Today, hybrid halide perovskite solar cells show high power conversion efficiencies (PCE >25%), but volatile organic cations (MA⁺, FA⁺) can limit their thermal stability. Therefore, inorganic perovskites such as CsPbI₃ appear as promising materials for perovskite PV devices ^[1]. In addition, their band gap, around 1.7 eV, is close to the ideal one for perovskite-on-silicon tandem applications. The inorganic perovskites are considered as good candidates for the development of stable and efficient single junction and tandem solar cells.

Despite many obvious advantages, the PCEs achieved with inorganic CsPbI₃ perovskites remain lower than those of their hybrid counterparts. The difference is mainly due to the difficulty of synthesising a stable CsPbI₃ perovskite phase. According to Goldschmidt tolerance factor calculations, CsPbI₃ is just at the limit of forming a stable perovskite phase due to the relatively small size of Cs⁺ cations compared to MA⁺ or FA⁺ cations. Therefore, the simultaneous formation of non-perovskite phases is often observed during the synthesis of CsPbI₃.

Here, we present our strategy to develop inorganic perovskite as an absorber, deposited by slot-die coating in ambient atmosphere, a relevant industrial large area deposition technique that has not yet been explored for this type of perovskite.

Firstly, we focused our attention on the ink preparation to obtain stable inorganic perovskite films. After investigating several additive strategies, dimethylammonium iodide (DMAI) ^[2] was chosen to stabilise CsPbI₃. Several parameters of the precursor solution such as the additive concentration and solvent ratio were further optimised. Furthermore, the composition was tuned by adding bromide ^[3]. Secondly, using this ink, the deposition of the inorganic perovskite thin film by slot-die coating, in ambient atmosphere, was optimized. Deposition parameters were refined based on morphological, structural, and optical characterizations of the deposited inorganic perovskite layers. Uniform thin films with a surface area of up to 5x10cm² were obtained (Fig.1).

Finally, to test our developed thin films, we implemented the optimised inorganic perovskite absorber in solar cells. A PCE of the same order of magnitude as the PCE obtained for solar cells deposited by spin-coating in ambient atmosphere on a small surface area of 2.5x2.5cm² was obtained on 5x10cm² (Fig.2). After further optimisations, we obtained a maximum power conversion efficiency of 12.7% for solar cells containing inorganic perovskite deposited by slot die coating in ambient atmosphere.