Ladder-like Conjugated Polymers with Substituent-Driven π–π* Interactions as Hole-Transporting Materials for Perovskite Solar Cells

Development of dopant-free HTMs is important for boosting the performance and durability of perovskite solar cells. Here, we report the design and synthesis of benzodithiophene-thiadiazole quinoxaline polymers (P1–P3) with OCH3, CH3, and CF3 substitutions, and are characterized by using GPC and XRD analysis. Further, we systematically investigated their hole mobility and its impact on the efficiency of perovskite solar cells. The presence of S··· O interactions provided backbone rigidity, promoting tighter molecular packing and enhancing charge transport properties. Density functional theory calculations, in conjunction with XRD analysis, revealed the presence of favorable noncovalent S···O contacts in P1, confirming the role of intramolecular interactions in enhancing backbone planarity and packing order. Additionally, alkyl side chains were strategically incorporated to increase solubility while maintaining planarity, resulting in an optimized HOMO energy level of approximately −5.25 eV. Among the synthesized polymers, the OCH3-substituted derivative exhibited the maximum power conversion efficiency (PCE) of 13.77%, significantly outperforming traditional dopant-free Spiro-OMeTAD-based systems. The strong π–π* interactions between the lone pair of electrons on the OCH3 group and sulfur atoms in the polymer backbone facilitate superior charge extraction and molecular packing, resulting in improved hole transport and prolonged operational stability under normal environmental conditions. These findings highlight the potential of rational molecular engineering in advancing dopant-free HTMs for next-generation perovskite photovoltaics.