Well-Designed Conjugation Groups in Bisphosphonic Acid Molecules Enhancing Flexible Perovskite Solar Cells

The hole-selective layer (HSL) combining nickel oxide (NiO_x) and self-assembled monolayers (SAMs) has garnered significant interest in thermally stable flexible perovskite solar cells (fPSCs). However, designing bisphosphonic acid molecules for SAMs that simultaneously ensure ordered molecular packing and efficient charge extraction and obtain high-quality perovskite films remains a challenge. Here, we introduce a series of rationally designed bisphosphonic acid molecules featuring different π-conjugated functional groups. By incorporating strategically designed benzene rings, we achieved enhanced molecular stacking of SAMs on the NiO_x surface, resulting in dense and highly ordered HSLs. The optimized π-conjugated structure not only facilitates efficient hole extraction and transport but also significantly enhances ultraviolet (UV) down-conversion efficiency of molecules. Moreover, the HSLs improve the quality of perovskite films, effectively suppressing nonradiative recombination losses. Consequently, the optimized fPSCs achieved a power conversion efficiency of 23.41% and exhibited an improved overall stability. This work provides a novel molecular design strategy for developing efficient and stable bisphosphonic acid molecules for SAMs.