31 research outputs found
On Voltage, Photovoltage, and Photocurrent in Bulk Heterojunction Organic Solar Cells
The interpretation of voltage, photovoltage, and photocurrent in polymer/fullerene bulk heterojunction (BHJ) solar cells is discussed in terms of fundamental device models and results of capacitance spectroscopy. First we establish the relationship between the applied voltage (which is the difference of Fermi levels) and the variation of electrostatic potential that governs the drift field. We then show the most common distribution of carriers and Fermi levels in the blend layer, supported on experimental results of impedance spectroscopy of P3HT:PCBM solar cells. We arrive at the conclusion that charge separation and charge transportation has very little to do with a built-in electric field between metal contacts, while kinetics plays a major role in photocurrent production. Finally, we discuss the key factors relevant to understand device properties and power conversion efficiencies of the BHJ solar cells: recombination, charge generation, and the currentāpotential curve, based on the suggested model that emphasizes mobile electrons and holes (that we term quasifree carriers) contributing to the respective Fermi levels
Simulation of Steady-State Characteristics of Dye-Sensitized Solar Cells and the Interpretation of the Diffusion Length
Quantitative modeling of the photovoltaic response of the dye-sensitized solar cell (DSC) is an important subject for improving both the understanding of operation mechanisms and the device performance. A range of experimental techniques indicates that nonlinear recombination of the form <i>U</i><sub><i>n</i></sub> = <i>k</i><sub>r</sub><i>n</i><sup>Ī²</sup>, with Ī² ā 1, is a property of DSCs. We show that the diffusion length <i>L</i><sub><i>n</i></sub> defined from the probability of collection is independent of the macroscopic perturbation for Ī²ā 1 only for a small perturbation, and in this case, it coincides with the value Ī»<sub><i>n</i></sub> = (<i>D</i><sub><i>n</i></sub>Ļ<sub><i>n</i></sub>)<sup>1/2</sup> that can be measured by impedance spectroscopy in homogeneous conditions. The increase of the diffusion length with the potential, usually observed experimentally, is attributed to the increase of the free carrier lifetime. We also discuss the modeling of real DSC devices under different conditions, and we conclude that the diffusionārecombinationāgeneration equation based on <i>U</i><sub><i>n</i></sub> = <i>k</i><sub>r</sub><i>n</i><sup>Ī²</sup> is a fundamental ingredient of the simulation tools
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
Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with āCoāPiā-Coated Hematite Electrodes
Uniform thin films of hematite (Ī±-Fe<sub>2</sub>O<sub>3</sub>) deposited by atomic layer deposition (ALD) coated
with varying
amounts of the cobalt phosphate catalyst, āCoāPi,ā
were investigated with steady-state and transient photoelectrochemical
measurements and impedance spectroscopy. Systematic studies as a function
of CoāPi thickness were performed in order to clarify the mechanism
by which CoāPi enhances the water-splitting performance of
hematite electrodes. It was found that under illumination, the CoāPi
catalyst can efficiently collect and store photogenerated holes from
the hematite electrode. This charge separation reduces surface state
recombination which results in increased water oxidation efficiency.
It was also found that thicker CoāPi films produced increased
water oxidation efficiencies which is attributed to a combination
of superior charge separation and increased surface area of the porous
catalytic film. These combined results provide important new understanding
of the enhancement and limitations of the CoāPi catalyst coupled
with semiconductor electrodes for water-splitting applications
Interpretation of Cyclic Voltammetry Measurements of Thin Semiconductor Films for Solar Fuel Applications
A simple model is proposed that allows
interpretation of the cyclic
voltammetry diagrams obtained experimentally for photoactive semiconductors
with surface states or catalysts used for fuel production from sunlight.
When the system is limited by charge transfer from the traps/catalyst
layer and by detrapping, it is shown that only one capacitive peak
is observable and is not recoverable in the return voltage scan. If
the system is limited only by charge transfer and not by detrapping,
two symmetric capacitive peaks can be observed in the cathodic and
anodic directions. The model appears as a useful tool for the swift
analysis of the electronic processes that limit fuel production
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
Role of ZnO Electron-Selective Layers in Regular and Inverted Bulk Heterojunction Solar Cells
Here the role of metal oxide (ZnO) electron-selective layers in the operating mechanisms of bulk-heterojunction polymerāfullerene solar cells is addressed. Inverted as well as regular structures containing ZnO layers at the cathode contact have been analyzed using capacitance methods in the dark and impedance spectroscopy under illumination. We systematically observed that the open-circuit voltage <i>V</i><sub>oc</sub> at 1 sun illumination results higher for inverted cells than that achieved by regular structures in Ī<i>V</i><sub>oc</sub> ā 30ā50 mV. This shift correlates with the displacement of the flat-band potential <i>V</i><sub>fb</sub> extracted from MottāSchottky capacitance analysis. A coherent picture is provided that states the hole Fermi level of the polymer highest occupied molecular orbital as an energy reference for both <i>V</i><sub>oc</sub> and <i>V</i><sub>fb</sub>. The study connects the position of the hole Fermi level to the <i>p</i>-doping character of the active layer that is influenced by the film morphology through vertical phase segregation
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 (ā¼1 mAcm<sup>ā2</sup>) and the
stability of the photocathode (stable hydrogen generation of 1.5 Ī¼mol
h<sup>ā1</sup> cm<sup>ā2</sup> for >3 h)
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 polyĀ(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 (Ī» ā 0.7 eV) are encountered.
Differences in the electronic coupling among polymer/fullerene blends
suffice to explain the slowest recombination exhibited by polyĀ(3-hexylthiophene)-based
solar cells. Our approach reveals how to directly connect photovoltaic
parameters as open-circuit voltage to molecular properties of blended
materials
Dye versus Quantum Dots in Sensitized Solar Cells: Participation of Quantum Dot Absorber in the Recombination Process
Inorganic quantum dots (QDs) show great potential as absorbers in sensitized solar cell, but there are open questions about the role of quantum dots, where primary electron and hole generation occurs, in the recombination process in this kind of solar cells. In opposition to the conventional dye-sensitized solar cell, here we show that inorganic QDs play a direct role in the recombination process. This fact has been determined by a fingerprint of QDs in the capacitance of the device, where the QDs surface states affect the density of states (DOS) distribution. It indicates that now surface states of QD contribute to the common DOS distribution of TiO<sub>2</sub>/QDs/ZnS, which behaves as a single entity, being impossible to distinguish between TiO<sub>2</sub> and QDs. This result highlights the necessity of treating (and optimizing) QD-sensitized solar cells from another perspective than dye-sensitized solar cells, considering the fundamental differences in their behavior