Improving the photovoltaic performance of ladder-type dithienonaphthalene-containing copolymers through structural isomerization

Abstract

A ladder-type angular-shaped dithienonaphthalene (aDTN), an isomer of ladder-type linear-shaped dithienonaphthalene (DTN), was designed and synthesized as an electron-rich unit to construct donor–acceptor copolymers with deep-lying highest occupied molecular orbital (HOMO) energy levels. Benzo[c][1,2,5]thiadiazole (BT) with various substituents were used as electron deficient units for synthesizing the target copolymers (PaDTNBTO, PaDTNBTH, and PaDTNBTF) via the Stille coupling reaction. Incorporating different substituents onto the BT moiety has significant effects on the photophysical and electrochemical properties of the copolymers, as well as on the roughness of the polymer/PC₇₁BM blends. With four solubilizing alkyl chains on the aDTN unit, all its three copolymers have good solubility in common solvents. The synthesized copolymers exhibit deep-lying HOMO energy levels, leading to high open circuit voltages (V_oc ≥ 0.90 V) of the resulting polymer solar cells. The bulk heterojunction solar cell based on the aDTN-containing copolymers (PaDTNBTO) shows an improved efficiency of 6.44% and an increased V_oc of 0.92 V compared to that based on the linear-shaped DTN containing counterpart (efficiency = 4.78%, V_oc = 0.86 V). Whereas, under the same device fabrication conditions, PaDTNBTH- and PaDTNBTF-based devices exhibit efficiencies of 5.22% and 1.73%, respectively. Our results demonstrate that aDTN is a better building block in constructing p-type copolymers for high open circuit voltage devices compared to the linear-shaped DTN.