SnO₂-based transparent conductive oxides (TCOs) are currently widely used in different applications, e.g. as electrodes for photovoltaic, water splitting and fuel cells, for their high performances in terms of carrier density and mobility, low costs, non-toxicity and high transparency in the visible spectrum. To maximize the performances of any device in which they are adopted, a proper balance between surface-to-volume ratio and crystallinity grade is required. Indeed the former enhances the active surface, the latter improves the carriers transport through the film. Moreover, a strong internal light scattering can be advantageous to enhance the absorption and, thus, the efficiency of the whole device. Despite some SnO₂-based TCO porous films have been already realized starting, for example, from a solution of nanoparticles,[1] hierarchical nanostructured contacts has not been done yet.

In this work Pulsed Laser Deposition (PLD) technique has been adopted to deposit a hierarchical nanostructured SnO₂ film. Due to the resulting tree-like nanostructures, the film can be tuned in a wide range of porosity, as confirmed by SEM images, maintaining excellent electrical properties. These quasi-1D nanostructures affects the electron transport, which is strongly different from the one in flat or randomly porous SnO₂-based TCOs. The vertical structure, indeed, drives the carriers from the branches to the main body of the nano-tree, while in the latter case a wider variety of paths can be followed. Electrochemical Impedance Spectroscopy measurements have been performed to characterize the behavior of these TCOs in electrochemical cells. Hierarchical nanostructures are very important for their high contact surface and also for their scattering properties. UV-vis measurements confirm that the optical path can be engineered altering the porosity of the film, turning into an advanced light management, useful for various applications, e.g. DSC. These kind of films are suitable for subsequent doping or functionalization with different techniques, such as ALD. In the present work a novel thermal doping process with Fluorine precursor (NH₄F_(aq)) has been performed. We have thus developed an efficient and facile technique to produce hierarchical nanostructured FTO films. These cheap and versatile films can be implemented as scaffolds for catalysts, as Pt in fuel cells, or active materials, e.g. Fe₂O₃ in water splitting, and the high haze makes them suitable also for DSC. The high surface-to-volume ratio of each single nanostructure opens the way for their implementation in highly sensitive sensors.

[1] R. Kou et al., J. Am. Chem. Soc., 135, p. 2541 (2011)