Impovement Of Electron Transfer Rate From CdSe/CdS/ZnS Core/Shell/Shell Quantum Dots To Metal Oxide Nanocrystal Films

Because of impressive size-tunable photoluminescence property, CdSe quantumdots （QDs） have been used as a light harvesting component, coupled with anothersemiconductor particles having favorable energetic band position, such as TiO2andZnO nanocrystals, in QDs sensitized solar cell （QDSSC）. The observation of multipleexciton generation in QDs opens the possibility of boosting the energy conversionefficiency of QD based solar cells to exceed the traditional thermodynamic limit forsingle junction bulk solar cell. However, currently the maximum power conversionefficiency of the QDSSCs is still lower than5%, which is dependent on the generationof electron-hole pairs, charge transfer dynamics and electron-hole recombinationprocesses in the devices. In this paper, we report a study on the improvement of theelectron transfer rate from CdSe QDs to metal oxide films as follows.（1） The effect of mercaptopropionic acid （MPA） molecules on the charge transferprocess from the CdSe QDs to ZnO nanocrystal films was studied. Three samples,ZnO/MPA/CdSe, ZnO/CdSe and CdSe QDs films， were prepared for themeasurements of the steady-state and time-resolved photoluminescence spectra.ZnO/MPA/CdSe were obtained by absorbing CdSe QDs on MPA linked ZnOnanocyrstal films, and ZnO/CdSe films were obtained by spin coating CdSe QDs onZnO nanocrystal films. The shortening of photoluminescence lifetimes of CdSe QDsdeposited on ZnO nanocrystal films with and without MPA molecules indicated theelectron transfer process happened in the interface between CdSe QDs and ZnOnanocrystals. But the decreased rate of electron transfer from CdSe QDs to ZnOnanocrystal films with MPA molecules was found, compared to that of the ZnO/CdSe sample. Considering the fact that MPA molecules did not improve the charge transferprocess from CdSe QDs to ZnO nanocrystal films, it was indicated that the MPAmolecule was not a good choice in CdSe QD sensitized solar cell. The experimentalresult suggested that the direct adsorption of CdSe QDs on metal oxide nanocrystalfilms would obtain QD sensitized solar cell with higher power conversion efficiency.（2） We studied the effect of thermal annealing on electron transfer fromCdSe/CdS/ZnS core/shell/shell QDs to metal oxide （TiO2and ZnO） nanocrystal filmsby steady-state and time-resolved photoluminescence spectroscopy. Less than onemonolayer CdSe QDs were spun cast onto well prepared flat ZnO and TiO2nanocrystal films, and SiO2nanocrystal films were used as references. The CdSecore/multi-shell QDs exhibited excellent thermal stability even after being annealed attemperature up to300℃. Also, the electron transfer rates were found to increase withincreasing the annealing temperature, both for TiO2/CdSe and ZnO/CdSe QD system.This is because thermal annealing enhanced electronic coupling between QDs andmetal oxides, resulting from the removal of excess surface ligands of QDs or bendingof the surface ligands. We successfully increaseed the elelctron transfer rate from3.2×10⁷s^-1to2.5×10⁸s^-1for TiO₂/CdSe QD system and from1.7×10⁷s^-1to1.7×10⁸s-1for ZnO/CdSe QD system in the annealing temperature range from100℃to300℃. We conclude that thermal annealing is an efficient way to improve the electrontransfer process between multi-shell QDs and metal oxide nanocrystal films.