Naphthodithiophene-Based
Conjugated Polymer with Linear, Planar Backbone Conformation and Strong
Intermolecular Packing for Efficient Organic Solar Cells
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Abstract
Two donor–acceptor copolymers,
PBDT and PNDT, containing 4,8-bis(2-ethylhexyloxy)benzo[1,2-b:3,4-b′]dithiophene
(BDT) and 4,9-bis(2-ethylhexyloxy)naphtho[1,2-b:5,6-b′]dithiophene
(NDT), respectively, as an electron-rich unit and 5,6-difluoro-2,1,3-benzothiadiazole
(2FBT) as an electron-deficient unit, were synthesized and compared.
The introduction of the NDT core into the conjugated backbone was
found to effectively improve both light harvesting and the charge
carrier mobility by enhancing chain planarity and backbone linearity;
the NDT copolymer has stronger noncovalent interactions and smaller
bond angles than those of the BDT-based polymer. Moreover, the introduction
of the NDT core brings about a drastic change in the molecular orientation
into the face-on motif and results in polymer:PCBM blend films with
well-mixed interpenetrating nanofibrillar bulk–heterojunction
networks with small-scale phase separation, which produce solar cells
with higher short-circuit current density and fill factor values.
A conventional optimized device structure containing PNDT:PC<sub>71</sub>BM was found to exhibit a maximum solar efficiency of 6.35%, an open-circuit
voltage of 0.84 V, a short-circuit current density of 11.92 mA cm<sup>–2</sup>, and a fill factor of 63.5% with thermal annealing,
which demonstrates that the NDT and DT2FBT moieties are a promising
electron-donor/acceptor combination for high-performance photovoltaics