Hybrid Perovskite Degradation from an Optical Perspective: A Spectroscopic Ellipsometry Study from the Deep Ultraviolet to the Middle Infrared

Abstract

A quantitative analysis of the thermally induced degradation of various device‐relevant multi‐cation hybrid perovskite films is performed using spectroscopic ellipsometry, for temperatures between 80 and 120 °C. The studied compositions are a triple cation perovskite Cs0.05(MA0.17FA0.83)0.95Pb(Br0.17I0.83)3, a Rb‐containing variant Rb0.05Cs0.05(MA0.17FA0.83)0.90Pb(Br0.17I0.83)3, and a methylammonium‐free Rb0.05Cs0.10FA0.85PbI3 composition. A very wide combined spectral range of 200 nm to 25 μm is covered by combining the data from two separate instruments. The relative changes in organic cation concentrations are quantified from the middle infrared molecular absorption bands, leveraging the use of point‐by‐point fitting for increased sensitivity. Additionally, the formation of PbI2 and non‐perovskite δ‐CsPbI3 phases is evidenced from Bruggemann effective medium fits to the visible and ultraviolet complex refractive indices. Methylammonium is almost completely depleted from the relevant compositions within 100 to 285 min of thermal annealing. The MA‐free perovskite degrades faster at intermediate temperatures, which is attributed to phase instability due to the formation of δ‐CsPbI3 in addition to PbI2.

Optical Simulations in Perovskite Devices: A Critical Analysis

“…Step 5: An accurate dispersion model for each layer is chosen based on the material considered. For example, while Lorentz oscillators can closely model the TiO 2 layers, the perovskite would require a number of Tauc-Lorentz oscillators additionally and the ITO would need to be modeled with a combination of new amorphous (based on Forouhi-bloomer) and Drude dispersion models . The suitability of models is readily available in literature for reference.…”

“…For instance, Tejada et al measured the optical constants of perovskite films deposited in a N 2 -purged glovebox as an indicator of their thermal excitation-induced degradation. To be able to neglect any degradation while measuring, the ellipsometer itself was purged with N 2 and the ambient conditions were maintained at 20 °C and less than 20% relative humidity . Subiao and co-workers showed that perovskite films left in controlled ambient conditions for a period of time underwent degradation which was duly reflected in the change in optical constants obtained from periodic ellipsometry data.…”

“…For example, while Lorentz oscillators can closely model the TiO 2 layers, the perovskite would require a number of Tauc-Lorentz oscillators additionally 141 and the ITO would need to be modeled with a combination of new amorphous (based on Forouhibloomer) 142 and Drude dispersion models. 143 The suitability of models is readily available in literature for reference. This step can be carried out in the commercial software available with the ellipsometry setup (e.g., CompleteEASE by JA Woollam) 144 or even built independently.…”

“…To be able to neglect any degradation while measuring, the ellipsometer itself was purged with N 2 and the ambient conditions were maintained at 20 °C and less than 20% relative humidity. 143 Subiao and co-workers showed that perovskite films left in controlled ambient conditions for a period of time underwent degradation which was duly reflected in the change in optical constants obtained from periodic ellipsometry data. By using the Tauc-Lorentz oscillator model, they could definitively exhibit an increase in roughness values and decrease in surface thickness as provided by EMA.…”

The Synthesis and the effect of Cu on optoelectronic qualities of β-In2S3 as a window layer for CIGS thin film solar cells

“…Moreover, the shallow light penetration depth and the surface roughness led to higher uncertainties in the measurements of the optical properties and constants [51].…”