Nanopatterned indium tin oxide as a selective coating for solar thermal applications

Indium tin oxide (ITO) coatings have been proposed to reduce thermal emission losses for solar thermal applications. Unfortunately, ITO also has a large amount of free charge carriers (∼1 × 1020 per cm3), which absorb sunlight. To address this issue, we propose a nano-patterned ITO-coated quartz exhibiting both anti-reflectivity (to maximize solar transmission) and low emissivity (to minimize long wavelengths radiative losses). A record small-size nanosphere (∼60 nm) etch mask was prepared via double self-assembly, followed by dry etching and characterisation. In parallel, alternative nanopattern geometries were modelled using the Lumerical FDTD software to optimise short wavelength transmission without diminishing the inherently low emissivity of unetched ITO. It was found that an inverted moth's eye pattern (height = 250 nm and spacing = 80 nm) gave the best results at various solar concentrations (1 sun @ 100 °C, 10 suns @ 400 °C, and 100 suns @ 600 °C), resulting in ∼7% improvement in the solar weighted transmission as well as a similar boost in the overall efficiency factor for selectivity. It was concluded that if the proposed deposition/etching processes can be cost-effectively scaled in a continuous process, it would provide a net performance boost for most solar thermal technologies.