A Dual Strategy of Steric Hindrance and Fluorination for Aggregation-Resistant and High-Performance Hole-Transport Monolayers

Carbazole phosphonic acids (PACzs), with 2PACz being the most prominent example, have emerged as promising monolayer hole-transporting layers (HTLs) in organic solar cells (OSCs) for modifying the work function (WF) of ITO electrodes. However, 2PACz tends to aggregate on ITO, resulting in significant surface inhomogeneity. To address this, we designed a series of phenyl-substituted PACzs. The phenyl incorporation introduces torsional steric hindrance that effectively suppresses molecular self-aggregation. By further introducing varying numbers of fluorine atoms, we precisely tuned the WF of the ITO/PACz interface. The resulting phenyl PACzs form uniform monolayers and serve as efficient HTLs. A fluorinated phenyl PACz-based OSC achieved a power conversion efficiency of 19.37%, outperforming the 18.53% obtained with 2PACz. Moreover, devices with these HTLs exhibit enhanced thermal stability, maintaining T₈₀ over 3000 h, far exceeding the 20 h observed for 2PACz-based OSCs. This study demonstrates the effectiveness of steric hindrance and fluorine incorporation in developing self-assembly monolayer for high-performance and stable OSCs.