Abstract
Tin doped indium oxide (ITO) thin films provide excellent transparency and conductivity for electrodes in displays and photovoltaic systems. Current advances in producing printable ITO inks are reducing the volume of wasted indium during thin film patterning. However, their applicability to flexible electronics is hindered by the need for high temperature processing that results in damage to conventional polymer substrates. Here, we detail the conditions under which laser heating can be used as a replacement for oven and furnace treatments. Measurements of the optical properties of both the printed ITO film and the polymer substrate (polyethylene terephthalate, PET) identify that in the 1.5–2.0 μm wavelength band there is absorption in the ITO film but good transparency in PET. Hence, laser light that is not absorbed in the film does not go on to add a deleterious energy loading to the substrate. Localization of the energy deposition in the film is further enhanced by using ultrashort laser pulses (~1 ps) thus limiting heat flow during the interaction. Under these conditions, laser processing of the printed ITO films results in an improvement of the conductivity without damage to the PET.
Highlights
Transparent conductive oxide (TCO) films are widely used in a variety of optoelectronic devices, such as solar cells[1,2], liquid crystal displays (LCD)[3], touch screens[4,5], and light-emitting diodes (LED)[6,7,8]
The optical absorption coefficients for the indium tin oxide (ITO) film and properties of the substrate (PET) substrate were measured in the wavelength range 250 nm to 2000 nm (Fig. 1)
It can be seen that in the visible wavelength region, there is excellent transparency for the PET and, the unirradiated ITO layer has an absorption coefficient of 3 × 104 cm−1, this corresponds to an absorption depth of 2.5 μm which is far in excess of the thickness
Summary
Transparent conductive oxide (TCO) films are widely used in a variety of optoelectronic devices, such as solar cells[1,2], liquid crystal displays (LCD)[3], touch screens[4,5], and light-emitting diodes (LED)[6,7,8]. The laser sintering approach heavily relies on the optical properties of nanoparticles in the film; one has to choose the radiation source according to their absorption spectrum. The emission spectrum of the laser was filtered to achieve maximum absorption of the laser light in the ITO particles whilst leaving the substrate undamaged and limiting heat flow due to its picosecond pulse duration.
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