Angular-Shaped 4,9-Dialkyl α- And β-Naphthodithiophene-Based Donor-Acceptor Copolymers: Investigation of Isomeric Structural Effects on Molecular Properties and Performance of Field-Effect Transistors and Photovoltaics

Two angular-shaped 4,9-didodecyl α-aNDT and 4,9-didodecyl β-aNDT isomeric structures have been regiospecifically designed and synthesized. The distannylated α-aNDT and β-aNDT monomers are copolymerized with the Br-DTNT monomer by the Stille coupling to furnish two isomeric copolymers, PαNDTDTNT and PβNDTDTNT, respectively. The geometric shape and coplanarity of the isomeric α-aNDT and β-aNDT segments in the polymers play a decisive role in determining their macroscopic device performance. Theoretical calculations show that PαNDTDTNT possesses more linear polymeric backbone and higher coplanarity than PβNDTDTNT. The less curved conjugated main chain facilitates stronger intermolecular π-π interactions, resulting in more redshifted absorption spectra of PαNDTDTNT in both solution and thin film compared to the PβNDTDTNT counterpart. 2D wide-angle X-ray diffraction analysis reveals that PαNDTDTNT has more ordered π-stacking and lamellar stacking than PβNDTDTNT as a result of the lesser curvature of the PαNDTDTNT backbone. Consistently, PαNDTDTNT exhibits a greater field effect transistor hole mobility of 0.214 cm² V^-1 s^-1 than PβNDTDTNT with a mobility of 0.038 cm² V^-1 s^-1. More significantly, the solar cell device incorporating the PαNDTDTNT:PC₇₁BM blend delivers a superior power conversion efficiency (PCE) of 8.01% that outperforms the PβNDTDTNT:PC₇₁BM-based device with a moderate PCE of 3.6%. Two new 4,9-dialkyl α- and β-naphthodithiophene-based D-A copolymers, PαNDTDTNT and PβNDTDTNT, are presented. With the better ordered structures in the solid state, PαNDTDTNT exhibits a greater field-effect transistor hole mobility of 0.214 cm² V^-1 s^-1 and a superior solar cell efficiency of 8.01% than PβNDTDTNT with a mobility of 0.038 cm² V^-1 s^-1 and a PCE of 3.6%.