Perovskite solar cells have attracted enormous interest since their discovery only a few years ago because they are able to combine the benefits of high efficiency and remarkable ease of processing over large areas. Whereas most of research has been carried out on glass, perovskite deposition and synthesis is carried out at low temperatures (<150 °C) to convert precursors into its final semiconducting form. Thus, developing the technology on flexible substrates can be considered a suitable and exciting arena both from the manufacturing view point (e.g. web processing, low embodied energy manufacturing) and that of the applications (e.g. flexible, lightweight, portable, easy to integrate over both small, large and curved surfaces). Research has been accelerating on flexible PSCs and has achieved notable milestones including PCEs of 15.6% on laboratory cells, the first modules being manufactured, ultralight cells with record power per gram ratios, and even cells made on fibres. Reviewing the literature, it becomes apparent that more work can be carried out in closing the efficiency gap with glass based counterparts especially at the large-area module level and, in particular, investigating and improving the lifetime of these devices which are built on inherently permeable plastic films. Here we review and provide a perspective on the issues pertaining progress in materials, processes, devices, industrialization and costs of flexible perovskite solar cells.

Atomic layer deposition is widely acknowledged as a powerful technique for the deposition of high quality layers for several applications including photovoltaics (PV). The capability of ALD to generate dense, conformal, virtually pinhole-free layers becomes attractive also for the emerging organo-metal halide perovskite solar cells (PSCs), which have garnered the interest of the PV community through their remarkable efficiency gains, now over 20%, in just a few years of research. Until now, the application of ALD layers in PSCs has almost exclusively been restricted to the stages of device fabrication prior to perovskite deposition. Researchers have mainly focused on fabricating efficient electron and hole transport layers (TiO₂, SnO₂, ZnO, NiO) and ultra-thin Al₂O₃ or TiO₂ passivation layers for several device configurations. The first section of this contribution reviews the current state-of-the-art ALD for perovskite solar cells. Then, we explore other potential opportunities, such as the fabrication of doped metal oxide selective contacts and transparent electrodes, also for use in tandem solar cell architectures, as well as barrier layers for encapsulation. Finally, we present our own experimental investigation of the challenges involved in depositing directly on perovskite absorbers in view of replacing organic electron and hole transport layers with ALD metal oxides (MOs). Therefore, the effects of temperature, oxidizing agents and metal precursors on perovskite are studied. A number of insights are gained which can lead to the development of ad hoc ALD processes that are compatible with the underlying perovskite, in this case, methylammonium lead iodide, MAPbI₃. The phase purity and surface chemistry of the perovskite were used as metrics to quantify the feasibility of depositing selected MOs which can be adopted as selective contacts and passivation layers.

The power conversion efficiency of NiO based perovskite solar cells has recently hit a record 22.1%. Here, the main advances are reviewed and the role of NiO in the next breakthroughs is discussed.