We also studied EIS data measured from a perovskite solar cell. The cell was made of fluorinedoped tin oxide, the electron-transport layer of SnO 2 , the transport-hole layer of Spiro-OMeTAD, and the absorber of (FAPbI 3 ) 0.85 (MAPbBr 3 ) 0.15 . The EIS spectrum was measured from 1 Hz to 5 MHz in a 1 atm atmosphere with a 100 mV voltage amplitude. The ECM was fitted against a model developed in , and the parameter values are given in Table . The impedance of this solar cell was regressed over the whole frequency range and was predicted below 1 Hz (Figure (c)). We found that GPs regressed precisely the experimental impedance (Figure (c) and S17).