Spiro-Linked Planar Core Small Molecule Hole Transport Materials Enabling High-Performance Inverted Perovskite Solar Cells

Small-molecule organic semiconductors have demonstrated significant potential for application in hole-transporting materials (HTMs) for perovskite solar cells (PSCs), thanks to their high reproducibility and convenient synthesis routes. Finely designed planar π-π stacking structures have emerged as one of the primary strategies for achieving high-performance small-molecule HTMs. In particular, the incorporation of a helical structure into HTM designs through a linearization approach has proven effective, leading to the development of novel materials with superior properties. In this study, the structure-property relationship of these small molecules has been systematically explored. The newly developed HTM, SPCF-MeTPA, based on a spiro[cyclopentane-1,9'-fluorene] core, features a rigid conjugated system with spiro-linked core. This design provides improved intermolecular charge extraction and transport, optimized energy levels, and effective surface passivation compared to SPTP-MeTPA, which has a nonplanar spatial arrangement. As a result, the champion device based on SPCF-MeTPA achieves efficiencies of 26.35% (certified 25.75%) and 24.55% for aperture areas of 0.07 and 1.01 cm², respectively. Additionally, these devices demonstrate exceptional long-term stability, further highlighting the potential of SPCF-MeTPA as a high-performance HTM.