Ultrafast Photoinduced Processes in Core and Core–Shell Quantum Dots for Solar Cell Applications “Tiny Crystals for Big Applications”

Abstract: The balance between our demands of energy and the energy we are consuming is not in equilibrium anymore. Therefore, the search for other energy resources is indispensable. Sustainable energy sources offer the alternative to the fossil fuels. Within many types of sustainable energy sources, solar energy offers more than the total global energy consumption. Solar cells are the smart conversion tools to harvest the incident photons and create electricity out of these photons. Solar cells have passed through different generations where two important factors control the solar cells market. The solar cell efficiency and price play the cornerstones in the solar cell marketing. Third generation of solar cells aims to maximize the price–performance equation by using cheap materials without compromising efficiency. Nanomaterials have emerged as the promising building blocks to harvest the solar light in the third generation of the solar cells. Among them, quantum dots (QDs) can be used as viable candidate due to their superb features such as high extinction coefficient, a tunable absorption edge, and the possibility to generate and collect multiple excitons by using single, high energy photon. Both the behaviors of photoexcited electrons and holes determine the overall efficiency of QD based solar cells. This thesis presents a systematic study of the ultrafast photoinduced charge dynamics in QD solar cell materials including the charge transfer, exciton migration, carrier trapping and their influence on real solar cell performance. The materials investigated start with conventional neat core CdSe QDs and extend to gradient Cd1-xSe1-yZnxSy core–shell (CS) QDs. The latter are used to obtain improved optical and device performance. The electron injection from CdSe into ZnO nanowires were first observed to be very fast (few ps). This fast electron injection encourages us to study the possibility to inject multiple electrons from a QD under high excitation conditions. We revealed that a competition between electron injection and Auger recombination occurs. 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.