Monodisperse cadmium sulphide (CdS) quantum dots (QDs) with a tunable size from 1.4 to 4.3 nm were synthesized by a non-injection method, and their surface states were characterized by photoluminescence spectroscopy and X-ray Photoelectron Spectroscopy (XPS). The steady state photoluminescence study identified that the proportion of the trap state emission increased with the QD size decrease, while from the photoluminescence decay study, it appeared that the trap state emission results from the emission via a surface deep trap state. The XPS measurements revealed the existence of surface Cd with sulfur vacancy sites which act as electron trap sites, and the population of these sites increases with the QD size decrease. These results are consistent to conclude that the trap state emission mainly originates from the surface deep trapped electrons at the surface Cd with sulfur vacancy sites.

No abstract

We have compared two dye anchoring functional groups, alkoxysilyl and carboxyl groups, to investigate their influence on the performance of dye sensitized solar cells. Dimethylaminoazobenzene was selected as a chromophore possessing a donor-accepter transition for the light absorption. Electrochemical and optical measurements were performed for 4-dimethylaminoazobenzene-4'-carboxylic acid and 4-dimethylaminoazobenzene-4'-triethoxysilane attached TiO 2 films. Electrochemical measurements and DFT calculation indicated almost identical potential energy levels and electron density of HOMO and LUMO states between these two dyes. Solar cell APCE spectra and charge recombination kinetics at the dye/TiO 2 interface revealed almost identical charge transfer rates/yields from and to the dye. The difference was observed on improvement of an open circuit photovoltage (Voc) by 60 mV and on the lifetimes of an electron in the TiO 2 conduction band for the dye with the alkoxysilyl functional group compared to the carboxyl group, suggesting that an alkoxysilyl functional group is more attractive to retard the charge recombination reaction between an electron in the TiO 2 conduction band and an oxidized form of an electrolyte redox couple. The highest solar energy conversion efficiency of 2.6 % was achieved for dye sensitized solar cells based on an azobenzene dye sensitizer under AM1.5G, one sun condition.