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Thesis Format

Integrated Article

Degree

Master of Science

Program

Chemistry

Supervisor

Song, Yang

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.

Summary for Lay Audience

The use of sustainable energy sources, in particular solar energy, has increased significantly over the years as a result of the increasing concerns of climate change. As such, research has heavily focused on two-dimensional hybrid organic-inorganic perovskites (HOIPs) for use in solar cells. 2D HOIPs are semiconducting materials consisting of an inorganic octahedra separated by organic spacers. The efficiency of perovskite solar cells has increased considerably over the years. However, optimization of their properties, in particular for solar cells applications, still requires more in-depth studies, thus modification of said properties is crucial. The goal is to permanently reduce the bandgap of 2D HOIPs to achieve the limit for maximum efficiency.

In this work we study the effects of high-pressure (HP) on 3 2D HOIPs, DPDAPbI4, CMA2MAPb2I7, and CMA2FAPb2I7 with differing organic spacers. We use Raman spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), photoluminescence (PL), and UV-Vis absorption measurements. Our findings provide critical information on the behavior of both the structures, in particular phase transitions, hydrogen bonding, distortion, and amorphization, and the optoelectronic properties, such as the PL emissions and the bandgap upon pressure treatment. Understanding the changes in the optoelectronic properties is crucial for applications such as solar cells. These findings in turn also provide a solid understanding of the behavior of 2D HOIPs with both soft and rigid ring spacers as well as a basis for modifying the structures through HP to achieve the desired properties for solar cell applications.

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