Deleterious Effect of Negative Capacitance on the Performance of Halide Perovskite Solar Cells Francisco Fabregat-Santiago,* ,† Michael Kulbak, ‡ Arava Zohar, ‡ Marta Valle ́ s-Pelarda, † Gary Hodes, ‡ David Cahen, ‡ and Iva ́ n Mora-Seró * ,† † Institute of Advanced Materials (INAM), Universitat Jaume I, 12006 Castelló , Spain ‡ Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel * S Supporting Information ABSTRACT: Negative capacitance in photovoltaic devices has been observed and reported in several cases, but its origin, at low or intermediate frequencies, is under debate. Here we unambiguously demonstrate a direct correlation between the observation of this capacitance and a corresponding decrease in performance of a halide perovskite (HaP; CsPbBr 3 )-based device, expressed as reduction of open-circuit voltage and fill factor. We have prepared highly stable CsPbBr 3 HaPs that do not exhibit any degradation over the duration of the impedance spectroscopy measure- ments, ruling out degradation as the origin of the observed phenomena. Reconstruction of current-voltage curves from the impedance spectroscopy provided further evidence of the deleterious role of negative capacitance on photoconversion performance. H alide perovskite (HaP)-based solar cells (HaP PV cells) constitute an intensely studied topic in photovoltaics because of the fast rise in photon to electrical energy conversion efficiencies, currently peaking at slightly above 22%. 1 In addition, the use of earth-abundant materials and of low-temperature solution-growth methods make these technologies even more attractive. 2-5 These features have boosted interest in this topic and at the same time allowed many laboratories around the world to make such devices, owing to the relatively low cost and availability of the facilities necessary to produce them. Despite this intensive work, HaP PV cells still surprise us with unconventional behaviors. In many cases, neither the origin of these behaviors nor their influence on the final device performance are (completely) understood. Probably the most extensively studied unconventional behavior is the hysteresis of current-potential curves 6-13 because of its implications for determining solar cell perform- ance. 14 At the same time, preconditioning (e.g., poling, illumination) was found to affect the performance of HaP devices. 15,16 Moreover, large capacitances have been observed at low frequencies for HaP thin films 17 and have been attributed to majority carrier accumulation at the interface(s) with the contacts. 18 This fact has important implications in carrier recombination, as accumulation implies a strong increase in charge density at the interfaces with consequent increase of recombination. 19 Even though many aspects of these phenomena are not completely understood, it is generally thought that ion migration plays an important role in them (even though direct proof of such migration, e.g., by isotope tracing, is still lacking). There are other unconventional phenomena in HaPs where interpretation is not straightforward, and doubts remain concerning their origin. This is the case of the inductive loops 20-24 and negative capacitance observed at intermediate and low frequencies by impedance spectroscopy (IS). It is likely that both phenomena would be related because both have been observed on the same sample but at different applied bias. 20 Nevertheless, the analysis of the relationship of both features is beyond the scope of the present Letter, which will focus instead on the negative capacitance at low frequencies. These phenomena are less-studied and less-understood than those mentioned earlier. It is worth highlighting that negative capacitance has been observed in different halide perovskite materials, with different configurations and different contacts pointing to a general behavior not reduced just to a concrete set of samples. 17,20,25-28 The effects of this phenomenon on the performance of the HaP PV cells was not clear. In this Letter, we show that negative capacitance has, in fact, a deleterious effect on the final solar cell performance, which should provide Received: June 25, 2017 Accepted: August 4, 2017 Published: August 4, 2017 Letter http://pubs.acs.org/journal/aelccp © 2017 American Chemical Society 2007 DOI: 10.1021/acsenergylett.7b00542 ACS Energy Lett. 2017, 2, 2007-2013