Synthesis and photovoltaic properties from inverted geometry cells and roll-to-roll coated large area cells from dithienopyrrole-based donor–acceptor polymers

Abstract

A series of donor–acceptor low band gap polymers composed of alternating dithienopyrrole or its derivative as donors and phthalimide or thieno[3,4-c]pyrrole-4,6-dione as acceptors (P1–P4) are synthesized by Stille coupling polymerization. All polymers show strong absorption in the visible region, for P2 and P4 possessing thieno[3,4-c]pyrrole-4,6-dione as an acceptor, their film absorption covers the region of 500–800 nm and 500–750 nm respectively, which makes them attractive as low band gap polymer solar cell (PSC) materials. With the incorporation of thiophene bridges, P3 and P4 have 0.24 and 0.21 eV higher HOMO energy levels than P1 and P2, respectively. A bandgap as low as 1.66 eV is obtained for P2. An up-scaling experiment is performed on bulk-heterojunction PSCs with an inverted device geometry fabricated on a small scale by spin coating and on a large scale using roll-to-roll (R2R) slot-die coating and screen printing. In both cases the best performing polymer is P2 with a V_oc of 0.56 V, a J_sc of −12.6 mA cm⁻², a FF of 40.3%, and a PCE of 2.84% for small spin coated devices, and a V_oc of 0.56 V, a J_sc of −8.18 mA cm⁻², a FF of 30.7%, and a PCE of 1.40% are obtained for R2R-fabricated devices with a significantly better performance than a standard P3HT/PCBM driven device.