Cotella, Giovanni Francesco; (2020) Innovative organic electroluminescent devices: diarylethenes as light-responsive switches and emissive graphene quantum dots. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The awarding in 2016 of the Nobel Prize in Chemistry to Prof. J. Sauvage, J. F. Stoddart and B. L. Feringa, “for the design and synthesis of molecular machines” proves the interest of the scientific community and high-tech industry towards stimuli-responsive multifunctional materials and devices. In this direction, the research activity described in this thesis is focused on light-responsive emissive devices, which can be controlled remotely and reversibly via irradiation with light of specific wavelengths. A range of photochromic diarylethene derivatives were judiciously selected in combination with commercially available organic semiconductors to generate the light-responsivity in our devices. At first, optically switchable green-emitting OLEDs and micro-OLEDs were investigated achieving the maximum ON/OFF ratio of ~20 and ~90 for current density and luminance, respectively. Additionally, through the studying of the performance of light-responsive single carrier devices, it was demonstrated that electrons are more affected than holes by the switching of the photo-active dopant. The device emissive area was further scaled-down working on the characterization of light-emitting transistors (OLETs) having channel length of 2.5 µm. For the first time such devices covered the whole visible spectrum and a maximum ON/OFF ratio exceed 500 was achieved for both drain current and luminance. Finally, another macro-trend of the display community was addressed, the constant search for innovative chromophores as alternative to poorly stable and highly expensive iridium-based phosphorescent materials or highly toxic cadmium-based colloidal quantum dots. In this direction, air stable and potentially non-toxic red-emitting graphene quantum dots (GQDs) were embedded in electroluminescent devices. The LEDs showed high colour purity (FWHM= 30 nm), maximum luminance of 1300 cd/m2 and EQE of 0.67 % which are among the best performance ever reported for LEDs based on red-emitting GQDs.

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