In this work, five
PTB7-Th-based conjugated polymers (PTB7-Th,
PTBSi20, PTBSi25, PTBSi33, and PTBSi100) with different contents of
siloxane-terminated pentyl side chain were synthesized, and properties
of corresponding blend films with narrow band gap nonfullerene IEICO-4F
acceptor were extensively investigated. According to the contact angle
testing, the PTB7-Th with 100% alkyl side chain and PTBSi100 100%
siloxane-terminated side chain on the benzodithiophene unit showed
surface energy values of 40.04 and 34.52 mJ/m2, respectively.
The results demonstrate that relative to alkyl side chain in PTB7-Th,
the siloxane-terminated side chain could effectively reduce the surface
energy of a resulting polymer. Based on Flory–Huggins interaction
parameter estimations, the miscibility between the polymer and IEICO-4F
would vary in an order of PTB7-Th > PTBSi20 > PTBSi25 > PTBSi33
>
PTBSi100, suggesting that siloxane-terminated side chain would afford
a tunable driving force for phase separation. Transmission electron
microscopy and Raman mapping could confirm large bulk domains inside
the PTBSi100:IEICO-4F blend film. In polymer solar cells, the blend
film of the PTBSi100 with the lowest miscibility to IEICO-4F showed
an undesirable power conversion efficiency (PCE) of 8.52%, which was
significantly lower than that of 11.23% for PTB7-Th, suggesting that
too large phase separation driving force is not beneficial for the
device performance. Side-chain random copolymers PTBSi20, PTBSi25,
and PTBSi33 for fine tuning could display PCEs of 11.94, 12.61, and
11.80%, respectively, all higher than that of PTB7-Th. Our results
not only reveal the big surface energy difference between the siloxane-terminated
side chain and the common alkyl side chain but also provide a guideline
for side chain engineering of conjugated polymer donors with tunable
morphology and optimal matching with a nonfullerene acceptor.
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