Almost everybody in the world has come across luminescence like fluorescence or phosphorescence, e.g. the hands of the good old analog watches. This is a phenomenon of a typical spectral downconversion of visible or UV-light whereby high-energy light is transformed into low-energy light by dissipation of energy. The opposite of this phenomenon is called upconversion whereby low-energy light is transformed into high-energy light by the combination of at least two photons into one photon of higher energy. This effect has barely been investigated as it is by far less likely to happen and its efficiency is usually low. However, solid-state materials that efficiently upconvert light at low intensities (< 100mW·cm-2) are slowly emerging as interesting applications like novel security inks offer a unique selling proposition. These upconversion phenomena could be achieved by several techniques like energy transfer upconversion (ETU), excited state absorption (ESA), both of which involving intermediary energy levels, but also simultaneous two-photon absorption (STRA) and second harmony light generation (SHG),which do not involve those intermediary energy levels. However, all of these mentioned techniques usually require high incident light densities which interfere with a common broad practical use. Another upconversion technique apart from the formerly mentioned is represented by triplet-triplet annihilation (TTA-UC), which can utilize low ambient solar irradiance, one requirement for a broader use in daily life applications, and was therefore, the chosen approach for this project. Efficient upconversion processes and a broad applicability of formulations and materials bear potential for energy conversion like photovoltaics and chemical reactions reducing the consumption and dependence of humanity on fossil-based usually CO2 emitting and non-renewable energy resources. However, access to such “easy to apply” and “easy to handle” upconverting formulations and materials is limited.
The aim of this project was to investigate, develop and prepare functional ink formulations capable of upconverting light by triplet-triplet annihilation (TTA-UC) therefore enabling unique new optical effects such as fluorescent patterns under near-infrared irradiation and a better applicability of upconversion in daily life. In order to obtain multicolor, designable and high efficiency upconverting functional inks, the goals of this action included investigations to find suitable printing techniques such as inkjet printing, developing coatable, printable and patternable solid-state light upconverting materials for ink formulations, such as modified rubbers or molecular glasses, but also analyzing the optical properties from the resulting specimens and develop new custom-made solid-state upconverter concepts for printing.
After the final period of the project the following conclusions can be drawn:
i) A stable and printable formulation capable of upconverting light by triplet-triplet annihilation (TTA-UC) can be obtained.
ii) The developed formulations can be used for screen-printing or bar coating and finally cured to obtain solid materials that show upconversion.
iii) Characterization and long-term stability tests of these specimen have shown that the upconversion is detectable for weeks but overall, the stability as well as the upconversion efficiencies are not sufficient to transfer it into an applicable product.
