Passivation of vapour deposited metal halide perovskites for high performance photovoltaic devices

Thesis

Metal-halide perovskites have exhibited excellent optoelectronic properties and are widely used as the photoactive layers in photovoltaics. While predominantly a research innovation which showed significant promise on a lab-scale, as of 2024, perovskite solar cells are moving towards large-scale commercialisation. While vapour phase deposition processes dominate the established thin-film manufacturing, perovskite solar cells are still predominantly developed using solution-based methods. To enable scalable fabrication and industry adoption, it is imperative to advance the understanding of metal-halide perovskite thin-films fabricated by the vapour deposition technique. This thesis advances the understanding of vapourdeposited perovskite thin films and establishes several key strategies to reduce bulk and interfacial defect densities, thereby enhancing the performance and stability of vapour-deposited devices.

A major advance presented in this work is the elucidation of how organic impurities in commercial formamidinium iodide precursors influence film formation in both solution- and vapour-deposited systems. It is demonstrated that, while such impurities can beneficially passivate defects in solution-processed films, they induce altered sublimation behaviour in vapour deposition, leading to off-stoichiometric compositions and the formation of non-photoactive polytype phases. These findings establish impurity control as a critical requirement for achieving phase-pure, highquality perovskite layers by vapour deposition.

A second key contribution is the introduction of a templating strategy that decouples perovskite nucleation from the underlying charge-transport substrate. This approach enables consistent film morphology, crystallographic orientation, and optoelectronic quality across diverse substrates. By providing a means to tune initial perovskite stoichiometry and suppress unwanted interfacial reactions, this templating method offers a scalable route toward reproducible, high-performance vapour-deposited perovskite devices.

Furthermore, this thesis demonstrates the use of aromatic ammonium halides as vapour-deposited passivation agents and reveals that their structural and functional properties depend strongly on the deposition method and post-deposition treatment. The work establishes that measurement atmosphere and thermal annealing can drive surface reconstruction and modulate passivation efficiency, thereby highlighting new levers for interface engineering in vapour-deposited perovskites.

Collectively, this research provides a comprehensive understanding of impurity effects, interfacial templating, and molecular passivation in vapour-deposited perovskite solar cells. These insights pave the way toward scalable, stable, and highefficiency perovskite photovoltaics compatible with industrial thin-film manufacturing.

Authors: Yan, S

• DOI: 10.5287/ora-7earengb2
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: photovoltaics; semiconductors; perovskite; Condensed Matter Physics; Passivation
• Subjects: Physics