Cadmium sulfide (CdS) quantum dots with a series of electron donor and acceptors were employed to investigate
thermodynamic and kinetic influences on photo-induced electron transfer reactions at CdS surface. Although the
potential energy levels of all electron donor and acceptors are located within the QD band gap, the QD
photoluminescence (PL) behavior is dependent upon the type of quencher. PL decreased into half, when one methyl
viologen or benzyl viologen molecule per one QD was added into the QD solution, implying that the molecule attaches
to the QD surface. In contrast, the dynamic quenching behavior was observed when thionine or o-tolidine was employed
as a quencher. PL quenching efficiency decreased, when the distance between the QD surface and the quencher was
increased by capping the QD with butylamine. Therefore, the PL quenching is mainly controlled kinetically rather than
thermodynamically.
Metal sulfide (CdS or PbS) quantum dots were synthesized in nanoporous TiO2 films for applications in solar energy
conversion devices. Sandwich type regenerative solar cells, based on the quantum dots sensitized TiO2 film, exhibit a
high IPCE over visible wavelengths by optimizing the polysulfide electrolyte composition. The CdS QD shows a higher
IPCE, compared to PbS, related to an increased light harvesting efficiency when the number and size of the QDs
intensified. In contrast, QD size dependence on the IPCE was observed for the PbS, likely resulting from the QD size
dependence on a conduction band edge potential (associated with quantum size effect) relative to the TiO2 conduction
band edge, or the kinetic competition between the hot electron injection and the electron relaxation in the PbS
conduction band. We also propose that an I3-/I- redox electrolyte, with NaSCN addition, can be employed to enhance the
solar cell performance. SCN- ions may attach to the QD surface forming a shell type structure to prevent the
photocorrosion reaction, and act as an intermediate electronic state to induce the sequential step electron transfer
reactions for the QD re-reduction.
Photocurrent improvement for the photoelectrochemical cell, based on a Cu2O/Cu electrode in hydrophobic ionic liquid electrolyte, was demonstrated by modifying the Cu2O surface using cyanide treatment, in this case the photocurrent amplitude drastically increased at least by a factor of 10 compared with the non-treatment. Passivation of the surface defect sites through the cyanide treatment process resulted in significantly raising the charge separation yield at the interface. From this experimentation the optimization of the cyanide treatment and the limitation of the photocurrent are considered.
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