Optical Mapping of Organohalide Lead Perovskite Films

Abstract

Although thin film methylammonium lead iodide (CH3NH3PbI3 also written as MAPbI3) perovskite photovoltaics (PV) are cheap to produce and have seen a quick rise in power conversion efficiency (PCE), from 2.19 in 2006 to 22.7 as of 2017, they have not been able to find a place on the world energy market due to the persistent problem of film degradation. In this thesis I looked at three optical characterization techniques, i) Spectral Ellipsometry (SE), ii) Confocal- Photoluminescence (c-PL), and iii) Raman Spectroscopy (RS), to determine their competency in characterizing thin film perovskite through the Z-axis. While interrogating the film with RS with high power to induce degradation we confirmed PbI2 to be the biproduct of degraded MAPbI3 films, as well as found evidence that heat conductivity of the substrate employed can result in increasing or decreasing the perovskites sensitivity heat. By forming film stacks of PbI2 and MAPbI3 we found that both RS and c-PL were able to distinguish between the PbI2 and MAPbI3 layers despite low depth resolution. C-PL also provided insight into the film formation (when using the two-step deposition process) by showing us that the MAI, even at low concentrations, appears to evenly diffuse throughout the, initially formed, PbI2 film layer. Lastly, using SE I was able to create optical models that accurately determined film composition as well as film thickness in stacked samples.