Pressure-Tuned Structures and Optoelectronic Properties of 2D Hybrid Organic-Inorganic Perovskites

Aditya Kutty Mr., Western University

Abstract

In recent years research interest and the use of 2D hybrid organic-inorganic perovskites (HOIPs) in optoelectronic devices, in particular solar cells, has increased substantially due to their high stability, relatively cheap costs and ease of fabrication, and intriguing optoelectronic properties, in comparison to their 3D counterparts. Optimization of these properties for solar cells applications however still requires more in-depth studies. Of the various methods used previously, high external pressure has emerged as a clean and effective method for inducing modifications to the structures of 2D HOIPs, in turn inducing modifications to the optoelectronic properties. In this study, we report the first high-pressure studies on three different 2D HOIPs: N,N-dimethylphenylene-p-diammonium (DPDA) lead iodide (DPDAPbI₄), cyclohexane methylamine (CMA) methyl ammonium (MA) lead iodide (CMA₂MAPb₂I₇), and cyclohexane methylamine (CMA) formamidinium (FA) lead iodide (CMA₂FAPb₂I₇).

The effects of high-pressure on both the structures and optoelectronic properties of DPDAPbI₄ were studied by performing Raman spectroscopy, Fourier-transform infrared (FTIR) spectroscopy X-ray diffraction (XRD), photoluminescence (PL), and UV-Vis absorption measurements. The vibrational spectroscopy in tandem with the XRD and PL measurements identifies a phase transition. Vibrational spectroscopy also indicates pressure-induced enhancement of the N–H^…I hydrogen bonding interactions between the NH₃⁺ groups and the lead iodide octahedra, indicating shortening of the Pb-I bond lengths, confirmed by the computational studies, thus providing critical information regarding the mechanism behind the bandgap reduction. Synchrotron XRD measurements confirm the phase transition with distortion analysis indicating enhancement of the octahedral distortion between 1-2 GPa. The enhancements in both the hydrogen bonding interactions and the octahedral distortion were found to constitute the origins behind the phase transition. PL measurements show a redshift of the free exciton (FE) peak along with broadening of the self-trapped exciton (STE) peak at low-pressure. UV-Vis absorption confirms the bandgap reduction, and all measurements indicate reversible structural modifications and pressure-induced optoelectronic properties. For CMA₂MAPb₂I₇ and CMA₂FAPb₂I₇ FTIR measurements suggest a possible phase transition between 3-5 GPa along with N–H^…I hydrogen bonding interactions between the NH₃⁺ groups and the lead iodide octahedra. Synchrotron XRD measurements confirm the phase transition while analysis of the LeBail refined cell parameters indicates negative linear compressibility for both compounds. PL measurements show a redshift for both major PL peak and UV-Vis absorption shows a bandgap reduction for both compounds. In general, the structural modifications and optoelectronic properties were found to be reversible upon decompression. Our results provide a solid understanding of the pressure effects on the structures and optoelectronic properties of both n =1 and n = 2 2D HOIPs with differing organic spacers, a crucial step for the use of 2D HOIPs in applications such as solar cells.