Coherent energy and charge carrier dynamics In quasi-2D lead halide perovskites

• Main Author: Ramesh, Sankaran
• Other Authors: Sum Tze Chien
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Science::Physics::Optics and light; Engineering::Materials::Photonics and optoelectronics materials
• Online Access: https://hdl.handle.net/10356/167902
• Institution: Nanyang Technological University

Halide Perovskites are versatile materials for photovoltaic and light emission applications thanks to their outstanding optical properties. While the three-dimensional halide perovskites evolved to become the archetypal system over the past decade, their two-dimensional (2D) and quasi-2D analogues have prompted interest recently, thanks to novel physics originating from their multiple quantum well structure. The ultrafast energy transfer or funnelling from high- to low-bandgap layers is a widely studied phenomenon in quasi-2D perovskites. Controlling funnelling by modulating the perovskite composition has been shown to enhance film properties and produce efficient light emitters and stable solar cells. However, the origin and mechanism behind the funnelling remains nebulous, with multiple reports assigning the process to F¨orster resonance energy transfer and electron/hole transfer with varying timescales. In this thesis, we investigate the physics of ultrafast funnelling in quasi-2D perovskites and devise new methods of manipulating the funnelling via material design. We examine in detail the role of the organic layer, which has hitherto been considered to play a passive role in the electronic properties of these materials. In Chapter 3, we theoretically model the archetypal Ruddlesden-Popper (RPP) class of quasi-2D perovskites as a superlattice composed of inorganic and organic layers as quantum wells (QW) and barriers. We discover that exciton delocalization across the organic barrier can lead to a coherent energy transfer in sub-ps timescales. We corroborate our model experimentally using pump-probe spectroscopy, explicating the role of organic barrier in tuning the inter-QW coupling and funnelling. We also uncover for the first time a coherent back-transfer of excitons in RPP. We extend our findings in Chapter 4 to demonstrate applications to materials design and devices based on RPP. First, we examine the strategy of mixing two spacer cations in the barrier layer. Via modelling, we prove that the cations are alloyed in the barrier layer rather than forming separate phases having only a single spacer. We also show efficient energy funnelling in the alloyed samples, implying an enhanced inter-QW coupling. Films made with this approach produced efficient, colour-tunable red light-emitting diodes (LEDs). Secondly, we tailor the funnelling in RPP to achieve slower excess energy loss times upto 40 ps. Through pump-probe spectroscopy of the RPP thin films interfaced with an electron extraction layer, we show that the slowly relaxing carriers are directly extracted from the low-n phases, preserving their excess energy. The extraction efficiency depends on the organic spacer. We propose this concept to realize solar cells that have reduced open-circuit voltage losses via the preservation of excess energy. In Chapter 5, Using two-dimensional electronic spectroscopy (2DES), we probe coherent coupling among the QWs that mediates the energy transfer. We uncover a delocalization of excitation among the layered phases that is governed by choice of the organic cation. The results help to rationalize the organic spacer as the crucial factor that tips the balance in the contest between funnelling and extraction. We also develop a versatile 2DES setup based on the pump-probe geometry using a pulse-shaping module powered by a hollow-core fiber compressor(HCFC). This setup presents an alternative to the Box-CARS geometry and does not require complex phase-matching schemes to detect the absorption signal. We also show the capability of this setup to reveal short-lived vibronic coherences in Nile Blue dye originating from its ring structure. For commissioning this setup for 2DES measurements on perovskites, we also perform preliminary measurements to reveal the early-time spectrally correlated response characteristic of excitation delocalization in RPP. This work reveals the role of the organic spacer in tuning the inter-QW coupling in quasi-2D perovskite systems. We uncover the physics of the ultrafast funnelling, building on which we develop applications of this concept to optoelectronic devices and the development of advanced spectroscopic probes that can directly reveal ultrafast coherent optical dynamics. By achieving a deeper understanding of the photophysics of quasi-2D perovskites and by using the organic barrier as an additional lever to tune its properties, greatly expands the scope for application of layered perovskites.