UNDERSTANDING INHOMOGENEITIES AND CARRIER DYNAMICS IN MIXED-HALIDE PEROVSKITES VIA STEADY-STATE AND TIME-RESOLVED PHOTOLUMINESCENCE IMAGING

The Photoluminescence (PL) technique, recognized for its non-destructive and rapid data acquisition attributes relative to other techniques, is pivotal in characterizing photovoltaic materials, especially perovskite absorbers. Quantifying lateral inhomogeneities is gaining interest in the Perovskite solar cells (PSC) community. This thesis harnesses a hyperspectral PL imaging system that has been calibrated to absolute photon numbers to spatially resolve key optoelectronic parameters such as the optical band gap, PL quantum yield, and quasi-Fermi level splitting, rather than relying on conventional averaged measurements. A critical investigation is conducted on the use of pulsed lasers, demonstrating that high repetition frequency pulsed lasers can yield comparable results to Continuous Wave (CW) lasers. A meticulous study is performed on absorbers of high-efficiency 22%-24 (FAPbI3)0.97(MAPbBr3)0.03 perovskite solar cells, uncovering spatial heterogeneities. These are characterized by luminescence variations, pinholes, and chemical inhomogeneity which is common in solution-processed perovskites, potentially affecting device performance. To decode the underlying causes of these heterogeneities, point Energy-Dispersive X-Ray spectroscopy (EDX) and high resolution Secondary Ion Mass Spectroscopy (SIMS) measurements are employed, revealing elemental variations originating from partial perovskite stack. Furthermore, high-resolution SIMS mapping exposes micrometer-scale regions lacking the organic component (FA+) and Br, yet rich in Pb and I, indicating PbI species. Such chemical variation then also affects the luminescence property of the absorber, showing spots of high and low emission. Apart from chemical heterogeneity, the partial perovskite stack features a stripes pattern that is a consequence of spin coating the TiO2 layer. Due to this, the charge extraction is spatially modulated by the TiO2 thickness. A detailed study of these stripes has been conducted in Chapter 5 of this thesis. When the measurement system is simple, such as an absorber layer on glass, interpretation of the luminescence measurement is easy. This thesis then discusses the complexities introduced by the electron transport layer in interpreting time-resolved luminescence measurements, such as quenching, transport, and interface recombination. To distinguish carrier dynamic processes for the perovskite layer in contact with the transport layer and using established models and simulation from literature, I employed transient PL measurements at Low level injection (LLI) using two geometries: one with illumination from the top surface of the perovskite and the other with illumination from the back interface. Assumptions of a Lambert-Beer generation profile allow the dissection of surface and interface recombination processes, as well as charge transfer mechanisms to the extraction layer. These insights are pivotal for future advancements in transport layer improvement as such methodology serves as a quality check of the interface as well as the surface.
Addressing surface heterogeneity, the thesis explores strategies, including the use of laser polishing on a CsMAFA perovskite surface, to achieve a smoothed surface with reduced heterogeneity as well as better Spiro-OMeTAD deposition. Atomic force and Kelvin probe force microscopy validate the reduced roughness and work function post-laser treatment. Reduction of surface work function means that the absorber will have better energy level alignment with the transport layer as well as enhanced charge collection. The thesis also ventures into the realm of perovskite film thickness modulation through laser ablation, a novel approach to tailoring absorber layer thickness for enhanced optoelectronic properties. This thesis offers insights into state-of-the-art perovskite absorbers, elucidating the intricate interplay between the perovskite and adjacent layers. It underscores the transformative impact of imaging techniques in understanding and refining the dynamic behavior of perovskite materials in response to interfaces.