Passivation Mechanism of (18-Crown-6) Potassium on Complex Defects in SnO₂ Electron Transport Layer of Solar Cells

In this study, first-principles calculations were employed to systematically investigate the interaction mechanisms between (18-crown-6) potassium (18C6-K⁺) and six typical defect sites on the SnO₂ (110) surface, including Sn_i + Sn_O, O_i + O_Sn, V_O + Sn_i, V_Sn + Sn_O, V_Sn + Sn_i, and Sn_i. Six intrinsic or complex defects universally coexist on the SnO₂ surface, and the defect states they introduced allow for precise tuning of material performance. The results demonstrated that the 18C6-K⁺ molecule can stably adsorb on all six defect sites and significantly increase defect formation energies, indicating its thermodynamic capability to suppress defect generation. A subsequent density of states (DOS) analysis revealed that the 18C6-K⁺ molecule exhibits strong defect passivation effects at Sn_i + Sn_O, V_O + Sn_i, V_Sn + Sn_i, and Sn_i sites, and partially mitigated the electronic disturbances induced by O_i + O_Sn and V_Sn + Sn_O defects. Furthermore, the incorporation of 18C6-K⁺ has been shown to reduce the electronic effective mass of defective systems, thereby enhancing surface carrier transport. A subsequent charge density difference (CDD) analysis revealed that the 18C6-K⁺ molecule forms Sn-ether and O-ether interactions through its ether bonds (C-O-C) with surface Sn and O atoms, inducing interfacial electronic reconstruction and charge transfer. The Bader charge analysis revealed that the H, C, and O atoms in 18C6-K⁺ lose electrons, whereas the Sn or O atoms at the surface defect sites gain electrons. This outcome is consistent with the CDD analysis and quantitatively confirms the extent of electron transfer from 18C6-K⁺ to the SnO₂ defect regions. These interactions effectively passivate defect states, thereby enhancing interfacial stability. The present study offers theoretical guidance and design insights for the development of molecular passivation strategies in SnO₂-based optoelectronic devices.