Enhanced Stability of Halide Perovskite by Strengthening Hydrogen Bonds

Abstract

The instability of metal–halide perovskites intrinsically stems from weak hydrogen bonding in their organic–inorganic hybrid structure. Here, we demonstrate that prolonged pressure treatment at ∼1.8 GPa for 12 h induces a permanent lattice modification in (FAPbI₃)₀._{9 5}(MAPbBr₃)₀._{0 5} polycrystalline films, characterized by strengthened hydrogen bonding between FA cations and iodide. Notably, such pressure-induced H-bond enhancement is preservable after decompression and yields dramatic stability improvements: Light-induced phase segregation is slowed by 12-fold, while resistance to illumination, heat, and moisture improves by four to six times. First-principles calculations reveal that the treatment raises the migration barrier for iodide, explaining the observed suppression of ion migration and phase segregation. These results establish that prolonged pressure treatment can enhance hydrogen bonding and produce substantially more stable perovskite materials and devices without altering their composition.

Introduction

Metal-halide perovskites (MHPs) have made great achievements in photovoltaics, with the power conversion efficiency (PCE) of perovskite solar cells (PSCs) rising at an unprecedented rate. − However, the stability issue remains a significant challenge that impedes the commercialization of PSCs. , Under exposure to moisture, heat, and light irradiation, MHP films undergo severe degradation, attributed to their soft lattices. − This intrinsic instability is fundamentally linked to the material’s organic–inorganic hybrid structure. The inorganic octahedral framework is stabilized by weak hydrogen bonds between the organic cations and the halide anions, making the strength of these interactions a key determinant of structural integrity. A variety of strategies have been explored to enhance the perovskite stability, including compositional engineering, morphological control, defect passivation, and strain engineering. − Common approaches rely on additives or dopants that modify intermolecular interactionsforming new hydrogen bonds or inducing lattice strainto strengthen the lattice. , However, it remains challenging to achieve substantial structural stabilization without altering the material’s chemical composition.

As a thermodynamic parameter, pressure provides a composition-preserving control parameter to effectively modify the perovskite lattice structures and optoelectronic properties. − Pressure-induced structure alterations, including lattice contraction and distortion, inevitably lead to adjustments to interatomic spacing and thereby can profoundly affect the strength of hydrogen bonding in the perovskite structure. Nevertheless, most high-pressure studies found that the structure and optoelectronic properties of MHP under high pressure tend to revert to the initial states after the pressure is released due to the elastic nature of the perovskite framework. In our prior work, we proposed a pressure aging strategy that extends the duration of pressure treatment (1.8 GPa, 12 h) and can induce recrystallization at grain boundaries and permanently improve carrier diffusivity and photoresponsivity in perovskite thin films. , The selection of 1.8 GPa is attributed to its industrial accessibility, endowing it with the potential for commercial-scale processing applications. This raises a critical, unanswered question: can such a time-extended pressure protocol also induce enduring modifications to the intrinsic lattice structure to confer enhanced stability?

Here, we demonstrate that prolonged pressure treatment produces a permanent structural modification that yields profound stability enhancement. By applying an industrially accessible pressure of ∼1.8 GPa for extended durations (>12 h), (FAPbI₃)_0.95(MAPbBr₃)_0.05 (FA: CH­(NH₂)₂; MA: CH₃NH₃) MHP polycrystalline films show a permanent structure change with remarkably strengthened hydrogen bonding between organic FA cations and iodide ions. The 12 h pressure treatment (12h PT) results in dramatically improved stability after decompression to ambient conditions, including ∼12 times slower phase segregation and four to six times higher resistance to light irradiation, thermal conditions, and moisture conditions. These findings not only introduce a new paradigm for stabilizing perovskites without altering their composition but also offer fresh insights into the intrinsic structure–stability relationship in these materials.

Results and Discussion

We first examined the structural change after 12 h of PT on the (FAPbI₃)_0.95(MAPbBr₃)_0.05 MHP thin film. The MHP thin films were held at 1.8 GPa in a diamond anvil cell (DAC) for 12 h using silicone oil as pressure-transmitting media (Figure S1), and then the pressure was released to ambient conditions. No obvious cracks or aggregation were observed in the MHP thin film after pressure treatment (Figure S2). At a relative macroscopic level, we found obvious morphological changes after 12 h of PT (Figures S3 and S4). Recounting the compositional intricacy of the MHP thin film, it remains a challenge to pinpoint the precise alterations in the lattice constant after 12 h of PT. Given that each grain within the MHP polycrystalline film behaves essentially as a single crystal (SC) structure, we employed FAPbI₃ SCs to take a deeper analysis of the structural variation within grains.

FAPbI₃ adopts a Pm-3m symmetry with the full disorder of the A-site FA organic molecules at ambient conditions. Upon meticulous analysis, we observed a consequential fortification of the hydrogen bond (N–H···I) between FA and I, demonstrated by the notable contraction in bond length from initial 3.3 and 3.4 Å to 2.8 and 2.6 Å after 12 h PT (Figure a,b). Furthermore, the bond angle of N–C–N of FA exhibits a significant decrease from 163 to 113°, mainly due to the strengthening and change of hydrogen bonding. Moreover, this enhanced hydrogen bonding effect is sustained persistently, which can be attributed to the 12 h high-pressure treatment that drives the irreversible conformational rearrangement of FA cations, thereby forming a stronger hydrogen bonding potential well. This strengthened interaction elevates the energy barrier for the reorientation of organic cations and induces local lattice distortions (e.g., the reduced N–C–N bond angle of FA cations), ultimately yielding a “locked” structure that inhibits the elastic recovery tendency of the inorganic framework and restricts the rotation of the inorganic octahedra. This enhanced hydrogen bonding is considered not to disrupt the overall long-range order of the lattice but induces a slight lattice contraction, as illustrated by XRD measurements (Figure S5 and Table S1). To confirm that these modifications arise from organic-cation reorganization rather than the halogen elements, we conducted control experiments that probed the inorganic sublattices without hydrogen bonds. The inorganic framework of CsPbBr₃ perovskite showed negligible change in unit-cell parameters, bond lengths, and angles after 12 h PT (Figure S6), confirming that the enhanced H-bonds primarily originate from the reorganization of organic cations. Therefore, the reported high-pressure treatment method is potentially applicable to mixed-cation perovskites (FA/MA-type) as well as organic-cation-containing mixed-halide perovskites. However, its efficacy may be subject to composition sensitivity, phase stability issues under high-pressure conditions, and the influence of pressure-transmitting media.

1.

Structural change of the FAPbI₃ SC after 12 h of pressure treatment. The crystal structure of FAPbI₃ SC before (a) and after (b) 12 h PT. Enhanced hydrogen bonding (N–H···I) and a noticeable change in the band angle of N–C–N in FA are observed. 3D charge density difference (isosurface = 0.002 e bohr^–3) of FAPbI₃ before (c) and after (d) pressure treatment. Red and blue show regions of space that are depopulated or populated by electrons, respectively. The hydrogen bond interactions are marked by red dashed lines. For clarity, one unit of the octahedral frameworks is shown in a close-up view.

To further emulate the hydrogen bonding in FAPbI₃ before and after 12 h PT, we performed calculations of the charge density difference (Figure c,d). As shown in Figure c,d, significant electron accumulation (blue region) is observed between the H atom of the FA cation and the iodide, while electron depletion (red region) occurs around the H atom. This directional charge transfer confirms the enhanced polarization and a stronger interaction. It is evident from our analysis that the bond lengths of FAPbI₃ within one of its octahedral frameworks decrease from 3.1–3.2 Å to 2.5–2.8 Å after pressure treatment (Figure d). A similar phenomenon, where pressure treatment has the potential to intensify hydrogen bonding concurrently modify bond angle, has been reported in the metal–organic frameworks. ,

To further verify the interactions of hydrogen bonds after 12 h PT, we carried out in situ high-pressure IR spectral measurements on FAPbI₃ SCs. The peaks at 1048, 1176, 1353, 1616, and 1708 cm^–1 in the IR spectra show a red shift compared to their initial positions (Figure S7), consistent with enhanced hydrogen bonding. , The specific changes in infrared characteristic peaks are listed in Table S2. The Raman spectrum of the treated MHP film exhibits an intensified Pb–I stretching mode near 110 cm^–1 (Figure S8), further supporting our conclusion that prolonged pressure treatment amplifies hydrogen bond interactions and induces a more ordered lattice.

Stability is the most critical issue that limits the long-term commercialization of halide perovskites, largely owing to their weak organic–inorganic interactions and consequent susceptibility to light, heat, and moisture–induced degradation. − Motivated by the pressure-induced structural modifications described above, we first probed the light, heat, and moisture stability of the MHP film by monitoring the evolution of the photoluminescence (PL) intensity. The 12 h PT sample showed markedly improved resilience under continuous illumination: Although both treated and untreated films exhibit transient photoinduced self-healing behavior, the pressure-treated films maintain higher PL intensity and display signatures of lower defect density , (Figure a–c). We extended these tests to thermal and humidity stress: under 85 °C (Figure d–f) and 85% relative humidity (Figure g–i), the pressure-treated films retained PL far better than the untreated films, corresponding to a four to six times improvement in resistance to light, heat, and moisture. These performance enhancements are consistent with the structural modifications of strengthened hydrogen bonding induced by prolonged pressure treatment. The magnitude of the stability enhancement in perovskite single crystals before and after pressure treatment is comparable to that in perovskite polycrystalline films, which further confirms that the improved stability is mainly attributed to hydrogen bond strengthening (Figure S9).

2.

Comparison of PL spectra and peak PL intensity between the MHP film before and after 12 h PT as functions of time under continuous 405 nm laser irradiation (4.5 mW/cm²) at ambient conditions (a–c), 85 °C (d–f), and 85% relative humidity (g–i).

To quantify the thermodynamic consequence of this structural tuning, we calculated the formation enthalpy of the MHP before and after pressure treatment. The formation enthalpy decreases from 20.19 meV atom^–1 in the untreated film to 8.98 meV atom^–1 after 12 h of PT, indicating a lower-energy, more thermodynamically stable structure. Taken together, the optical, environmental-stress, and energetic data indicate that pressure treatment yields a more robust perovskite lattice and offers a practical route to improve the operational stability of perovskite optoelectronic devices.

We further investigated the light-induced phase segregation of the film using time-of-flight secondary-ion mass spectrometry (TOF-SIMS), which provides high surface sensitivity. The 2D TOF-SIMS maps show that after continuous illumination, the pressure-treated film maintains a markedly more homogeneous I^– distribution than the untreated film (Figure a and b). In the untreated sample, surface I^– intensities decline with illumination, consistent with migration of iodide from the surface into the film interior; by contrast, this surface-to-bulk migration is largely suppressed in the 12 h PT sample (Figure b). Quantitative analysis of the depth profile of the I^– distribution indicates that the phase segregation rate in the pressure-treated film is reduced by ∼12-fold relative to the untreated film, demonstrating substantially enhanced photostability (Figure S10).

3.

2D TOF-SIMS mapping of I^– in untreated (a) and pressure-treated MHP films (b). Potential diffusion pathways of an interstitial I^– ion migration in FAPbI₃ before (c) and after (d) pressure treatment.

Mechanistically, iodide migration in the perovskite lattice proceeds by hopping of I^– between neighboring octahedral sites, a process that involves local rotation and relaxation of the Pb–I octahedra and requires local rotation and relaxation of Pb–I octahedra. To probe how the pressure treatment affects this migration, we calculated the diffusion barriers for an interstitial I^– in FAPbI₃ before and after pressure treatment (Figure c,d). The calculated activation energy increases from 0.73 eV in the as-prepared structure to 1.33 eV after pressure treatment, unequivocally showing that the strengthened hydrogen bonding increases the diffusion barrier for I^– migration. This substantial increase underscores that the enhanced environmental stability is governed predominantly by kinetic stabilization via suppressed ion migration, rather than the relatively modest thermodynamic stabilization. These combined experimental and computational results indicate that the enhanced H-bonding induced by extended pressure treatment effectively impedes iodide migration and thereby retards light-induced phase segregation.

Conclusions

In summary, we demonstrated that a prolonged pressure treatment can induce a permanent, stability-enhancing modification to the perovskite films. By applying 1.8 GPa for 12 h to (FAPbI₃)₀._{9 5}(MAPbBr₃)₀._{0 5} films, we achieved a lasting enhancement of the hydrogen bonds between FA cations and iodide. This retained modificationwhich is contingent on the presence of intrinsic hydrogen bonds in the materialconfers exceptional stability, slowing phase segregation by 12-fold and boosting resilience to light, heat, and moisture by four to six times. Our findings also revealed that the persistence of enhanced hydrogen bonding requires inherent hydrogen bonds within perovskite materials. These findings underscore hydrogen bonding as a key lever for controlling perovskite stability and present prolonged pressure treatment as a versatile, composition-preserving route to fabricate more stable perovskite materials and devices.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.5c01704.

• Experimental details, supplementary notes, and additional measurements (PDF)

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Y.Y., Y.Z., and H.X. contributed equally to this work.

The authors declare no competing financial interest.