Vacuum deposition is a solvent-free method suitable for growing thin films of metal-halide perovskites (MHPs) [1] and charge-transport layers. Vacuum-based methods offer a diverse array of advantages, including precise control of layer thickness, excellent uniformity and homogeneity of the formed thin film, choice over a wide range of materials and compositions, and the flexibility to grow multi-layer structures and larger-scale modules without the need to rely on complex choices of “orthogonal solvents”.  In particular, we have elucidated a four-source co-evaporation process of formamidinium-caesium (FACs)-based perovskite thin films. Excellent device performance of >19% power conversion efficiency was obtained in an “inverted” p-i-n solar cell architecture, owing to reduced defect density and improved charge-carrier lifetimes of such vapour-deposited perovskite intrinsic layer [2].

However, most reports of high-efficiency solar cells based on vacuum-deposited MHP films often employ solution-processed hole transport layers (HTLs) such as PTAA or Spiro-OMeTAD. Not only are these HTLs relatively expensive, and the additional solution-processing step complicates the overall fabrication process, but also more critically, materials such as Spiro-OMeTAD are prone to degradation under different environmental stressors of temperature and humidity, and thus curtailing the operational lifetime of these devices.

We investigated organometallic copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc), deposited via the thermal-evaporation method, as alternative, low cost, and durable HTLs in all-vacuum-deposited solvent-free [CH(NH₂)₂]_0.83Cs_0.17PbI₃ (FACsPbI₃) perovskite solar cells. We reveal that the CuPc HTL demonstrated improved compatibility in a p-i-n photovoltaic device, in comparison with ZnPc. Furthermore, we thoroughly examined the long-term stability of these all-vacuum-processed devices under a range of testing conditions. Importantly, unencapsulated devices as large as 1 cm² exhibited outstanding thermal durability, demonstrating no observable degradation in efficiency after more than 5000 hours in storage and 3700 hours under 85 °C heat-stressing in N₂ atmosphere [3].

In addition, we uncover the striking differences in the sticking, adhesion, and nucleation dynamics of the organic perovskite precursor, formamidinium iodide (FAI), to various HTLs. We highlight the impact of varying sticking characteristics to the stoichiometry of the formed perovskite thin films from co-evaporation. We believe that this finding epitomises the importance of optimising growth parameters specific to individual charge transport layer if FAI is to be used as a precursor in any co-evaporated perovskite [3].