Mapping the thermal processing window for optoelectronic-grade Cs₃Bi₂I₉ thin films fabricated via flow reactor synthesis
摘要
The influence of mixing temperature in a microreactor (70–150 °C) on the crystallinity, surface morphology, and local conductivity of Cs₃Bi₂I₉ thin films was systematically investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and conductive AFM (C-AFM). XRD analysis confirmed the formation of the Cs₃Bi₂I₉ phase in all samples, with increasing mixing temperature leading to sharper diffraction peaks, indicating enhanced crystallinity and larger crystallite size. The Scherrer equation revealed a progressive increase in crystallite size, suggesting improved long-range ordering at higher processing temperatures. In contrast, AFM measurements demonstrated a reduction in surface grain size from ~ 1.6 µm (70 °C) to ~ 0.3 µm (150 °C), accompanied by non-monotonic changes in surface roughness. Optimal mixing at 110 °C yielded the smoothest morphology (RMS ~ 43 nm, Rpv ~ 250 nm), indicative of grain coalescence and densification. However, further heating to 150 °C in the microreactor induced surface instability, increasing roughness (RMS ~ 54 nm, Rpv ~ 700 nm), likely due to thermal stress or iodide volatilization. C-AFM analysis revealed a strong correlation between grain structure and local conductivity. At 90 °C, current maxima aligned with grain peaks, suggesting efficient charge transport. In contrast, films processed at 150 °C exhibited disrupted conduction pathways, with minimal height–current correlation, indicating electronic degradation. The 110 °C sample displayed uniform conductivity, consistent with its compact morphology. These findings highlight 110 °C as the optimal mixing temperature in the microreactor, balancing crystallinity, surface smoothness, and electrical performance, while higher temperatures induce structural and functional deterioration. This study provides critical insights for optimizing Cs₃Bi₂I₉ films for optoelectronic applications.