Resolving Oxidation States and X–site Composition of Sn Perovskites through Auger Parameter Analysis in XPS

Abstract

MA + ), formamidinium (FA + ) and X = I − , Br − , Cl − . Furthermore, they are classified by their perovskite crystal structure (Figure 1a). Their lower toxicity compared to their more established APbX 3 counterparts and relatively narrower yet tunable bandgap make them attractive candidates for a range of optoelectronic applications. [5] However, the facile oxidation of the Sn(II) species to Sn(IV) compounds severely limits their lifetime, especially under ambient conditions. [6] The development of Sn-based semiconductors necessitates a reliable determination of the chemical state of the constituent elements. X-ray photoelectron spectroscopy (XPS) is an important tool in this regard to conduct research on novel energy materials and devices. [7,8] Typically, these studies are based on the observation of the core level photoelectron emission features and their position on the binding energy (E b ) scale. To counteract charging effects, referencing based on adventitious carbon is often performed. However, this results in a typical inaccuracy of ±0.2 eV, whereas even errors of >1 eV have been observed. [9] For many Sn compounds with different Sn oxidation states and compositions, the main Sn 3d core level emission exhibits only minimal E b shifts, which lie below this error range. Consequently, probing degradation as well as assigning the effects of compositional changes and additives to the surface chemistry of Sn-based perovskites with conventional XPS can be challenging.In addition, shifts on the binding energy scale not originating from chemical bonds are especially pronounced for semiconductors (Figure 1b). Similar to effects observed in UPS, [10] excitation during measurement with an X-Ray source may induce photovoltages, [11] resulting in surface band bending. Surface charges, e.g., induced by ionic additives, may lead to similar effects that originate from the resulting space charge and not the bonding to the additive itself. [12] Consequently, a probing depth-dependent shift is induced, which may further inhibit reliable charge referencing. Similarly, changes in the materials' work function (e.g., due to differences in doping) would also result in changes of the absolute binding energies for an otherwise identical material.For chemical state analysis of Sn, XPS analysis based on the modified Auger parameter (AP) concept offers an effective alternative to distinguish different chemical states. [13] This approach relies on the observation of both the photoelectrons (i.e., Sn 3d) and Auger electrons (i.e., Sn MNN). Reliable chemical state analysis ofSn semiconductors by XPS is hindered by the marginal observed binding energy shift in the Sn 3d region. For hybrid Sn-based perovskites especially, errors associated with charge referencing can easily exceed chemistry-related shifts. Studies based on the modified Auger parameter α ′ provide a suitable alternative and have been used previously to resolve different chemical states in Sn alloys and oxides. However, the meaningful interpretation of Auger parameter var...

Intrinsic Formamidinium Tin Iodide Nanocrystals by Suppressing the Sn(IV) Impurities

“…Likewise, the α′ values of the FASnI 3 NC films made from purified and nonpurified SnI 2 significantly differ (922.3 eV vs 921.6 eV), with the first value matching precisely the one reported for polycrystalline FASnI 3 films. 71 These data demonstrate the enhanced sensitivity of the AP analysis for detecting minuscule differences in the local chemical state of Sn. In agreement with XPS data, we have not observed any signs of the oxidized FA 2 SnI 6 form in 119 Sn solid-state NMR, which is expected at −4818 ppm.…”

“…The modified Auger parameter, α′ (AP), 72 allows one to cancel out shifts related to charging and band bending effects and was shown to be useful for resolving the oxidation state and chemical environment of Sn in perovskite-type structures, including polycrystalline FASnI 3 films. 71 The value of α′ corresponds to the sum of the binding energy (BE) of the Sn 3d 5 / 2 photoelectron line and the kinetic energy (KE) of the corresponding Sn M 4 N 4,5 N 4,5 Auger line. Any shift in α′ can be directly related to a change in the local electronic polarizability, 72 which is extremely sensitive to changes in the local chemical state of the atom in the compound.…”

“…XPS combined with inert-gas transfer was used to collect the Sn 3d core level spectra . The acquired spectra for Sn 3d 5/2 and 3d 3/2 do not show apparent asymmetry and can be well-fitted with a single component, indicating Sn is mainly present in one oxidation state (Figure d).…”

“…This is especially the case with low Sn 2+ or Sn 4+ contents. The modified Auger parameter, α′ (AP), allows one to cancel out shifts related to charging and band bending effects and was shown to be useful for resolving the oxidation state and chemical environment of Sn in perovskite-type structures, including polycrystalline FASnI 3 films . The value of α′ corresponds to the sum of the binding energy (BE) of the Sn 3d 5 / 2 photoelectron line and the kinetic energy (KE) of the corresponding Sn M 4 N 4,5 N 4,5 Auger line.…”

Intrinsic formamidinium tin iodide nanocrystals by suppressing the Sn(IV) impurities

“…. XPS spectra of the purified and non-purified SnI2 and reference peaks for SnI2 28 and SnI4. We note that there are no available references for pure SnI4.…”