Understanding the role of organic ligands on charge transport and photocurrent generation in layered perovskites

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Halide perovskites show great potential for solar energy applications as perovskite solar cells now achieve efficiencies as high as 25.7% for single junction devices.[1] Unfortunately, the rise of perovskite solar cells is hindered by their strong sensitivity to moisture, oxygen, illumination, thermal and bias stresses.[2] Progress in encapsulation technologies alone will not suffice to allow market entry of perovskite solar cells. It is therefore crucial to tackle the question of stability.In order to protect halide perovskite films from moisture, a key trigger of degradation in perovskite-based devices, larger and hydrophobic organic cations (ligands) have been introduced in the crystal lattice.[3] As a consequence, the crystal phase is modified to form a 2D- or layered perovskite structure with slabs of inorganic octahedra separated by ligands. While this modification offers higher environmental stability, it comes at the cost of performance with a drastic loss in solar cell efficiency, in part due to a significant disruption of charge transport and exciton dissociation.[4]In view of this challenge, two research directions are currently being explored: 1) creating a mixed 3D-2D perovskite structure to find a reasonable balance between stability and efficiency;[5] and 2) finding the appropriate ligand to limit its negative impact on efficiency.[6] In this project we will focus on the second aspect and will aim at improving our understanding of the impact of ligands on charge transport and exciton dissociation in layered perovskites. In particular, we will tune the length of the organic ligand and follow its impact on various transport and optoelectronic properties of the semiconducting film. We will also explore the influence of the Van der Waals gap between organic ligands present in the Ruddlesden-Popper phase by comparing its properties with the Dion-Jacobson phase. The goal of this study will be to determine experimentally a maximum ligand size to ensure sufficient coupling between inorganic slabs. In addition, we will ascertain whether the Dion-Jacobson phase should be preferred to the Ruddlesden-Popper one, which is currently the main focus of research efforts in 2D-3D perovskite solar cells.[1] NREL, National Renewable Energy Laboratory. Best Research-Cell Efficiency Chart, 2021. [2] L. Schmidt-mende, et al., APL Mater. 2021, 9, 109202. [3] T. L. Leung, et al., Commun. Mater. 2022, 3, 1. [4] M. S. Holanda, et al., EcoMat 2021, 3, e12124. [5] A. Caiazzo, R. A. J. Janssen, Adv. Energy Mater. 2022, 2202830. [6] H. Fu, J. Mater. Chem. C 2021, 9, 6378.