Influence of Covalent and Noncovalent Backbone Rigidification Strategies on the Aggregation Structures of a Wide-Band-Gap Polymer for Photovoltaic Cells

With
the purpose of improving the backbone planarity and thus the
charge transport property of the wide-band-gap ester-modified polymer,
the covalent and noncovalent (F···S conformational
lock) backbone rigidification strategies are, respectively, employed
to design two new benzodithiophene and ester-modified oligothiophene-based
copolymers (PBDE-TT and PBDE-DFDT). Although thieno­[3,2-b]­thiophene (TT) and difluorinated 2,2′-bithiophene (DFDT)
possess planar conformations with similar conjugation length, polymer
PBDE-DFDT shows much stronger aggregation effects in diluted solution
and more compact and ordered π–π stacking in thin
film compared to the polymer PBDE-TT, which are associated with the
existence of several nontraditional hydrogen-bonding interactions
for the DFDT-based oligothiophene unit. Thus, the hole mobility of
polymer PBDE-DFDT is over two times higher than that of its counterpart.
The photovoltaic device based on PBDE-DFDT:IT-4F blend achieves a
distinctly higher power-conversion efficiency of 14.16%, which is
mainly attributed to the higher ordering of aggregates and more symmetric
charge transport in the blend film. The results reveal that optimizing
the aggregation structures via the modulation of intermolecular interactions
is of great importance for enhancing the charge mobility and photovoltaic
performance of materials, which shall be considered for further designing
high-performance donor polymers with simple chemical structures.