REVEALING NON-RADIATIVE LOSSES IN HALIDE PEROVSKITES WITH PHOTOLUMINESCENCE MICRO-SPECTROSCOPY

Abstract

Perovskites are a class of semiconductors that found their place in many fields of applications, most researched application of halide perovskites is in making perovskite solar cells and perovskite light emitting diodes. Perovskite-based solar cells have reached device efficiency which is equivalent to the efficiency of crystalline silicon solar cells. However, researchers still need to reach a consensus on the exact reason behind the phenomenal success of these materials even in the polycrystalline phase. Halide perovskites are strange materials having greatly complex photo-physical processes. Now it is very well established that the nature of electrical conduction in perovskites is mixed ionic-electronic, which is one of the reasons for the complexity of understanding the fundamental physical process undergoing in these materials. Mixed ionic-electronic conduction is responsible for many intriguing properties of halide perovskite. The Ionic-electronic conduction gives rise to many peculiar phenomena, such as the coexistence of photo-brightening and photo-degradation under continuous wave LASER illumination,1 photoluminescence blinking/flickering,2–4 and negative capacitance.5 However, for transient PL measurements, the impact of ion migration is “supposed to be” negligible due to the large activation time for ion migration. Spatial, spectral, and temporal PL measurements can help us in better understanding the fate of excited carriers which ultimately guides us for better optimization of the material properties and device design to achieve maximum device performance. Although, resolving the spectroscopic signatures is extremely challenging due to the many intertwined processes. Halide perovskites (HPs) also suffer from halide segregation which results in spectral and mechanical instabilities. Perovskite films/devices have been found to degrade under heat, exposure to moisture, oxygen, and UV irradiation. The main reason behind several degradation mechanisms could be the low lattice energy of the perovskite crystals. Owing to the interdependence of material properties, their respective characteristic features are entangled in a highly complex manner. Perovskite materials are very sensitive to their fabrication steps, due to their skyscraping sensitivity on fabrication steps. Therefore, it is critically important to understand the sample-to-sample variation in these materials, which is a big headache for the perovskite community worldwide.6 Properties of perovskites are so sensitive to the environment as well as to the measurement conditions that the simplest questions as simple as ‘what is the quantum yield of these materials?’ does not have a straightforward answer. In the literature, the carrier dynamics in these samples are usually fitted either by ABC model or by the Shockley–Read–Hall (SRH) model. However, Scheblykin and coworkers showed that both the ABC and SRH models fail to predict the photoluminescence quantum yield (PLQY) maps as a function of LASER repetition rates and LASER excitation power over a broad range of repetition rates. Particularly these models fail at high average excitation power density, due to their inability to explain the decrease in PLQY at such excitation power densities. Neither ABC nor SRH model is capable of satisfactorily fitting the PLQY maps and decay kinetics together. They suggested that the inclusion of Auger-assisted electron trapping and Auger recombination in the SRH model can explain the PLQY maps with better accuracy, however, this model still fails to fit/explain the PL transients and PLQY maps together. In this thesis, the growth of the solution-processed perovskite films, their degradation, and the carrier dynamics in these materials at nanosecond regime is discussed. The investigation of growth mechanism, degradation, and carrier dynamics in halide perovskites is carried out in this dissertation and grouped in the 5 chapters.