Energy-Level Engineering with Co-Sensitization of Organic Molecules on NiO_x for Sn-Based Perovskite Solar Cells

A cooperative self-assembly of organic small molecules made of diester/diacid-bithiophene units [BT^2D-CE (1), BT^2D-CA (2)] with hexylpyridine- or phosphonic acid-functionalized dithienopyrrole units [DTP^2D-Py (3), DTP^2D-PA (4)] was induced atop Nickel Oxide (NiO_x) hole-transport-layer for lead-free tin-based perovskite solar cells. The pyridine functionality of DTP^2D-Py (3) in conjunction with the carboxyl group of BT^2D-CA (2) altered the NiO_x/Perovskite interfacial energetics and accelerates selective hole extraction by passivating the buried interfacial defects, resulting in a power conversion efficiency of 8.1% due to enhanced photovoltage (0.560 to 0.593 V), alongside remarkable operational stability. The crucial role of synergistic energy-level modulation, especially the Fermi level, was unraveled using ultraviolet photoelectron spectroscopy, demonstrating that combining complementary conjugated organic backbones with targeted functionalities offers versatility for enhancement of overall device performance and stability.