant band-broadening in the solid state and relatively higher oxidation potential are characteristic of the BDPT-based polymers. Poly(benzodipyrrolothiophene-alt-benzothiadiazole) (PBDPTBT) achieved the highest field-effect hole mobility of up to 0.02 cm ² V ^-1 s ^-1. The photovoltaic device using the PBDPTBT/PC ₇₁BM blend (1:3, w/w) exhibited a V _oc of 0.6 V, a J _sc of 10.34 mA cm ^-2, and a FF of 50 %, leading to a decent PCE of 3.08 %. Encouragingly, the device incorporating poly(benzodipyrrolothiophene-alt- thienopyrrolodione) (PBDPTTPD)/PC ₇₁BM (1:3, w/w) composite delivered a highest PCE of 3.72 %. The enhanced performance arises from the lower-lying HOMO value of PBDPTTPD to yield a higher V _oc of 0.72 V.