Pressure-Induced Modifications to the Structural and Optoelectronic Properties of 2D Hybrid Organic-Inorganic Perovskites
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.