Copolymers Based on π‑Conjugated Asymmetric Naphthalene Diimide Building Blocks: Synthesis, Crystallography, and Structure–Property–Charge Transport/Photovoltaic Correlations

Symmetric and new asymmetric electron-transporting naphthalene
diimide (NDI) copolymers comprising five-member ring
heterocyclic (Het) donor units [Het = furan (Fu), thiophene (Th),
and selenophene (Se)] were synthesized via anion radical polymerization
of dibrominated Het-NDI-Het monomers, which avoids the
use of toxic reagents. Product polymers and monomeric building blocks
were characterized by a battery of techniques including nuclear magnetic
resonance, optical absorption, and Raman spectroscopy, as well as
cyclic voltammetry and single-crystal X-ray diffraction. DFT computations
were carried out for trimer models of the polymers. These comprehensive
data indicate that furan unit incorporation results in more planar
blocks/polymer backbones with elevated HOMO energies but limited Fu-NDI
skeletal π-connectivity versus those of the Th and Se analogues.
The resulting polymers were employed to fabricate organic field-effect
transistors (OFETs) and all-polymer solar cells (APSCs) affording
electron mobilities ranging from 0.012 cm2 V–1 s–1 to 0.24 cm2 V–1 s–1 and power conversion efficiencies from 1.71%
to 6.41%. In all cases, the Fu-containing polymers exhibit the lowest
performance, and PCEs within the series of polymers based on asymmetric
NDI building blocks outperform those with symmetric building blocks.
Films of the pristine polymers and their blends with the polymer donor PBDB-T were characterized by AFM, TEM, and GIWAXS and, for
the blends, also by space-charge limited current measurements. Together
these results provide important geometry–electronic structure–performance
correlations, illuminating the reasons underlying the key device performance
trends.