Negative capacitance has been in the news twice this week; first as a potentially enabling technology for more efficient computing and second as a way to improve the efficiency and stability of perovskite solar cells.
With a little physics ingenuity, an international group of scientists have designed a way to redistribute electricity on a small scale, potentially opening new avenues of research into more energy-efficient computing. The image above shows the movement of the domain wall (a-c and b-d) in a capacitor when a charge is added to one side (c). The resulting redistribution of the domain wall causes a negative capacitive effect. (Image by Argonne National Laboratory.)
In a new study, researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory, together with collaborators in France and Russia, have created a permanent static ”negative capacitor,” a device thought to have been in violation of physical laws until about a decade ago.
While previously proposed designs for negative capacitors worked on a temporary, transient basis, the new Argonne-developed negative capacitor concept works as a steady-state, reversible device.
The researchers found that by pairing a negative capacitor in series with a positive capacitor, they could locally increase the voltage on the positive capacitor to a point higher than the total system voltage. In this way, they could distribute electricity to regions of a circuit requiring higher voltage while operating the entire circuit at lower voltage.
“The objective is to get electricity where it is needed while using as little as possible in a controlled, static régime,” said Argonne materials scientist Valerii Vinokur, the corresponding author of the study.
In traditional capacitors, the electric voltage of the capacitor is proportional to their stored electrical charge — increasing the amount of stored charge increases the voltage. In negative capacitors, the opposite happens — increasing the amount of charge decreases the voltage. Because the negative capacitor is a part of the larger circuit, this does not violate conservation of energy.
“One way you can think about it is like having a refrigerator,” said University of Picardie (France) scientist Igor Lukyanchuk, the first author of the paper. ”Inside the refrigerator, of course, it is much colder than the outside environment, but that is because we are heating up the rest of the environment by expending energy to cool the refrigerator.”
A prime component of the negative capacitor put forward by Vinokur and his colleagues involves a filling made of a ferroelectric material, which is similar to a magnet except that it has an internal electric polarization, rather than a magnetic orientation.
“In a ferroelectric nanoparticle, on one surface you will have a positive charge, and at the other surface you will have negative charges,” Vinokur said. ”This creates electric fields that try to depolarize the material.”
By splitting a nanoparticle into two equal ferroelectric domains of opposite polarization, separated by a boundary called a domain wall, Vinokur and his colleagues were able to minimize the effect of the total depolarizing electric field. Then, by adding charge to one of the ferroelectric domains, the researchers shifted the position of the domain wall between them.
Because of the cylindrical nature of the nanoparticle, the domain wall began to shrink, causing it to displace beyond the new electric equilibrium point. ”Essentially, you can think of the domain wall like a fully extended spring,” said Lukyanchuk. ”When the domain wall displaces to one side because of the charge imbalance, the spring relaxes, and the released elastic energy propels it further than expected. This effect creates the static negative capacitance.”
On the verge of outcompeting current thin-film solar cells, perovskite solar cells seem to embody an ideal solar cell: highly efficient and low-cost – if there was not the issue of a weak long-term stability, which remains a challenge. Related to this are peculiar phenomena occurring in perovskite materials and devices, where very slow microscopic processes can furnish them with a kind of “memory effect”.
For instance, measuring the efficiency of a perovskite solar cell can depend on things like how long the device is illuminated prior to measurement or how the voltage was applied. A few years ago, this effect, known as current-voltage hysteresis, led to disputes on how to accurately determine the efficiency of perovskites. Another example of these obscure processes is a (partial) recovery of a previously degraded solar cell during day-night cycling.
Such effects are a concern when measuring the solar cells’ performance as a function of frequency, which is a typical measurement for characterizing these devices in more detail (impedance spectroscopy). They lead to large signals at low frequencies (Hz to mHz) and giant capacitance values for the (mF/cm2), including strange, “unphysical” negative values that are still a puzzle to the research community.
Now, chemical engineers from the lab of Anders Hagfeldt at EPFL have solved the mystery. Led by Wolfgang Tress, a scientist in Hagfeldt’s lab, they found that the large perovskite capacitances are not classical capacitances in the sense of charge storage, but just appear as capacitances because of the cells’ slow response time.
The researchers show this by measurements in the time domain and with different voltage scan rates. They find that the origin of the apparent capacitance is a slow modification of the current passing the contact of the solar cells, which is regulated by a slow accumulation of mobile ionic charge. A slowly increasing current appears like a negative capacitance in the impedance spectra.
The work sheds light onto the interaction between the photovoltaic effect in these devices and the ionic conductivity of perovskite materials. Gaining such in-depth understanding contributes to the endeavor to tailored, stable perovskite solar cells.
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