Surface oxidation of quantum dots
(QDs) is one of the biggest challenges
in quantum dot-sensitized solar cells (QDSCs), because it introduces
surface states that enhance electron–hole recombination and
degrade device performance. Protection of QDs from surface oxidation
by passivating the surface with organic or inorganic layers can be
one way to overcome this issue. In this study, solid-state QDSCs with
a PbS QD absorber layer were prepared from thin mesoporous TiO<sub>2</sub> layers by the successive ionic layer adsorption/reaction
(SILAR) method. Spiro-OMeTAD was used as the organic p-type hole transporting
material (HTM). The effects on the solar cell performance of passivating
the surface of the PbS QDs with the tripeptide l-glutathione
(GSH) were investigated. Current–voltage characteristics and
external quantum efficiency measurements of the solar cell devices
showed that GSH-treatment of the QD-sensitized TiO<sub>2</sub> electrodes
more than doubled the short circuit current and conversion efficiency.
Impedance spectroscopy, intensity-modulated photovoltage and photocurrent
spectroscopy analysis of the devices revealed that the enhancement
in solar cell performance of the GSH-treated cells originates from
improved charge injection from PbS QDs into the conduction band of
TiO<sub>2</sub>. Time-resolved photoluminescence decay measurements
show that passivation of the surface of QDs with GSH ligands increases
the exciton lifetime in the QDs