Because of the excellent optoelectronic properties of hybrid organic–inorganic metal halide perovskites, [1-3] the photovoltaic field has undergone a rapid progress in the last decade. Perovskites were first introduced in the field as sensitizers in dye-sensitized solar cells with promising results but very poor stability because of the dissolution of perovskites in liquid electrolytes.[4] Stability and efficiency were dramatically improved by introducing a solid-state hole conductor and a mesoporous TiO₂/perovskite layer.[5] Currently, the certified power conversion efficiency of perovskite solar cells (25.2%)[6] is comparable to the photovoltaic performance of other thin-film photovoltaic technologies (Si, CdTe, and GaAs). Nevertheless, in spite of this progress, the poor-term stability of photovoltaic perovskites complicates their commercialization. Degradation processes in these devices are not only related to the intrinsic properties that determine the thermal and/or electrical stability (device configuration and perovskite composition)[7] but also strongly affected by environmental factors (moisture, light, oxygen, and temperature).[8-9] Specifically, under moisture exposure, as a consequence of, mainly, the reaction between water molecules and methylammonium cations (CH₃NH₃⁺), which acts as a Brønsted/Lewis base, the perovskites tend to be hydrolyzed back to the precursors giving rise to morphological and crystal structural changes, optical absorption decay, and the deterioration of the electronic properties that determine the photovoltaic performance of perovskite solar cells. [8-11]

Different strategies have been employed to prevent degradation and improve the device stability because of the sensitivity of perovskite materials toward ambient moisture. Many of them imply the modification of the perovskite composition by the insertion of ions to achieve a more stable crystal structure,[7] the employment of buffer layers between perovskite films and electron- or hole-selective layers,[12] or even the substitution of the spiro-OMeTAD layer by other more hydrophobic hole-selective materials.[13] On the other hand, different materials have also been employed to encapsulate complete perovskite solar devices and avoid moisture exposure, such as hydrophobic polymers,[14] atomic layer-deposited Al₂O₃ films,[15] or even using sealing glass as a barrier layer.[16] Although successful, many of these approaches involve expensive and complex deposition processes and even restrict the photovoltaic performance of perovskite solar devices.

Here,[17] we investigate a simple solvent-free polymer encapsulation method for perovskite solar cells using a conformal plasma polymer thin film. This organic layer is formed by the remote plasma-assisted vacuum deposition of the solid precursor adamantane (ADA). The synthesis is carried out at room temperature and under low-power plasma activation to avoid energetic species or UV radiation of the plasma to reach the substrate surface. This methodology has been successfully applied in recent articles for the fabrication of photonic films based on organic dyes and small functional molecules working as optical sensors, optical filters, tunable photoluminescence emitters, and lasing gain media.[18-20] This deposition process is compatible with opto and microelectronic components and can be scaled to large deposition areas and to wafer-scale fabrication.[21-22] To our best knowledge, this is the first report where the deposition of an organic plasma nanocomposite thin film is employed to encapsulate perovskite solar cells. The ADA precursor molecules (C₁₀H₁₆) consist of a single C–C bond with four connected cyclohexane rings arranged in the armchair configuration. This material is very effectively plasma-polymerized under remote conditions allowing the deposition of homogenous and compact ADA thin films characterized for being insoluble in water and thermally stable up to 250 °C. Additionally, these deposited films show a high transmittance (≈90%) in the low-energy region of the visible spectrum (λ > 300 nm).