Abstract
This research intends to use the semiconductor quantum dots to replace those expensive ruthenium dyes in the traditional photoelectrochemical solar cells. By varying the quantum dot sizes, there is a potential to expand the absorption spectrum to cover all ranges of the sunshine, without changing materials. Under proper nano-structure design, the quantum dot sensitized solar cells (QDSSC) may have higher quantum efficiency than the dye-sensitized solar cells (DSSC). The energy gaps of variable sized quantum dots (QDs) dominate how the electrons are excited from the valence band to the conduction band by variable frequencies of input photons. The positions of both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) determines the ability of electron injection to the metal oxide semiconductors. Based on first principles calculation, the molecular structures of (CdSe)n (n=1~16) clusters and the anatase TiO2 (101) surface have been configured and optimized. Both time independent and time dependent density functional theory (DFT), which chose B3LYP (Becke, 3-parameter, Lee-Yang-Parr) and PBE (Perdew-Burke-Ernzerhof) exchange correlation functionals, are employed. Photoelectric properties, such as: electron orbitals, density of states (DOS), HOMO and LUMO (and resultant band gaps) are predicted. They are used to study the solvent effect of CdSe clusters in cyclohexane, binding energy between the CdSe cluster and the TiO2 surface, and the energy spectrum shift after adhesion. An UV/VIS spectrophotometer was used to measure the absorption spectra of variable sized CdSe quantum dots. The band gaps calculated from the experiment are close to the predictions from the quantum simulation. It is concluded that we can enhance the sunlight absorption by mixing different diameters of CdSe clusters, and a more suitable bonding structure between the CdSe clusters and the TiO2 surface is proposed to promote the electron injection efficiency.