a–d, Examples of some of the most stable PSCs reported so far, demonstrating a variety of possible PCE time profiles upon ageing under 1 sun illumination. e,f, Common practices in the T80 estimations. The black curves show schematically how PCE evolves with ageing time in the case of ‘burn-in’ effect (e) and in the case of nonmonotonic PCE (f). The three strategies depicted to estimate T80 differ in how the initial PCEs (‘100%’) are estimated and the choice of starting point of the ageing timer (shown in the figure as circles of corresponding colours). In the first strategy, 80% PCE is counted from the initial PCE at t = T0 (shown in orange). In the second strategy, 80% PCE is counted from the back-extrapolated ‘after burn-in’ PCE in panel e (red dashed line indicates the extrapolation) or from the PCE maximum in panel f (red dashed line used to guide the eyes); in both cases the timer starts at t = T0 (shown in green). In the final strategy, 80% PCE is counted from the PCE at t = Tmax in panel f or at t = Tburn-in in panel e, and the ageing timer starts at corresponding timestamps (shown in blue). Panel a is adapted with permission from ref. 49, Elsevier; panel b is adapted with permission from ref. 148, Elsevier; panel c is adapted with permission from ref. 72, Springer Nature Ltd; panel d is adapted with permission from ref. 65, IEEE.