Innovative Approaches for Light-Emitting Electrochemical Cells

Lorenzo Pietro Mardegan
Thesis name in original language
Innovative Approaches for Light-Emitting Electrochemical Cells
Abstract & Cover

In the last two decades, light-emitting diodes (LEDs) and organic light-emitting diodes (OLEDs) have driven the development of lighting technology and systems in terms of efficiency, performance and new applications. The market for these technologies is expected to keep rising in the next decades as a result of the large energy and climate crisis that our modern society is facing. However, the possibilities of integration of LED sources are very limited, because OLEDs rely on an expensive fabrication process, consisting of multiple low-pressure and high-temperature sequential layers.  Light-emitting electrochemical cells (LECs) are another class of thin film light-emitting devices based on the same type of organic semiconductors as those used in OLEDs but with a fundamentally different working mechanism. The simultaneous presence of electronic and ionic charge carriers makes LECs independent of the work function of the electrodes and can consist, in their simplest form, in a single active layer sandwiched between two electrodes. Thanks to these properties, LECs truly represent a promising alternative as cost-effective sources for general lighting applications.  In this thesis, various novelties are introduced in LEC devices and in their fabrication such as a new ionic transporting polymer, new emitters, and finally the use of novel characterization methods new to the field of LECs, that give important insight in the functioning and shortcomings of these devices. In this Thesis, we demonstrate the introduction of a new ionic transporting polymer for polymer LECs. The concentration of the ionic transporting polymer and salt were optimized allowing to obtain state-of-the-art devices with long lifetime and brightness (over 1600 operational hours above 300 cd/m2). A new characterization tool was also used to probe the photoluminescence signal under electrical bias of a device. Thanks to this setup, it was possible to link the photoluminescence decay with the different phases of the turn-on and the recovery after turn-off.  Secondly, in the field of semitransparent optoelectronics, we also developed efficient semitransparent LECs with a unique SnO2/ITO-based top cathode fabricated with atomic layer deposition and pulsed laser deposition techniques. The high transparency of the cathode resulted in a peak transmission of 82% corresponding at the electroluminescence peak (563 nm). Interestignly, the two sides of the devices show a different luminance response to the electrical bias. The down side (anode side) shows higher luminance and longer lifetime than the up side (cathode side). We concluded that few possible reasons of this behavior can be associated with the different refractive indices of the substrate/anode and cathode, internal reflections and electroluminescence quenching. To prove this, photoluminescence measurements were done by irradiating either the down or up sides. The results indicate that the photoluminescence intensity is lower when measured exciting from the top side, suggesting that anode and cathode quench the photoluminescence by non-radiative recombination in different levels and that the additional damage might be caused by the cathode deposition techniques.  Finally, a series of copper(I) and platinum(II) complexes are used into working LECs. New emitters for light-emitting devices are necessary in order to mitigate the high costs of the most common iridium(III) compounds. In the last few years, Cu(I) complexes have rapidly grown in interest inside the LEC field showing fast progresses, on the other hand, Pt(II) complexes have only found application in LECs only very recently. Here, first we focus on how different anions affects copper(I)-LECs and second, on the fine-tuning of the ligands to achieve for the first time blue/green electroluminescence from platinum(II)-LECs.  In summary, supported by comprehensive electrical device characterization and photoluminescence studies, this work demonstrates the applicability of these novelties to LECs and more in general to solid-state light-emitting devices. 

Universitat de ValĂȘncia
(Valencia, Spain)
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