Layered two-dimensional organo-metal halide perovskites are currently in the limelight, largely because their versatile chemical composition offers the promise of tunable photophysical properties. We report here on (time-dependent) density functional theory [(TD)DFT] calculations of alkyl-ammonium lead iodide perovskites, where significant changes in the electronic structure and optical properties are predicted when using long versus short alkyl chain spacers. The mismatch between the structural organization in the inorganic and organic layers is epitomized for dodecyl chains that adopt a supramolecular packing similar to that of polyethylene, at the cost of distorting the inorganic frame and, in turn, opening the electronic band gap. These results rationalize recent experimental data and demonstrate that the optoelectronic properties of layered halide perovskite semiconductors can be modified through the use of electronically inert organic saturated chains.

Low-dimensional lead halide hybrid perovskites are nowadays in the spotlight because of their improved stability and extensive chemical flexibility compared to their 3D perovskite counterparts, the current challenge being to design functionalized organic cations. Here, we report on the synthesis and full characterization of a perovskite-like hybrid where the 1D lead iodide layout is patterned with a donor-acceptor charge transfer complex (CTC) between pyrene and tetracyaniquinodimethane. By combining multiple structural analysis and spectroscopic techniques with ab initio modelling, we show that the electronic, optical and chargetransport properties of the hybrid materials are dominated by the organic CTC, with the inorganic backbone primarily acting as a template for the organization of the donor and acceptor molecules. Interestingly, time-resolved microwave conductivity (TRMC) measurements show an enhanced photocurrent generation in the 1D hybrid compared to the pure organic charge-transfer salt, likely associated with transient localization of the holes on the lead-iodide octahedra. This observation is line with the close energy resonance between the valence crystal orbitals of the lead-iodide lattice and the frontier occupied molecular orbitals of pyrene predicted by the DFT calculations. Therefore, it paves the way toward the design of new hybrid low-dimensionality perovskites offering a synergic combination of organic and inorganic functionalities.

Low-dimensional metal halide perovskites are being intensively investigated because of their higher stability and chemical versatility in comparison to their 3D counterparts. Unfortunately, this comes at the expense of the electronic and charge transport properties, limited by the reduced perovskite dimensionality. Cation engineering can be envisaged as a solution to tune and possibly further improve the material’s optoelectronic properties. In this work, we screen and design new electronically active A-site cations that can promote charge transport across the inorganic layers. We show that hybridization of the valence band electronic states of the perovskite inorganic sublattice and the highest occupied molecular orbitals of the A-site organic cations can be tuned to exhibit a variety of optoelectronic properties. A significant interplay of A-cation size, electronic structure, and steric constraints is revealed, suggesting intriguing means of further tuning the 2D perovskite electronic structure toward achieving stable and efficient solar cell devices.

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