SPO tracking on the 98:2 TH device and corresponding PCE and J SC at different scan speeds before and after the aging in the inset. The measurements demonstrate that the observed degradation stems largely from ionic losses while the ion-freeze PCE remains nearly unchanged. The dashed line is a guide to the eye, connecting the experimentally measured initial and final ion-freeze PCE (stars), while the gradient shading highlights the expected emergence of the ionic losses. The steady-state and ion-freeze PCE are marked by the diamonds and stars, respectively. The red arrows highlight the ionic losses after the degradation. The latter are determined from the difference between the steady-state and ion-freeze PCE, as exemplified by the yellow horizontal lines in the insets. The experimental conditions for the SPO and FH measurements can be found in Methods and all the remaining FH data in Supplementary Fig. 14 .
