Shape Control of Semiconductor Nanocrystals and Their Applications to Optoelectronic Devices

Abstract

Shape- and size-controlled semiconductor nanocrystals have recently drawn keen attention due to their tunable optical and electrical properties. Precise control in their dimension and spatial chemical composition (i.e., dot, rod, or tetrapod structure with homogeneous or multi-compositions) enables us to manipulate their electronic states, exciton generation/relaxation dynamics, and transport phenomena, which determines light absorption, fluorescence, or carrier transport within nanocrystals or nanocrystals arrays. In order to realize their full potentials to practical applications based on these promising materials, deeper understanding on their physical and chemical properties as well as systematic utilization should be progressed. In these regards, design principles on semiconductor nanocrystals to control optical or electrical properties and their applications to optoelectronic devices are discussed. For effective generation of photons from other energy sources, spherical-shaped InP quantum dots with ZnSeS chemical composition gradient are synthesized by wet chemistry. The composition gradient of Se in the shell phase is easily achieved by one-pot approach with a simple addition of both Se and S precursors at the same time, where Se is dominantly located close to the InP cores while S content increases along the radial direction of the shell. The introduction of the ZnSeS composition gradient shells ensures high quantum efficiency above 70 % due to the successful passivation of surface states with reduced lattice strain and also improves chemical stability against chemical and physical stress. In order to facilitate the exciton dissociation for photovoltaic devices, tetrapod-shaped cadmium selenide nanocrystals are synthesized by continuous precursor injection approach. The continuous precursor injection implies the successive injection of precursor solution and branching agent using a syringe pump with a controlled injection rate. The CdSe tetrapods prepared with new synthetic route exhibit exceptional shape selectivity above 90 % as well as finely tunable dimensions varying from 1 to above 40 in aspect ratio simply by controlling the reaction temperature and injection rate. Moreover, these high quality tetrapods can be easily scaled-up in multi-gram quantity without reduction in morphological uniformity. Using the InP quantum dots with ZnSeS chemical composition gradient shells, highly efficient and bright green light emitting diodes are demonstrated. On the basis of an inverted device structure, solution-processed interfacial dipole was introduced for minimizing electron injection barrier between an electron transport layer to InP quantum dots. Optimized carrier transport energy levels allows direct injection of charge carriers within quantum dots, which leads to the considerable improvement in external quantum efficiency of the devices in entire operation range. To utilize CdSe tetrapods in organic-inorganic hybrid solar cells in reproducible and reliable manner, sequential fabrication is employed to realize bicontinuous hybrid morphology. The sequential fabrication solves the poor colloidal stability of ligand-less tetrapods as well as the inseparable fabrication variables participating in nanoscopic morphology and surface state of nanocrystals. It is found that primary amines could effectively passivate surface state of CdSe nanocrystal phase and contribute to the enhancement of device performance. The improved device performance is rationalized by the reduced bimolecular recombination of charged carriers at the trap sties. The optimized device shows considerable power conversion efficiency up to 2.2 % with high reproducibility. This thesis demonstrates the practical approaches to exploit semiconductor nanocrystals into the practical devices, covering the tailored synthesis of nanocrystals, suitable device structure, and novel processing method. It is believed that this comprehensive approach will make a success on the realization of emerging nanoscience into the practical optoelectronic applications.