Synergistic Hybrid‐Ligand Passivation of Perovskite Quantum Dots: Suppressing Reduced‐Dimensionality and Enhancing Optoelectronic Performance

In terms of surface passivation for realizing efficient CsPbI₃-perovskite quantum dot (CsPbI₃-PQD)-based optoelectronic devices, phenethylammonium iodide (PEAI) is widely used during the ligand exchange. However, the PEA cation, due to its large ionic radius incompatible with the 3D perovskite framework, acts as an organic spacer within polycrystalline perovskites, leading to the formation of reduced dimensional perovskites (RDPs). Despite sharing the identical 3D perovskite framework, the influence of PEAI on the structure of CsPbI₃-PQDs remains unexplored. Here, it is revealed that PEAI can induce the formation of high-n RDPs (n > 2) within the CsPbI₃-PQD solids, but these high-n RDPs undergo an undesirable phase transition to low-n RDPs, leading to the structural and optical degradation of CsPbI₃-PQDs. To address the PEAI-induced issue, we employ triphenylphosphine oxide (TPPO) as an ancillary ligand during the ligand exchange process. The incorporation of TPPO prevents H₂O penetration and regulates the rapid diffusion of PEAI, suppressing the formation of low-n RDPs. Moreover, TPPO can passivate the uncoordinated Pb²⁺ sites, reducing the nonradiative recombination. This hybrid-ligand exchange strategy using both PEAI and TPPO enables realizing efficient and stable CsPbI₃-PQD-based light-emitting diode (external quantum efficiency of 21.8%) and solar cell (power conversion efficiency of 15.3%) devices.