Controlled Syntheses Of Colloidal Metal Oxide Nanocrvstals And Their Applications In Solution-Processed Light Emitting Diodes

Colloidal metal oxide nanocrystals have exhibited great potential as charge-transporting materials in high-efficiency solution-processed optoelectronic devices due to their excellent solution processibility, controllable optical and electrical properties, and good chemical stability. Solution-processed optoelectronic devices involve various device structures and abundant active materials so that the band energy levels and optoelectronic properties of a charge-transporting layer need to be carefully tailored to fully meet the requirements of a specific device.In this thesis, n-type ZnO and p-type NiO based colloidal nanocrystals, which are the typical electron-and hole-transporting materials, respectively, were selected as the representative systems to study the optoelectronic properties manipulation of the colloidal oxide nanocrystals. Based on the comprehensive understanding of chemical reactions and innovative design of reaction pathways, doped (or alloyed) ZnO and NiO nanocrystals synthesized by nonaqueous approaches were realized. The enhancement of hole-transporting properties owing to controllable incorporation of foreign atoms in NiO nanocrystals was demonstrated in the solution-processed light emitting diodes.Understanding the synthetic chemistry of colloidal oxide nanocrystals is the foundation of tuning the optoelectronic properties and developing potential applications, which is also the main purpose of our work. For ZnO based materials, chemical reactivity of cationic precursors and alcoholysis pathways were fount out the decisive factors in dopant incorporation and avoiding phase separation. Combined with the concept of "bandgap engineering", CdxZni-xO alloyed nanocrystals were successfully aquired. Then we summarized the optical bandgaps of intrinsic ZnO, MgxZm-xO and CdxZm-xO alloyed nanocrystals with different dimensions and concluded that bandgap engineering could be decided by quantum comfinement effects and dopant substitution simutaniously. Finally, the bandgap of ZnO based nanocrystals ranging from 3.3 eV to 3.9 eV was accomplished. For the study of NiO based nanocrystals, we successfully introduced Co2+and Cu2+into NiO lattice through two distinguished approaches, obtaining CoxNi1-xO alloyed oxide nanocrystals with Co content from 0 to 100%and CuxNi1-xO nanocrystals with fixed Cu concentration. The reation passways were based on the distinguished chemical reactivity of precursors containing cations. "Surface doping" was demonstrated effective in overcoming the difficulty in doping systems including host and dopant prcursors with huge reactivity difference.Afterwards, CuxNi1-xO nanocrystals were used as an example to explore the effects of optoelectronic properties of charge-transporting layers on the performance of solution-processed light emitting diodes. First, we found out that the conductivity of NiO nanocrystals films can be enhanced by Cu2+ incorporation. Then both NiO and CuxNi1-xO nanocrystals were employed as the hole-transporting layers in the quantum dots light emitting diodes (QLEDs). Compared with the devices using NiO nanocrystals, the devices using CuxNi1-xO nanocrystals exhibited lower turn-on voltage and higher power efficiency, indicating that CuxNi1-xO nanocrystals hole-transporting layers can probably play a crucial role in the high-performance QLEDs.Our achievement will boost the development of synthetic chemistry of doped or alloyed colloidal oxide nanocrystals and their further applications as charge-transporting interlayers in solution-processed optoelectronic devices.