Triple Design Strategy for Quinoxaline-Based Hole Transport Materials in Flexible Perovskite Solar Cells

Molecular design strategies such as noncovalent conformational locks, self-assembly, and D-A molecular skeletons have been extensively used to devise efficient and stable hole transport materials. Nevertheless, most of the existing excellent examples involve only single or dual strategies, and triple strategies remain scarcely reported. Herein, we attempt to develop two quinoxaline-based hole transport materials (DQC-T and DQ-T-QD) through a triple strategy encompassing an S···N noncovalent conformational lock, D-A molecular skeletons, and self-assembly or conjugate engineering. The S···N noncovalent conformational lock formed by thiophene sulfur atoms and quinoxaline nitrogen atoms improves molecular planarity, further inducing the formation of high-quality perovskite films and enhancing hole transport ability; the asymmetric D-A molecular backbone endows the material with a larger dipole moment (μ = 5.80 D) to promote intramolecular charge transfer; and the carboxyl group, methoxy, and sulfur atom establish strong interactions between the NiO_x and perovskite layers, including self-assembly and defect passivation, which mitigates the occurrence of detrimental interfacial charge recombination and reactions. Thus, the 2-thiophenecarboxylic acid derivative DQC-T, featuring an asymmetric D-A molecular backbone, exhibits superiority in terms of good interface contact, hole extraction, and transport compared to DQ-T-QD with a D-A-π-A-D type structure. Naturally, the optimal power conversion efficiency of NiO_x/DQC-T-based p-i-n flexible perovskite solar cells is 18.12%, surpassing that of NiO_x/DQ-T-QD-based devices (16.67%) and NiO_x-based devices with or without DQC (a benzoic acid derivative without a noncovalent conformational lock) as co-HTMs (16.75% or 15.52%). Our results reflect the structure-performance relationship well, and provide a referable triple strategy for the design of new hole transport materials.