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

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

doi:10.3389/fchem.2019.00946

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

Front Chem. 2020 Jan 28:7:946. doi: 10.3389/fchem.2019.00946. eCollection 2019.

Authors

Inés García-Benito¹, Claudio Quarti^{2

3}, Valentin I E Queloz¹, Yvonne J Hofstetter⁴, David Becker-Koch⁴, Pietro Caprioglio^{5

6}, Dieter Neher⁵, Simonetta Orlandi⁷, Marco Cavazzini⁷, Gianluca Pozzi⁷, Jacky Even⁸, Mohammad Khaja Nazeeruddin¹, Yana Vaynzof⁴, Giulia Grancini^{1

9}

Affiliations

¹ Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, EPFL Valais Wallis, Sion, Switzerland.
² Laboratory for Chemistry of Novel Materials, Department of Chemistry, Université de Mons, Mons, Belgium.
³ Univ Rennes, ENSCR, CNRS, ISCR (Institut des Sciences Chimiques de Rennes) - UMR 6226, Rennes, France.
⁴ Integrated Centre for Applied Physics and Photonic Materials and Centre for Advancing Electronics Dresden (CFAED), Technical University of Dresden, Dresden, Germany.
⁵ Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany.
⁶ Young Investigator Group Perovskite Tandem Solar Cells, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany.
⁷ CNR - Istituto di Scienze e Tecnologie Chimiche "G. Natta" (CNR-SCITEC), Milan, Italy.
⁸ Univ Rennes, INSA Rennes, CNRS, Institut FOTON - UMR 6082, Rennes, France.
⁹ Dipartimento di Chimica Fisica, University of Pavia, Pavia, Italy.

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.

Keywords: 2D perovskites; excitonic materials; fluorinated organic spacer; phase transition; temperature dependence.