Alternating polymers based on fluorinated alkoxyphenyl-substituted benzo[1,2-b:4,5-b′]dithiophene and isoindigo derivatives for polymer solar cells

Abstract

To effectively modulate the molecular energy levels of isoindigo-based polymers through side-chain modification on the donor segments, three alkoxyphenyl-modified benzo[1,2-b:4,5-b′]dithiophene (BDT) were synthesized and used as electronic-donor units, which were copolymerized with the electron-withdrawing segment 2-ethylhexyl-substituted isoindigo (IID) to construct donor-acceptor (D-A) polymers. The obtained three polymers are named as P-IID, mF-IID and oF-IID, which possess 4-(2-octyldodecaneoxy)-phenyl, 2-fluoro-4-(2-octyldodecaneoxy)-phenyl and 3-fluoro-4-(2-octyldodecaneoxy)-phenyl side-chains in the BDT unit, respectively. All polymers display broad absorption regions with the film absorption edges of 759 nm (P-IID), 742 nm (mF-IID) and 726 nm (oF-IID). The fluorination on the phenyl side-chains of BDT unit results in obvious blue-shift in the absorption spectra, especially for the ortho-position fluorinated polymer oF-IID because of the enlarged dihedral angles between the phenyl rings and the BDT backbones. Gradient reduction in the highest occupied molecular orbital (HOMO) energy levels are observed for the three polymers, with HOMO values of −5.35 eV for P-IID, −5.48 eV for mF-IID and −5.53 eV for oF-IID, separately. As the result of the HOMO energy levels, polymer solar cells (PSCs) with inverted device configuration by blending the polymers with [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) show continually increased open circuit voltage (Voc) from 0.84 V for P-IID to 0.88 V (mF-IID) and then a notable value of 0.96 V for oF-IID. However, the fluorination leads to intensively reduced short-circuit current density (Jsc) of the PSCs, 9.68 mA cm−2 for P-IID, 5.61 mA cm−2 for mF-IID and a very low value of 1.61 mA cm−2 for oF-IID, respectively. Consequently, moderate power conversion efficiency (PCE) of 5.23% is achieved for the non-fluorinated polymer P-IID as well as a decreased value of 2.50% for the meta-fluorinated polymer and a very low PCE of 0.93% for the ortho-fluorinated polymer. These results imply that although fluorine substituted side-chain in the donor unit can effectively deepen the HOMO energy levels of the polymers to obtain enlarged Voc, the absorption losses of the fluorinations caused low Jsc should be taken into consideration to design high efficient electron-donating alternating polymers for polymer solar cells.