Importance of Optimal Composition in Random Terpolymer-Based
Polymer Solar Cells
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Abstract
A new series of donor–acceptor
(D–A) conjugated random
terpolymers (PBDTT–DPP–TPD) were synthesized from electron-rich
thienyl-substituted benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene (BDTT), in conjugation with two electron-deficient
units, pyrrolo[3,4-<i>c</i>]pyrrole-1,4-dione (DPP) and
thieno[3,4-<i>c</i>]pyrrole-4,6-dione (TPD), of different
electron-withdrawing strengths. The optical properties of these random
terpolymers can be easily controlled by tuning the ratio between DPP
and TPD; an increase in TPD induced increased absorption between 400
and 650 nm and a lower highest occupied molecular orbital energy level,
while higher DPP contents resulted in stronger absorption between
600 and 900 nm. The best power conversion efficiency (PCE) of 6.33%
was obtained from PBDTT–DPP75–TPD25 with [6,6]-phenyl-C<sub>71</sub>-butyric acid methyl ester (PC<sub>71</sub>BM) due to the
improved light absorption and thus a short-circuit current density
(<i>J</i><sub>SC</sub>) higher than 16 mA/cm<sup>2</sup>. Interestingly, the trend observed in the PCE values differed from
that of optical behavior of the PBDTT–DPP–TPD in terms
of the DPP to TPD ratio, showing nonlinear compositional dependence
from 2 to 6%. Density functional theory calculations showed that the
small portions of strong electron-withdrawing DPP in PBDTT–DPP25–TPD75
and PBDTT–DPP10–TPD90 could provide trap sites, which
suppress efficient charge transfer. In contrast, for PBDTT–DPP90–TPD10
and PBDTT–DPP75–TPD25, the effect of minor portions
of TPD on electron density distribution was found to be minimal. In
addition, the polymer packing and nanomorphology were investigated
by grazing-incidence X-ray scattering and atomic force microscopy.
The findings suggested that controlling the ratio of electron-deficient
units in the random terpolymers is critical for optimizing their performance
in polymer solar cells because it affects the polymer packing structure,
the optical and electrical properties, and the electron distribution
in the polymers
