(401g) Bandgap Engineering of Cs-Pt Halide Perovskites By Modifying Solvent and Pt Oxidation State

Metal halide perovskites is a burgeoning material family with excellent optoelectronic properties including but not limited to broad range of visible-light absorption, tunable bandgap, high extinction coefficient, long carrier diffusion length/lifetime, and high defect tolerance. Of the wide range of perovskite compositions, APbX₃ (A = Cs, MA, FA; X = I, Br, Cl) are the most explored due to promising performance for applications like photovoltaics (PV), photocatalytic devices (water splitting), X-ray detectors, light emitting diodes, etc. However, toxicity of lead (Pb) is a serious concern as Pb²⁺ ions are highly bioavailable and can reach the human food chain. Due to toxicity concerns, there is an ongoing search for Pb-free halide perovskites (such as based on Sn, Ge, Bi, Sb, Cu, Ag, Ti, etc.), which can demonstrate promising optoelectronic properties as the Pb counterparts.

Pt-based halide perovskites are interesting for photocatalytic devices for generation of green fuels. Pt being a high Z, d-transition element is stable in both 2+ and 4+ oxidation states, that can potentially provide an extra lever of dimensionality control to tailor the optoelectronic properties.^[1],[2] Here, we demonstrate the change in dimensionality and bandgap of Pt-based halide perovskites with change of Pt-oxidation state and solvent engineering.

A’_mA_n-1B_nX_3n+1 type of quasi-2D perovskite structure is expected with Pt²⁺ based precursor, which shows many special properties associated with characteristic chemisorption reaction between the Pt²⁺ and dimethyl sulfoxide (DMSO). Sub-bandgap absorption in the films is observed resulting from d-d* transitions within the split d levels of Pt due to Pt-DMSO chemisorption and ligand-metal charge transfer. Additionally, the strongly bonded DMSO molecules act as organic spacer in the “quasi-2D” perovskite phase. We will also demonstrate DMSO/N,N-dimethyl formamide (DMF) solvent engineering as a lever to tune the perovskite bandgap (by changing “n” value of the “quasi-2D” perovskite phase). The σ-Ï€ orbital mixing due to spin-orbital coupling on the Pt-DMSO bonding enables the intersystem crossing of excitons from singlet state to triplet states, strongly enhanced due to high atomic number of Pt. As a result, the specific triplet emission/decay is observed with the characteristics of large “Stokes Shift” and longer lifetime in the order of nanoseconds. The stability properties of this material will also be presented. The unique Pt-based “quasi-2D” perovskite seems to have high potential in the application of photocatalysis attributed to the long charge carrier lifetime and excellent charge transfer/separation properties.

In the case of Pt⁴⁺ as the precursor, the phase structure of the material is significantly different from that of the Pt²⁺ based perovskite, which is possibly attributed to the higher oxidation state and the effective ion size. A vacancy ordered double perovskite Cs₂PtI₆ phase is proposed, wherein, the octahedra single crystals in μm sizes are formed. Also, the reaction between Pt⁴⁺ and DMSO is assumed to be different in intensity from that of Pt²⁺ due to the difference in Lewis’ acidity of PtI₄ and PtI₂. As a result, solvent engineering seems to hardly tune the bandgap of Cs₂PtI₆.

The reported research provides one of the first experimental observations of tunability for transition metal hybrid layered double perovskites demonstrating their compositional and structural diversity.

[1] https://pubs.acs.org/doi/pdf/10.1021/jacs.1c12760

[2] https://onlinelibrary.wiley.com/doi/abs/10.1002/ange.202016185