Now, a collaborative research group led by Professor Xiao at Peking University unlock the air-processing limitations and report high-performance α-FAPbI₃ PSCs processed at a wide relative humidity (RH) range (from 20% to 60%), achieving state-of-the-art PCEs over 24.5% (Science, https://www.science.org/doi/10.1126/science.adn9646)^[14]. The key innovation behind this milestone is the formation of a hydrophobic capping layer, which is self-assembled by a chlorinated organic molecule (Fig. 1(a)), (4-N-benzo[c]carbazolyl-2,6-dichlorophenyl)bis(2,4,6-trichlorophenyl) methyl radical (TTM-7BCz). Driven by surface energy minimization, the hydrophobic organic molecules accumulate on the surface to form a capping layer^[15]. Similar to a previous report^[16], such a hydrophobic capping layer significantly prevents the moisture ingress whilst persevering the D-complex. Consequently, an orderly crystallization of α-FAPbI₃ is enabled in ambient air with a RH up to 80%. This advancement represents a significant step towards overcoming the challenges associated with hygroscopic coordinating solvents in air-processed α-FAPbI₃ PSCs.

With the crystallization pathways revealed, the authors set out to tackle the critical dilemma for air-processed α-FAPbI₃, that is, the D-complex, crucial for the direct conversion to α-FAPbI₃, also facilitates moisture ingress, leading to unwanted δ-FAPbI₃ formation. Through introducing a hydrophobic TTM-7BCz into the PbI₂ precursor solution, a crystal capping layer is formed atop the PbI₂ intermediate film owing to the TTM-7BCz self-accumulation and strong chlorine-PbI₂ coordination. Fig. 1(d) shows that for the intermediate film with the capping layer (target intermediate), the D-complex maintains with 300 s of thermal annealing, starkly contrasting with that of the pristine intermediate. DCA measurements show that the target intermediate, when subjected to 60 s of thermal annealing, shows an improved hydrophobicity, but it still allows moisture ingress, confirmed by the initial contact angle decreasing from 83^o to 76^o. This is consequent with δ-FAPbI₃ formation upon the subsequent FAI deposition (Fig. 1(e)). However, with a prolonged thermal annealing (300 s), the contact angle becomes larger (~86^o) and consistent over time. This enhanced moisture resistance is attributed to the thickening of the hydrophobic capping layer during prolonged annealing, with the D-complex remaining abundant deeper within the film, as verified by time-of-flight secondary ion mass spectroscopy (TOF-SIMS). As such, direct conversion of α-FAPbI₃ without the presence of its δ-phase is achieved in humid air (Figs. 1(e) and 1(f)). In-situ UV−vis absorption spectroscopy is further performed to monitor the crystallization dynamics regulated by the capping layer. As shown in Figs. 1(g) and 1(h), when the pristine intermediate is annealed, its absorption edge gradually redshifts to 525 nm, indicating the D-complex loss and PbI₂ formation, while the absorption edge of the target intermediate is almost pinned at 450 nm, corresponding to D-complex. Thanks to the stabilized D-complex, the target intermediate exhibits remarkably retarded perovskite crystallization as compared to that of the pristine intermediate (14.5 vs 2.6 s).

In summary, the development of a chlorinated organic molecule to self-assemble a hydrophobic capping layer magically enables the PbI₂ intermediate film to resist moisture intrusion while preserving the essential D-complex needed for the conversion to α-FAPbI₃. This breakthrough overcomes the limitations of air-processed α-FAPbI₃ caused by hygroscopic coordinating solvents, facilitating the reproducible manufacture of high-performance α-FAPbI₃ PSCs under varied environmental conditions. Despite the achievements, further efforts are necessary. Firstly, instead of relying on trial-and-error methods, leveraging molecular dynamic simulations and high-throughput machine learning can provide deeper insights into molecular interactions and self-assembly behaviors, which will ultimately rationalize the design principles of the capping layer. Secondly, the chlorinated π-radicals proposed by the authors feature electron-drawing groups and a π-electron delocalization structure, potentially exerting an electrical doping effect on the perovskite^[17]. Therefore, it is crucial to investigate how the molecular structure correlates with interfacial energetic alignment and device performance. Lastly, the feasibility of self-assembling these organic molecules into a capping layer using scalable processes (e.g., slot die and doctor blading) remains uncertain. Thus, efforts should focus on evaluating and demonstrating the compatibility of this capping layer strategy for air-processing large-area perovskite. Achieving this milestone would make a solid step towards the real-world applications of α-FAPbI₃ PSCs.

The black-phase formamidine-lead iodide (α-FAPbI₃), boasting an optimal bandgap of 1.5 eV, stands out as a premier choice for narrow-bandgap perovskite solar cells (PSCs), achieving a certified power conversion efficiency (PCE) of 26.1%^[1−5]. This impressive performance hinges on the orderly and homogeneous crystallization of α-phase pure FAPbI₃, facilitated by coordinating solvents such as dimethyl sulfoxide (DMSO) to form intermediates like PbI₂-DMSO complex (D-complex). The D-complex plays a pivotal role in crystallization thermodynamics, enabling the direct formation of α-FAPbI₃ without the photoinactive δ-phase^[6−9]. However, DMSO, a commonly used coordinating solvent, is highly hygroscopic and prone to hydration upon moisture exposure. This tendency leads to incomplete perovskite crystallization and accelerates the transformation of α-FAPbI₃ into its δ-phase^{[2, 10]}. Consequently, the best-performing α-FAPbI₃ PSCs must be processed in an inert atmosphere with strictly controlled relative humidity (RH) and suffers from relatively poor reproducibility. Given the hard-to-control atmosphere at industrial scale, it is challenging yet imperative to eliminate the negative effects stemming from hygroscopic coordinating solvents^[11−13].

In addition to the device performance, the authors delve into the pathways of FAPbI₃ crystallization steered by the D-complex and moisture in a two-step spin-coating process. Initially, PbI₂ intermediate film is deposited using DMF/DMSO cosolvents and subjected to thermal annealing. X-ray diffraction (XRD) measurements reveal that with DMSO evaporation, the D-complex gradually converts to crystalline PbI₂. After 180 s of thermal annealing, the D-complex disappears (Fig. 1(b)), but residual DMSO molecules persist, confirmed by XRD and proton nuclear magnetic resonance (¹H NMR). To understand how D-complex affects FAPbI₃ crystallization in humid air, Formamidinium Iodide (FAI) is spin-coated on the intermediate films annealed by 60 and 300 s, respectively. It turns out that theformer with D-complex is immediately transformed to photoinactive δ-FAPbI₃, while for the latter without D-complex, α- and δ-FAPbI₃ coexists. Despite thermal annealing promoting the α-FAPbI₃ formation in both cases, the final films suffer from δ-FAPbI₃ and PbI₂ impurities (Fig. 1(c)). Interestingly, by simply altering the atmosphere to nitrogen, the intermediate film containing D-complex undergoes a direct transformation to α-phase pure FAPbI₃ through intermolecular exchange between DMSO and FAI. In contrast, δ-FAPbI₃ still occurs for the intermediate film without D-complex since FAI is difficult to be intercalated into PbI₂. Dynamic water contact angle (DCA) measurements further corroborate moisture intrusion induced by the hygroscopic DMSO, which lower the energy barrier for δ-FAPbI₃ formation^[2].

Benefiting from the retarded crystallization, the air-processed perovskite derived from the target intermediate (300-s thermal annealing) shows excellent optoelectronic properties, including a carrier lifetime of 6120 ns, a carrier mobility of 1.0 × 10⁻² cm²∙V⁻¹∙s⁻¹, and a trap density as low as 4.5 × 10⁻¹⁵ cm⁻³. Inspiringly, the PSCs are fabricated in humid air, and their photovoltaic performance are compared, as shown in Fig. 1(i). It is found that the target PSCs offer PCEs over 24.5% within the RH ranging from 20% to 60%, and even at a high RH of 80%, the PCE remains as high as 23.4%, obviously superior than the counterparts. Equal importantly, the reproducibility is greatly improved particularly when the RH exceeds 60%. In addition, the target PSCs without any encapsulation demonstrate great stability with less than 5% PCE degradation after continuously working for 500 h (Fig. 1(j)).