6 research outputs found
Effects of Oxide Contact Layer on the Preparation and Properties of CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> for Perovskite Solar Cell Application
In perovskite solar cells, oxide
electron transport layers (ETL)
and their interface with the organometal trihalides are key to achieve
efficient and stable devices. In the present work we investigate ZnO
and TiO<sub>2</sub> ETLs and their influence on the preparation of
CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> film by two different
techniques. In the âone-stepâ technique, a solution
is used for the deposition of a precursor layer which is dripped and
subsequently annealed. In the âtwo-stepâ sequential
technique, a PbI<sub>2</sub> precursor layer is converted into perovskite.
We show that, on ZnO, the annealing treatment of the âone-stepâ
deposited layer is optimum for a duration time of only 2 min. This
duration is much less critical for the TiO<sub>2</sub> underlayer.
Long annealing times produce the degradation of the pigment and formation
of PbI<sub>2</sub>. It is also shown that the âone-stepâ
technique gives better results for the sensitization of smooth oxide
underlayers whereas the âtwo-stepâ one must be utilized
for rough or structured underlayer sensitization. The best solar cell
performances were achieved by combining a low-overvoltage electrodeposited
ZnO layer, a planar architecture, and a perovskite layer prepared
by a âone-stepâ deposition-dripping route. A maximum
overall conversion efficiency of 15% was measured for the ZnO-based
perovskite solar cell. Cell impedance spectra have been measured over
a large applied voltage range. Their analysis, using an ad-hoc equivalent
circuit, shows that charge recombinations are reduced for the âone-stepâ
perovskite and that a better interface with the oxide is produced
in that case
Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells
The partial replacement
of methylammonium by formamidinium and
cesium in organolead trihalide materials is of great importance to
improve the performance and stability of photovoltaic solar cells.
However, the effect of multiple cations on the cell functioning and
their electrical characteristics remains to be clarified. By using
the impedance spectroscopy technique, we have investigated the electrical
response to a small ac perturbation applied to solar cells implementing
hybrid perovskites with various compositions, polarized over a large
potential range. The solar cell preparation protocols have been optimized
to reach power conversion efficiencies higher than 17%. The impedance
response has been investigated both under light and in the dark to
discriminate the light sensitive parameters. The spectra have been
carefully analyzed using an <i>ad hoc</i> equivalent circuit,
and the data have been discussed in the light of the existing literature.
The spectra showed no intermediate frequency inductive loop due to
the absence of multistep charge transfer involving surface states.
A large inductive loop is found to be the signature of poorly functioning
solar cells. Except for the high frequency capacitance, which is the
bulk response of perovskite, the other parameters are influenced by
interface and contact phenomena, ionic conductivity and charge accumulations.
The scaling of the low frequency capacitance with the hysteresis amplitude
is clearly stated by our comprehensive study. Moreover, no diffusion
impedance due to the diffusion of ionic species is observed. However,
ion mobility results in a strong effect on recombinations and has
a strong influence on the low frequency impedance response of the
system
Impact of Organic Hole Transporting Material and Doping on the Electrical Response of Perovskite Solar Cells
The
hole transport material (HTM) layer is a key component of the
perovskite solar cells (PSCs) that must be optimized to reach high
efficiency. The development of new HTMs alternative to Spiro-OMeTAD
and the understanding of the role of doping agents on these layers
are important research axes in the field. It requires the use of appropriate
characterization tools enabling us to discriminate the bulk and interface
effects. In the present paper, we fully analyze the effect of HTM
doping and of the material on the impedance response of PSCs. The
approach has been implemented on two different molecular HTMs, Spiro-OMeTAD
and a new molecular carbazole HTM, called B186, and with various doping
levels. We show that limitations by poor doping are characterized
by an extra high frequency impedance loop for which capacitance and
resistance analysis gives the dielectric constant and conductivity
of the material, respectively. However, the low-frequency part of
the spectra provides important information on the charge accumulation/outflow
and on the recombination levels. More generally, the presented approach
is of high practical interest for the development of new organic HTMs
and for the optimization of the layer doping
Modeling Dye-Sensitized Solar Cells: From Theory to Experiment
Density functional theory (DFT) and
time-dependent DFT are useful
computational approaches frequently used in the dye-sensitized solar
cell (DSSC) community in order to analyze experimental results and
to clarify the elementary processes involved in the working principles
of these devices. Indeed, despite these significant contributions,
these methods can provide insights that go well beyond a purely descriptive
aim, especially when suitable computational approaches and methodologies
for interpreting and validating the computational outcomes are developed.
In the present contribution, the possibility of using recently developed
computational approaches to design and interpret the macroscopic behavior
of DSSCs is exemplified by the study of the performances of three
new TiO<sub>2</sub>-based DSSCs making use of organic dyes, all belonging
to the expanded pyridinium family
Low-Temperature Preparation of Ag-Doped ZnO Nanowire Arrays, DFT Study, and Application to Light-Emitting Diode
Doping ZnO nanowires (NWs) by group
IB elements is an important
challenge for integrating nanostructures into functional devices with
better and tuned performances. The growth of Ag-doped ZnO NWs by electrodeposition
at 90 °C using a chloride bath and molecular oxygen precursor
is reported. Ag acts as an electrocatalyst for the deposition and
influences the nucleation and growth of the structures. The silver
atomic concentration in the wires is controlled by the additive concentration
in the deposition bath and a content up to 3.7 atomic % is reported.
XRD analysis shows that the integration of silver enlarges the lattice
parameters of ZnO. The optical measurements also show that the direct
optical bandgap of ZnO is reduced by silver doping. The bandgap shift
and lattice expansion are explained by first principle calculations
using the density functional theory (DFT) on the silver impurity integration
as an interstitial (Ag<sub>i</sub>) and as a substitute of zinc atom
(Ag<sub>Zn</sub>) in the crystal lattice. They notably indicate that
Ag<sub>Zn</sub> doping forms an impurity band because of Ag 4d and
O 2p orbital interactions, shifting the Fermi level toward the valence
band. At least, Ag-doped ZnO vertically aligned nanowire arrays have
been epitaxially grown on GaN(001) substrate. The heterostructure
has been inserted in a light emitting device. UV-blue light emission
has been achieved with a low emission threshold of 5 V and a tunable
red-shifted emission spectrum related to the bandgap reduction induced
by silver doping of the ZnO emitter material