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Negative capacitance at the low-frequency domain and inverted hysteresis are familiar features in perovskite solar cells, where the origin is still under discussion. Here we use Impedance Spectroscopy to analyse these responses in methylammonium lead bromide cells treated with lithium cation at the electron selective layer/perovskite interface and in iodide devices exposed to different relative humidity conditions. Employing the Surface Polarization Model, we obtain a time constant associated to the kinetics of the interaction of ions/vacancies with the surface, τkin, in the range of 10 0 -10 2 s for all the cases exhibiting both features. These interactions lead to a decrease in the overall recombination resistance, modifying the low-frequency perovskite response and yielding to a flattening of the cyclic voltammetry. As consequence of these results we find that that negative capacitance and inverted hysteresis lead to a decrease in the fill factor and photovoltage values.

We demonstrate highly efficient, stable, and flexible perovskite solar cells of large areas, utilizing a carbon back-contact electrode in a p–i–n cell configuration. We enabled good electronic contact at the interface with carbon by inserting an ultrathin buffer layer before the carbon coating. Solar cells of such structure reach a power conversion efficiency of 15.18% on PET foil (device area of 1 cm2). We performed impedance spectroscopy and transient decay measurements to understand the electron transport characteristics. Furthermore, we demonstrate excellent operational (maximum power point) and thermal (85 °C) stability of these devices over 1000 h of aging.

niques involve optical perturbations. [9,10] IS has been used to analyze transport and recombination processes together through the response of the device to an electrical perturbation. [11,12] IMPS and IMVS techniques have been widely applied to study dye-sensitized solar cells, [13,14] photoelectrodes for water oxidation, [15][16][17] and recently are gaining attention in the field of perovskite solar cells (PSCs). [18,19] The analysis of the data obtained with these techniques is generally made through the characteristic time constants. [20][21][22] While such an analysis is very useful in certain cases, this strategy limits the amount of information that can be obtained from these techniques. With regard to IS, experimental data are usually analyzed by modeling the internal electrochemical processes of the device using an equivalent circuit model (EC), [23] built from passive electrical elements such as resistances, capacitances, and inductances. [24][25][26] These parameters in turn can provide key information regarding the nature of the operation of the device and its limitations. However, the choice of the correct EC could be a difficult task, because several ECs can reproduce the same experimental spectra. Therefore, there exists a need to combine the analysis methods of these three techniques to overcome their individual limitations and extract the maximum knowledge from the experimental data.Before discussing how these techniques are related, we present a scheme of a photosensitive device showing the basic light to electricity conversion mechanism in Figure 1. In these systems, when a photon flux (f) reaches the device, part of these photons can be absorbed, generating electronhole pairs (represented by absorbed current, j a ). The pairs that do not recombine and are successfully separated, provide the free-photogenerated current (j ph ) in the light absorber material. Part of this current is lost as a recombination current (j rec ), and the remaining current ( j e = j ph − j rec ) is extracted at the external contacts of the device. j rec , and thus j e , are functions of the external voltage (V e ).In Figure 2, the basic operation of IS, IMPS, and IMVS is shown. Figure 2b-d shows the excitation and measurement signals involved in IS, IMPS, and IMVS, respectively. Note that all variables with a tilde (  ) indicate AC perturbations, while variables with overbars ( ) represent steady-state signals. Thus, the steady-state at which the measurements are performed is defined by φ j , , eV and e j . As shown in Figure 2b, for IS (in potentiostatic mode), an AC small-perturbation in voltage Small-perturbation techniques such as impedance spectroscopy (IS), intensity-modulated photocurrent spectroscopy (IMPS), and intensity-modulated photovoltage spectroscopy (IMVS) are useful tools to characterize and model photovoltaic and photoelectrochemical devices. While the analysis of the impedance spectra is generally carried out using an equivalent circuit, the intensity-modulated spectroscopies are often analyzed th...