Ladder-Type Dithienonaphthalene-Based Donor-Acceptor Copolymers for Organic Solar Cells

A novel ladder-type dithienonaphthalene (DTN) was designed and synthesized as an electron-rich unit for constructing donor-acceptor copolymers. Different acceptor moieties, including benzo[c][1,2,5]thiadiazole (BT), 5,6-bis(hexyloxy)-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (TBT), and 2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,SH)-dione (TDPP), were used as electron-deficient units for the target copolymers. These copolymers (PDTNBT, PDTNTBT, and PDTNTDPP) were obtained via the Stile coupling reaction and characterized by H-1 NMR spectroscopy, UV-vis absorption spectroscopy, cyclic voltammetry, and gel permeation chromatography (GPC). Owing to the four solubilizing alkyl chains on the DTN unit, all the three copolymers have good solubility in common solvents. Among these polymers, PDTNTBT exhibits the highest space-charge limit current (SCLC) hole mobility of 2.13 x 10(-5) cm(2) V-1 s(-1), which is beneficial for achieving high performance solar cells. Under the simulated AM 1.5G illumination condition (100 mW/cm(2)), solar cells based on PDTNTBT:PC71BM (1:3, w/w) exhibit a power conversion efficiency (PCE) of 4.8% with a current density of 10.3 mA cm(-2), an open-circuit voltage of 0.86 V, and a fill factor of 54%. With the same device fabrication method, PDTNTDPP:PC71BM (1:3, w/w) and PDTNBT:PC71BM (1:3, w/w) based devices exhibit efficiencies of 1.52% and 2.79%, respectively. Furthermore, inverted solar cells based on these copolymer blends are also fabricated. The inverted devices based on PDTNTDPP:PC71BM (1:2, w/w) and PDTNBT:PC71BM (1:2, w/w) exhibit PCEs of 1.60% and 2.89%, respectively, which are similar to their corresponding conventional devices. And the inverted devices based on PDTNTBT:PC71BM (1:2, w/w) show a higher PCE of 5.0%, and more importantly, they are quite stable as demonstrated by the 4.75% PCE after ambient storage for two months.