Quantum Dot-Sensitized Solar Cells - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 91

The use of CQDs allows the production of material with a high degree of
crystallinity a high control of QD properties (as size, shape, bandgap…) but in the
first reports relatively low QD loading was obtained [ 16 , 22 ]. In QDSSCs, it is
widely reported historically that directly grown QDs on the electrode surface show
relatively high performance than that of indirectly attached CQDs, due to a higher
sensitizer loading and consequently higher harvesting efficiency [ 16 , 33 ]. Using
direct assembled approach, the QDs were attached to mesoporous framework
robustly and charge transfer resistance was possibly reduced [ 34 , 35 ]. But it was
demonstrated that potentially QDSSCs based on CQDs exhibit a higher potentiality
for the development of efficient devices [ 16 ]. Semiconductor grown directly on the
surface of the mesoporous electrode present poor crystallinity and broader quantum
dot size dispersion than CQDs. In addition, deleterious effects in the grain
boundaries could arise [ 7 ]. Recently, the group of Zhong has developed a method
based in a ligand exchange of the capping molecules in CQDs [ 36 - 38 ]. With this
method higher QD loading is attained and consequently higher efficiency in fact the
record efficiency for QDSSCs has been reported by this group with CdSe x Te 1-x
CQDs [ 38 ].
Taking into account these considerations here we review different sensitization
methods for both CQDs and semiconductors grown directly on the electrode
surface. The following coating methods were identified as low cost methods
compared to physical and vacuum techniques:
3.1 Sensitization with Colloidal Quantum Dots
For the sensitization with CQDs two consecutive steps are required. In first step,
presynthesized CQDs are grown with an accurate control of the crystalline quality,
size, and shape (and consequently bandgap) distribution or capping [ 39 ]. Second,
CQDs should be attached to the nanostructured electrode. However, there is no
single procedure to attach CQDs to the photoanode and it is possible to differ-
entiate between assisted and direct sensitization. In the assisted sensitization
process, bifunctional linker molecules are used to anchor CQDs to the nano-
structured photoanode [ 8 , 23 ]. These linker molecules generally have a functional
carboxylic group which first attach to one side of the TiO 2 and the other side of the
linker (generally a thiol group) was connected to the CQD [ 8 , 23 ]. In addition to
carboxyl linkers, other functional groups can also be applied as bifunction linkers
for fixing CQD to the nanoporous electrode (as amine group in cysteine), and is
realized that this linker molecules play an important role in PEC performance [ 21 ].
On the other hand, CQDs can be directly attached to the electrode without the use
of any specific linker. For example, if toluene solvent for CdSe CQDs is substi-
tuted by dichoromethane, CQDs can be directly absorbed on TiO 2 surface simply
by dipping the substrate in the dichloromethane solution with CQDs [ 22 , 24 ]. The
use of linker molecules is benefit during the CQDs synthesis which capped with,
generally, long organic molecules (oleic acid), thus offer controlling QD size and