Undesirable role of remnant PbI2 layer on low temperature processed planar perovskite solar cells

Abstract

Two-step deposition methods for preparing CH₃NH₃PbI₃ films are becoming increasingly competitive when considering fabricating perovskite solar cells (PSCs) under ambient conditions, along with possessing the ability to better control their morphology. The surplus PbI₂ phase can be detected within the formed CH₃NH₃PbI₃ films due to partial conversion of PbI₂ to CH₃NH₃PbI₃, which causes variation in the PbI₂ stoichiometry of the resultant films. In this work, we carefully study the influence of the remnant PbI₂ on the performance of planar PSCs including efficiency and thermal stability by varying the PbI₂ stoichiometry of the resultant CH₃NH₃PbI₃. Through a further comparative study, the undesirable role of the remnant PbI₂ layer on planar PSC performance is exposed, indicating that the remnant PbI₂ layer not only greatly impedes carrier extraction and transport, but also accelerates the degradation of the CH₃NH₃PbI₃ film. To further eliminate the interference of this phenomenon, low temperature processed planar PSCs approaching 13% efficiency are attained in ambient atmosphere by the modified two-step method with enhanced thermal stability, which has significant potential for future mass production of PSCs and provides insight into the correlation between device performance and the PbI₂ stoichiometry of CH₃NH₃PbI₃ films.