17 research outputs found
Dynamics of Back Electron Transfer in Dye-Sensitized Solar Cells Featuring 4-<i>tert</i>-Butyl-Pyridine and Atomic-Layer-Deposited Alumina as Surface Modifiers
A series
of dye-sensitized solar cells (DSCs) was constructed with
TiO<sub>2</sub> nanoparticles and N719 dye. The standard I<sub>3</sub><sup>–</sup>/I<sup>–</sup> redox shuttle and the CoÂ(1,10-phenanthroline)<sub>3</sub><sup>3+/2+</sup> shuttle were employed. DSCs were modified
with atomic-layered-deposited (ALD) coatings of Al<sub>2</sub>O<sub>3</sub> and/or with the surface-adsorbing additive 4-<i>tert</i>-butyl-pyridine. Current–voltage data were collected to ascertain
the influence of each modification upon the back electron transfer
(ET) dynamics of the DSCs. The primary effect of the additives alone
or in tandem is to increase the open-circuit voltage. A second is
to alter the short-circuit current density, <i>J</i><sub>SC</sub>. With dependence on the specifics of the system examined,
any of a myriad of dynamics-related effects were observed to come
into play, in both favorable (efficiency boosting) and unfavorable
(efficiency damaging) ways. These effects include modulation of (a)
charge-injection yields, (b) rates of interception of injected electrons
by redox shuttles, and (c) rates of recombination of injected electrons
with holes on surface-bound dyes. In turn, these influence charge-collection
lengths, charge-collection yields, and onset potentials for undesired
dark current. The microscopic origins of the effects appear to be
related mainly to changes in driving force and/or electronic coupling
for underlying component redox reactions. Perhaps surprisingly, only
a minor role for modifier-induced shifts in conduction-band-edge energy
was found. The combination of DSC-efficiency-relevant effects engendered
by the modifiers was found to vary substantially as a function of
the chemical identity of the redox shuttle employed. While types of
modifiers are effective, a challenge going forward will be to construct
systems in ways in which the benefits of organic and inorganic modifiers
can be exploited in fully additive, or even synergistic, fashion
Determining the Conduction Band-Edge Potential of Solar-Cell-Relevant Nb<sub>2</sub>O<sub>5</sub> Fabricated by Atomic Layer Deposition
Often
key to boosting photovoltages in photoelectrochemical and
related solar-energy-conversion devices is the preferential slowing
of rates of charge recombinationî—¸especially recombination at
semiconductor/solution, semiconductor/polymer, or semiconductor/perovskite
interfaces. In devices featuring TiO<sub>2</sub> as the semiconducting
component, a common approach to slowing recombination is to install
an ultrathin metal oxide barrier layer or trap-passivating layer atop
the semiconductor, with the needed layer often being formed via atomic
layer deposition (ALD). A particularly promising barrier layer material
is Nb<sub>2</sub>O<sub>5</sub>. Its conduction-band-edge potential <i>E</i><sub>CB</sub> is low enough that charge injection from
an adsorbed molecular, polymeric, or solid-state light absorber and
into the semiconductor can still occur, but high enough that charge
recombination is inhibited. While a few measurements of <i>E</i><sub>CB</sub> have been reported for conventionally synthesized,
bulk Nb<sub>2</sub>O<sub>5</sub>, none have been described for ALD-fabricated
versions. Here, we specifically determine the conduction-band-edge
energy of ALD-fabricated Nb<sub>2</sub>O<sub>5</sub> relative to that
of TiO<sub>2</sub>. We find that, while the value for ALD-Nb<sub>2</sub>O<sub>5</sub> is indeed higher than that for TiO<sub>2</sub>, the
difference is less than anticipated based on measurements of conventionally
synthesized Nb<sub>2</sub>O<sub>5</sub> and is dependent on the thermal
history of the material. The implications of the findings for optimization
of competing interfacial rate processes, and therefore photovoltages,
are briefly discussed
Inhibiting Metal Oxide Atomic Layer Deposition: Beyond Zinc Oxide
Atomic
layer deposition (ALD) of several metal oxides is selectivity inhibited
on alkanethiol self-assembled monolayers (SAMs) on Au, and the eventual
nucleation mechanism is investigated. The inhibition ability of the
SAM is significantly improved by the in situ H<sub>2</sub>-plasma
pretreatment of the Au substrate prior to the gas-phase deposition
of a long-chain alkanethiol, 1-dodecanethiol (DDT). This more rigorous
surface preparation inhibits even aggressive oxide ALD precursors,
including trimethylaluminum and water, for at least 20 cycles. We
study the effect that the ALD precursor purge times, growth temperature,
alkanethiol chain length, alkanethiol deposition time, and plasma
treatment time have on Al<sub>2</sub>O<sub>3</sub> ALD inhibition.
This is the first example of Al<sub>2</sub>O<sub>3</sub> ALD inhibition
from a vapor-deposited SAM. The inhibitions of Al<sub>2</sub>O<sub>3</sub>, ZnO, and MnO ALD processes are compared, revealing the versatility
of this selective surface treatment. Atomic force microscopy and grazing-incidence
X-ray fluorescence further reveal insight into the mechanism by which
the well-defined surface chemistry of ALD may eventually be circumvented
to allow metal oxide nucleation and growth on SAM-modified surfaces
Engendering Long-Term Air and Light Stability of a TiO<sub>2</sub>‑Supported Porphyrinic Dye via Atomic Layer Deposition
Organic
and porphyrin-based chromophores are prevalent in liquid-junction
photovoltaic and photocatalytic solar-cell chemistry; however, their
long-term air and light instability may limit their practicality in
real world technologies. Here, we describe the protection of a zinc
porphyrin dye, adsorbed on nanoparticulate TiO<sub>2</sub>, from air
and light degradation by a protective coating of alumina grown with
a previously developed post-treatment atomic layer deposition (ALD)
technique. The protective Al<sub>2</sub>O<sub>3</sub> ALD layer is
deposited using dimethylaluminum isopropoxide as an Al source; in
contrast to the ubiquitous ALD precursor trimethylaluminum, dimethylaluminum
isopropoxide does not degrade the zinc porphyrin dye, as confirmed
by UV–vis measurements. The growth of this protective ALD layer
around the dye can be monitored by an in-reactor quartz crystal microbalance
(QCM). Furthermore, greater than 80% of porphyrin light absorption
is retained over ∼1 month of exposure to air and light when
the protective coating is present, whereas almost complete loss of
porphyrin absorption is observed in less than 2 days in the absence
of the ALD protective layer. Applying the Al<sub>2</sub>O<sub>3</sub> post-treatment technique to the TiO<sub>2</sub>-adsorbed dye allows
the dye to remain in electronic contact with both the semiconductor
surface and a surrounding electrolyte solution, the combination of
which makes this technique promising for numerous other electrochemical
photovoltaic and photocatalytic applications, especially those involving
the dye-sensitized evolution of oxygen
Distance-Engineered Plasmon-Enhanced Light Harvesting in CdSe Quantum Dots
Improvement
of light harvesting in semiconductor quantum dots (QDs) is essential
for the development of efficient QD-based solar energy conversion
systems. In this study, plasmon-enhanced light absorption in CdSe
QDs sensitized on silver (Ag) nanoparticle (NP) films was examined
as a function of interparticle (QD to Ag NP) distance. Up to 24-fold
plasmonic enhancement of fluorescence from QDs was observed when the
particle separation distance was ≥5 nm. The enhancement effect
was observed to largely sustain the exciton lifetimes in QDs and to
strongly depend on the incident photon wavelength following the plasmon
resonant strength of Ag NPs, confirming that the enhanced photoluminescence
was mainly due to the enhancement in photoabsorption in CdSe QDs by
the plasmon of Ag NPs. This study suggests applications of Ag NPs
in QD-based solar energy conversion for significantly improving light
harvesting in QDs
Low-Temperature Atomic Layer Deposition of CuSbS<sub>2</sub> for Thin-Film Photovoltaics
Copper
antimony sulfide (CuSbS<sub>2</sub>) has been gaining traction
as an earth-abundant absorber for thin-film photovoltaics given its
near ideal band gap for solar energy conversion (∼1.5 eV),
large absorption coefficient (>10<sup>4</sup> cm<sup>–1</sup>), and elemental abundance. Through careful in situ analysis of the
deposition conditions, a low-temperature route to CuSbS<sub>2</sub> thin films via atomic layer deposition has been developed. After
a short (15 min) postprocess anneal at 225 °C, the ALD-grown
CuSbS<sub>2</sub> films were crystalline with micron-sized grains,
exhibited a band gap of 1.6 eV and an absorption coefficient >10<sup>4</sup> cm<sup>–1</sup>, as well as a hole concentration of
10<sup>15</sup> cm<sup>–3</sup>. Finally, the ALD-grown CuSbS<sub>2</sub> films were paired with ALD-grown TiO<sub>2</sub> to form
a photovoltaic device. This photovoltaic device architecture represents
one of a very limited number of Cd-free CuSbS<sub>2</sub> PV device
stacks reported to date, and it is the first to demonstrate an open-circuit
voltage on par with CuSbS<sub>2</sub>/CdS heterojunction PV devices.
While far from optimized, this work demonstrates the potential for
ALD-grown CuSbS<sub>2</sub> thin films in environmentally benign photovoltaics
High-Surface-Area Architectures for Improved Charge Transfer Kinetics at the Dark Electrode in Dye-Sensitized Solar Cells
Dye-sensitized solar cell (DSC) redox
shuttles other than triiodide/iodide
have exhibited significantly higher charge transfer resistances at
the dark electrode. This often results in poor fill factor, a severe
detriment to device performance. Rather than moving to dark electrodes
of untested materials that may have higher catalytic activity for
specific shuttles, the surface area of platinum dark electrodes could
be increased, improving the catalytic activity by simply presenting
more catalyst to the shuttle solution. A new copper-based redox shuttle
that experiences extremely high charge-transfer resistance at conventional
Pt dark electrodes yields cells having fill-factors of less than 0.3.
By replacing the standard Pt dark electrode with an inverse opal Pt
electrode fabricated via atomic layer deposition, the dark electrode
surface area is boosted by ca. 50-fold. The resulting increase in
interfacial electron transfer rate (decrease in charge-transfer resistance)
nearly doubles the fill factor and therefore the overall energy conversion
efficiency, illustrating the utility of this high-area electrode for
DSC
Fast Transporting ZnO–TiO<sub>2</sub> Coaxial Photoanodes for Dye-Sensitized Solar Cells Based on ALD-Modified SiO<sub>2</sub> Aerogel Frameworks
A doubly coaxial photoanode architecture based on templated SiO<sub>2</sub> aerogels was fabricated on transparent conducting oxides for use in dye-sensitized solar cells (DSSCs). These templates were coated with ZnO <i>via</i> atomic layer deposition (ALD) to yield an electronically interconnected, low-density, high-surface-area, semiconductor framework. Addition of a thin conformal layer of a second metal oxide (alumina, zirconia, or titania) <i>via</i> ALD was found to suppress the dissolution of ZnO that otherwise occurs when it is soaked in alcohol solutions containing acidic dyes used for sensitization or in acetonitrile solutions containing a pyridine derivative and the iodide/tri-iodide (I<sup>–</sup>/I<sup>–</sup><sub>3</sub>) redox shuttle. Electron transport in SiO<sub>2</sub>–ZnO–TiO<sub>2</sub> photoelectrodes was found to be nearly 2 orders of magnitude faster than in SiO<sub>2</sub>–TiO<sub>2</sub> structures, implying that the interior ZnO sheath serves as the primary electron conduit. In contrast, rates of electron interception by the oxidized form of the redox shuttle were observed to decrease when a TiO<sub>2</sub> shell was added to SiO<sub>2</sub>–ZnO, with the decreases becoming more significant as the thickness of the titania shell increases. These effects lead to improvements in efficiency for DSSCs that utilize I<sup>–</sup>/I<sup>–</sup><sub>3</sub>, but much larger improvements for DSSCs utilizing ferrocene/ferrocenium, a notoriously fast redox shuttle. For the former, overall energy conversion efficiencies maximize at 4.0%. From a variety of experiments, the primary factor limiting aerogel-based DSSC performance is light loss due to scattering. Nevertheless, variants of the doubly coaxial structure may prove useful in devising DSSCs that can achieve excellent energy conversion efficiencies even with fast redox shuttles
Atomic Layer Deposition of the Quaternary Chalcogenide Cu<sub>2</sub>ZnSnS<sub>4</sub>
Atomic layer deposition (ALD) is a layer-by-layer synthesis
method
capable of depositing conformal thin films with thickness and compositional
control on subnanometer length scales. While many materials have been
synthesized by ALD, the technologically important metal sulfides are
underexplored, and homogeneous quaternary metal sulfides are absent
from the literature. We report an ALD process to synthesize Cu<sub>2</sub>ZnSnS<sub>4</sub> (CZTS), a potentially low cost semiconductor
being explored for photovoltaic applications. Two strategies are reported:
one in which a trilayer stack of binary metal sulfides (i.e., Cu<sub>2</sub>S, SnS<sub>2</sub> and ZnS) is deposited and mixed by thermal
annealing, as well as a supercycle strategy that is similar to the
conventional ALD procedure for forming nanolaminates. Both routes
rely on the facile solid state diffusion of chalcogenides for mixing.
For this ALD route to the CZTS system, the challenges are nucleation,
ion-exchange between the film and the volatile chemical precursors,
and phase-stability of binary SnS<sub>2</sub>. The thin films were
made with no sulfurization step. The X-ray diffraction and Raman spectra
were consistent with the formation of CZTS. X-ray fluorescence measurements
revealed that the films contained the expected amount of sulfur based
on the target oxidation states. Photoelectrochemical measurements
under simulated AM1.5 illumination using Eu<sup>3+</sup> as an electron
acceptor demonstrated that the films were photoactive and had an average
internal quantum efficiency (IQE) of 12%
Porphyrins as Templates for Site-Selective Atomic Layer Deposition: Vapor Metalation and in Situ Monitoring of Island Growth
Examinations
of enzymatic catalysts suggest one key to efficient
catalytic activity is discrete size metallo clusters. Mimicking enzymatic
cluster systems is synthetically challenging because conventional
solution methods are prone to aggregation or require capping of the
cluster, thereby limiting its catalytic activity. We introduce site-selective
atomic layer deposition (ALD) on porphyrins as an alternative approach
to grow isolated metal oxide islands that are spatially separated.
Surface-bound tetra-acid free base porphyrins (H<sub>2</sub>TCPP)
may be metalated with Mn using conventional ALD precursor exposure
to induce homogeneous hydroxide synthetic handles which acts as a
nucleation point for subsequent ALD MnO island growth. Analytical
fitting of in situ QCM mass uptake reveals island growth to be hemispherical
with a convergence radius of 1.74 nm. This growth mode is confirmed
with synchrotron grazing-incidence small-angle X-ray scattering (GISAXS)
measurements. Finally, we extend this approach to other ALD chemistries
to demonstrate the generality of this route to discrete metallo island
materials