Chiral Aza-Helicene Phosphonic Acids for Stabilizing Efficient Perovskite-Silicon Tandem Solar Cells

The popular planar carbazole-based hole-selective self-assembled molecules (SAMs) for perovskite solar cells (PSCs) suffered from intrinsic instability toward electric potential, heat, and light illumination. To address this issue, herein, we report a kind of chiral helically shaped SAM, aza-helicene phosphonic acid A5HPA, and A7HPA, featuring their self-assembly attributed to the extended nonplanar π-conjugated system of aza-helicene with highly intrinsic stability toward thermal aging, light soaking, and electrical oxidation. Due to the increased torsion degree of the helicene backbone and the improved helical chiral molecular self-consistency, P and M enantiomers of A7HPA molecules tend to stack in an alternating pattern similar to "gear mesh," leading to reinforced intermolecular π-π interactions and conjugation effect to rigidify the hole transport layer. Consequently, the A7HPA-based single-junction wide bandgap PSC and perovskite-silicon tandem solar cell exhibited impressive long-term stability under both damp-heat and light-thermal synergetic stress tests and provided 23.41% and 33.06% (certified as 32.57%) power conversion efficiencies, respectively.