title: Kinetics of Ion Transport in Perovskite Active Layers and Its Implications for Active Layer Stability.
abstract: Solar cells fabricated using alkyl ammonium metal halides as light absorbers have the right combination of high power conversion efficiency and ease of fabrication to realize inexpensive but efficient thin film solar cells. However, they degrade under prolonged exposure to sunlight. Herein, we show that this degradation is quasi-reversible, and that it can be greatly lessened by simple modifications of the solar cell operating conditions. We studied perovskite devices using electrochemical impedance spectroscopy (EIS) with methylammonium (MA)-, formamidinium (FA)-, and MA(x)FA(1-x) lead triiodide as active layers. From variable temperature EIS studies, we found that the diffusion coefficient using MA ions was greater than when using FA ions. Structural studies using powder X-ray diffraction (PXRD) show that for MAPbI3 a structural change and lattice expansion occurs at device operating temperatures. On the basis of EIS and PXRD studies, we postulate that in MAPbI3 the predominant mechanism of accelerated device degradation under sunlight involves thermally activated fast ion transport coupled with a lattice-expanding phase transition, both of which are facilitated by absorption of the infrared component of the solar spectrum. Using these findings, we show that the devices show greatly improved operation lifetimes and stability under white-light emitting diodes, or under a solar simulator with an infrared cutoff filter or with cooling.
authors.affiliation: Department of Chemistry, University of Massachusetts Amherst , Amherst, Massachusetts 01003-9303, United States.
authors.forename: Monojit
authors.initials: M
authors.lastname: Bag
authors.affiliation: Department of Chemistry, University of Massachusetts Amherst , Amherst, Massachusetts 01003-9303, United States.
authors.forename: Lawrence A
authors.initials: LA
authors.lastname: Renna
authors.affiliation: Department of Physics, University of Massachusetts Amherst , Amherst, Massachusetts 01003-9303, United States.
authors.forename: Ramesh Y
authors.initials: RY
authors.lastname: Adhikari
authors.affiliation: Department of Chemistry, University of Massachusetts Amherst , Amherst, Massachusetts 01003-9303, United States.
authors.forename: Supravat
authors.initials: S
authors.lastname: Karak
authors.affiliation: Materials Science Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720-1730, United States.
authors.forename: Feng
authors.initials: F
authors.lastname: Liu
authors.affiliation: Department of Chemistry, University of Massachusetts Amherst , Amherst, Massachusetts 01003-9303, United States.
authors.forename: Paul M
authors.initials: PM
authors.lastname: Lahti
authors.affiliation: Materials Science Division, Lawrence Berkeley National Laboratory , Berkeley, California 94720-1730, United States.|Department of Polymer Science & Engineering, University of Massachusetts , Amherst, Massachusetts 01003-9303, United States.
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Russell
authors.affiliation: Department of Physics, University of Massachusetts Amherst , Amherst, Massachusetts 01003-9303, United States.
authors.forename: Mark T
authors.initials: MT
authors.lastname: Tuominen
authors.affiliation: Department of Chemistry, University of Massachusetts Amherst , Amherst, Massachusetts 01003-9303, United States.
authors.forename: D
authors.initials: D
authors.lastname: Venkataraman