13 research outputs found
IRāSpectrophotoelectrochemical Characterization of Mesoporous Semiconductor Films
A combined IRāspectroscopic and electrochemical
approach
for the study of photo- and bias-induced reactions at the semiconductor/electrolyte
interface is presented. Information on the electronic structure of
a mesoporous semiconductor nanoparticle network, concretely the density
and distribution of band gap states, as well as the nature of solution
species are analyzed in situ. It has been shown that under appropriate
conditions the electrode potential determines the quasi-Fermi level
throughout the mesoporous film and thus the occupation of IR-active
band gap states, independently of the type of external perturbation,
i.e., application of a bias voltage or electrode exposure to photons
exceeding the semiconductor band gap at open circuit. Importantly,
electronic properties of the semiconductor and vibrational properties
of solution species can be addressed simultaneously by IRāspectroscopy.
In addition, electrochemical methods provide a means for the active
manipulation (in potentiostatic measurements) or the passive tracking
(during open circuit potential decay) of the quasi-Fermi level in
the mesoporous film together with the possibility of electron quantification
(by charge extraction experiments)
Charge-Transfer Reductive in Situ Doping of Mesoporous TiO<sub>2</sub> Photoelectrodes: Impact of Electrolyte Composition and Film Morphology
Some material properties
not only depend on synthesis and processing
parameters but also may significantly change during operation. This
is particularly true for high-surface-area materials. We used a combined
electrochemical and spectroscopic approach to follow the changes in
the photoelectrocatalytic activity and in the electronic semiconductor
properties of mesoporous TiO<sub>2</sub> films upon charge-transfer
reductive doping. Shallow donors (i.e., electron/proton pairs) were
introduced into the semiconductor by the application of an external
potential or, alternatively, by band gap excitation under open circuit
conditions. In the latter case, the effective open circuit doping
potential depends critically on the electrolyte composition (e.g.,
the presence of electron or hole acceptors). Transient charge accumulation
(electrons and protons) in nanoparticle electrodes results in a photocurrent
enhancement that is attributed to the deactivation of recombination
centers. In nanotube electrodes, the formation of a spaceācharge
layer results in an additional decrease of charge recombination at
positive potentials. Doping is transient in nanoparticle films but
turns out to be stable for nanotube arrays
Electrons in the Band Gap: Spectroscopic Characterization of Anatase TiO<sub>2</sub> Nanocrystal Electrodes under Fermi Level Control
Macroscopic properties of semiconductor nanoparticle
networks in
functional devices strongly depend on the electronic structure of
the material. Analytical methods allowing for the characterization
of the electronic structure in situ, i.e., in the presence of an application-relevant
medium, are therefore highly desirable. Here, we present the first
spectral data obtained under Fermi level control of electrons accumulated
in anatase TiO<sub>2</sub> electrodes in the energy range from the
MIR to the UV (0.1ā3.3 eV). Band gap states were electrochemically
populated in mesoporous TiO<sub>2</sub> films in contact with an aqueous
electrolyte. The combination of electrochemical and spectroscopic
measurements allows us for the first time to determine both the energetic
location of the electronic ground states as well as the energies of
the associated optical transitions in the energetic range between
the fundamental absorption threshold and the onset of lattice absorption.
On the basis of our observations, we attribute spectral contributions
in the vis/NIR to dād transitions of Ti<sup>3+</sup> species
and a broad MIR absorption, monotonically increasing toward lower
wavenumbers, to a quasi-delocalization of electrons. Importantly,
signal intensities in the vis/NIR and MIR are linearly correlated.
Absorbance and extractable charge show the same exponential dependence
on electrode potential. Our results demonstrate that signals in the
vis/NIR and MIR are associated with an exponential distribution of
band gap states
Origin of Nonlinear Recombination in Dye-Sensitized Solar Cells: Interplay between Charge Transport and Charge Transfer
Electron transfer between nanostructured semiconductor
oxides and
redox active electrolytes is a fundamental step in many processes
of technological interest, such as photocatalysis and dye-sensitized
solar cells. It has been shown that the transfer kinetics in the dye-sensitized
solar cell are determined simultaneously by trap-limited transport
and by the relative energetics of donor and acceptor states in the
semiconductor and electrolyte. In this work, the transport and recombination
mechanisms of photogenerated electrons in dye-sensitized solar cells
are modeled by random walk numerical simulations with explicit description
of the electron transfer process in terms of the MarcusāGerischer
model. The recombination rate is computed as a function of Fermi level
in order to extract the electron lifetime and its influence on the
electron diffusion length. The simulation method allows one to relate
the recombination reaction order to the trap distribution parameter,
the band edge position, and the reorganization energy. The results
show that a model involving electron transfer from both shallow and
deep traps, coupled with transport via shallow states, adequately
reproduces all the experimental phenomena, including the dependence
of the electron lifetime and the electron diffusion length on the
open-circuit voltage when either the conduction band or the redox
potential are displaced. Nonlinear recombination is predicted when
the electron diffusion length increases with Fermi level, which is
correlated with a reaction order different from one, in an open-circuit
voltage decay āexperimentā. The results reported here
are relevant to the understanding of the effect of using new electrolyte
compositions and novel redox shuttles in dye-sensitized solar cells
How Important is Working with an Ordered Electrode to Improve the Charge Collection Efficiency in Nanostructured Solar Cells?
The collection efficiency of carriers in solar cells
based on nanostructured
electrodes is determined for different degrees or morphological one-dimensional
order. The transport process is modeled by random walk numerical simulation
in a mesoporous electrode that resembles the morphology of nanostructured
TiO<sub>2</sub> electrodes typically used in dye-sensitized solar
cells and related systems. By applying an energy relaxation procedure
in the presence of an external potential, a preferential direction
is induced in the system. It is found that the partially ordered electrode
can almost double the collection efficiency with respect to the disordered
electrode. However, this improvement depends strongly on the probability
of recombination. For too rapid or too slow recombination, working
with partially ordered electrodes will not be beneficial. The computational
method utilized here makes it possible to relate the charge collection
efficiency with morphology. The collection efficiency is found to
reach very rapidly a saturation value, meaning that, in the region
of interest, a slight degree of ordering might be sufficient to induce
a large improvement in collection efficiency
The Redox Pair Chemical Environment Influence on the Recombination Loss in Dye-Sensitized Solar Cells
Reduction of recombination losses
in dye-sensitized solar cells
(DSC) is vital to fabricate efficient devices. The electron recombination
lifetime depends on the relative energetics of the semiconductor and
the redox pair and on the chemical nature of the electrolyte (hole
conductor). In this work, the behavior of the electron lifetime in
DSC devices prepared with various solvents (acetonitrile, valeronitrile,
ethylene carbonate, pure ionic liquids), additives (lithium ions,
TBP), and redox pairs (iodide/iodine, CoĀ(II)/CoĀ(III)) is thoroughly
studied using high-extinction dyes. Lifetimes were extracted by means
of small-perturbation electrochemical techniques (impedance spectroscopy,
intensity-modulated photovoltage spectroscopy) and open-circuit voltage
decays. To ensure a safe inner comparison and a proper interpretation,
all devices were constructed using the same type of TiO<sub>2</sub> electrode and the same dyes (C101 and Z907 for iodide/iodine and
cobalt-based electrolytes, respectively). Furthermore, small-perturbation
techniques and voltage decay provided consistent results. The lifetime
shows a clear change of behavior when iodide/iodine electrolytes in
organic solvents are compared to iodide/iodine in ionic liquids and
with cobalt electrolytes. In the first case, the lifetimeāvoltage
semilogarithmic plot exhibits a curvature, whereas in the second case
the behavior is purely exponential. This observation is consistent
with previous theoretical predictions based on the multiple-trapping
model and the MarcusāGerischer theory, which predict an exponential
law for large reorganization energies and a curvature for small ones.
The obtained results show that solvents or ligands that interact strongly
with the redox mediator originate larger reorganization energies and
lead to devices with shorter lifetimes. This can be interpreted as
an enhancement of extra routes for electron recombination as a consequence
of a wider overlap in energies between donor and acceptor states for
strongly interacting chemical environments
Elucidating Transport-Recombination Mechanisms in Perovskite Solar Cells by Small-Perturbation Techniques
Solar cells using perovskite as semiconducting
pigment have recently
attracted a surge of interest owing to their remarkable solar-to-electric
conversion efficiencies and ease of processing. In this direction
various device architectures and materials have been employed, and
attempts were made to elucidate the underlying working principles.
However, factors governing the performance of perovskite devices are
still obscure. For instance, the interpretation of electrochemical
impedance spectroscopy (EIS) is not straightforward, and the complexity
of the equivalent circuits hinders the identification of transport
and recombination mechanisms in devices, especially those that determine
the performance of the device. Here in we carried out a comprehensive
and complementary characterization of perovskite solar cells by using
an array of small-perturbation techniques: EIS and intensity-modulated
photocurrent and photovoltage spectroscopy (IMPS/IMVS). The employment
of IMPS allowed us to identify two transport times separated by 2
orders of magnitude and with opposite voltage dependences. For recombination,
well agreement was found between lifetimes obtained by IMVS and EIS.
The feature associated with recombination and charge accumulation
in an impedance spectrum through correlation to the IMVS response
was experimentally identified. This correlation paves the way to reconstruct
the currentāvoltage curve using a continuity equation model
for transport and recombination in the working device. The adopted
methodology demonstrates that complementary techniques facilitate
the interpretation of EIS results in perovskite solar cells, allowing
us for the identification of the transport-recombination mechanisms
and providing new insights into the efficiency-determining steps
Universal Features of Electron Dynamics in Solar Cells with TiO<sub>2</sub> Contact: From Dye Solar Cells to Perovskite Solar Cells
The
electron dynamics of solar cells with mesoporous TiO<sub>2</sub> contact
is studied by electrochemical small-perturbation techniques.
The study involved dye solar cells (DSC), solid-state perovskite solar
cells (SSPSC), and devices where the perovskite acts as sensitizer
in a liquid-junction device. Using a transport-recombination continuity
equation we found that mid-frequency time constants are proper lifetimes
that determine the currentāvoltage curve. This is not the case
for the SSPSC, where a lifetime of ā¼1 Ī¼s, 1 order of
magnitude longer, is required to reproduce the currentāvoltage
curve. This mismatch is attributed to the dielectric response on the
mid-frequency component. Correcting for this effect, lifetimes lie
on a common exponential trend with respect to open-circuit voltage.
Electron transport times share a common trend line too. This universal
behavior of lifetimes and transport times suggests that the main difference
between the cells is the power to populate the mesoporous TiO<sub>2</sub> contact with electrons
Universal Features of Electron Dynamics in Solar Cells with TiO<sub>2</sub> Contact: From Dye Solar Cells to Perovskite Solar Cells
The
electron dynamics of solar cells with mesoporous TiO<sub>2</sub> contact
is studied by electrochemical small-perturbation techniques.
The study involved dye solar cells (DSC), solid-state perovskite solar
cells (SSPSC), and devices where the perovskite acts as sensitizer
in a liquid-junction device. Using a transport-recombination continuity
equation we found that mid-frequency time constants are proper lifetimes
that determine the currentāvoltage curve. This is not the case
for the SSPSC, where a lifetime of ā¼1 Ī¼s, 1 order of
magnitude longer, is required to reproduce the currentāvoltage
curve. This mismatch is attributed to the dielectric response on the
mid-frequency component. Correcting for this effect, lifetimes lie
on a common exponential trend with respect to open-circuit voltage.
Electron transport times share a common trend line too. This universal
behavior of lifetimes and transport times suggests that the main difference
between the cells is the power to populate the mesoporous TiO<sub>2</sub> contact with electrons
Comparison of TiO<sub>2</sub> and ZnO Solar Cells Sensitized with an Indoline Dye: Time-Resolved Laser Spectroscopy Studies of Partial Charge Separation Processes
Time-resolved laser spectroscopy
techniques in the time range from
femtoseconds to seconds were applied to investigate the charge separation
processes in complete dye-sensitized solar cells (DSC) made with iodide/iodine
liquid electrolyte and indoline dye D149 interacting with TiO<sub>2</sub> or ZnO nanoparticles. The aim of the studies was to explain
the differences in the photocurrents of the cells (3ā4 times
higher for TiO<sub>2</sub> than for ZnO ones). Electrochemical impedance
spectroscopy and nanosecond flash photolysis studies revealed that
the better performance of TiO<sub>2</sub> samples is not due to the
charge collection and dye regeneration processes. Femtosecond transient
absorption results indicated that after first 100 ps the number of
photoinduced electrons in the semiconductor is 3 times higher for
TiO<sub>2</sub> than for ZnO solar cells. Picosecond emission studies
showed that the lifetime of the D149 excited state is about 3 times
longer for ZnO than for TiO<sub>2</sub> samples. Therefore, the results
indicate that lower performance of ZnO solar cells is likely due to
slower electron injection. The studies show how to correlate the laser
spectroscopy methodology with global parameters of the solar cells
and should help in better understanding of the behavior of alternative
materials for porous electrodes for DSC and related devices