Suppression of the Efficiency Roll-Off Characteristics in Solution-Processed Lanthanide-based Organic Light-Emitting Diodes
Organic light-emitting diodes (OLEDs) have gained a tremendous amount of interest, due to their applications in display and solid-state lighting technologies. This interest mainly arises from the easy and inexpensive manufacturing processes for the fabrication of lightweight, transparent, flexible, and large-area devices. While OLEDs have now reached the stage of commercialization, there are still some serious issues associated with exciton-exciton and exciton-polaron annihilation mechanisms, which reduce their efficiency particularly under the high current densities required to achieve a high level of brightness. This phenomenon is known as efficiency roll-off. On the other hand, since most luminescent organic semiconductors are in principle four-level lasing systems, color-tunable and easy-to-manufacture optically pumped organic solid-sate lasers (OSLs) have attracted a lot of attention, due to their applications in biological sensing, spectroscopy, and data communication. However, highly demanded portable electrically driven continuous-wave (CW) OSLs (organic diode lasers) have not been realized to date. This is mainly because of the above-mentioned efficiency roll-off issue under the high current densities required to achieve lasing threshold in OLEDs. Visible-emitting trivalent lanthanide ions (Ln3+) are promising emitters for OLED applications. They are also known for their excellent lasing properties, owing to their long excited-state lifetimes, favoring population inversion in optically pumped plastic lasers and amplifiers. Furthermore, near infrared (NIR)-emitting trivalent lanthanide ions are important emitters in telecommunication applications in the 900-1600 nm spectral range. Despite the fact that lanthanide-based emitters have been studied in the NIR-emitting OLEDs, their efficiency is still much lower than in their visible-emitting counterparts. Moreover, like other types of OLED, lanthanide-based devices suffer from the aforementioned efficiency roll-off mechanisms. In the present thesis, we show enhanced efficiency from solution-processed NIR-emitting lanthanide-based OLEDs. We also demonstrate unprecedented low efficiency roll-off characteristics in both visible and NIR-emitting lanthanide-based devices.