| 초록 |
Quantum dots (QDs) are capable of controlling the emission and absorption wavelength due to the bandgap widening effect of nanometer-sized particles. Many efforts have been made to increase the efficiency of QDs by using a core/shell structure. However, the conventional method of creating the core followed by shelling has the disadvantage of repeated processing. In this study, we synthesize composition gradient quaternary ZnCdSSe QDs of high efficiency (quantum yield = 88.96%, full width at half maximum = 28.20 nm) through one-pot synthesis. The X-ray diffraction peak for the (111) plane in ZnCdSSe QDs was shifted 2.64° compared to that for pure CdSe. From Cs-corrected STEM-EDS line scan results, it can be seen that the center of the QDs consists for more than 40% of Cd, clearly showing that a CdSe-rich core was formed, while the amount of Zn increases significantly toward the outer area. In addition, the interface was modified by increasing the molar concentration of Se. QDs at the modified interface were applied to QD-sensitized solar cells, which showed a 25.5% increase in photoelectric conversion efficiency owing to the reduced electron confinement effect. The increase seems to be caused by the excited electrons being relatively easily transferred to the level of TiO2 owing to the reduced electron confinement effect. Consequently, the electron confinement effect was observed to be reduced by increasing the ZnSe (or Zn1-xCdxSe)-rich phase at the interface. This means that, based on the thermodynamic simulation, the interface between the core QDs and the surface of the QDs can be controlled. The improvement of optical and electronic properties by controlling interfaces and surfaces during the synthesis of QDs, as reported in this work, can be useful for many applications beyond solar cells. |
