Credible evidence for the passivation effect of remnant PbI2 in CH3NH3PbI3 films in improving the performance of perovskite solar cells

Abstract

The role of remnant PbI₂ in CH₃NH₃PbI₃ films is still controversial, some investigations have revealed that the remnant PbI₂ plays a passivation role, reduces the charge recombination in perovskite solar cells (PSCs), and improves the performance of PSCs, but the opposing views state that remnant PbI₂ has no passivation effect and it would deteriorate the stability of the devices. In our investigation, the CH₃NH₃PbI₃ films have been prepared by a two-step spin-coating method and the content of the remnant PbI₂ in CH₃NH₃PbI₃ films has been tuned by varying the preparation temperature. It has been found that increasing the heating temperature could increase the coverage of spin-coated PbI₂ films, which has led to high coverage CH₃NH₃PbI₃ films and more remnant PbI₂ in CH₃NH₃PbI₃ films, and as a result, the performance of PSCs was enhanced obviously and the maximum power conversion efficiency of 14.32 ± 0.28% was achieved by the PSCs prepared at 130/120 °C (PbI₂ films were heated at 130 °C and CH₃NH₃PbI₃ films were heated at 120 °C). Furthermore, the dark current, electrochemical impedance spectroscopy and time-resolved fluorescence emission decay measurements revealed that the charge recombination in PSCs has been gradually suppressed and the fluorescence emission lifetime has gradually increased with the content of remnant PbI₂ increasing. Thus, the passivation effects of the unreacted and decomposed PbI₂ in improving the performance of PSCs have been confirmed unquestionably.