From 0D to 2D—Dimensional Tuning of Structural and Optoelectronic Properties of Pt Halide Perovskites

Metal halide perovskites, especially lead halide perovskites, have attracted extensive attention for their potential in solar energy conversion. However, issues including lead toxicity and instability impede their commercial viability. Platinum halide perovskites offer an eco-friendly alternative. Current research predominantly focuses on 0D vacancy-ordered double perovskites (A₂PtX₆). Dimensional engineering is a powerful strategy for tuning the structural and optoelectronic properties of these materials. In this presentation we will report our newly discovered 2D CsPtI₃(DMSO) perovskite analog and compare its properties with 0D Cs₂PtI₆.

Energy dispersive X-ray spectrometry (EDS) analysis revealed the element ratio of Cs:Pt:I:S as 1:1:3:1, and Fourier transform infrared spectroscopy (FTIR) confirmed the DMSO-Pt ordination via k-S mode, indicating the formation of a layered structure. Based on this information, we simulated the 2D structure of CsPtI₃(DMSO) using density functional theory (DFT), showing that Pt (II) coordinates with the sulfur long-pair electrons in DMSO through a κ-S mode in this layered 2D structure. DMSO molecules replace the iodide ions in the octahedra, acting as small ligand spacer molecules that separate the 2D layers. The 2D layered CsPtI₃(DMSO) structure exhibits smaller interlayer distance, distinct from the typical “quasi-2D perovskite structure A₂A’_n-1B_nX_3n+1”, where 1 < n < ∞. This structure is further confirmed by comparing the experimental Grazing Incidence Wide-Angle X-ray Scattering (GIWAXS) pattern with the simulated pattern based on the DFT calculation.

We then compared the band structure of 2D CsPtI₃(DMSO) and 0D Cs₂PtI₆ using UV-vis absorption spectrum, X-ray Photoelectron Spectroscopy (XPS), and Ultraviolet Photoelectron Spectroscopy (UPS). It is indicated that 0D Cs₂PtI₆ exhibits n-type behavior, consistent with previous reports, while the 2D perovskite is p-type. The oxidation state of Pt in CsPtI₃(DMSO) is 2+, aligning with the chemical formula. Thermogravimetric analysis (TGA) and XRD analysis revealed that 2D CsPtI₃(DMSO) is thermally stable up to 181.5°C, beyond which it transforms into a mixture of Cs₂PtI₆ with interstitial Pt, which changes the conductivity of Cs₂PtI₆ from n-type to p-type, as confirmed by UPS. This observation aligns with our DFT calculations, which suggest that Pt can act as an interstitial defect (electron sink) in Cs₂PtI₆, thereby tuning the pristine n-type Cs₂PtI₆ into slightly p-type.

The 2D CsPtI₃(DMSO) phase exhibits broad emission around 850nm with a low photoluminescence quantum yield (PLQY) of less than 0.1% when excited at 532nm. This is accompanied by a significant Stokes shift from the absorption edge, approximately 600 nm. Additionally, the excitation transition is forbidden under 785 nm laser excitation. To investigate the emission mechanism, we conducted an in-depth examination of the photoluminescent properties of 2D CsPtI₃(DMSO), including temperature-dependent photoluminescence (PL) and time-resolved PL (TRPL) studies. A large value of Huang-Rhys factor (S_2D ~16.8), obtained by fitting the temperature-dependent FWHM of PL peaks against temperature, and the short lifetime (τ ~ 0.23 ns) suggests that the 850 nm broad emission is likely due to self-trapped excitons (STEs). In contrast, 0D Cs₂PtI₆ showed no significant PL emissions upon excitation (even at low temperature, 80K) but strong Raman signal, likely due to excessively strong electron-phonon coupling. This strong coupling can cause the excited- and ground-state curves to intersect in the configuration diagram, allowing excited electrons and holes to recombine non-radiatively and emit several phonons.

In summary, we report our first synthesis of 2D Pt halide perovskite CsPtI₃(DMSO) and demonstrate how dimensional engineering influences the structural and optoelectronic properties of platinum halide perovskites, highlighting significant differences between 0D and 2D configurations.