Publication date: 15th December 2025
Two-dimensional (2D) halide perovskites are attractive for improving device stability and performance, yet their electronic structure and energy level alignments remain insufficiently understood. We therefore investigate a series of alkylammonium-based Ruddlesden–Popper perovskites (n = 1, A'₂PbI₄) with spacer cation lengths from propylammonium (C₃) to decylammonium (C₁₀), combining X-ray diffraction, optical spectroscopy, and ultraviolet photoelectron spectroscopy (UPS).
UPS reveals systematic, chain-length–dependent variations in the measured density of states. These changes align well with density functional theory (DFT) when considering the shallow probing depth of UPS. Notably, the ionization energy remains nearly constant across all samples and comparable to MAPbI₃, indicating that the widening of the band gap in 2D perovskites is dominated by an upward shift of the conduction band rather than changes in the valence band.
Optical measurements show only modest band gap variations (up to ~90 meV). Instead of a monotonic trend, an odd–even effect emerges: perovskites with odd-numbered alkyl chains exhibit a blue-shifted absorption onset relative to even-numbered ones. DFT attributes this to Pb–I–Pb bond distortions in the inorganic layer, driven by differences in alkyl-chain packing.
Finally, we use reflection electron energy loss spectroscopy (REELS) to probe electronic transitions with tunable probing depth (~1–10 nm). This enables us to assess surface-related gap modifications and is particularly useful for studying the formation and properties of 2D surface layers on 3D perovskites. Several representative examples will be presented.
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