Mystery of negative capacitance in perovskite solar cells solved

We investigated the puzzling low-frequency phenomena in the impedance spectra of perovskite solar cells and found that the measured capacitances are not related to charge accumulation but just appear as giant (positive and negative) capacitances due to an electron injection current that is modified by slowly redistributed ions.
Mystery of negative capacitance in perovskite solar cells solved

Research in perovskite solar cells is so exciting because it offers working in a relevant field of applied research with high-performance devices and at the same time with a material that provides many puzzling phenomena.

Almost two years ago I was interested in looking deeper into the role of impurities in the perovskite material that can act as defects. By adding small amounts (ppm to %) of Bismuth to the perovskite we indeed succeeded in making devices really worse. We performed a nice study on the role of these defects, which, however, was not highly appreciated by the reviewers giving us a hard time to finally publish that work.

While characterizing these devices, I also tried impedance spectroscopy. I was not expert in this technique but knew that it is supposed to give some information on defects as well. Instead of finding the classical signatures of defects, I discovered that we could systematically tune the so-called negative capacitance effect. This effect had been reported to be deleterious for solar cell performance but its origin seemed to remain a complete mystery; similar to the extremely high positive capacitances, which had been attributed to a giant dielectric constant and later to a combined accumulation of electronic and ionic charge.

I thought it would be interesting to look more into this topic and gave it to Firouzeh Ebadi, a visiting PhD student who just arrived. The results of her measurements made us dive into the effects of high capacitances. We measured in the time domain to achieve some distance of thought from the idea of classical impedances. Varying the voltage scan rate and the temperature, we saw that what looks like a capacitive response is in fact a slowly changed electrical current that is controlled by a slow process at the interface between perovskite layer and the adjacent charge transport layer. This change in injection current could be due to accumulation of ionic charges, as the perovskite is also an ion conductor.

Having found and verified this explanation, we were excited to publish it but at the same time surprised that it had not been put forward previously. However, detailed literature research showed that in other material systems similar effects had been reported already even decades ago. Also in perovskite devices some valid explanations had been proposed. Nevertheless, a lot of confusion remained, which made us conclude that a focused publication on this effect is urgently required. Luckily, following controversial discussions with one reviewer, editors and reviewers found our work sufficiently interesting. At the same time two further excellent publications by colleagues on "The two faces of capacitance" and on "Ionic-to-electronic current amplification" appeared that discuss the high capacitances in similar terms to our publication.   

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Electrical and Electronic Engineering
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