Metal iodide thin films have gained a lot of attention during the recent decade. Although their applications cover a variety of technological fields, majority of the research is motivated by photovoltaics. Halide perovskites are the most studied metal iodides for photovoltaics. Halide perovskite solar cells are made from abundant and low-cost materials, yet they show high solar conversion efficiencies. Unfortunately, some of the halide perovskites show poor chemical and thermal stability. In addition, the presence of lead in the best-performing materials causes concern. These issues have recently drawn attention to other types of materials including Ag₂BiI₅ and Cs₃Bi₂I₉, for example. Halide perovskites are expected to find their main application in tandem solar cells with silicon, and the same may be true also for Ag₂BiI₅, Cs₃Bi₂I₉ and related materials.

Large-scale applications of halide perovskite and other metal iodide thin films require scalable and well-controllable deposition methods. The currently used methods are simple and low-cost but are difficult to scale up for industrial mass production of solar cells. Atomic layer deposition (ALD) is well known for its unique controllability and excellent scalability and has therefore a lot to give also in the field of metal iodide films. We have developed, as the first team in the world, ALD processes for various metal iodides. We started by developing processes for the binary iodides PbI₂ [1], CsI [2], and SnI₂ [3]. All these processes use metal silylamides as the metal precursors and SnI₄ as the iodine precursor. The binary processes can be combined to make more complex materials: so far we have made the inorganic halide perovskites CsPbI₃ [2] and CsSnI₃ [3] by combining CsI with PbI₂ and SnI₂, respectively. CH₃NH₃PbI₃ can be prepared as well by exposing PbI₂ to CH₃NH₃I vapor [1]. We recently designed a new iodine source that produces anhydrous HI vapor on-site and overcomes thus the limitations of SnI₄ such as high cost and tin contamination in the deposited films. We have demonstrated the feasibility of the source by depositing CsI.

Our most recent efforts are directed towards Ag₂BiI₅, Cs₃Bi₂I₉ and related materials. As the first steps, we have developed ALD processes for the silver halides AgCl, AgBr and AgI. We are currently working on an ALD process for bismuth iodide BiI₃. Also, the first experiments aiming to combine AgI and BiI₃ to ternary iodides are underway.