Effect of Incorporated
Nitrogens on the Planarity and Photovoltaic Performance of Donor–Acceptor
Copolymers
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Abstract
Systematic control of the chemical structure of conjugated
polymers is critically important to elucidate the relationship between
the conjugated polymer structures and properties and to optimize their
performance in bulk heterojunction (BHJ) polymer solar cell (PSC)
devices. Herein, we synthesized three new copolymers, i.e., <b>P0</b>, <b>P1</b>, and <b>P2</b>; these copolymers
contain the same benzodithiophene donor unit but have different acceptor
units with different numbers of nitrogen atoms in the range of 0–2.
The effects of the introduced nitrogen atoms on the structural, optical,
electrical, and photovoltaic properties of the conjugated polymers
were investigated; the structural properties of the polymers, in particular,
were studied using both experimental (grazing-incidence X-ray scattering
(GIXS) measurements) and computational methods (molecular simulation).
As the number of introduced nitrogen atoms increased, the planarity
of the main chain conformation increased in the order of <b>P0</b> < <b>P1</b> < <b>P2</b>. Additionally, the <b>P0</b>, <b>P1</b>, and <b>P2</b> polymers showed increased
interlayer domain spacings of 1.61, 1.72, and 1.78 nm, respectively,
with increased intermolecular ordering. These results were in excellent
agreement with the simulation results. In addition, the enhanced planarity
resulted in a red-shifting at the onset of absorption in the polymer
film from 544 to 585 nm, a downshift in the lowest unoccupied molecular
orbital (LUMO) energy level from −3.02 to −3.26 eV,
and an increase in the hole mobility from 2.33 × 10<sup>–6</sup> to 3.78 × 10<sup>–5</sup> cm<sup>2</sup>/(V s). As a
result, we observed dramatically enhanced performance of the PSCs
in the order of <b>P0</b> < <b>P1</b> < <b>P2</b>. For example, the <b>P2</b>:PC<sub>61</sub>BM device exhibited
a 3.5-fold improvement in power conversion efficiency (PCE) compared
to that of <b>P0</b>:PC<sub>61</sub>BM. The further optimization
of <b>P2</b> with PC<sub>71</sub>BM showed the PCE of 3.22%
