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Thesis Format

Integrated Article

Degree

Master of Science

Program

Physics

Supervisor

Song, Yang

Abstract

Recently, 2D hybrid organic-inorganic perovskites (HOIP) have garnered lots of research interest for their applications in optoelectronic devices, especially in solar cells. The optoelectronic properties of 2D HOIPs have yet to be optimized for these applications. High external pressure is well known to induce structural modifications to 2D HOIPs, and thus modify their optoelectronic properties. Herein, we report a study of the effects of high pressure (HP) on the structures and optoelectronic properties of cyclohexane methylamine (CMA) lead iodide (CMA2PbI4) and the structures of N,N-dimethylphenylene-p-diammonium (DPDA) lead iodide (DPDAPbI4).

High pressure measurements of CMA2PbI4 were performed using Raman spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), photoluminescence (PL), and UV-Vis absorption. Results from vibrational spectroscopy demonstrate pressure-enhanced hydrogen bonding between the NH3 group within the CMA and the I ions in the inorganic octahedra. Synchrotron XRD measurements provided critical information upon the structural modifications, such as a negative linear compressibility in phase III. Photoluminescence measurements showed a redshift of the emission profile and a phase transition-induced peak splitting. UV-Vis absorption measurements showed a pressure-induced bandgap reduction upon compression. All measurements displayed reversible modifications of the structure and optoelectronic properties. Similarly, HP measurements of DPDAPbI4 were carried out by Raman spectroscopy, FTIR spectroscopy, and XRD. Vibrational spectroscopy measurements showed H-I bonding on both ligands as well as on the ring and a phase transition near 2 GPa. In-situ synchrotron XRD measurements provided further critical insights upon the pressure-induced structural modifications of DPDAPbI4. DPDAPbI4 was found to be surprisingly soft as its bulk modulus is among the softest 2D lead-iodide HOIPs. Structural modifications were found to be reversible upon decompression to atmospheric pressure. Overall, our findings reveal important structural information involving the spacer softness of these two different 2D HOIPs, which substantially enhance our understanding of the structure-property relationship, which is critical for applications of 2D HOIPs in optoelectronic devices.

Summary for Lay Audience

Two-dimensional hybrid organic-inorganic perovskites (HOIP) are made by layers of semiconducting inorganic octahedra separated by insulating organic spacers. These materials have garnered lots of attention in recent times due to their favorable properties for electronic devices, especially solar cells, for which the perovskite solar cell’s efficiency has grown tremendously in the last couple decades. However, their properties are not yet fully optimized for such applications. As a result, modification of the properties of these materials is extremely important to reach maximal efficiency in electronic devices. In particular, the bandgap of 2D HOIPs needs to be reduced to the Shockley-Queisser limit for maximal efficiency in solar cells.

Herein, we report the investigation of the effects of high pressure (HP) on two 2D HOIPs, CMA2PbI4 and DPDAPbI4, through the use of vibrational spectroscopy (Raman and Fourier-transform infrared (FTIR) spectroscopy), X-ray diffraction (XRD), UV-Vis absorption, and photoluminescence (PL). Vibrational spectroscopy gave access to information about the chemical bonding, distortion, and deformation as responses to external compression. Structural modifications and phase transitions were identified through vibrational spectroscopy and XRD. UV-Vis absorption was used to determine the pressure-induced modification of the bandgaps and PL was used to measure PL emission, which is useful for applications such as light emitting diodes. These findings allow us to better predict the behavior of other 2D HOIPs with ring spacers at HP and to better understand what changes to 2D HOIPs are necessary to obtain the desired properties.

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