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Over the past few years quantum dots(QDs) have attracted widespread and intensive attention due to their particular opto-electronic properties. Size of the QDs can tailor their absorption spectrum, which makes them attractive for photovoltaic(PV) application. Meanwhile, quantum-dot-sensitized solar cells(QDSCs) have attracted an intensive attention as promising third-generation PV devices. The common feature of QDSCs is quantum confinement of the exciton in the semiconductor sensitizer, leading to a size-dependent absorption spectrum. Moreover, their theoretical thermodynamic efficiency of about 44 % has been reported, higher than the traditional 32.9 % calculated ceiling. This is because third-generation PV devices overcome the Shockley-Queisser limit which photons with energy less than the bandgap are not absorbed, while photons with energy greater than the bandgap lose excess energy unnecessarily via emission of phonons (thermalization).
In this study, aiming at high efficiency of QDSCs with CdS QDs/mesoporous-TiO2 photoanodes, physical properties of CdS QDs/mp-TiO2 NPs grown by Sono-Chemical SILAR(SC-SILAR) method were studied. It was found that Sono-Chemical Successive Ionic Layer Adsorption and Reaction (SC-SILAR) method has less growth time and larger absorbance of CdS QDs, compared with the conventional SILAR method. The sono-chemical energy of the SC-SILAR method has an extra energy of acoustic cavitation which owns its ability to concentrate acoustic energy in micro bubbles. These bubbles have temperatures around 5,000 K, pressures of roughly 1,000 atm, and heating and cooling rates above 1010 K/s. Such sono-chemical effects increase the chemical activity in the solution due to the formation of chemical radical reactions and creation relatively stable chemical species. Moreover, to investigate the advantages of the sono-chemical technique, Sono-chemical-CdS (SC-CdS) QDs were synthesized as a function of the intensity of ultrasonic waves.
The investigations via UV-Vis absorbance confirmed that a strong and broad absorption peaks appeared from 325 to 500 nm for the CdS QDs, revealing the optical properties of a quantum-sized CdS. The absorbances of UV-vis spectra increase with the number of cycles and the powers of sono-chemical as well as the red shifts of the broad absorption peak. It should be noted that the absorbances of the CdS QDs synthesized via the SC-SILAR were higher than those synthesized via the SILAR. This fact implies that SC-SILAR method is a faster process for growing QDs with large absorbance. By calculating the energy band gap, it was also proved that the band gap of QDs decreased as the particle size increased. Energy-dispersive X-ray spectroscopy showed that the atomic ratio of Cd and S was almost 1:1. This might be originated from the fact that an equal molar concentration of the precursors was used in the processes. As the cycles increased, the atomic percent of Cd and S uniformly increased in each method. Comparing the atomic ratios of Cd and S of SC-SILAR method with those of the SILAR mehtod, it was surely confirmed that the sono-chemical energy is of help to synthesize the CdS QDs onto mp-TiO2 films. X-ray diffraction data suggested that the crystal characteristic changed from cadmium sulfate hydrate to cadmium sulfide by injecting sono-chemical waves, resulting in growth of well-crystallized CdS QDs. In addition, the QDs size was estimated by the Scherrer’s equation. Finally, incident photon-to-current conversion efficiency(IPCE) data showed a higher and broader efficiency spectra for SC-CdS QDs with increasing cycles and sono-chemical powers of method. Conclusively, a sono-chemical SILAR method has proven to be a more efficient growing process in synthesizing QDs.
Thesis Advisor: Prof. Sang-Ho Sohn