Synthesis and Characterization of Functionalized Spacer Cations for the Incorporation in Layered Perovskites

Layered hybrid organic-inorganic perovskite (LHOIP) materials are an emerging class of semiconductors endorsed as a more stable alternative compared to the more widely investigated 3D hybrid organic-inorganic halide perovskites (HOIP). Consisting of alternating slabs of inorganic layers separated by layers of organic spacer cations, their optoelectronic properties are determined by quantum and dielectric confinement effects resulting in materials with high exciton binding energies and charge carrier anisotropy. Considering aliphatic spacer cations, such as n-butylammonium(BA) or phenylethylammonium (PEA), the spacer cations act mainly structurally, and the optoelectronic properties are determined by the distortions of the inorganic octahedral Pb-I-network. Recent reports have discovered that the incorporation of organic semiconducting spacer cations into the layered perovskite structure is an interesting approach to tuning the optoelectronic properties. While aliphatic spacer cations create a quantumwell structure due to a mismatch between their HOMO and LUMO energy levels and the valence band (VB) and conduction band (CB) of the inorganic layer, respectively, semiconducting organic chromophores can contribute to the electronic structure, thus potentially breaking the quantum confinement. For example, by incorporating electron-accepting spacer cations, type II organic-inorganic nano-heterostructures can be formed, where the excited electrons are transferred from the inorganic layer to the organic spacer chromophore. The possibility to modify the localization of the electrons and holes opens the door for a wide range of applications, such as in LED and solar cell devices. However, incorporating organic spacer cations remains challenging due to their large size. In this work, novel spacer cations for the formation of layered perovskite materials are investigated. Focusing on novel spacer cations based on naphthalene diimide (NDI) chromophore and NDI derivatives, critical aspects for the layered perovskite formation are analyzed. We further investigate the electron transfer mechanismfrom the inorganic layer to the NDI spacer cation by time-resolved photoluminescence and transient absorption (TA) spectroscopy showing electron transfer on a sub-picosecond timescale. The formation of long-lived charge-separated states and their potential implication in the exciton dissociation were studied by time-resolved microwave conductivity (TRMC) measurements. Modifying NDI spacer cations and synthesizing a core substituted derivative, a visible-light absorbing spacer cation was incorporated into a layered perovskite structure. Selectively exciting the spacer cation and tuning the CB and VB of the inorganic layer by forming a mixed halide octahedral perovskite network, we were able to further study electronic interactions between the organic and inorganic layers in the semiconducting material.