Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV25)
DOI: https://doi.org/10.29363/nanoge.hopv.2025.056
Publication date: 17th February 2025
Two-dimensional (2D) halide perovskites have gained significant attention for enhancing both the performance and stability of perovskite devices. While band gap changes and crystal structure variations are well established, the energy level positions and band alignments at interfaces remain less understood. To address this, we systematically investigate alkylammonium-based Ruddlesden–Popper perovskites (n = 1, A'₂PbI₄) with varying alkyl chain lengths (from propylammonium C₃, to decylammonium C₁₀) using X-ray diffraction (XRD), optical analysis and ultraviolet photoelectron spectroscopy (UPS).
UPS reveals systematic changes in the density of states (DOS), depending on the length of these spacer cations. These variations can be well explained by comparison to calculations of the density of states done by density functional theory (DFT) when considering the low probing depth of UPS. Surprisingly, the ionization energy (i.e., VB position) remains nearly constant across all samples. and are in a similar range as the 3D forming MAPbI3, suggesting that the increase in band gap for 2D perovskites seems predominantly due to an upward shift in the conduction band.
When performing photoluminescence and reflection spectroscopy measurements on these various 2D layers, only minor band gap variations (up to 90 meV) are observed as a function of spacer cation length. Here, no linear trend is found, rather an intriguing odd-even effect emerges, where perovskites with odd-numbered alkyl chains exhibit a blue-shifted absorption onset compared to their even-numbered counterparts. DFT simulations attribute this effect to Pb-I-Pb bond distortions within the inorganic framework, influenced by the packing efficiency of the alkyl chains.
Finally, I will present reflection electron energy loss spectroscopy (REELS) measurements, which probe inelastic electron scattering to provide insights into electronic transitions from conduction to valence band states. By tuning the excitation energy, it is possible to adjust the probing depth from ~1 to 10 nm, enabling us to assess surface modifications and their impact on the optical gap. This is particularly relevant for investigating 2D perovskite formation on 3D layers, for which I will present several examples.
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International Conference on Hybrid and Organic Photovoltaics
International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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