Donor-acceptor type conjugated copolymers based on alternating BNBP and oligothiophene units: from electron acceptor to electron donor and from amorphous to semicrystalline

Donor-acceptor (D-A) type conjugated polymers are widely used in high-performance organic opto-electronic devices. In this work, we develop a series of D-A type conjugated polymers, P-nT (n = 1-5), based on alternating double B <- N bridged bipyridine (BNBP) unit and oligothiophene units of various length. In P-1T, P-2T, P-3T, P-4T and P-5T, the D units are thiophene, bi-thiophene, tri-thiophene, tetra-thiophene and penta-thiophene, respectively. We find that the opto-electronic properties and thin film morphologies of the polymers can be substantially changed by the length of the oligothiophene units. From P-1T to P-5T, both the HOMO and LUMO energy levels of the polymers gradually upshift because of the electron-donating nature of oligothiophene units. P-1T, P-2T and P-3T can be used as electron acceptors in all-polymer solar cells, while P-4T and P-5T can work as electron donors in fullerene-based organic solar cells. In addition, with the increasing length of the oligothiophene units, the crystallinity of the polymers increased. P-1T and P-2T are amorphous, while P-3T, P-4T and P-5T show semicrystalline property in solid state. All-polymer solar cell with P-3T as the electron acceptor exhibits a power conversion efficiency (PCE) of 6.52% and the fullerene-based organic solar cells with P-5T as the electron donor exhibits a PCE of 5.79%. These results provide a molecular design guideline to rationally tune optical, electronic and crystalline properties of D-A type conjugated polymers.