Colloidal semiconductor nanocrystals for light-emitting devices : from materials to device perspectives

• Main Author: Shendre, Sushant
• Other Authors: Hilmi Volkan Demir
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
• Online Access: https://hdl.handle.net/10356/89264; http://hdl.handle.net/10220/48039
• Institution: Nanyang Technological University

Colloidal semiconductor nanocrystals are highly promising materials as active luminophores for making efficient lighting-emitting devices (LEDs). In this thesis, by developing an advanced heterostructure design made of CdSe/CdS@Cd1-xZnxS core/crown@shell, the emission efficiency of the newly introduced 2D NPL materials has been increased. The robustness of the surface passivation achieved is evident from the successful, hitherto challenging, aqueous dispersions for these hetero-NPLs having a photoluminescence (PL) quantum yield (QY) up to 90%. A combination of the peripheral edge passivation through crown growth and a gradiently alloyed shell achieving flat surface capping, while also suppressing Auger recombination by smoothening the interface electrostatic potential, leads to the increased PL QY and makes surface chemical robustness possible. The novel architecture of this material offers a high level of performance in LED applications to achieve external quantum efficiency (EQE) of 5%, which is among the best achieved by the NPLs to date. Also, the aqueous dispersion of these NPLs paves the way for achieving a controlled assembly of these hetero-NPLs in patterned depositions of varied shapes on a scale of few hundred nanometers. At the same time to further the development of QLEDs, in this thesis, using the recently introduced CdSe@ZnS gradient composition QDs as a working model, the limiting factors of QLEDs have been probed to observe the effect of active device environment on the excitonic recombinations of the luminophores under electrical excitation. The results highlight the role of charging induced Auger recombination in quenching the emitter efficiency, which is much more predominant than the effect of electric field in the test conditions. The results found in this work nudge the direction of future research towards the mitigation of the disadvantages posed by Auger processes in QLED operation. The findings of this thesis indicate that the synthesis of advanced nanocrystals and understanding of their active device operation will enable us to make high-efficiency colloidal devices to compete as an alternative to their thin-film counterparts.