Synthesis
of Eco-Friendly CuInS<sub>2</sub> Quantum
Dot-Sensitized Solar Cells by a Combined Ex Situ/in Situ Growth Approach
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Abstract
A cadmium-free CuInS<sub>2</sub> quantum
dot (QD)-sensitized solar
cell (QDSC) has been fabricated by taking advantage of the ex situ
synthesis approach for fabricating highly crystalline QDs and the
in situ successive ionic-layer adsorption and reaction (SILAR) approach
for achieving high surface coverage of QDs. The ex situ synthesized
CuInS<sub>2</sub> QDs can be rendered water soluble through a simple
and rapid two-step method under the assistance of ultrasonication.
This approach allows a stepwise ligand change from the insertion of
a foreign ligand to ligand replacement, which preserves the long-term
stability of colloidal solutions for more than 1 month. Furthermore,
the resulting QDs can be utilized as sensitizers in QDSCs, and such
a QDSC can deliver a power conversion efficiency (PCE) of 0.64%. Using
the SILAR process, in situ CuInS<sub>2</sub> QDs could be preferentially
grown epitaxially on the pre-existing seeds of ex situ synthesized
CuInS<sub>2</sub> QDs. The results indicated that the CuInS<sub>2</sub> QDSC fabricated by the combined ex situ/in situ growth process exhibited
a PCE of 1.84% (short-circuit current density = 7.72 mA cm<sup>–2</sup>, open-circuit voltage = 570 mV, and fill factor = 41.8%), which
is higher than the PCEs of CuInS<sub>2</sub> QDSCs fabricated by ex
situ and in situ growth processes, respectively. The relative efficiencies
of electrons injected by the combined ex situ/in situ growth approach
were higher than those of ex situ synthesized CuInS<sub>2</sub> QDs
deposited on TiO<sub>2</sub> films, as determined by emission-decay
kinetic measurements. The incident photon-to-current conversion efficiency
has been determined, and electrochemical impedance spectroscopy has
been carried out to investigate the photovoltaic behavior and charge-transfer
resistance of the QDSCs. The results suggest that the combined synergetic
effects of in situ and ex situ CuInS<sub>2</sub> QD growth facilitate
more electron injection from the QD sensitizers into TiO<sub>2</sub>