Investigation of the Optoelectronic Properties of Formamidinium Lead Bromide Perovskites Using Time-Resolved Infrared Spectroscopy
Open Access
- Author:
- Swartzfager, John
- Graduate Program:
- Chemistry
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- July 26, 2019
- Committee Members:
- John B Asbury, Thesis Advisor/Co-Advisor
Benjamin James Lear, Committee Member
Mark Maroncelli, Committee Member
Noel Christopher Giebink, Committee Member - Keywords:
- Time-resolved spectroscopy
Formamidinium lead bromide
Lead halide perovskites
Time-resolved infrared spectroscopy
Optoelectronic
Photovoltaics
Fano resonance
Transient absorption
Polaron
Electron-phonon coupling
Excited state dynamics
Charge carriers - Abstract:
- Halide perovskite photovoltaics have garnered a significant amount of interest over the last ten years. With favorable optoelectronic properties including high open circuit voltages, efficient PL quantum yields, and low non-radiative recombination rates, combined with solution processability and recent efficiencies of 24.2%, perovskites are serious contenders to compete with silicon photovoltaics. However, an in-depth understanding of the materials excited state properties and why they can achieve such high efficiencies is missing. Which is why previous work by our lab sought to better understand the excited state dynamics of methylammonium lead iodide (MAPbI3), using time-resolved infrared spectroscopy. On the basis of that work it was discovered that charge carriers present in MAPbI3 interact with the phonon modes of the material, which leads to the formation of large polarons. These large polarons are responsible for the long lived excited state lifetimes and modest charge carrier mobility’s of the material. Upon completion of the MAPbI3 study we decided to study a different halide perovskite, with the hopes of determining our labs previous observations were unique to MAPbI3, or if they were a universal property of all halide perovskites. The material decided upon was formamidinium lead bromide (FAPbBr3). This choice was influenced by recent interest in formamidinium perovskites, as it had been discovered they possessed a superior thermal stability to that of methylammonium perovskites. The focus of this thesis is on the study of the optoelectronic properties of FAPbBr3, utilizing time-resolved infrared spectroscopy. From our investigation it was discovered that the charge carriers of FAPbBr3 form large polarons, which have a similar binding energy to those of MAPbI3. Also, while probing the structural dynamics of FAPbBr3, a unique form of coupling was revealed. It was discovered that the transient C-N stretch of the organic cation, and the higher energy Drude-like absorption tail of the polaron formed a Fano resonance. This unique resonance occurs due to coupling between a discrete and continuum state, which to our knowledge is the first time such a phenomenon has been observed for FAPbBr3. Fitting the Fano resonance allowed for the extraction of the transient line shape of the C-N stretch, the breadth of which was discovered to increase at higher temperature. This phenomenon was previously observed for the transient N-H bend of MAPbI3, implying that there is an increased dynamic disorder at higher temperature, leading to an increased population of large polarons. However, the center frequency of the transient C-N stretch did not change with temperature, a characteristic not observed for the transient N-H bend of MAPbI3. Implying that the dynamics of the two different cations may respond differently to temperature, which we hope to address in future studies using polarization selective infrared pump-probe spectroscopy.