Engineered Hybrid Organic-Inorganic Perovskite/Ag Contact for Preventing Ag Diffusion using Buffer Barriers

CH₃NHPbI₃(MAPbI₃), one of the organic-inorganic halide perovskite materials, has attracted strong interest in various optoelectronic devices due to its high absorption coefficient, tunable band gap, and fast charge transport. [1] However, halide perovskite not only causes reliability issues in temperature and humidity environments but also is decomposed into CH₃NH₃⁺ and PbI₃ by an electron-induced reduction reaction at the metal/MAPbI₃ interface. [2] Although various candidates such as oxide thin films, 2D perovskites, and polymers were proposed as protective layers to prevent the deformation of MAPbI₃, [3] the effects of diffused metal ions on structural deformation and phase transition in the metal/MAPbI₃ contact have not been clearly understood yet.<br/>Here, we directly confirmed the diffusion of metal ions and phase deformation in Ag/MAPbI₃ contact using transmission electron microscopy and energy dispersive x-ray spectroscopy. The PbI₃ phase is formed in the MAPbI₃ matrix contacted with metal and gradually transformed into PbI₂ and I^- due to Gibbs free energy, which is confirmed by diffracted peaks of the PbI₂ phase at 12.6°, 28.2°, and 29.4°. Furthermore, thermodynamically stable Ag-I bonding is generated by energy barrier, forming an AgI interfacial layer at the Ag/MAPbI₃. Because non-protective MAPbI₃ with the damaged surface shows reduced grain size by 20% and the formation of deep pinholes, we inserted various protective layers to passivate the surface and maintain the MAPbI₃ matrix. In order to prevent penetration of Ag, SnO₂ thin films with a thickness of 30 nm were deposited onto MAPbI₃ by atomic layer deposition. Because anions of the SnO₂ precursor are bonded with the most reactive MA⁺ at the early stage, only hexagonal and orthorhombic PbI₂ phases were identified in the as-deposited structure. The physically deposited C₆₀ layer with a 30 nm thickness damages the MAPbI₃ matrix, resulting in the formation of PbI₂ phases and a 15% reduction in the grain size of MAPbI₃. PMMA of the 30 nm thickness was coated on the MAPbI₃ layer for surface protection and prevention of diffused Ag ions. The penetrated Ag ions through the PMMA layer generate AgI phase inside the MAPbI₃ matrix though the MAPbI₃ was effectively protected for 30 days without the formation of PbI₂ phase. Two types of structures composed of C₆₀ and PMMA are suggested for the anti-diffusion barrier and passivation layer, respectively. Although the C₆₀/PMMA bilayer structure effectively prevents the formation of PbI₂ and AgI phases for 50 days, the grain size of MAPbI₃ decreased by 5%. Compared to the bilayer structure, MAPbI₃ matrix protected by C₆₀-PMMA complex is well maintained in its original structure even after 60 days without phase deformation and grain shrinking. Because the uniform and cohesive hydrophobic PMMA film not only effectively protects the MAPbI₃ structure but also the diffusion of Ag ions is controlled by the C₆₀ layer, our findings suggest advanced buffer layers to protect the surface of MAPbI₃ and prevent structural deformation caused by metal diffusion in various metal/MAPbI₃ contact.<br/>[1] A. Babayigit et al., Nat. Mater. <b>15, </b>247 (2016)<br/>[2] K. Hong et al., J. Mater. Chem. C. <b>9, </b>15212 (2021)<br/>[3] Abd. Rashid et al., Energy Environ. Sci. <b>14</b>, 2906 (2021)