Role of Alkali Cations in Stabilizing Mixed-Cation Perovskites to Thermal Stress and Moisture Conditions

Abstract

Perovskite solar cells (PSCs) based on organic–inorganic hybrid perovskites containing a small fraction of substituted alkali-metal cations have shown remarkable performance and stability. However, the role of these cations is unclear. The thermal- and moisture-induced degradation of FA1–x Cs x PbI3 and (FA1–x Cs x )1–y Rb y PbI3 (where FA represents formamidinium, x, y = 0.1, 0.05) is investigated using in situ photoelectron spectroscopy (PES). Both compositions exhibit superior moisture stability compared with methylammonium lead iodide under 9 mbar of water vapor. Ga Kα hard X-ray PES is used to investigate the composition of the perovskites at depths up to 45 nm into the surface. This allows more accurate quantification of the alkali-metal distribution than is possible using conventional X-ray PES. The addition of RbI results in a fairly homogeneous distribution of both Cs+ and Rb+ in the surface layers (in contrast to surface Cs depletion seen in its absence), together with a marked reduction in surface iodide vacancies. Overall, RbI is found to play a critical role in increasing the thermal stability of FA1–x Cs x PbI3 by providing a source of I– that fills iodine vacancy sites in the perovskite lattice, while Rb+ is not substantially incorporated into the perovskite. We suggest that the concomitant increase in ion migration barriers in the surface layers is key to improved PSC performance and long-lasting stability.

Simultaneous Optimization of Charge Transport Properties in a Triple-Cation Perovskite Layer and Triple-Cation Perovskite/Spiro-OMeTAD Interface by Dual Passivation

“… 38 The N 1s core-level binding energy is observed at 400.7 eV and assigned to the N atoms in the FA cation in the mixed cation perovskite film. 78 The N 1s peaks in the perovskite structure modified with the three molecules shifted to lower binding energies compared to the control sample ( Figure S8 ). The shift in N 1s binding energies is the same for 2I-Ac and 2Cl-Ac molecules (400.2 eV) in the modified thin film, while it is greater for 2Br-Ac (400.1 eV).…”

Surface stability of ionic-liquid-passivated mixed-cation perovskite probed within situphotoelectron spectroscopy

“…The high-resolution I 3d core level spectra are used to investigate the X-site (halogen) component of the ABX 3 lattice. 13 Relative to FACs, the I 3d core level in IL-FACs also showed a shi to low binding energy (À0.2 eV), similar to the Pb 4f core level. Shis to low binding energy on IL incorporation have been attributed to bonding interaction between the perovskite halide lattice and imidazolium in the IL structure; we return to this topic below.…”

“…23 In previous work, we have assessed the extent of X-ray-induced degradation of MAPI-based perovskites. 13,25 This indicates that, using lab-based sources such as those used here, beam-induced degradation is not signicant over the timescale of our experiments, compared (for example) with the degradation induced by annealing. The sample position was moved by 0.5 mm to a fresh position (as the X-ray spot size is 300 mm) for each new condition.…”

“…3(b)(ii) and (iii)), consistent with the decomposition of perovskite into PbI 2 during sample heating under UHV conditions. 13,23 There was no signicant change in the I 3d BE of IL-FACs on heating as shown in Fig. 3…”

“…13,23 In addition, there is a small additional doublet at lower binding energy (137 AE 0.1 eV and 141.9 AE 0.1 eV BE), which can be assigned to metallic Pb (herein denoted as Pb 0 ). 13 This is an indicator either of incomplete conversion to perovskite from the precursor solution 32 or of sample degradation. 33 In contrast the spectrum of the IL-incorporated sample (Fig.…”