**1. Introduction**

Transparent conducting oxide (TCO) film is one of the essential components in various state-of-the-art optoelectronic devices, including liquid crystal displays (LCDs), organic solar cells, touch screens, and organic light emitting devices (OLEDs). Among TCOs, indium-tin-oxide (ITO) is a well-known wide bandgap semiconductor that has metal-like electrical properties and high optical transmission in the visible region [1, 2]. Indeed, ITO possesses high electrical conductivity (∼10<sup>−</sup><sup>4</sup> Ω-cm) and high transmission (∼90%) in the visible range [3, 4]. In OLED applications, surface properties of ITO films, such as their electron affinity and work function, play a key role in determining the characteristics of OLEDs, owing to their direct contact with the organic materials, as a hole injection layer [5]. ITO has also been extensively used as a good ohmic contact material owing to the excellent surface conductivity in GaN-base light emitting diodes (LEDs) [6].

Recently, material processing by femtosecond (fs) laser irradiation has attracted a great deal of attention because the energy of fs pulses can be precisely and rapidly transferred to the film without thermal effects. This is so-called femtosecond laser annealing (FLA). FLA demonstrates as a good method to induce crystallization within a thin layer of materials and avoid thermal melting in materials. Thus, FLA can be employed in flexible technology, where thin films are coated on polymer substrates with low glass transition temperature (*T*g). For example, Pan et al. reported near-infrared femtosecond laser-induced crystallization in amorphous silicon [7]. Recently, laser-induced periodic surfaces structures (LIPSS) or ripples have been observed for various materials, under pulsed laser illumination near their ablation thresholds [8–14]. Materials with LIPSSs also possess modified physical properties, such as the friction [15], the hydrophobicity/hydrophilicity [16], the conductivity [11–13] and the absorptance [16–18].

This chapter reviews recent advances on fabrication of various nanostructures on the surface of ITO films by using femtosecond laser processing. In addition, it reports detailed property modification of fs-laser treated ITO films, including enhanced electrical conductivity, anisotropic optical transmission, laser-colorized effect, and performance of the organic photovoltaic devices using ITO with LIPSSs. This chapter also gives an insight into the formation mechanisms for LIPSSs, the relationship between fs-laser processing and properties of fs laser-treated ITO films, and potential applications of nanostructuring ITO films.
