8 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
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
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
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 capacitanceāvoltage 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 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
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 metalāorganic 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 ā¼2 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 ā¼1:1, 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
Operating Mechanisms of Mesoscopic Perovskite Solar Cells through Impedance Spectroscopy and <i>J</i>ā<i>V</i> Modeling
The
performance of perovskite solar cell (PSC) is highly sensitive
to deposition conditions, the substrate, humidity, and the efficiency
of solvent extraction. However, the physical mechanism involved in
the observed changes of efficiency with different deposition conditions
has not been elucidated yet. In this work, PSCs were fabricated by
the antisolvent deposition (AD) and recently proposed air-extraction
antisolvent (AAD) process. Impedance analysis and <i>J</i>ā<i>V</i> curve fitting were used to analyze the
photogeneration, charge transportation, recombination, and leakage
properties of PSCs. It can be elucidated that the improvement in morphology
of perovskite film promoted by AAD method leads to increase in light
absorption, reduction in recombination sites, and interstitial defects,
thus enhancing the short-circuit current density, open-circuit voltage,
and fill factor. This study will open up doors for further improvement
of device and help in understanding its physical mechanism and its
relation to the deposition methods
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>ā1</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>ā1</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