Study on photo-induced carrier dynamics of perovskite materials using ultrafast transient absorption spectroscopy

Abstract

Since the first introduction of perovskite semiconducting materials to solar cells, perovskite materials have emerged as promising candidates for alternative energy conversion due to their inherent advantages, including low processing costs, rapid manufacturing, simplicity of processing, lightweight properties and high power conversion efficiency. However, perovskite materials face various challenges, particularly regarding stability. Perovskite materials are susceptible to degradation when exposed to moisture and oxygen, which can lead to a decline in their performance over time. For these reasons, this dissertation focuses on understanding the photo-induced charge carrier dynamics of perovskite materials with degradation using ultrafast time-resolved spectroscopy. The performance of perovskite solar cells (PSCs) hinges on the generation, recombination, and separation of photo-generated carriers. In essence, the key mechanisms revolve around the movement of carriers within perovskite materials. To gain insights into these critical mechanisms, a femtosecond transient absorption (TA) spectrometer emerges as an indispensable tool. Chapter 2 presents the fundamental principles of time-resolved spectroscopy and provides a comprehensive exploration of the experimental setup employed for femtosecond TA spectroscopy through a pump-probe setup. The chapter encompasses the origin of TA signals and the methodologies for the collection of TA data, including the calculation of excitation density and fluence, essential for investigating excitation-dependent carrier dynamics. In Chapter 3, the focus shifts to the ultrafast photo-induced carrier dynamics of mixed perovskite films, specifically formamidinium lead triiodide (FAPbI3) and methylammonium lead tribromide (MAPbBr3). The chapter delves into the influence of solution additive methods and hole transport materials (HTMs) on the photo-induced carrier dynamics. Additive methods mitigate the impact of trap sites, resulting in a slower rise in TA signals due to increased carrier density. This delay is attributed to the hot phonon bottleneck induced by the augmented carrier density. In contrast, the use of HTMs accelerates hole transport, leading to the rapid separation of photo-induced electron-hole pairs. These findings underscore the potential of optimizing photovoltaic performance by enhancing carrier dynamics with HTMs and increasing carrier population while mitigating trap effects through additive methods. Chapter 4 delves into a detailed analysis of the ultrafast photo-induced carrier dynamics within perovskite films subjected to degradation from exposure to atmospheric conditions. Specifically, perovskite films composed of Cs0.05(FAPbI3)0.85(MAPbBr3)0.15 are exposed to atmospheric conditions throughout experiments, inducing degradation. The study introduces three decay modes—carrier trapping, carrier extraction, and carrier recombination—to explain the changes in carrier dynamics with prolonged exposure to atmospheric conditions. As the perovskite films degrade due to atmospheric exposure, the decay rates of carrier trapping and carrier extraction decrease. With increasing exposure time, the carrier density of carrier extraction declines, while the carrier density of carrier recombination increases. Importantly, it is noted that increased carrier density within perovskite films counteracts the decreased decay rate of the carrier extraction mode, suggesting that heightened carrier density accelerates both carrier trapping and carrier extraction. The research also reveals that carrier extraction predominantly involves bi-molecular processes, while carrier trapping is dominated by tri-molecular processes. These findings illustrate that atmospheric exposure leads to a deceleration and reduction in the carrier extraction mode, adversely affecting the optical characteristics and light-harvesting properties of perovskite films. Chapter 5 meticulously investigates the ultrafast photo-induced carrier dynamics within perovskite quantum dots (PQDs). The core-shell CsPb(Br0.25I0.75)3@SiO2 PQDs solution is exposed to a polar solvent BuOH, resulting in structural degradation. Three decay modes—carrier trapping, radiative carrier recombination, and trap-assisted non-radiative recombination—are introduced to analyze the carrier dynamics of PQDs. Exposure to BuOH leads to the migration of I- within PQDs, resulting in the formation of trap states due to defects within the perovskite crystal lattice. These trap states increase the number of carriers trapped, subsequently reducing carriers undergoing radiative carrier recombination. Furthermore, BuOH exposure accelerates both carrier trapping and radiative carrier recombination. Given that the light-emitting property of PQDs primarily depends on radiative carrier recombination, these changes hinder carriers from remaining in excited states, leading to a decline in the light-emitting property of PQDs. However, it's noteworthy that increasing the excitation fluence can mitigate the impact of carrier trapping and enhance radiative carrier recombination, because the trap states become occupied by the increased carrier density. This dissertation significantly enhances our understanding of perovskite materials, the impact of degradation on their light-harvesting and light-emitting characteristics. The findings contribute to advancing the field of perovskite photovoltaics and offer insights into enhancing the structural stability of PSCs and PQDs.