Semester

Summer

Date of Graduation

2012

Document Type

Dissertation

Degree Type

PhD

College

Statler College of Engineering and Mineral Resources

Department

Lane Department of Computer Science and Electrical Engineering

Committee Chair

Xian-An Cao.

Abstract

Monodispersed colloidal quantum dots (QDs) synthesized by low-cost solution methods have many attractive properties, including high wavelength tunability, efficient luminescence, narrow emission bandwidth, and strong broadband absorption. They therefore offer a new class of materials for efficient optoelectronic conversions in various devices, including light-emitting diodes (LEDs), photodetectors, and thin-film solar cells. Despite the recent rapid progress, the development of QD-based devices is still in its infancy. For instance, both the external quantum efficiency of QD-LEDs and power conversion efficiency of QD solar cells reported in the literature are only a few percent, far below those of state-of-the- art inorganic and organic semiconductor devices. The performance of QD-based devices is largely limited by the insulating and bulky organic ligands of QDs, which are necessary for colloidal synthesis, but create large interparticle spacing and interfacial energy barriers, impeding charge transport and injection as well as exciton dissociation and energy transfer.;This work aims to enhance the energy conversion efficiencies in QD-based optoelectronic devices through surface modification of QDs and device structure optimization, which can enable efficient electronic coupling and energy transfer between QDs and surrounding materials. First, an in-house capability of colloidal QD synthesis by the hot-injection method was developed. High-quality CdSe, CdS and CdTe core QDs with sharp excitonic absorption features and narrow PL bandwidths were synthesized. Growing a ZnS shell surrounding the CdSe core QDs via the SILAR method increased the photoluminescence quantum yield (PL QY) from 10% to ∼50%. Furthermore, the ZnS shell was replaced by a graded CdS/Zn0.5Cd0.5S/ZnS multishell, leading to reduced interfacial defects and an improved PL QY ∼65%. Second, CdSe and CdSe/ZnS QDs with inorganic metal chalcogenide ligands (SnS4 4-) were synthesized by an organic-to-inorganic ligand exchange process. The SnS4 ligands significantly enhanced the inter-QD electronic coupling in QD solids, but caused a substantial reduction in the PL efficiency. Finally, the applicability of CdSe QDs with organic and inorganic ligands for optoelectronic applications is evaluated through detailed optical, electrical and optoelectronic characterization. LEDs based on a QDs/organic materials hybrid structure were fabricated and characterized. The best result was obtained from the LEDs based on organically-capped CdSe/ZnS QDs with a layer of blue phosphorescent FIrpic dyes as efficient exciton harvesters and energy donors. Precise control of the concentration of the donors and their distance from the QD layer led to complete exciton energy transfer and efficiency enhancement by a factor of 2.5. Meanwhile, the SnS4-capped QDs were found to retain strong excitonic absorption. Under a 150 W Xe lamp illumination, the photocurrent response of an ITO/QDs/Al structure increased by several orders of magnitude after the ligand exchange. These findings bode well for the applicability of colloidal QDs with metal chalcogenide ligands to efficient energy conversion in low-cost thin-film solar cells.

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