Publication date: 16th July 2025
Two-dimensional (2D) halide perovskites have garnered significant attention for their ability to enhance both the performance and stability of perovskite devices. While changes in band gap and crystal structure in 2D halide perovskites are widely agreed upon, understanding energy level positions and band alignments at the interfaces remains elusive. To bridge this knowledge gap, we present a systematic investigation of alkylammonium-based Ruddlesden–Popper perovskites (n = 1, A'2PbI4) with varying alkyl chain lengths of spacer cations (from propylammonium, C3, to decylammonium, C10), using X-ray diffraction (XRD), UV-vis spectroscopy, and ultraviolet photoelectron spectroscopy (UPS).
The XRD results indeed confirm the expected increase in d-spacing with the alkyl chain length, while (UPS) measurements show consistent changes in the density of states (DOS) depending on the length of these spacer cations. These variations can be well explained by comparison to DOS calculations 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, however, a slight odd-even variation is observed in UPS data. The extracted ionization energy values are only marginally larger than what we find for 3D MAPbI3, suggesting that the increase in band gap for 2D perovskites seems predominantly due to an upward shift in the conduction band.
The mentioned odd-even effect is even more pronounced when investigating the optical band gaps. Perovskites with odd-numbered alkyl chains exhibit blue-shifted absorption onset compared to their even-numbered counterparts by up to 90 meV. DFT simulations reveal that this effect arises from distortions in the Pb-I-Pb bond angles within the inorganic framework, influenced by the packing efficiency of the alkyl chains. Specifically, odd-numbered cations lead to larger structural distortions, resulting in higher bond angle deviations and larger band gaps.
In conclusion, our study reveals that the observed variations in optical properties stem from an odd-even effect driven by the packing of spacer cations, rather than changes in interplane distance.
nanoGe is a prestigious brand of successful science conferences that are developed along the year in different areas of the world since 2009. Our worldwide conferences cover cutting-edge materials topics like perovskite solar cells, photovoltaics, optoelectronics, solar fuel conversion, surface science, catalysis and two-dimensional materials, among many others.
Previously nanoGe Spring Meeting (NSM) and nanoGe Fall Meeting (NFM), MATSUS is a multiple symposia conference focused on a broad set of topics of advanced materials preparation, their fundamental properties, and their applications, in fields such as renewable energy, photovoltaics, lighting, semiconductor quantum dots, 2-D materials synthesis, charge carriers dynamics, microscopy and spectroscopy semiconductors fundamentals, etc.
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.
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
The main topics of the Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP) are discussed every year in Asia-Pacific for gathering the recent advances in the fields of material preparation, modeling and fabrication of perovskite and hybrid and organic materials. Photovoltaic devices are analyzed from fundamental physics and materials properties to a broad set of applications. The conference also covers the developments of perovskite optoelectronics, including light-emitting diodes, lasers, optical devices, nanophotonics, nonlinear optical properties, colloidal nanostructures, photophysics and light-matter coupling.
The International Conference on Perovskite Thin Film Photovoltaics Perovskite Photonics and Optoelectronics (NIPHO) is the best place to hear the latest developments in perovskite solar cells as well as on recent advances in the fields of perovskite light-emitting diodes, lasers, optical devices, nanophotonics, nonlinear optical properties, colloidal nanostructures, photophysics and light-matter coupling.