Elucidating the role of templates on the stability of perovskites using X-ray photoelectron spectroscopy

Abstract

Perovskite solar cells (PSCs) based on organometal halides have gained widespread attention due to their high efficiency and cost-effective manufacturing. However, their commercial viability is significantly limited by poor stability under environmental stressors such as heat and moisture. This thesis investigates how different template additives can be used to stabilise perovskites, with particular emphasis on their role in preventing degradation, as revealed through advanced X-ray photoelectron spectroscopy (XPS) techniques. Chapter 4 investigates the self-healing mechanism in hydrogel-lead halide perovskite (PVK) composites for photovoltaic applications. Poly(2-hydroxyethyl methacrylate) (pHEMA) is added using a two-step method to enhance PVK thermal and moisture stability. Devices with pHEMA-PVK composites achieve a 17.8% power conversion efficiency (PCE) and retain 95.4% of initial efficiency after 1500 hours at 35% relative humidity (RH), outperforming pure PVK. XPS shows a ~3 nm pHEMA surface layer, decreasing with temperature. Nitrogen-containing species trapped in this layer under high humidity are reincorporated as humidity decreases, confirming pHEMA’s role in the self-healing properties. Chapter 5 focuses on water-resistant, tunable perovskite absorbers achieved by incorporating peptide hydrogel templates in one-step method. The peptide templates stabilise perovskites by passivating defect sites and encapsulating crystallites, leading to improved optical and structural properties. In situ XPS and near-ambient pressure XPS (NAP-XPS) results show that peptide hydrogels mitigate thermally induced degradation and reduce ion migration under heating and moisture stress. Devices with peptide hydrogels exhibit enhanced performance and maintain 81% of their initial efficiency after 480 hours at 35% RH. Chapter 6 examines the use of Poly(4-styrenesulfonic acid-co-maleic acid) sodium salt (PSSMA) as a polymer template additive to enhance perovskite stability and performance. In situ HT-XPS and NAP-XPS reveal reduced degradation in PSSMA-doped films, with more stable I/Pb2+ and N/Pb2+ ratios and less nitrogen loss under thermal stress. PSSMA-treated films retain organic components, reducing defects under thermal and humidity stress. Devices incorporating PSSMA achieve higher PCE and retain 89.1% efficiency after 500 hours of humidity exposure, demonstrating improved resilience. Chapter 7 explores the role of poly(4-vinylpyridine) (P4VP) as an ultra-thin hydrophobic interlayer to improve the stability of PSCs. HT-XPS and NAP-XPS analyses reveal severe degradation in perovskite films without P4VP, including shifts in Pb 4f binding energy and metallic lead (Pb0) formation. P4VP-modified films show enhanced stability, with minimal binding energy shifts and reduced Pb0 formation, suggesting P4VP interacts with Pb2+ ions. Devices with P4VP retain 89.8% of their efficiency after 500 hours in humid conditions, compared to rapid degradation in unmodified devices. This thesis provides valuable insights into the design of more stable perovskite materials with stabilising templates for photovoltaic applications, offering promising strategies to overcome current limitations in PSCs stability.

Date of Award	7 Jan 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andrew Thomas (Main Supervisor) & Stephen Edmondson (Co Supervisor)