Molecular-Level Design of Li-Free Dopants for Space-Grade Perovskite Solar Cells

Perovskite solar cells are attractive for space photovoltaics because of their high efficiency and lightweight nature, yet their stability is challenged by proton irradiation and repeated thermal shocks. In particular, Li-containing dopants in conventional hole-transport layers (HTLs) can promote ion migration and interfacial degradation. Here, density functional theory (DFT) calculations and molecular dynamics (MD) simulations were employed to elucidate the molecular mechanisms of Li-free ionic-liquid dopants. A series of X-phenethylammonium bis(trifluoromethanesulfonyl)imides (X-PEA-TFSI; X = H, F, Cl, Br, and OCH3) was evaluated in terms of molecular polarization, perovskite interfacial binding, and proton-transfer behavior with 4-tert-butylpyridine (tBP). Among the candidates, F-PEA⁺ exhibited the strongest polarization and the most favorable binding to the perovskite surface, supporting enhanced interfacial stabilization. F-PEA⁺ also promoted proton transfer to tBP, reducing free tBP that can induce de-doping in the HTLs. DFT-based MD simulations further confirmed the stability of the proton-exchanged state over a broad temperature range. Unlike conventional Li-TFSI, F-PEA-TFSI stabilized tBP through a proton-transfer-mediated pathway. In addition, F-PEA⁺ increased iodine and formamidinium vacancy formation energies on the perovskite surface and showed a favorable reprotonation pathway, suggesting reversible passivation after proton irradiation. These results identify that strong molecular polarization, interfacial binding, proton transfer, and vacancy suppression explain the enhanced radiation and thermal-shock stability of F-PEA-TFSI-based perovskite solar cells.