Effect of diazine isomers on photovoltaic properties of simple and efficient wide band gap polymer donor materials

In this paper, we designed and synthesized six new wide band gap donor-acceptor (D-A) copolymers, PZ2-PZ7, with diazine isomers (pyrazine, pyridazine, pyrimidine) as the A unit and alkylthiophene- or alkylthiothienyl-substituted BDT as the D unit. The effect of diazine isomeric A units and side-chain substituents on molecular structure, optoelectronic properties and photovoltaic performances of the resulting D-A copolymers were investigated in details. Six low-cost polymers were easily obtained by minimal reaction steps and cheap raw materials. All the polymers exhibited wide band gaps (WBGs) (>1.9 eV) and deep-lying HOMO energy levels (<−5.48 eV), which is convenient for paring with low band gap nonfullerene acceptors (NFAs). Theoretical simulation calculation indicates that diazine isomeric A unit has great impact on the planarity of polymer backbone. Bulk heterojunction polymer solar cells (PSCs) were fabricated with these polymers as donor materials and representative NFAs (Y6 and ITIC) as acceptor materials. After optimization, pyrazine-based polymers PZ2 and PZ3 exhibited higher PCEs than those of pyridazine- and pyrimidine-based ones (PZ4-PZ7) regardless of the acceptors. Finally, the PZ2:Y6-based PSC achieved the highest power conversion efficiency (PCE) up to 11.21 %, which could be attributed to more balanced carrier mobility, increased charge dissociation probability, reduced charge recombination, and superior active layer morphology. This work suggests that diazine-based WBG polymers, especially for pyrazine, would become as very promising donor candidates for constructing low-cost and efficient NFA-based PSCs.