Atomic layer deposition for perovskite solar cells:research status, opportunities and challenges

Atomic layer deposition is widely acknowledged as a powerful technique for the deposition of high qualitylayers 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 halideperovskite solar cells (PSCs), which have garnered the interest of the PV community through theirremarkable efficiency gains, now over 20%, in just a few years of research. Until now, the application ofALD layers in PSCs has almost exclusively been restricted to the stages of device fabrication prior toperovskite deposition. Researchers have mainly focused on fabricating efficient electron and holetransport layers (TiO2, SnO2, ZnO, NiO) and ultra-thin Al2O3or TiO2passivation layers for several deviceconfigurations. Thefirst section of this contribution reviews the current state-of-the-art ALD forperovskite solar cells. Then, we explore other potential opportunities, such as the fabrication of dopedmetal 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 thechallenges involved in depositing directly on perovskite absorbers in view of replacing organic electronand hole transport layers with ALD metal oxides (MOs). Therefore, the effects of temperature, oxidizingagents and metal precursors on perovskite are studied. A number of insights are gained which can leadto the development ofad hocALD processes that are compatible with the underlying perovskite, in thiscase, methylammonium lead iodide, MAPbI3. The phase purity and surface chemistry of the perovskitewere used as metrics to quantify the feasibility of depositing selected MOs which can be adopted asselective contacts and passivation layers.