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.