(Invited) Achieving Stable Black-CsPbI3 Thin-Films for Opto-Electronic Applications
Authors: Saha Rafikul Ali , Degutis Giedrius , Steele Julian , Roeffaers Maarten 
Published: February 2024

Halide perovskite materials, particularly CsPbI3, have emerged as promising candidates for optoelectronic applications, including solar cells, due to their favorable bandgap of ~1.7 eV for tandem solar cells with silicon-based counterparts. However, their widespread adoption is hindered by their instability and reactivity under ambient conditions, especially to moisture and oxygen. The black phase CsPbI3 polymorphs (α, β, and γ) are only stable at elevated temperatures, and at room temperature (RT), they transform to a yellow, nonperovskite phase (δ) with a wider bandgap of ~2.8 eV, leading to a loss of device performance. To address this challenge, various approaches have been explored to increase the stability of the black phase of CsPbI3 thin films under ambient conditions.[1]
One approach is based on interface engineering to induce crystal strain, which increases the stabilization time of the black phase. Thin films of CsPbI3 are prepared on substrates with different thermal expansion coefficients, leading to interfacial strain when quenched from high temperatures (> 320 °C) to RT. This interfacial strain increases the stability of the black phase from a few seconds in bulk polycrystalline samples to a few minutes in thin films, as shown in Fig. 1(a).[2] Additionally, black phase stabilization has been enhanced to a few hours by introducing minimal amounts (< 5%) of dopant elements in the Pb site of CsPbI3, which modifies the texture and generates microstrain in the polycrystalline perovskite films (see Fig. 1(b)).[3] The dopants locally disrupt the normal bonding arrangement and manipulate strains/distortions, and structural relaxation around the dopants is mediated by dynamic disorder and the relatively soft elastic constants of halide perovskite, resulting in the stabilization of the required crystallographic phase.
Furthermore, annealing of CsPbI3 thin films in the presence of oxygen has been explored as another approach to increase the stability of the black phase (stable for a few days). Heating the films under oxygen leads to surface oxidation and the formation of Pb-O bonds, resulting in the gathering of Pb and O atoms and the formation of lead oxide. The replacement of I atoms in the inorganic frameworks by O atoms tends to form volatile iodine (I2), but PbI2 is typically detected as the dominant product of degradation during heating in a dry air environment (Fig. 1(c)).[4] The oxidation products on the surface could potentially serve as a protective layer to inhibit further oxidation.
Another highly effective approach to enhance the stability of the black phase CsPbI3 in perovskite thin films and devices is through the implementation of coarse photolithography to embed a PbI2-based interfacial microstructure.[5] The resulting films, adorned with a tessellating microgrid ((Fig. 1(d))), display remarkable resistance to moisture-induced decay and demonstrate prolonged stability of the black phase, persisting beyond 2.5 years even in dry environments. This remarkable stability can be attributed to the significant increase in the phase transition energy barrier and the effective limitation of potential yellow phase formation to structurally isolated domains within the microgrid.
In summary, our laboratory has effectively implemented various methods to enhance the durability of black phase CsPbI3 thin films when exposed to ambient conditions. These methods comprise inducing interfacial strain, introducing bulk strain through doping, embedding interfacial microstructures, and surface oxidation. These techniques have demonstrated encouraging outcomes in maintaining the black phase of CsPbI3 for extended durations, ranging from a few minutes to several months, and exhibit significant potential for the advancement of practical applications of CsPbI3-based perovskite solar cells. Further exploration and advancement in this direction are anticipated to contribute to achieving long-lasting and efficient perovskite devices.
[1] Acc. Mater. Res., 2020, 1, 1, 3–15
[2] Science, 2019, 365, 679-684
[3] J. Am. Chem. Soc., 2021, 143, 28, 10500–10508
[4] unpublished results
[5] Nature Commun., 2022, 13, 7513.

Figure 1