Negative Capacitance and Inverted Hysteresis: Matching Features in Perovskite Solar Cells.

Negative capacitance at low frequency domain and inverted hysteresis are familiar features in perovskite solar cells, which origin is still under discussion. Here we use Impedance Spectroscopy to analyse the evolution of these features in methylammonium lead bromide cells (MAPbBr3) treated with extrinsic cations at the cathode/perovskite interface and in iodide devices (MAPbI3) exposed under different relative humidity (%RH) conditions. To process the impedance spectra, an equivalent circuit based on the Surface Polarization Model (SPM) is implemented, obtaining the same result for all sample, i.e. large kinetic relaxation times and high recombination values correspond to those samples exhibiting strong inverted hysteresis and negative capacitance. A scan rate analysis of the cyclic voltammetry curves confirms the recombinant nature of both features and the direct relationship between them, providing experimental evidence of the recombination activated by ionic surface interaction as the main origin of inverted hysteresis and negative capacitance responses.