Enhanced Photovoltaic Performance of Amorphous Copolymers
Based on Dithienosilole and Dioxocycloalkene-annelated Thiophene
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Abstract
Organic
photovoltaics (OPVs) have attracted considerable attention
due to their potential for generating renewable energy. The power
conversion efficiency (PCE) of the OPVs largely depends on the organic
semiconducting materials. Thus, the elucidation of structure–property
OPV performance relationships is important for the rational improvement
of OPVs. Here, low-bandgap copolymers comprising dithieno[3,2-<i>b</i>:2′,3′-<i>d</i>]silole as a donor
unit and dialkyl-substituted naphtho[2,3-<i>c</i>]thiophene-4,9-dione
as an acceptor unit were synthesized to investigate the influence
of the polymer molecular weight and the alkyl chain length in the
acceptor unit on the polymer properties and photovoltaic performance.
All the prepared copolymers are amorphous in the solid state. Both
the increase of polymer molecular weight and variation of the alkyl
side chains in the acceptor unit subtly affected molecular properties.
However, these structural modifications showed significant impact
on the photovoltaic performance in bulk heterojunction (BHJ) solar
cells based on copolymer/[6,6]-phenyl-C<sub>71</sub>-butyric acid
methyl ester (PC<sub>71</sub>BM), with PCEs that range between 2.35
and 5.21%. Furthermore, the optimization of thin-film fabrication
by use of a ternary solvent system led to the appearance of improved
morphology accompanied by subtly ordered states of the copolymer in
the BHJ films and, hence, improved carrier mobility and charge-separation
efficiency. Consequently, the BHJ solar cell can achieve a PCE of
7.85%, which is the highest performance among the amorphous copolymers
in the conventional device structure. This result highlights the importance
of fine-tuning both the molecular structure and device fabrication
in the construction of high-performance organic photovoltaics based
on amorphous copolymers and PC<sub>71</sub>BM