Tracking the formation of methylammonium lead triiodide perovskite

Abstract

The formation mechanism of perovskite methylammonium lead triiodide (CH3NH3PbI3) was studied with in situ X-ray photoelectron spectroscopy (XPS) on successive depositions of thermally evaporated methylammonium iodide (CH3NH3I) on a lead iodide (PbI2) film. This deposition method mimics the “two-step” synthesis method commonly used in device fabrication. We find that several competing processes occur during the formation of perovskite CH3NH3PbI3. Our most important finding is that during vapour deposition of CH3NH3I onto PbI2, at least two carbon species are present in the resulting material, while only one nitrogen species is present. This suggests that CH3NH3I can dissociate during the transition to a perovskite phase, and some of the resulting molecules can be incorporated into the perovskite. The effect of partial CH3NH3 substitution with CH3 was evaluated, and electronic structure calculations show that CH3 defects would impact the photovoltaic performance in perovskite solar cells. The possibility that not all A sites in the APbI3 perovskite are occupied by CH3NH3 is therefore an important consideration when evaluating the performance of organometallic trihalide solar cells synthesized using typical approaches.

Substrate-dependent electronic structure and film formation of MAPbI3 perovskites

“…As a result, the peak positions for C 1 s are centered at 286.7 eV (redox state +1), for N 1S at 402.6 eV (redox state +1), and for I 3d 5/2 at 619.5 eV (redox state −1), all of which are in good agreement with previously published values [see e.g. refs 28,31,38,39]. The colored peaks in Fig.…”

“…Other research looked at the sequential evaporation of precursor materials, e.g. methylammonium iodide (MAI) on top of PbI 2 28 or lead halides on top of MAI29. Regarding the influence of substrate work function on the position of the perovskite conduction band (CB) and valence band (VB), a pinning of the Fermi energy at the CB is commonly observed for n-type substrates like TiO 2 , ZnO, and Al 2 O 3 , with a Wf around 4 eV223031.…”

Investigation of Thermally Induced Degradation in CH3NH3PbI3 Perovskite Solar Cells using In-situ Synchrotron Radiation Analysis

“…To confirm the thermal decomposition of the MAPbI 3 perovskite surface, we utilised in-situ HR-XPS to determine the composition ratio, similar to Liu et al ., who confirmed the growth of MAPbI 3 39 . In-situ HR-XPS measurements were conducted under ultra-high vacuum conditions, as the analysis of MAPbI 3 components changes depending on different temperatures (Fig.…”

Dissociation of Methylammonium Cations in Hybrid Organic–Inorganic Perovskite Solar Cells

“…5(b,c), also reveals the presence of two carbon species in agreement with previous reports. 40 Although XPS involves ionizing radiation, the incident intensity in a laboratory source (Al Kα, used herein) is one or two orders of magnitude lower than the synchrotron radiation used for XE and XA, so radiation-induced degradation should be minimized (although, as previously noted, we believe radiation-induced degradation is minimal in our synchrotron XE and XA measurements of pristine MAPbI 3 ). If, for example, the defects are more concentrated near the surface, such as the first 30 nm of our ∼ 300 nm-thick MAPbI 3 film contains a defect concentration of 50% of the MA sites, then owing to the different probe attenuation lengths ( 1 nm for XPS, 200 nm for XE and XA, and several mm for NMR), we would expect to see 50% of the carbon 1s XPS, 15% of the carbon XA and XE, and 5% of the CH 3 NMR spectral intensities should be due to the defects, in reasonable agreement with the data shown in Fig.…”