Fig. 6: Diagram illustrating a mechanism consistent with the observed differences in the behavior of lead-tin perovskite solar cells with PTAA or PEDOT:PSS HTLs after aging.

a Devices using PTAA are proposed to mainly experience an increase in low mobility p-doping defects (such as the Sn2+ vacancies shown here) during aging, resulting in an increased background hole density p₀ but a constant mobile ion density. b Devices using PEDOT:PSS are proposed to additionally experience an increase in mobile I⁻ vacancy density. This counteracts the effect of the p-doping defects on the Fermi level, keeping the background hole density p₀ low.