Tuning
the Organic Solar Cell Performance of Acceptor
2,6-Dialkylaminonaphthalene Diimides by Varying a Linker between the
Imide Nitrogen and a Thiophene Group
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Abstract
Core-substituted naphthalene diimides
(NDI) are promising candidates
as acceptors for organic solar cells. To study their structure–property
relationships, a series of 2,6-dialkylamino-NDI compounds with various
substituents were synthesized, characterized, and tested in bulk heterojunction
solar cells by blending with regioregular poly(3-hexylthiophene) (P3HT).
The imide substituents consisted of a linker connected to a thiophene
group, where the linker was phenyl, methyl, or ethyl. The core substituents
were cyclohexylamino or 2-ethylhexylamino. While the various substituents
had little effect on the optoelectronic properties in solution, they
strongly affected device performance and blend morphology. Under the
conditions studied, the best performance was obtained with the methyl
linker combined with the cyclohexylamino core substituent, with a
power conversion efficiency of 0.48% and a high open circuit voltage
of 0.97 V. For blends of P3HT with modified NDI non-fullerene acceptors,
the methyl linker promoted larger phase-separated domains than the
ethyl or phenyl linkers. DFT calculations showed that the linker determines
the orientation of the thiophene conjugated plane with respect to
the NDI conjugated plane. That angle was 114°, 45°–61°,
and 8° for the methyl, phenyl, and ethyl linkers, respectively.
Using thiophene at the end of the imide substituent adds a unique
dimension to tune morphology and influence the molecular heterojunction
between donor and acceptor