Two-dimensional halide perovskites incorporate larger, less volatile, and hydrophobic organic cations which break up the 3D perovskite structure. This leads to the formation of inorganic lead/iodide containing sheets, separated by the spacer cations. 2D perovskites have gained widespread interest as they turned out to be a key factor in unlocking high efficiencies in perovskite solar cells. They are usually employed as a capping or passivating layer and are either deposited subsequently on top of the 3D film or mixed into the precursor solution. Previous studies have extensively investigated the structural and optical properties of layered perovskites, particularly in respect to their role within perovskite based solar cells.

However, the role of energy level positions and band alignments at the interfaces remains a subject of ongoing debate. Notably, contradicting results have been published with respect to their electronic structure and additional work is needed to understand the role of 2D layers for optoelectronic applications. This work presents a step in this direction, by providing a systematic investigation of various alkylammonium-based Ruddlesden–Popper perovskites (n = 1, A'2PbI4) with varying chain length of the spacer cations, by X-ray diffraction (XRD), UV-vis spectroscopy, and ultraviolet photoelectron spectroscopy (UPS) and DFT calculations.

When changing the carbon chain length of the spacer molecules from C3 (propylammonium) all the way up to C10 (decylammonium) our results show a linear relationship between the alkyl chain length and the d-spacing of the 2D perovskite crystal. However, in contrast to our initial expectations, the optical and electronic properties are barely affected throughout this sample series; for example, all samples show a band gap of 2.4 ± 0.1 eV. Intriguingly, there is a slight odd-even effect, specifically we find that spacers with even number of carbon atoms show slightly smaller values here compared to the odd-numbered ones. DFT calculations corroborated this trend for the band gaps. By careful analysis of the calculated crystal structures, we identify that the modulation of the band gap is attributed to the packing efficiency of cations: those with an odd number of carbon atoms demonstrate significantly less efficient packing compared to even-numbered cations. This leads to enhanced distortions of the inorganic layers, which in turn weakens the orbital hybridization strength and results in an increased band gap.