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Mystery of negative capacitance in perovskite solar cells solved

solar cells

On the verge of outcompeting current thin-film solar cells, perovskite solar cells seem to embody an ideal solar cell with high efficiency and low cost. However, they have poor long-term stability, which remains a challenge. Related to this are peculiar phenomena occurring in perovskite materials and devices, where very slow microscopic processes cause a kind of “memory effect.”

For instance, measuring the efficiency of a 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 accurately determining 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 solar cell 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 values (mF/cm2), including strange, “unphysical” negative values that are still a puzzle to the research community.

Now, 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 ‘ 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 , 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 .

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