Light-emitting Electrochemical Cells: Temperature Dependence and Host-guest-systems




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Light-emitting electrochemical cells (LECs) yield high efficiency and long-lasting performance in a simple device architecture. Due to the ionic nature of these devices, electric double layer formation occurs at the electrodes in response to an applied field, producing efficient charge injection and recombination for light emission. LECs from perovskites, nanoparticles, or organic small molecules have potential as thin, conformable, lost-cost solutions for seamless integration in light-emitting applications. Understanding the interplay of electronic and ionic processes of LECs is necessary to improve the luminance, efficiency, stability, and lifetimes for this potential to be actualized. This dissertation focuses on temperature-dependent studies of ionic, electronic, and optical properties of LECs and host-guest systems to improve stability, increase efficiency, and control color. We established that iridium LECs show superior temperature stability to their ubiquitous ruthenium counterparts, resisting radiant flux loss until 67 °C (152 °F). To understand the mechanistic origin of this superior stability, the temperature-dependence of the photoluminescence of iridium and ruthenium complexes was measured. Although textbooks have asserted that iridium complex stability superiority is due to energetic suppression of non-luminescent and antibonding states, it was alternatively found that this superiority was due to favorable radiative recombination rates relative to nonradiative transitions. We implemented novel small-molecule ionic hosts based on carbazole derivatives with ionic transitional metal complex (iTMC) guestsfor use in LECs. These hosts demonstrated wide, tunable bandgaps and accessible oxidation and reduction features, consistent with our design parameters. LECs with a PBI-CzH host demonstrated superior performance, where PBI is 4- bromophenylbenzimidazole, and CzH is an unsubstituted carbazole unit. These LECs achieved 624 cd/m2 luminance at 3.80% external quantum efficiency, competitive in the field of iTMCs. Xray diffraction suggested that host packing caused the superior performance of the PBI-CzH host and offered a key design insight for future LEC hosts. A novel ionic iridium complex guest was prepared to be used in conjunction with a CsPbBr3 perovskite host and a polyelectrolyte to achieve high performance in a perovskite light-emitting device with a single-layer structure. Maximum luminance (10600 cd/m2 ), current efficiency (11.6 cd/A), and power efficiency (9.04 Lm/W) were achieved at a 14% weight fraction of the guest, and voltage-tunable color was demonstrated. These results show improvement for all reported metrics for perovskite host devices, demonstrating the potential for a host-guest approach with radiationally designed ionic emitters in perovskite LECs. Finally, we followed the low-temperature performance of current, electroluminescence, and photoluminescence of perovskite LECs to reveal the effects on ionic transport, electronic transport, and optical properties. Initially, lowering the temperature increased device efficiency. However, the efficiency was found to decrease as the temperature decreased from 240 K and below. Differential scanning calorimetry was used to assess morphological changes induced by the polyelectrolyte. Ultimately, it was revealed that the interplay of these factors greatly depends on the mechanical properties of the polyethylene oxide electrolyte, and the suppression of the glass transition could substantially improve low-temperature device performance.



Physics, Condensed Matter