Fluorination of Organic Spacer Impacts on the Structural and Optical Response of 2D Perovskites.

Inés García-Benito,Yvonne J Hofstetter,Pietro Caprioglio,Dieter Neher,Mohammad Khaja Nazeeruddin,Jacky Even,Marco Cavazzini,Giulia Grancini,Gianluca Pozzi,David Becker-Koch,Claudio Quarti,Simonetta Orlandi,Yana Vaynzof,Valentin I E Queloz

doi:10.3389/fchem.2019.00946

Abstract

Low-dimensional hybrid perovskites have triggered significant research interest due to their intrinsically tunable optoelectronic properties and technologically relevant material stability. In particular, the role of the organic spacer on the inherent structural and optical features in two-dimensional (2D) perovskites is paramount for material optimization. To obtain a deeper understanding of the relationship between spacers and the corresponding 2D perovskite film properties, we explore the influence of the partial substitution of hydrogen atoms by fluorine in an alkylammonium organic cation, resulting in (Lc)2PbI4 and (Lf)2PbI4 2D perovskites, respectively. Consequently, optical analysis reveals a clear 0.2 eV blue-shift in the excitonic position at room temperature. This result can be mainly attributed to a band gap opening, with negligible effects on the exciton binding energy. According to Density Functional Theory (DFT) calculations, the band gap increases due to a larger distortion of the structure that decreases the atomic overlap of the wavefunctions and correspondingly bandwidth of the valence and conduction bands. In addition, fluorination impacts the structural rigidity of the 2D perovskite, resulting in a stable structure at room temperature and the absence of phase transitions at a low temperature, in contrast to the widely reported polymorphism in some non-fluorinated materials that exhibit such a phase transition. This indicates that a small perturbation in the material structure can strongly influence the overall structural stability and related phase transition of 2D perovskites, making them more robust to any phase change. This work provides key information on how the fluorine content in organic spacer influence the structural distortion of 2D perovskites and their optical properties which possess remarkable importance for future optoelectronic applications, for instance in the field of light-emitting devices or sensors.

Highlights

Hybrid organic–inorganic perovskites (HP) are currently one of the most attractive fields of research due to their large potential for photovoltaic applications
Low dimensional perovskites belong to a large family of structures which read as R2(A)n−1MnX3n+1, where R is the bulky organic molecule, A the small organic cation, M the divalent metal, X the halogen and n the number of inorganic layers (Mitzi, 2001)
Despite the fact that the organic component does not contribute to the frontier orbitals of the 2D perovskite (Traore et al, 2018), we found a stark blue-shift of the excitonic resonance as a result of the presence of fluorine in the organic spacer

Summary

Introduction

Hybrid organic–inorganic perovskites (HP) are currently one of the most attractive fields of research due to their large potential for photovoltaic applications. The electronic structure of the frontier orbitals is fully related to atomic contributions from the inorganic PbX4 frame, with no direct contribution on the electronic and optical response of the organic spacer It has been widely reported how changing the halogen as well as the size and nature of the organic cations offer interesting features in 2D perovskites, such as superior hydrophobicity and high tunability of chemical composition and physical properties (Yu et al, 2007; Misra et al, 2017; Chen et al, 2018; Shi et al, 2018; Zheng et al, 2018). A decrease of 0.3 eV in the ionization potential (IP) was found as a result of incorporating fluorinated spacer in the 2D network

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