Ion Movement Explains Huge Voc 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 a tenfold increase in PL intensity is expected for every 60 mV rise in VOC (at 300 K). However, the solar cells investigated show a reversible VOC increase from as low as 0.5 up to 1.05 V during light soaking, whereas the PL intensity hardly changes. A model is developed based on mobile ions in combination with a nonideal contact. It reproduces the decoupling of the VOC and PL as well as the transient behavior in great detail. Using state-of-the-art materials and passivation layers shows that light soaking is still a relevant feature of high-efficiency perovskite solar cells. The ionic liquid additive 1-butyl-3-methylimidazolium tetrafluoroborate slows down the light-soaking behavior, giving an example of how ionic motion in perovskite solar cells can be influenced.