Spectroscopy study of hot carrier cooling dynamics in halide perovskites

• Main Author: Lim, Melvin Jia Wei
• Other Authors: Sum Tze Chien
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Physics::Optics and light; Engineering::Materials::Photonics and optoelectronics materials
• Online Access: https://hdl.handle.net/10356/163272
• Institution: Nanyang Technological University

Hot carrier solar cells (HCSCs) hold one of the greatest promises to break the fundamental limit behind the power conversion efficiencies (PCE) of single-junction solar cells. A theoretical limit known as the Shockley-Queisser limit places a barrier of about 33 % PCE for a single-junction solar cell. A class of materials known as halide perovskites (HPs) have recently emerged as frontrunner materials for several optoelectronic applications such as solar cells and light-emitting diodes. PCEs of perovskite solar cells (PSCs) have seen unprecedented growth in the past decade but recent increments have decelerated considerably as the PCEs creep towards the fundamental limit. However, the slow hot carrier cooling (HCC) properties reported in HPs sparked renewed interest towards the possibility in developing a perovskite HCSC, as such slow HCC properties are one of the prerequisites for potential HC absorber materials. This thesis aims to advance the understanding of the HCC dynamics in HPs using various optical spectroscopy methods. One other aim of this thesis is proposing methods for systematic determination of HC metrics that will aid the development of HCSCs. The first part of this thesis re-establishes the reported HCC dynamics from ultrafast transient absorption (TA) spectroscopy. We show that the well-established method of extracting the carrier temperature, which is one of the most important HC metrics, has fundamental flaws and compromises comparability of values between studies. We evaluate the underlying assumptions behind this method and found them to be principally inappropriate. We propose an alternative full-spectrum fit approach to reproduce the TA spectrum of HPs and show that the extracted HC metrics reproduce the expected physics behind HCC of HPs. Our full-spectrum fit method that is systematic and free from ambiguities can describe compositions of HPs well. In the next part, we study the HCC dynamics from the photoluminescence (PL) spectra of HPs. PL-based techniques that are less commonly employed, can also be used to monitor the HCC dynamics of HPs. The procedure for extracting relevant HC metrics from the PL spectra of HPs is largely like that adopted for the TA spectra. Motivated by the ambiguities of the standard fitting method, we sought to develop a full-spectrum fitting method for the complementary PL-based techniques. A full-spectrum fitting procedure based on the generalized Planck's law for radiation from semiconductors is proposed for extracting the HC metrics. By adopting our approach, the PL spectra of several compositions of HPs were studies and we show that the slow HCC effects also manifest under steady-state conditions in the form of low thermalization coefficients. The results provide the connection between the widely reported ultrafast slow HCC phenomena and the steady state. An evaluation of the viability of halide perovskites for HCSCs operating under continuous illumination is also presented. One other approach to achieve slower HCC rates is through quantum confinement. Halide perovskite nanocrystals (HPNCs) possess a unique property in that the quantum confinement induced cooling bottleneck is retained in these materials compared to conventional group II-VI semiconductors like CdSe and PbSe. However there remains an unresolved controversy regarding the existence of such a bottleneck in size-dependent studies of HPNCs. The final part of the thesis examines the influence of quantum confinement effects on the HCC rates of HPNCs in a bid to resolve the existing controversy. Through a series of independent ultrafast spectroscopy techniques, we study the hot carrier cooling properties of weakly and strongly confined HPNCs to identify any quantum confinement induced changes. Our findings provide an overall picture on the evolution of quantum confinement effects on HPNCs. The findings in this thesis advance the understanding behind the HCC behaviour of HPs on the ultrafast and the steady state. The proposed procedures for determining relevant HC metrics in this thesis pushes towards their systematic and unambiguous determination, facilitating their comparability between studies. These findings are valuable in the direction towards the development of next-generation perovskite photovoltaic technologies.