Transparent electrodes based on spray coated fluorine-doped tin oxide with enhanced optical, electrical and mechanical properties†
Transparent electrodes are key components of optoelectronic devices such as solar cells, light emitting diodes and touchscreen displays. Despite great progress, there remain several challenges to be addressed in transparent conducting coatings, especially for achieving high-performance materials with scalable and inexpensive production. Here we present the deposition of fluorine-doped tin oxide (FTO) thin films with enhanced optoelectronic and mechanical properties using ultrasonic spray pyrolysis. We systematically investigate the influence of deposition temperature and doping amount on the structural, electrical, optical and mechanical properties of FTO films using a suite of characterization techniques including X-ray diffraction, scanning electron microscopy, optical spectroscopy, Hall effect measurement and nanoindentation. We discovered that at a given dopant level, grain size dictates the electrical and mechanical properties, with larger grain sizes and the reduction in grain boundaries – achieved at higher deposition temperatures – promoting an increase in electron mobility and in mechanical strength due to the inverse Hall-Petch effect. The effect of fluorine doping is complementary to that of deposition temperature: the amount of fluorine does not significantly affect grain size and mechanical properties, however it has a great effect on the optical and electrical properties of FTO. Using optimized deposition conditions we achieved FTO coatings with excellent optoelectronic properties (electrical resistivity ∼7 × 10−4 Ω cm; visible transparency ∼90%) and enhanced mechanical properties (elastic modulus ∼170 GPa; hardness ∼16 GPa). These results are some of the best ever reported for FTO films, and provide a solid base for their direct implementation within optoelectronic devices.