Time-of-flight secondary ion mass spectrometry (TOF-SIMS) is one of the few techniques that can specifically distinguish between organic cations such as methylammonium and formamidinium. Distinguishing between these two species can lead to specific insight into the origins and evolution of compositional inhomogeneity and chemical gradients in halide perovskite solar cells, which appears to be a key to advancing the technology. TOF-SIMS can obtain chemical information from hybrid organic-inorganic perovskite solar cells (PSCs) in up to three dimensions, while not simply splitting the organic components into their molecular constituents (C, H, and N for both methylammonium and formamidinium), unlike other characterization methods. Here, we report on the apparently ubiquitous A-site organic cation gradient measured when doing TOF-SIMS depth-profiling of PSC films. Using thermomechanical methods to cleave perovskite samples at the buried glass/transparent conducting oxide interface enables depth profiling in a reverse direction from normal depth profiling (backside depth profiling). When comparing the backside depth profiles to the traditional front side profiled devices, an identical slight gradient in the A-site organic cation signal is observed in each case. This indicates that the apparent A-site cation gradient is a measurement artifact due to beam damage from the primary ion beam causing a continually decreasing ion yield for secondary ions of methylammonium and formamidinium. This is due to subsurface implantation and bond breaking from the 30 keV bismuth primary ion beam impact when profiling with too high of a data density. Here, we show that the beam-generated artifact associated with this damage can mostly be mitigated by altering the measurement conditions. We also report on a new method of depth profiling applied to PSC films that enables enhanced sensitivity to halide ions in positive measurement polarity, which can eliminate the need for a second measurement in negative polarity in most cases.
Keywords: HPSC; PSC; TOF-SIMS; cation migration; degradation; interface; interface chemistry; passivating additive.