Quantum dot sensitized solar cells (QDSCs) are receiving a lot of attention as promising third generation solar cells, being virtually all of them based on sensitized photoanodes. Finding efficient QD-sensitized photocathodes would pave the way toward the implementation of tandem QDSCs. In this context, NiO photocathodes have been sensitized with colloidal CdSe quantum dots directly attached to the semiconductor oxide surface. The emission spectra indicate effective hole injection from the excited state of the quantum dots to the valence band of the NiO. A maximum incident current to photon conversion efficiency of 17% at 420 nm has been achieved. For the sake of comparison, other ways to prepare and anchor the QDs have been tested. Sensitization routes based on presynthesized colloidal quantum dots show better results than in situ growth techniques such as successive ionic layer adsorption and reaction. Electrochemical impedance measurements have identified transport resistance in NiO as one of the limiting factors in the performance of the system under study. Interestingly, surface treatments based on the deposition of very thin films of either SiO 2 or Al 2 O 3 can diminish recombination at the NiO/CdSe/electrolyte interface. This work also identifies a number of possible routes for the improvement of this kind of electrodes, unveiling their potential use in tandem quantum dot solar cells.
We quantify the rate and efficiency of picosecond electron transfer (ET) from PbS nanocrystals, grown by successive ionic layer adsorption and reaction (SILAR), into a mesoporous SnO2 support. Successive SILAR deposition steps allow for stoichiometry- and size-variation of the QDs, characterized using transmission electron microscopy. Whereas for sulfur-rich (p-type) QD surfaces substantial electron trapping at the QD surface occurs, for lead-rich (n-type) QD surfaces, the QD trapping channel is suppressed and the ET efficiency is boosted. The ET efficiency increase achieved by lead-rich QD surfaces is found to be QD-size dependent, increasing linearly with QD surface area. On the other hand, ET rates are found to be independent of both QD size and surface stoichiometry, suggesting that the donor-acceptor energetics (constituting the driving force for ET) are fixed due to Fermi level pinning at the QD/oxide interface. Implications of our results for QD-sensitized solar cell design are discussed.
A hybrid quantum dot sensitized solar cell (QDSC) composed of CdSe quantum dots (QDs) as light harvesters and TiO(2) and 3,3'''-didodecyl-quaterthiophene (QT12) as electron and hole conductors, respectively, has been fully processed in air. The sensitizer has been introduced into the TiO(2) nanoporous layer either by the successive ionic layer adsorption and reaction method or by attaching colloidal QDs either directly or through molecular cables (linkers). As previously observed for QDSCs based on liquid electrolytes, the efficiency depends on the way of QD attachment, the direct adsorption of QDs being the procedure yielding the best results. Thermal annealing was applied in order to enhance the device response under illumination. Remarkable open circuit potentials are attained (close to 1 V), leading to an efficiency of 0.34% (AM 1.5G) in initial tests. Although low, it ranks as one of the highest values reported for solid state QDSCs based on titanium dioxide and colloidal quantum dots.
TiO2 nanotubes (NTs) have been widely used
for a number
of applications including solar cells, photo(electro)chromic devices,
and photocatalysis. Their quasi-one-dimensional morphology has the
advantage of a fast electron transport although they have a relatively
reduced interfacial area compared with nanoparticulate films. In this
study, vertically oriented, smooth TiO2 NT arrays fabricated
by anodization are decorated with ultrathin anatase nanowires (NWs).
This facile modification, performed by chemical bath deposition, allows
to create an advantageous self-organized structure that exhibits remarkable
properties. On one hand, the huge increase in the electroactive interfacial
area induces an improvement by 1 order of magnitude in the charge
accumulation capacity. On the other hand, the modified NT arrays display
larger photocurrents for water and oxalic acid oxidation than bare
NTs. Their particular morphology enables a fast transfer of photogenerated
holes but also efficient mass and electron transport. The importance
of a proper band energy alignment for electron transfer from the NWs
to the NTs is evidenced by comparing the behavior of these electrodes
with that of NTs modified with rutile NWs. The NT-NW self-organized
architecture allows for a precise design and control of the interfacial
surface area, providing a material with particularly attractive properties
for the applications mentioned above.
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