Atomic Layer Deposition of CsI and CsPbI₃

Abstract

Cesium iodide (CsI) is a well-established scintillator material that also serves as a precursor for all-inorganic halide perovskite solar absorbers, such as CsPbI 3 . However, the lack of conformal and scalable methods to deposit halide perovskite thin films remains a major challenge on their way to commercialization. In this work, we employ atomic layer deposition (ALD) as the key method due to its inherent scalability to large areas and complex-shaped surfaces. We demonstrate two new ALD processes for the deposition of CsI and CsPbI 3 thin films. The CsI process relies on cesium bis(trimethylsilyl) amide (Cs(btsa)) and tin(IV) iodide (SnI 4 ) as precursors and yields high-purity, uniform, and phase-pure thin films. This process works in a wide temperature range (140−350 °C) and exhibits a large growth per cycle value (GPC) of 3.3 Å (85% of a CsI monolayer). Furthermore, we convert CsI into CsPbI 3 perovskite by exposing a CsI film to our earlier PbI 2 ALD process. We demonstrate the deposition of phase-pure γor δ-CsPbI 3 perovskite thin films, depending on the applied deposition temperature and number of PbI 2 cycles. We believe that the ALD-based approach described in this work will offer a viable alternative for depositing perovskite thin films in applications that involve complex high aspect ratio structures or large substrate areas.

Atomic layer deposition of PbCl2, PbBr2 and mixed lead halide (Cl, Br, I) PbXnY2−n thin films

“…An 81 nm thick film deposited at 75 °C, had an 11 nm surface roughness ( R q ) which is 13% of the film thickness and is in the range of what we typically observe for crystalline halide films made via ALD. 10,11…”

“…Such roughness is typical for crystalline metal halide films deposited with ALD. 10,11 As the deposition temperature increases the relative roughness starts to increase exponentially, because the GPC decreases, causing films deposited with the same number of cycles to become thinner and discontinuous (Fig. 3e).…”

“…9 In the halide perovskite case, high aspect ratio structures either enable key functionality of the device or enhance performance. 5 Previously we have developed ALD processes for PbI 2 10 and CsI 11 based on metal bis(trimethylsilyl)amides (btsa) and tin tetraiodide. In these processes a ligand exchange reaction between the metal (btsa) and SnI 4 yields the desired metal iodide and volatile heteroleptic tin by-products (Sn(btsa) x I 4−x ).…”

“…The PbI 2 and CsI processes also allowed us to deposit thin films of CH 3 NH 3 PbI 3 (MAPbI 3 ) 10 and CsPbI 3 . 11 However, most halide perovskite applications rely on mixed halide perovskites i.e. , partial substitution of ions in the perovskite composition, for example substitution of iodine with bromine or chlorine in MAPbI 3 .…”

“…, partial substitution of ions in the perovskite composition, for example substitution of iodine with bromine or chlorine in MAPbI 3 . 5–8,12 Currently (June 2022), to our knowledge, our iodide ALD processes, 10,11 the work of Natarajan et al on copper chloride 13 as well as the recent work of Vagott et al on PbI 2 14 constitute the entirety of existing literature on direct ALD of metal chlorides and iodides. 15 No metal bromide ALD processes have been reported.…”