Stabilizing black-phase CsPbI3 under over 70% humidity

Tian Tian; Meifang Yang; Jianyu Yang; Wuqiang Wu; Liming Ding

doi:10.1088/1674-4926/43/3/030501

Recently, all-inorganic perovskites have attracted attention due to good thermal stability^[1-12]. Among them, CsPbI₃ has the most desirable optical bandgap (~1.7 eV) for applications in optoelectronic devices^[13-16]. In general, making black-phase CsPbI₃ film requires a high-temperature annealing up to 320 °C^{[17, 18]}, which inevitably raises energy consumption. Though being made at high temperature, the resulting black-phase (α or β phase) CsPbI₃ film still suffers from an undesirable phase transition under ambient conditions^{[19, 20]}. Several strategies have been developed to lower the annealing temperature (90–100 °C)^[20-26], it is still challenging to stabilize black-phase CsPbI₃ under ambient condition with high humidity and without a tedious annealing process. Herein, we developed a simple crystal redissolution (CR) strategy to make stable black-phase CsPbI₃ film in ambient air with high humidity and without post-annealing. 4-N,N-dimethylamino-4ʹ-Nʹ-methyl-stilbazolium tosylate (DAST) can chemically interact with CsPbI₃ to reduce the formation energy of black-phase and inhibit CsPbI₃ to undergo black-to-yellow phase transition.

Fig. 1(a) shows the CR approach. By using the perovskite precursor consisting of PbI₂, CsI and HI, a light-yellow film was obtained in ambient air, which is due to the existence of both yellow-phase δ-CsPbI₃ and PbI₂, as evidenced in XRD pattern (Fig. 1(b))^[22]. In contrast, by using CR-derived perovskite precursor (Fig. S1), a mirror-like black CsPbI₃ film was obtained even under 70% relative humidity, which uniformly covered the entire substrate (inset in Fig. 1(c)). Compared with the control sample (Fig. 1(b)), there is no PbI₂ signal (12.6°) in XRD pattern (Fig. 1(c))^[27], which is due to a more direct conversion and rapid self-assembly from CsPbI₃ crystals to CsPbI₃ film, rather than the complicated competition among Pb²⁺, Cs⁺, I^– ions and solvent molecules^{[27, 28]}. The diffraction peaks at 14.98° and 29.20° are the typical (100) and (200) planes of black-phase β-CsPbI₃. Meanwhile, the absorbance of the control film sharply declined after 450 nm, while CR-derived black CsPbI₃ film presents an absorption onset at 733 nm (Fig. S2), which agrees with the previous report on β-CsPbI₃ film^[12]. For the control film, inferior surface coverage was observed (Figs. S3(a) and S3(c)). And CR-derived film shows better surface coverage (Figs. S3(b) and S3(d)).

Figure 1.(Color online) (a) The ambient air-processed black-phase CsPbI₃ film via CR strategy. The XRD patterns of the control (b) and CR-derived CsPbI₃ film (c). Note: the hash key represents the signal from δ-CsPbI₃; the square symbol represents the signal from PbI₂; the diamond symbol represents the signal from β-CsPbI₃; the circular pattern represents the signal from CsI and the asterisk represents the signal from FTO glass substrate. (d) The structure of DAST. (e) Schematic for the molecular interaction and CsPbI₃ film formation. (f) The XRD pattern for DAST-modified CsPbI₃ film after being stored in air for one month.

Black-phase CsPbI₃ film gradually degraded and underwent phase transition when stored in air for one week, as evidenced by the gradual decrease of absorbance (Fig. S4). To further improve phase stability and optoelectronic properties of β-CsPbI₃ film prepared by CR strategy, we introduced the DAST additive (Fig. 1(d)). DAST not only maintains black-phase CsPbI₃ structure, but also slightly enhances the crystallinity and promotes the crystal growth orientation along (100) and (200) planes (Fig. S5). DAST also helps to reduce the grain sizes (100–200 nm) and improve the surface coverage of the resultant β-CsPbI₃ film (Fig. S6). DAST molecules can interact with CsPbI₃via robust bidentate coordination, thus impeding grain growth due to the steric hindrance effect (Fig. 1(e))^[11]. The interaction between DAST molecule and β-CsPbI₃ was studied by FTIR (Fig. S7). The pure DAST shows characteristic signals at 1023 and 1666 cm^–1, corresponding to C=C bond and benzene group, respectively. DAST-modified CsPbI₃ film also shows similar peaks, but with a slight shift, suggesting possible interaction between zwitterion and ions in perovskites^[20]. The DAST-modified CsPbI₃ film was stored at room temperature in air with a relative humidity of ~35%. There was no obvious degradation observed even after one month, as proved by XRD pattern (Fig. 1(f)).

In short, by using the CR strategy, we successfully stabilized the black-phase CsPbI₃ film in ambient air with >70% humidity. DAST can further stabilize the black phase. The approaches in this work will be useful for developing efficient perovskite solar cells.

We appreciate the National Natural Science Foundation of China (22005355) and Guangdong Basic and Applied Basic Research Foundation (2019A1515110770). L. Ding thanks the National Key Research and Development Program of China (2017YFA0206600) and the National Natural Science Foundation of China (51773045, 21772030, 51922032, 21961160720) for financial support.

Supplementary materials to this article can be found online at https://doi.org/10.1088/1674-4926/43/3/030501.

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