94 research outputs found
Ultrathin Buffer Layers of SnO<sub>2</sub> by Atomic Layer Deposition: Perfect Blocking Function and Thermal Stability
This
study pinpoints the advantages of ultrathin electron selective
layers (ESL) of SnO<sub>2</sub> fabricated by atomic layer deposition
(ALD). These layers recently caught attention in planar perovskite
solar cells and appear as powerful alternatives to other oxides such
as TiO<sub>2</sub>. Here, we carry out a thorough characterization
of the nature of these ultrathin ALD SnO<sub>2</sub> layers providing
a novel physical insight for the design of various photoelectrodes
in perovskite and dye-sensitized solar cells and in photoelectrochemical
water splitting. We use a combination of cyclic voltammetry, electrochemical
impedance spectroscopy, Hall measurements, X-ray photoelectron spectroscopy,
atomic force microscopy, and electron microscopy to analyze the blocking
behavior and energetics of as-deposited (low-temperature) and also
calcined ALD SnO<sub>2</sub> layers. First, we find that the low-temperature
ALD-grown SnO<sub>2</sub> layers are amorphous and perfectly pinhole-free
for thicknesses down to 2 nm. This exceptional blocking behavior of
thin ALD SnO<sub>2</sub> layers allows photoelectrode designs with
even thinner electron selective layers, thus potentially minimizing
resistance losses. The compact nature and blocking function of thin
SnO<sub>2</sub> films are not perturbed by annealing at 450 °C,
which is a significant benefit compared to other amorphous ALD oxides.
Further on, we show that amorphous and crystalline ALD SnO<sub>2</sub> films substantially differ in their flatband (and conduction band)
positionsî—¸a finding to be taken into account when considering
band alignment engineering in solar devices using these high-quality
blocking layers
The Transient Photocurrent and Photovoltage Behavior of a Hematite Photoanode under Working Conditions and the Influence of Surface Treatments
Hematite (α-Fe<sub>2</sub>O<sub>3</sub>) is widely
recognized
as a promising candidate for the production of solar fuels via water
splitting, but its intrinsic optoelectronic properties have limited
its performance to date. In particular, the large electrochemical
overpotential required to drive the water oxidation is known as a
major drawback. This overpotential (0.4 – 0.6 V anodic of the
flat band potential) has been attributed to poor oxygen evolution
reaction (OER) catalysis and to charge trapping in surface states
but is still not fully understood. In the present study, we quantitatively
investigate the photocurrent and photovoltage transient behavior of
α-Fe<sub>2</sub>O<sub>3</sub> photoanodes prepared by atmospheric
pressure chemical vapor deposition, under light bias, in a standard
electrolyte, and one containing a sacrificial agent. The accumulation
of positive charges occurring in water at low bias potential is found
to be maximum when the photocurrent onsets. The transient photocurrent
behavior of a standard photoanode is compared to photoanodes modified
by either a catalytic or surface passivating overlayer. Surface modification
shows a reduction and a cathodic shift of the charge accumulation,
following the observed change in photocurrent onset. By applying an
electrochemical model, the values of the space charge width (5–10
nm) and of the hole diffusion length (0.5–1.5 nm) are extracted
from photocurrent transients’ amplitudes with the sacrificial
agent. Characterization of the photovoltage transients also suggests
the presence of surface states causing Fermi level pinning at small
applied potential. The transient photovoltage and the use of both
overlayers on the same electrode enable differentiation of the two
overlayers’ effects and a simplified model is proposed to explain
the roles of each overlayer and their synergetic effects. This investigation
demonstrates a new method to characterize water splitting photoelectrodesî—¸especially
the charge accumulation occurring at the semiconductor/electrolyte
interface during operation. It finally confirms the requirements of
nanostructuring and surface control with catalytic and trap passivation
layers to improve iron oxide’s performance for water photolysis
Temperature Dependence of Transport Properties of Spiro-MeOTAD as a Hole Transport Material in Solid-State Dye-Sensitized Solar Cells
The internal transport and recombination parameters of solid-state dye-sensitized solar cells (ssDSCs) using the amorphous organic semiconductor 2,2′,7,7′-tetrakis(<i>N,N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) as a hole transport material (HTM) are investigated using electrical impedance spectroscopy. Devices were fabricated using flat and nanostructured TiO<sub>2</sub> and compared to systems using nanostructured ZrO<sub>2</sub> to differentiate between the transport processes within the different components of the ssDSC. The effect of chemically p-doping the HTM on its transport was investigated, and its temperature dependence was examined and analyzed using the Arrhenius equation. Using this approach the activation energy of the hole hopping transport within the undoped spiro-MeOTAD film was determined to be 0.34 ± 0.02 and 0.40 ± 0.02 eV for the mesoporous TiO<sub>2</sub> and ZrO<sub>2</sub> systems, respectively
Optically Transparent FTO-Free Cathode for Dye-Sensitized Solar Cells
The woven fabric containing electrochemically
platinized tungsten wire is an affordable flexible cathode for liquid-junction
dye-sensitized solar cells with the I<sub>3</sub><sup>–</sup>/I<sup>–</sup> redox mediator and electrolyte solution consisting
of ionic liquids and propionitrile. The fabric-based electrode outperforms
the thermally platinized FTO in serial ohmic resistance and charge-transfer
resistance for triiodide reduction, and it offers comparable or better
optical transparency in the visible and particularly in the near-IR
spectral region. The electrode exhibits good stability during electrochemical
loading and storage at open circuit. The dye-sensitized solar cells
with a C101-sensitized titania photoanode and either Pt–W/PEN
or Pt–FTO cathodes show a comparable performance
Co(III) Complexes as p‑Dopants in Solid-State Dye-Sensitized Solar Cells
Following our recent work on the
use of CoÂ(III) complexes as p-type
dopants for triarylamine-based organic hole-conductors, we herein
report on two new CoÂ(III) complexes for doping applications. With
the aim of ameliorating the dopant’s suitability for its use
in solid-state dye-sensitized solar cells, we show how the properties
of the dopant can be easily adjusted by a slight modification of the
molecular structure. We prove the eligibility of the two new dopants
by characterizing their optical and electrochemical properties and
give evidence that both of them can be used to oxidize the molecular
hole-transporter spiro-MeOTAD. Finally, we fabricate high-performance
solid-state dye-sensitized solar cells using a state-of-the-art metal-free
organic sensitizer in order to elucidate the influence of the type
of dopant on device performance
Mesoscopic Oxide Double Layer as Electron Specific Contact for Highly Efficient and UV Stable Perovskite Photovoltaics
The
solar to electric power conversion efficiency (PCE) of perovskite
solar cells (PSCs) has recently reached 22.7%, exceeding that of competing
thin film photovoltaics and the market leader polycrystalline silicon.
Further augmentation of the PCE toward the Shockley–Queisser
limit of 33.5% warrants suppression of radiationless carrier recombination
by judicious engineering of the interface between the light harvesting
perovskite and the charge carrier extraction layers. Here, we introduce
a mesoscopic oxide double layer as electron selective contact consisting
of a scaffold of TiO<sub>2</sub> nanoparticles covered by a thin film
of SnO<sub>2</sub>, either in amorphous (a-SnO<sub>2</sub>), crystalline
(c-SnO<sub>2</sub>), or nanocrystalline (quantum dot) form (SnO<sub>2</sub>-NC). We find that the band gap of a-SnO<sub>2</sub> is larger
than that of the crystalline (tetragonal) polymorph leading to a corresponding
lift in its conduction band edge energy which aligns it perfectly
with the conduction band edge of both the triple cation perovskite
and the TiO<sub>2</sub> scaffold. This enables very fast electron
extraction from the light perovskite, suppressing the notorious hysteresis
in the current–voltage (<i>J–V</i>) curves
and retarding nonradiative charge carrier recombination. As a result,
we gain a remarkable 170 mV in open circuit photovoltage (<i>V</i><sub><i>oc</i></sub>) by replacing the crystalline
SnO<sub>2</sub> by an amorphous phase. Because of the quantum size
effect, the band gap of our SnO<sub>2</sub>-NC particles is larger
than that of bulk SnO<sub>2</sub> causing their conduction band edge
to shift also to a higher energy thereby increasing the <i>V</i><sub><i>oc</i></sub>. However, for SnO<sub>2</sub>-NC there
remains a barrier for electron injection into the TiO<sub>2</sub> scaffold
decreasing the fill factor of the device and lowering the PCE. Introducing
the a-SnO<sub>2</sub> coated mp-TiO<sub>2</sub> scaffold as electron
extraction layer not only increases the <i>V</i><sub><i>oc</i></sub> and PEC of the solar cells but also render them
resistant to UV light which forebodes well for outdoor deployment
of these new PSC architectures
Fine-tuning the Electronic Structure of Organic Dyes for Dye-Sensitized Solar Cells
A series of metal-free organic dyes exploiting different combinations of (hetero)cyclic linkers (benzene, thiophene, and thiazole) and bridges (4<i>H</i>-cyclopenta[2,1-<i>b</i>:3,4-<i>b′</i>]dithiophene (CPDT) and benzodithiophene (BDT)) as the central π-spacers were synthesized and characterized. Among them, the sensitizer containing the thiophene and CPDT showed the most broad incident photon-to-current conversion efficiency spectra, resulting in a solar energy conversion efficiency (η) of 6.6%
Analysis of Electron Transfer Properties of ZnO and TiO<sub>2</sub> Photoanodes for Dye-Sensitized Solar Cells
Mesoporous TiO<sub>2</sub> nanoparticle films are used as photoanodes for high-efficiency dye-sensitized solar cells (DSCs). In spite of excellent photovoltaic power conversion efficiencies (PCEs) displayed by titanium dioxide nanoparticle structures, the transport rate of electrons is known to be low due to low electron mobility. So the alternate oxides, including ZnO, that possesses high electron mobility are being investigated as potential candidates for photoanodes. However, the PCE with ZnO is still lower than with TiO<sub>2</sub>, and this is typically attributed to the low internal surface area. In this work, we attempt to make a one-to-one comparison of the photovoltaic performance and the electron transfer dynamics involved in DSCs, with ZnO and TiO<sub>2</sub> as photoanodes. Previously such comparative investigations were hampered due to the morphological differences (internal surface area, pore diameter, porosity) that exist between zinc oxide and titanium dioxide films. We circumvent this issue by depositing different thicknesses of these oxides, by atomic layer deposition (ALD), on an arbitrary mesoporous insulating template and subsequently using them as photoanodes. Our results reveal that at an optimal thickness ZnO exhibits photovoltaic performances similar to TiO<sub>2</sub>, but the internal electron transfer properties differ. The higher photogenerated electron transport rate contributed to the performances of ZnO, but in the case of TiO<sub>2</sub>, it is the low recombination rate, higher dye loading, and fast electron injection
Effect of Posttreatment of Titania Mesoscopic Films by TiCl<sub>4</sub> in Solid-State Dye-Sensitized Solar Cells: A Time-Resolved Spectroscopy Study
Posttreatment of mesoporous titanium dioxide films by
TiCl<sub>4</sub> solutions is commonly applied during the fabrication
of solid-state
dye-sensitized solar cells (ssDSCs), as this operation markedly improves
the performance of the photovoltaic device. The effect of the posttreatment
upon the charge carrier dynamics was scrutinized in an ssDSC aiming
at unraveling its mechanism. Kinetic studies carried out using femtosecond
and nanosecond transient absorption spectroscopies showed that a biphasic
electron injection from the dye excited state is observed, for both
treated and nontreated films, whose kinetics is not significantly
affected by the surface modification step. However, hole injection
in the hole transport material (HTM) spiro-OMeTAD and charge recombination
were found to be markedly slower in TiCl<sub>4</sub>-treated films.
These findings are rationalized by a model describing the interaction
at the interface between TiO<sub>2</sub>, the dye-sensitizer, and
spiro-OMeTAD. Rather than resulting from a modification of the energetics
of the conduction band of the oxide, the effect of the TiCl<sub>4</sub> posttreatment appears to be associated with a subtle change of the
film morphology. Results emphasize the importance of controlling the
contact at the heterojunction between the HTM and the sensitized semiconductor
oxide network
Heterogeneous Electron Transfer from Dye-Sensitized Nanocrystalline TiO<sub>2</sub> to [Co(bpy)<sub>3</sub>]<sup>3+</sup>: Insights Gained from Impedance Spectroscopy
Dye-sensitized
solar cells (DSCs) employing the [CoÂ(bpy)<sub>3</sub>]<sup>3+/2+</sup> redox mediator have recently attained efficiencies
in excess of 12%, increasing the attractiveness of DSCs as an alternative
to conventional photovoltaics. Heterogeneous electron transfer from
dye-sensitized nanocrystalline TiO<sub>2</sub> to [CoÂ(bpy)<sub>3</sub>]<sup>3+</sup> ions in solution, a process known as recombination
in the context of DSC operation, is an important loss mechanism in
these solar cells. Here, we employ impedance spectroscopy over a range
of temperatures to characterize electron storage, transport, and recombination
in efficient DSCs based on the [CoÂ(bpy)<sub>3</sub>]<sup>3+/2+</sup> redox mediator, with either the amphiphillic ruthenium sensitizer
Z907 or the state-of-the-art organic sensitizer Y123. The temperature
dependence of the electron-transport resistance indicates that transport
occurs via states at energies lower than commonly assumed for the
TiO<sub>2</sub> conduction band edge. We show that a non-exponential
dependence of capacitance, transport resistance, and recombination
resistance on photovoltage can be interpreted as evidence for partial
unpinning of the TiO<sub>2</sub> energy levels. We also find that
the nature of the sensitizing dye determines the predominant recombination
route: via the conduction band for Y123 and via band gap states for
Z907, which is the main reason for the superior performance of Y123.
The different mechanisms appear to arise from changes in electronic
coupling between TiO<sub>2</sub> donor states and [CoÂ(bpy)<sub>3</sub>]<sup>3+</sup> acceptor states,
as opposed to changes in the density of TiO<sub>2</sub> states or
their energetic matching with the acceptor-state distribution. These
findings have implications for modeling heterogeneous electron transfer
at dye-sensitized semiconductor–solution interfaces in general
and for the optimization of DSCs
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