Ion Movement Explains Huge <i>V</i> _OC Increase despite Almost Unchanged Internal Quasi?Fermi?Level Splitting in Planar Perovskite Solar Cells

Light soaking under “1 sun” is performed on planar p–i–n perovskite solar cells with a Cs0.05MA0.10FA0.85Pb(I0.95Br0.05)3 absorber while measuring current and voltage transients simultaneously to spectral photoluminescence (PL). From theory tenfold increase in PL intensity expected for every 60 mV rise VOC (at 300 K). However, the investigated show reversible from as low 0.5 up 1.05 V during light soaking, whereas hardly changes. A model developed based mobile ions combination nonideal contact. It reproduces decoupling of well transient behavior great detail. Using state-of-the-art materials passivation layers shows that still relevant feature high-efficiency cells. The ionic liquid additive 1-butyl-3-methylimidazolium tetrafluoroborate slows down light-soaking behavior, giving an example how motion can be influenced.