Ultrafast Photoinduced Processes in Core and Core–Shell Quantum Dots for Solar Cell Applications “Tiny Crystals for Big Applications”
Author
Summary, in English
Compared with electrons, the photoinduced holes are more likely to be trapped. However, such trap states sometimes can be radiative with long lifetime up to tens of microseconds in oleic acid capped CdSe QDs. In this scenario, the hole injection in p-type QD solar cells are proved to be less efficient (<10%) compared with electron injection in n-type counterparts. It is highly affected by the surface trapping sites induced by the linker exchange process. The hole injection can then be improved by passivating the surface trap sites using core shell structures. Besides electron or hole injection, exciton migration can also occur via Förster resonant energy transfer (FRET). We found that FRET between QDs would enable to make use of the absorption of light by the indirectly attached QDs in QD-sensitized metal oxide (MO) anodes. In well-organized multi-sized QD mixtures, the energy transfer is even more pronounced. We experimentally observed the FRET process in randomly arranged multi-sized QD assembly and tandem stacked QD layers by using time-resolved and steady-state spectroscopies. Theoretical simulations where dipole distribution model was introduced for coupling calculations complies well with the experimental results. In order to minimize the effect of surface defects and improve the photostability of QD solar cells, we investigated the core–shell QD system where the surface trapping of carriers can be well passivated by shell materials with enhanced optical properties and device performance. Herein, a wider band gap semiconductor is employed as a shield shell around the active core in gradient growth, known as gradient Cd1-xSe1-yZnxSy CSQDs. Such QDs offer higher photostability, higher fluorescence quantum yield, and less interfacial defects than the conventional step-like CSQDs. We first characterized the gradient CSQDs using steady-state optical spectroscopy and HR-TEM images in order to determine their dimensions and to evaluate the shell thickness. Then XRD and EDX were used to characterize the chemical composition and the crystal structures.
The photodynamic of these CSQDs in photovoltaic systems was also studied. We first found that the electron injection from the active core to n-type MO showed relatively larger exponential shell thickness dependence compared with step-like CSQDs. We established that the highest electron injection efficiency (~ 80%) can be found with shell thickness up to 1.3 nm. Such shell also allows high surface passivation providing optimal conditions for charge collection in solar cells. Finally, we integrated our knowledge about the electron and hole behaviors to explain the solar cell performances according to the core–shell structure. We confirmed that the hole trapping is the critical factor for QD-sensitized solar cell efficiency. The trapping can be well repaired by using optimal core–shell structure.
Department/s
Publishing year
2015
Language
English
Full text
Document type
Dissertation
Publisher
Division of Chemical Physics, Department of Chemistry, Lund University
Topic
- Atom and Molecular Physics and Optics
Keywords
- Quantum dots
- core–shell
- electron injection
- hole injection
- hole trapping
- exciton migration
- solar cells
- ultrafast dynamics
- time-resolved spectroscopy.
Status
Published
Research group
- Pullerits
Supervisor
- Tõnu Pullerits
ISBN/ISSN/Other
- ISBN: 978-91-7422-388-0
Defence date
13 March 2015
Defence time
09:15
Defence place
lecture hall C, at the Center of Chemistry and Chemical Engineering, Getingevägen 60, Lund
Opponent
- Prashant Kamat (Professor)