|Ph.D Student||Diana Yanover|
|Subject||Semiconductor Quantum Dots Characterization and|
Application in Photovoltaic Devices
|Department||Department of Chemistry||Supervisor||Full Professor Lifshitz Efrat|
|Full Thesis text|
Colloidal semiconductor quantum dots (QDs) are crystals of nanoscale dimensions capped with organic molecules, which makes them soluble in various solvents. Many physical (e.g., electronic and optical) properties of nanosize crystals are different from those of the bulk materials. This “quantum size effect” is accounted for by the three-dimensional quantum confinement of the electrons, which results in size-dependent quantization (discretization) of their energy levels. Colloidal QDs can be easily prepared by wet chemistry methods, which makes them attractive materials for low-cost and scalable solution-processed optoelectronic devices. In particular, QD-based solar cells are considered to be most promising due to the expected great increase in the device efficiency.
The present research was focused on IV-VI materials (PbSe, PbS), mainly, because they show tunable and broadband absorption in the UV-near IR range, which makes them suitable for harvesting a wide spectral range of the solar spectrum. In this work, we have developed high-yield synthesis of air-stable small-sized (~3-3.5 nm in diameter) PbSe/PbS core/shell QDs, characterized by the band-gap energy in the range of 1.1-1.4 eV. For this purpose, we have developed high-yield synthesis (30-60%) of ultra-small (~2-2.5 nm in diameter) PbSe QDs with relatively narrow size distribution (~15%). Next, we have developed the method of epitaxial coating of a large number (0.2-0.3 μmol) of QDs by PbS layer and thus achieved high-yield production of PbSe/PbS core/shell QDs.
Optical studies of the synthesized ultra-small PbSe QDs have revealed exceptionally large Stokes shifts, which are strongly dependent on the particle size. The measured exciton relaxation lifetime () decreases with increasing the PbSe QD size and with increasing the PbS shell thickness. Moreover, it has been found that PbSe/PbS QDs have larger values than those of the PbSe QDs of the same size, both materials exhibiting high photoluminescence (PL) quantum yields (~60%). In the experiments at different temperatures, it has been demonstrated that in PbSe QDs of less than 2 nm in diameter, the band-gap temperature coefficient changes its sign from positive to negative and, in both PbSe and PbSe/PbS QDs, the value increases 2-3-fold at 4 K as compared to the room temperature. Detailed room temperature and temperature-dependent optical study has revealed that PbSe/PbS QDs are more stable to oxidation than PbSe QDs.
Finally, PbSe and PbSe/PbS QDs were used to construct a solar cell. The PV cell constructed of PbSe/PbS QDs had the highest conversion efficiency of as high as ~4%.