טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentGeorgy Maikov
SubjectColloidal Semiconductor Quantum Dots for
Photo-Voltaic Applications
DepartmentDepartment of Chemistry
Supervisor Full Professor Lifshitz Efrat
Full Thesis textFull thesis text - English Version


Abstract

The thesis presents optical spectroscopy studies of various semiconductor colloidal quantum dots (CQDs) that differ by composition, size, and structural architecture. Using these nanometric scale materials is associated with a three dimensional confinement of the charge carriers, splitting the bulk energy bands into size-dependent discrete states rendering the CQDs with tunable optical and electronic properties. The research was motivated, in part, by the special interest and importance in integrating these materials in next generation photo-voltaic cell (PVSC) devices. The CQDs present a high degree of reproducibility, narrow size distribution, uniform shape and high quantum efficiency. However, several important applications, such as PVSCs, impose a limit on their size, yet demand tunability of the band-gap. This thesis discusses the development and the investigation of a new type of CQDs consisting of a core, coated by epitaxial shell, when either the core or the shell has an alloy composition.

The basic measurements were carried out on an ensemble of CQDs, from the IV-VI group, by following the continuous-wave and transient photoluminescence measurements over a wide temperature range. The study revealed distinct behaviors of the hetero-structures with respect to that of pure cores: (i) increase of the emission quantum yield; (ii) Reduction of an exchange interaction; (iii) tuning of the radiative lifetime with shell width and composition.

Multiple exciton generation (MEG) in IV-VI CQDs is a timely topic, with anticipated benefit in PVSCs. The MEG was studied with a time-resolved optical pump and probe spectroscopy. Remarkably, it was found that the measured quantum yield of MEG, the hot exciton cooling rate and the Auger recombination rate of bi-excitons are similar for the core and the hetero-structured CQDs with the same core size. Based on the optical studies, the hetero-structures were tested in a prototype PVSC device. The preliminary work showed that the Schottky PVSC based on hetero-structured CQDs provided higher power conversion efficiency compared to a pure core-based device.

The highlight of the research was the investigation of the CQDs' properties on a single dot level. The thesis describes a detailed investigation of excitonic-vibrational coupling by following exceptionally narrow emission lines in the micro-photoluminescence spectrum of a single CQD. The observations are explained as vibronic transitions consisting of band-edge zero-phonon electronic transitions coupled to individual ligands. The vibronic coupling is significant in controlling single- exciton and multiple-exciton relaxation processes with a strong influence on the performance of PVSCs and other opto-electronic applications.