Optoelectronic Properties and Temporal Stability of AZO/Al/Cu/Al/AZO Multilayer Films

To investigate how the position and thickness of ultrathin Al interfacial layers regulate the optoelectronic properties and temporal stability of AZO/Cu/AZO multilayer films, AZO/Cu/AZO (ACA), AZO/Al/Cu/AZO (AACA), and AZO/Al/Cu/Al/AZO (AACAA) multilayers were deposited on glass substrates by magnetron sputtering. For clarity, the stack notation is given from the film surface toward the substrate. AACA contains a 1 nm Al interfacial layer above Cu, whereas AACAA-1 and AACAA-2 contain Al layers on both sides of Cu with top/bottom thicknesses of 1/1 and 2/1 nm, respectively. The effects of Al layer insertion position and thickness on the microstructure, optoelectronic properties, and temporal stability were systematically investigated. The ACA and AACA films exhibited ZnO and Cu phases with preferred ZnO (002) and Cu (111) diffraction, respectively. The AACA film showed the best initial optoelectronic performance, with the average transmittance increasing from 75.9% to 85.7% and the sheet resistance decreasing from 18.5 to 6.7 Ω/sq, yielding a figure of merit (F_OM) of 3.2 × 10^-2 Ω^-1. After the additional Al layer was introduced beneath Cu, the Cu (111) signal became very weak and the sheet resistance increased markedly, indicating a substantial change in the structural and interfacial state of the ultrathin Cu layer. After two years of air exposure, pronounced Cu-O-rich particles were observed on the ACA surface, and the relative changes in average transmittance and sheet resistance reached 10.7% and 95.7%, respectively. In contrast, the corresponding changes for AACAA-2 were only 1.2% and 2.7%, demonstrating the best temporal stability. These results reveal a clear trade-off between initial optoelectronic performance and long-term stability and show that dual Al interfacial modification is an effective route for stabilizing AZO/Cu/AZO multilayer electrodes.