posted on 2020-08-12, 19:49authored byIlia M. Pavlovetc, Michael C. Brennan, Sergiu Draguta, Anthony Ruth, Taylor Moot, Jeffrey A. Christians, Kyle Aleshire, Steven P. Harvey, Stefano Toso, Sanjini U. Nanayakkara, Jonah Messinger, Joseph M. Luther, Masaru Kuno
Ion migration represents
an intrinsic instability of metal halide
perovskite solar cells. Here we show that triple-cation FAxMAyCs1–x–yPbI3 [FA+ = (NH2)2CH+, MA+ = CH3NH3+] active layers with mixed
orthorhombic, post-perovskite (δortho-CsPbI3), and cubic perovskite (α) phases (i.e., α/δ-phase
FAxMAyCs1–x–yPbI3) exhibit improved cation stability against applied bias relative
to pure α-phase perovskites (i.e., FA0.85Cs0.15PbI3 and FA0.76MA0.15Cs0.09PbI3). Infrared photothermal heterodyne imaging and time-of-flight
secondary ion mass spectrometry are used to visualize exclusive α-phase
perovskite lateral device A+ cation accumulation (depletion)
at perovskite negative (positive) electrode interfaces. The resulting
compositional heterogeneities lead to degradation. Operational stability
testing of solar cells reveals similar degradation behavior; α/δ-phase
FAxMAyCs1–x–yPbI3 lateral devices/solar cells, by contrast, show improved stabilities.
Enhanced α/δ-FAxMAyCs1–x–yPbI3 stability is rationalized by δortho-phase inclusions, acting as barriers through which A+ cations do not easily migrate. This study thus provides new
insights into cation migration in FAxMAyCs1–x–yPbI3 perovskites and suggests a materials
design strategy toward suppressing cation instabilities in hybrid
perovskites.