Structure-Property Relationship of Naphthalene Diimide-Fused Thiophene Donor-Acceptor Copolymers for Organic...

Naphthalene diimide (NDI)-based donor-acceptor (D-A) copolymers have potential to be used as ambivalent materials for single-material organic solar cells (SMOSCs). Herein, we systematically explored NDI-fused-thiophene D-A copolymers by varying the donor $\pi$ -conjugation length by changing the number of fused- thiophene rings. Increase in $\pi$ -conjugation length of donor-fused thiophene rings significantly decreases the bandgap, from 1.95 to 1.64 eV due to the upshift of the highest occupied molecular orbital (HOMO) energy level, from -5.84 to -5.54 eV. The calculation of electron and hole reorganization energies indicates that increasing the fused-thiophene ring in donor unit may lead to a balanced electron-hole mobility promoting ambipolar transport and also enhances optical properties and photovoltaic performance. The morphological study indicates that increasing the $\pi$ conjugation of donor moieties leads to improved flexibility in the copolymers accompanied by an initial decrease in crystallinity followed by an increase in crystallinity for NDI-4fTh. Consequently, NDI-4fTh exhibits enhanced charge transport properties and, thus, better performance for organic solar cells. Overall, this study highlights the structure-property relationship relevant to next-generation organic photovoltaic devices through careful tuning of the $\pi$ -conjugation length of the donor moieties in NDI-basedcopolymers.