Fluorination of Benzothiadiazole–Benzobisthiazole
Copolymer Leads to Additive-Free Processing with Meliorated Solar
Cell Performance
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Abstract
Processing
solvents and conditions have unique importance in the
performance of bulk heterojunction organic solar cells. In the present
work, we have investigated the role of a primary solvent and solvent
additive in the device performance of two benzobisthiazole (BBTz)-based
push–pull type polymers. In an inverted cell structure, the
BBTz-<i>co</i>-fluorinated benzothiadiazole polymer (PBBTzFT)
with a PC<sub>71</sub>BM acceptor showed additive-free enhanced performance
with a power conversion efficiency (PCE) of 6.4% from a 1,2-dichlorobenzene
solvent, while the BBTz-<i>co</i>-pyridylthiadiazole polymer
(PBBTzPT) showed maximum performance from a chlorobenzene (CB) solution
with a 1,8-diiodooctane (DIO) additive (PCE = 2.3%). The detailed
investigation by atomic force microscopy and two-dimensional grazing
incidence X-ray diffraction corroborates that the fluorination of
benzothiadiazole brought about optimal morphology without a solvent
additive, the PCE of which is comparable with the previous nonfluorinated
analogue (PCE = 6.5%) processed from CB with DIO