TOF-SIMS Analysis of Hybrid Perovskite Materials—Elucidating Dopant Incorporation vs Measurement Artifacts and Investigating Ion Migration via In Situ Bias During TOF-SIMS Analysis

Metal Halide Perovskite (MHP) Photovoltaics have the potential to be a game-changing technology in photovoltaics, with low cost solution processing inherent to the technology and a rapid progress in device efficiency and stability. Time of flight SIMS is one of the few analytical techniques able to quickly measure the distribution of both the organic and inorganic components of a MHP device stack (both through the depth as well as laterally with 2-D and 3-D imaging).

Due to the gains in efficiency and stability, surface treatment of MHP films with other organic materials after film formation is becoming increasingly common. Results from a recent detailed study profiling a MHP film with a piperazinium-iodide (PI) surface treatment (m/z 84.1) will be covered. Due to the size of the PI molecule, it was expected the dopant remains at the surface, and little is incorporated into the film bulk. The results obtained for PI when depth profiling could easily be mis-interpreted as a potential SIMS artifact due to either beam damage and/or cascading. The results of a focused study separating out these artifacts from actual depth distribution of the PI molecule are given, and are applicable to a broad range of these larger surface treatment species. A series of MHP films were prepared on polished silicon wafers with lower surface roughness (10-15nm rms) with PI surface treatment or an ALD SnO_x surface layer as a marker to compare knock-on effects vs. PI incorporation, including on films where a thin PMMA layer was put ontop of the HOIP film before PI/SnO_x. Profiles were taken with a 1KeV Cs+ source (5nA), as well as a 20KeV argon GCIB (4nA) at 40eV/atom on two different TOF-SIMS systems. The samples prepared with a configuration of Si/MHP/PMMA/(PI or SnOx) proved the most insightful, indicating very little knock-on artifacts in the samples with PMMA interlayer, even though the surface roughness was similar to the neat MHP films. The films prepared with a configuration of Si/MHP/(PI or SnOx) show incorporation of BOTH Sn and PI, but with more PI incorporation. This is not unexpected as Sn can substitute for Pb in the structure, and the results show the unexpected nuances of SIMS measurements on these unique defect tolerant materials.

A more complete understanding of ion migration in MHP materials could lead to improvements in both efficiency and reliability, and further understanding of these phenomena is of great importance. We will briefly present past work where an in situ electrical bias was placed on a perovskite device while under investigation with TOF-SIMS. This was completed with simple commercial off the shelf components in an ION-TOF TOF-SIMS V instrument and could be easily implemented on other instruments. A device stack of glass / ITO / Me-4PACz / DMA0.1FA0.6Cs0.3Pb(I0.8Br0.2)3 / LiF (1 nm) / C60 (30 nm) / SnOx (15 nm)/Au (20 nm) was used for this study. An electrical bias was applied between the top gold contact and the bottom ITO contact during TOF-SIMS measurements. By applying a +0.75V and -0.75V forward and reverse bias to the device, a driving force for negatively charged halide ions is created to migrate towards the back or front of the device, respectively. The in-situ data shows the halide ion migration towards the back ITO contact after the forward bias is applied. The negative bias was then applied and the halide ions migrate back towards the front of the device and return to the original unbiased state. In both cases the formamidinium and lead traces do not show similar migration, showing only the charged species in the device are affected by the bias. The results show a framework that can be used for further study. Potential complications with the analysis of this type of data will be discussed.