8 research outputs found
Elucidating Operating Modes of Bulk-Heterojunction Solar Cells from Impedance Spectroscopy Analysis
We discuss the progress and challenges
in the application of impedance
spectroscopy analysis to determine key processes and parameters in
organic bulk-heterojunction solar cells. When carrier transport or
outer interface extraction do not severely influence the solar cell
performance, a simple method to quantify the open-circuit voltage
loss caused by the kinetics of charge carrier recombination is provided,
based on the determination of chemical capacitance and recombination
resistance. This easily allows distinguishing between energetic and
kinetic effects on photovoltage, and establishes a benchmark for the
performance comparison of a set of different cells. A brief discussion
of impedance analysis in the much less studied case of collection-limited
solar cells is introduced
Additional file 1: Table S1. of Amyotrophic lateral sclerosis modifies progenitor neural proliferation in adult classic neurogenic brain niches
Antibodies used in the immunohistochemical study. Table S2. Summary of patient characteristics. Table S3. Immunohistochemical studies used in ALS diagnosis. Values for TDP-43 and ubiquitin are expressed in inclusions per field; %pTDP-43 represents the percentage of phosphorylated TDP inclusions out of the total. Table S4. Description of neurogenesis patient to patient. Table S5. Neurogenesis findings in the subgranular zone of the hippocampal dentate gyrus, by patient. Table S6. Summary of results in the SVZ. Table S7. Summary of results in the hippocampus. (DOC 302 kb
Recombination in Organic Bulk Heterojunction Solar Cells: Small Dependence of Interfacial Charge Transfer Kinetics on Fullerene Affinity
We investigate the causes for obtaining higher open-circuit
voltage
in solar cells that use a fullerene with a smaller electron affinity.
Using impedance spectroscopy technique, we show that the change of
fullerene LUMO energy has very little influence on the kinetic rate
of charge transfer across the interface. In terms of the Marcus theory,
large reorganization energy values govern the recombination kinetic
rate, which is only slightly dependent on the fullerene LUMO energy,
and also depends weakly on the energy location of recombining carriers
within the broad density of states. Since the recombination rate is
very similar in the different devices, we conclude that the larger
open-circuit voltage is due to the larger donor HOMO/acceptor LUMO
offset
Toward Stable Solar Hydrogen Generation Using Organic Photoelectrochemical Cells
Organic photoactive materials are
promising candidates for the
generation of solar fuels in terms of efficiency and cost. However,
their low stability in aqueous media constitutes a serious problem
for technological deployment. Here we present organic photocathodes
for the generation of hydrogen in aqueous media with outstanding stability.
The device design relies on the use of water-resistant selective contacts,
which protect a P3HT:PCBM photoactive layer. An insoluble cross-linked
PEDOT:PSS hole-selective layer avoids delamination of the film, and
an electron-selective TiO<sub><i>x</i></sub> layer in contact
with the aqueous solution electrically communicates the organic layer
with the hydrogen-evolving catalyst (Pt). We developed a novel method
for the synthesis of the TiO<sub><i>x</i></sub> layer compatible
with low-temperature conditions. Tuning the thickness of the TiO<sub><i>x</i></sub>/Pt layer leads to a trade-off between the
achievable photocurrent (∼1 mAcm<sup>–2</sup>) and the
stability of the photocathode (stable hydrogen generation of 1.5 μmol
h<sup>–1</sup> cm<sup>–2</sup> for >3 h)
Interplay between Fullerene Surface Coverage and Contact Selectivity of Cathode Interfaces in Organic Solar Cells
Interfaces play a determining role in establishing the degree of carrier selectivity at outer contacts in organic solar cells. Considering that the bulk heterojunction consists of a blend of electron donor and acceptor materials, the specific relative surface coverage at the electrode interfaces has an impact on the carrier selectivity. This work unravels how fullerene surface coverage at cathode contacts lies behind the carrier selectivity of the electrodes. A variety of techniques such as variable-angle spectroscopic ellipsometry and capacitance–voltage measurements have been used to determine the degree of fullerene surface coverage in a set of PCPDTBT-based solar cells processed with different additives. A full screening from highly fullerene-rich to polymer-rich phases attaching the cathode interface has enabled the overall correlation between surface morphology (relative coverage) and device performance (operating parameters). The general validity of the measurements is further discussed in three additional donor/acceptor systems: PCPDTBT, P3HT, PCDTBT, and PTB7 blended with fullerene derivatives. It is demonstrated that a fullerene-rich interface at the cathode is a prerequisite to enhance contact selectivity and consequently power conversion efficiency
Molecular Electronic Coupling Controls Charge Recombination Kinetics in Organic Solar Cells of Low Bandgap Diketopyrrolopyrrole, Carbazole, and Thiophene Polymers
Low-bandgap
diketopyrrolopyrrole- and carbazole-based polymer bulk-heterojunction
solar cells exhibit much faster charge carrier recombination kinetics
than that encountered for less-recombining poly(3-hexylthiophene).
Solar cells comprising these polymers exhibit energy losses caused
by carrier recombination of approximately 100 mV, expressed as reduction
in open-circuit voltage, and consequently photovoltaic conversion
efficiency lowers in more than 20%. The analysis presented here unravels
the origin of that energy loss by connecting the limiting mechanism
governing recombination dynamics to the electronic coupling occurring
at the donor polymer and acceptor fullerene interfaces. Previous approaches
correlate carrier transport properties and recombination kinetics
by means of Langevin-like mechanisms. However, neither carrier mobility
nor polymer ionization energy helps understanding the variation of
the recombination coefficient among the studied polymers. In the framework
of the charge transfer Marcus theory, it is proposed that recombination
time scale is linked with charge transfer molecular mechanisms at
the polymer/fullerene interfaces. As expected for efficient organic
solar cells, small electronic coupling existing between donor polymers
and acceptor fullerene (<i>V</i><sub>if</sub> < 1 meV)
and large reorganization energy (λ ≈ 0.7 eV) are encountered.
Differences in the electronic coupling among polymer/fullerene blends
suffice to explain the slowest recombination exhibited by poly(3-hexylthiophene)-based
solar cells. Our approach reveals how to directly connect photovoltaic
parameters as open-circuit voltage to molecular properties of blended
materials
How the Charge-Neutrality Level of Interface States Controls Energy Level Alignment in Cathode Contacts of Organic Bulk-Heterojunction Solar Cells
Electronic equilibration at the metal–organic interface, leading to equalization of the Fermi levels, is a key process in organic optoelectronic devices. How the energy levels are set across the interface determines carrier extraction at the contact and also limits the achievable open-circuit voltage under illumination. Here, we report an extensive investigation of the cathode energy equilibration of organic bulk-heterojunction solar cells. We show that the potential to balance the mismatch between the cathode metal and the organic layer Fermi levels is divided into two contributions: spatially extended band bending in the organic bulk and voltage drop at the interface dipole layer caused by a net charge transfer. We scan the operation of the cathode under a varied set of conditions, using metals of different work functions in the range of ∼2 eV, different fullerene acceptors, and several cathode interlayers. The measurements allow us to locate the charge-neutrality level within the interface density of sates and calculate the corresponding dipole layer strength. The dipole layer withstands a large part of the total Fermi level mismatch when the polymer:fullerene blend ratio approaches ∼1:1, producing the practical alignment between the metal Fermi level and the charge-neutrality level. Origin of the interface states is linked with fullerene reduced molecules covering the metal contact. The dipole contribution, and consequently the band bending, is highly sensitive to the nature and amount of fullerene molecules forming the interface density of states. Our analysis provides a detailed picture of the evolution of the <i>potentials</i> in the bulk and the interface of the solar cell when forward <i>voltage</i> is applied or when photogeneration takes place
Amorphous Iron Oxyhydroxide Nanosheets: Synthesis, Li Storage, and Conversion Reaction Kinetics
We
present a facile approach to synthesize amorphous iron oxyhydroxide
nanosheet from the surfactant-assisted oxidation of iron sulfide nanosheet.
The amorphous iron oxyhydroxide nanosheet is porous and has a high
surface area of 223 m<sup>2</sup> g<sup>–1</sup>. The lithium
storage properties of the amorphous iron oxyhydroxide are characterized:
it is a conversion-reaction electrode material, and it demonstrates
superior rate capabilities (e.g., discharge capacities as high as
642 mAh g<sup>–1</sup> are delivered at a current density of
2 C). The impedance spectroscopy analysis identifies a <i>RC</i> series subcircuit originated by the conversion-reaction process.
Investigation of the conversion-reaction kinetics through the <i>RC</i> subcircuit time constant reproduces the hysteresis in
the discharge/charge voltage profile. Hysteresis is then connected
to underlying thermodynamics of the conversion reaction rather than
to a kinetic limitation