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 Voc of 0.56 V, a Jsc of -12.6 mA cm-2, a FF of 40.3%, and a PCE of 2.84% for small spin coated devices, and a Voc of 0.56 V, a Jsc 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. © 2013 The Royal Society of Chemistry.
Bibliographical noteThe authors gratefully acknowledge supports from the Danish
National Research Foundation (DNRF) and the National Natural
Science Foundation of China (NSFC, Grant no. 51011130028)
for the Danish-Chinese Center for Organic based Photovoltaic
Cells with Morphological Control, within which this work was
performed. Support from Sino-Danish Centre for Education and
Research (SDC) is also fully acknowledged.
- Energy gap
- Solar cells
- Plastic coatings