Proceedings of nanoGe September Meeting 2017 (NFM17)
Publication date: 20th June 2016
Due to their the narrow band emission, high photoluminescence quantum yields (PLQYs), color tunability, and solution processability, all inorganic perovskite quantum dots (APQDs) have attracted much attention for use in thin film displays and solid-state lighting applications. Despite their impressive metrics, however, poor stability—particularly with respect to temperature, moisture, and light exposure—remains a ubiquitous impediment for virtually all perovskite materials and devices. The lack of stability has prevented the practical and commercial utilization of perovskite optoelectronics. Typically, APQDs are capped by long alkyl ligands such as oleic acid and oleylamine to achieve high PLQY and high dispersibility in nonpolar solvents such as toluene and octane. APQD light-emitting diode (LED) – so far stabilized and processed with the relatively insulating ligands – display poor efficiency. The poor carrier transport in these films, as a result of the inability to utilize shorter conducting ligands to stabilize APQDs, has been the major bottleneck preventing their realization in efficient devices. Here we overcome this limitation by designing a two-step ligand exchange strategy to replace the long carbon chain ligands on APQD with halide ion-pair ligands (e.g., di-dodecyl dimethyl ammonium bromide, DDAB). We show that attempts to directly exchange APQD with halide ion-pair ligands results in the severe degradation of their luminescence. Only by devising an intermediate step to desorb protonated oleylamine, can APQDs be exchange with a quaternary ammonium halide ion pair. APQDs capped with halide ion pair enabled us to fabricate LEDs with high external quantum efficiency (EQE). We achieved the highest efficiencies for green and blue APQD LEDs, exhibiting EQEs of 3.9% and 1.9% respectively. With the new ligand passivation technique, perovskite QDs present high photoluminescence quantum yield (PLQY) with remarakably high operational stability in ambient conditions (60±5% lab humidity) and high pump fluences, thus overcoming one of the major challenges impeding the development of perovskite-based applications.
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