Photovoltaic systems based on metal halide perovskites (MHPs) represent one of the most promising routes to overcoming the performance limits of current silicon cells, particularly through their integration within silicon/perovskite tandem structures. However, MHP materials suffer from significant stability limitations of both intrinsic and extrinsic origin. Among the latter, moisture is one of the most critical degradation agents for the majority of MHP systems. Yet the standard encapsulation solutions developed for silicon technologies (typically polymer coatings topped with glass covers) prove unsuitable for durably protecting these devices: the low permeation resistance of the polymer allows excessive lateral moisture diffusion. In this context, encapsulation approaches based on the deposition of nanolaminate thin films have emerged, offering high ultra-barrier properties (water vapour transmission rates (WVTR) on the order of 10⁻⁴ to 10⁻⁶ g·m⁻²·d⁻¹) while remaining transparent and potentially flexible depending on the materials used.

This doctoral thesis aims to establish selection criteria for flexible barrier materials deposited by temporal atomic layer deposition (ALD) and spatial atomic layer deposition (SALD), intended for the thin-film encapsulation of MHP-based photovoltaic systems. These methods offer the notable advantages of being deployable over large areas (m²) and of enabling deposition temperatures below 100 °C, thereby preventing thermal degradation of MHP systems. An in-depth study of several candidate materials, primarily metal oxides and organic-inorganic hybrid materials deposited by ALD-MLD (molecular layer deposition), is carried out in order to characterise their key physicochemical properties: mechanical properties, gas permeation resistance (helium, which is faster to measure and representative, and water vapour), and the presence of through-defects liable to compromise barrier performance.

A specific focus of this work concerns the study of through-defects such as pinholes within ALD thin films. A method for determining these defects by electrochemical means, adapted from the literature, enables sensitive and quantitative characterisation of the areal density of defects present in the barrier layers.

In addition, three-dimensional (3D) simulation models of gas permeation within nanolaminate multilayer architectures are proposed. Based on the finite element method, these models allow a more accurate and realistic prediction of gaseous species transport through the thin-film stacks than conventional one-dimensional (1D) analytical approaches, by accounting for the actual geometry of the defects and the multilayer architecture.

Finally, the highest-performing materials and architectures identified during this work are implemented to carry out thin-film encapsulation directly on MHP-based photovoltaic devices. The stability of the encapsulated devices is evaluated by means of accelerated ageing tests, making it possible to assess the real effectiveness of the developed encapsulation solutions under conditions representative of environmental operating constraints.

Taken together, this work contributes to the understanding of permeation mechanisms in multilayer encapsulation systems and proposes an integrated approach, from material selection through to device application, for the development of high-performance, transparent and flexible thin-film encapsulation adapted to the requirements of emerging perovskite-based photovoltaic technologies.

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