Probing the stability of halide perovskite films using electron spectroscopy techniques.

Abstract

Halide perovskites have attracted considerable attention from the photovoltaic research community due to the rapid advancements in reported device efficiencies. This attention is also fuelled by their easy solution processability compared to the extensive and costly crystallisation process required for traditional silicon solar cells. However, the structural instability of perovskites, particularly under high-temperature and high-humidity conditions, hampers their potential for commercialisation. This thesis attempts to explore the underlying mechanisms responsible for this instability and aims to address it through modification of the perovskite with additives.

The response of two halide perovskites, CH3NH3PbI3 and CH3NH3PbBr3, to high-temperature exposure is initially discussed utilising X-ray Photoelectron Spectroscopy (XPS). The primary objective is to understand the role of halide ions in the structural stability of perovskites. Analysis reveals that CH3NH3PbBr3 exhibits greater resistance to heating compared to CH3NH3PbI3, and a proposed degradation mechanism explains this difference in thermal stabilities. The incorporation of ionic liquids (ILs) BMIMBF4 and BMIMCl into a mixed-halide perovskite is investigated, employing a range of analytical techniques such as XPS, work function measurements, Near Edge X-ray Absorption Fine Structure (NEXAFS), Angle-resolved Hard X-ray Photoelectron Spectroscopy (Ar-HAXPES), and Near-Ambient Pressure XPS (NAP-XPS) to study the thermal and moisture resistance of the films. The role of the incorporated ILs, as well as the addition of a SnO2 electron transport layer (ETL), is investigated. Results indicate that the combined effect of ILs and SnO2 enhances the stability of the perovskite against heat and moisture. Finally, an investigation into perovskite modification using the two zwitterionic methylamines, TMAO and betaine, utilising XPS, Ar-HAXPES, and NAP-XPS is presented. The study demonstrates that TMAO-modified samples exhibit notable heat tolerance, while betaine-modified samples showcase superior moisture endurance.

Overall, this thesis provides new insights into the intrinsic mechanisms governing the degradation of perovskites and the role of additives in stabilising perovskite materials. These results could inform the future design of perovskite solar cells.