What determines the iodine oxidation reaction kinetics in halide perovskite solar cells?

Abstract

Perovskite solar cells (PSCs) have garnered significant attention owing to their solution fabrication, cost-effectiveness, and high power conversion efficiency, yet their practical application has been hindered by instability issues. Under operational conditions, redox reactions have been found to be prevalent in perovskite devices, but the underlying mechanism remains very unclear. Here, we systematically investigated the impact of light irradiation, atmosphere and interfacial structures on the redox kinetics in PSCs. Our results show that oxygen acts as a predominant factor driving device degradation, other than the transport layer or spectral components. Spectroscopic results reveal that the formation of iodine-related defects, e.g., triiodide ion (I₃⁻) and iodine (I₂), are dramatically boosted under oxygen-rich conditions, and can be further accelerated by light exposure. These findings provide critical insights into the redox reaction mechanism of perovskite-based materials, and offer a potential direction for enhancing the long-term stability of PSCs.

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Introduction

PSCs have attracted considerable interest in the photovoltaics research community due to their high efficiency, low production cost, and exceptional optoelectronic properties.^1–6 Recently, PSCs have achieved a certified efficiency of 26.95%,⁷ which is comparable to that of the silicon-based photovoltaics. However, numerous factors contribute to the instability of PSCs, ultimately leading to significant deterioration in their performance.^8–10

Perovskites have been shown to be intrinsically unstable, as manifested by phase instability, low formation energy, and ion migration.^11–13 The constituents of perovskites, like iodide (I⁻) and lead(II) ions (Pb²⁺), often undergo redox reactions under operation. External factors, such as light exposure, temperature, humidity, oxygen, tensile stress, and electrical bias, have been substantiated to accelerate these redox-related degradation processes in perovskite-based devices.^14–18 For example, a hydration process disrupts the three-dimensional perovskite structure, leading to its decomposition into lead iodide (PbI₂) and hydroiodic acid (HI). These degradation products then undergo photoactivated redox reactions with atmospheric oxygen, ultimately producing metallic lead (Pb⁰) and iodine (I₂) vapor through a series of photochemical oxidation processes.^19–21 Oxygen-induced reversible degradation has also been identified as a unique phenomenon in some PSCs. B. Purev-Ochir et al. found that exposure to oxygen can initially degrade the perovskite material, yet the reaction is reversible under certain conditions, such as in a vacuum environment.²² When interfaced with metal oxides (such as titanium dioxide, TiO₂), it may trap charge carriers, and facilitate the generation of reactive oxygen species under illumination, which accelerates the photochemical degradation of perovskite materials.²³ First-principles calculation and spectroscopic analysis by Ambrosio et al. revealed that oxide-induced defect states promote the formation of self-trapped excitons, which enhance the interaction between charge carriers and molecular oxygen and the generation of superoxide radicals.²⁴ More recently, Li et al. reported that photo-induced redox reactions play a key role in the degradation of tin–lead perovskite solar cells. Illumination generates superoxide (O₂⁻) that oxidizes Sn²⁺ to Sn⁴⁺, triggering Pb²⁺ accumulation and irreversible phase changes. This process leads to lattice damage and performance loss, underscoring the importance of controlling redox chemistry for device stability.²⁵ To date, several strategies have been developed to suppress the redox activity of perovskites,²⁶ including stress release, and defect passivation.^27–29 One representative approach is the utilization of Eu³⁺/Eu²⁺ redox pairs, which continuously oxidize Pb⁰ and reduce I⁰ defects in FAPbI₃-based PSCs, resulting in enhanced power conversion efficiency (PCE) and device durability.³⁰ Despite this progress,^31–34 several factors contribute to the redox reaction, and are interwoven with each other under operational conditions. Therefore, it is essential to scrutinize the redox mechanisms under diverse operational conditions, which would help us improve the long-term stability of the solar cell devices.

In this study, we investigate the redox reaction mechanism occurring in PSCs fabricated on different substrates and under varying environments. We demonstrate that oxygen is the primary factor driving the oxidation degradation of PSCs, other than the spectral component of irradiation or transport layers. Specifically, in an oxygen-rich environment, perovskites undergo accelerated defect formation, including I₂ and I₃⁻ species, which severely compromise device performance. In contrast, under oxygen-deficient conditions, prolonged external stimuli lead to negligible degradation, highlighting the significant role of surface oxygen in driving the degradation process. Our findings underscore that oxygen is one of the key factors responsible for perovskite degradation in oxygen-containing conditions, shedding light on the design and assemble of stable PSCs.

Results and discussion

We spin-coated perovskite films onto various types of transport layers to analyze the impact of interfacial structure on the redox kinetics, including oxide layers (NiO_x, SnO₂, TiO₂), self-assembled layers: [2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl] phosphonic acid (MeO-2PACz), and organic layers: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) for characterization. Subsequently, the films were immersed in a toluene solution and exposed to four distinct conditions for 10 hours: (1) nitrogen environment (N₂), (2) nitrogen environment with light exposure (light-N₂), (3) oxygen environment (O₂), and (4) oxygen environment with light exposure (light-O₂). To establish a pure oxygen environment, high-purity O₂ gas was introduced into a custom-fabricated transparent chamber until complete atmospheric replacement was achieved. This sealed system enabled precise control of the gaseous environment during the treatment period. The perovskite composition investigated in this work is Cs_0.05FA_0.81MA_0.14PbI_2.85Br_0.15. Further details of the materials and experimental procedures are provided in the SI.

As shown in Fig. 1(a), the solution remained colorless under N₂, while a slight discoloration was observed under light-N₂. Notably, a visible magenta coloration developed under O₂, and this color change became significantly more intense under light-O₂. These results clearly demonstrate that perovskite films undergo substantial degradation in oxygen-rich environments, with light exposure further accelerating this process. This pronounced trend was consistently observed across different charge transport layers, highlighting oxygen as a critical factor in the redox reaction of perovskite solar cells.

Fig. 1 (a) Perovskite films coated with different transport layers immersed in toluene under four distinct conditions for 10 hours: N₂, light-N₂, O₂ and light-O₂. A white LED of 100 mW cm⁻² was used for the experiment. “FTO” refers to perovskite film coated on fluorine-doped tin oxide (FTO) without transport layers. (b) UV-vis spectra of the toluene extracts from perovskite films aged under four different environmental conditions. (c) Absorbance of I₂ signals determined by the UV-vis spectra of toluene solution, which has immersed the perovskite films for 10 hours under the four specified conditions. “Pristine” refers to freshly prepared perovskite films with various transport layers, directly immersed in toluene without any treatment. (d) Absorbance of I₂ signals of the toluene solutions, which were obtained by immersing perovskite film with different transport layers under certain conditions for 10 hours.

The degradation mechanism should be related to the oxidation of I⁻ to I₂. Under oxygen-rich conditions, the perovskite film reacts with oxygen, leading to the oxidation of the I⁻ ions present in the perovskite structure. This process results in the formation of molecular I₂, which is responsible for the magenta color observed in the solution.³⁵ The light exposure further accelerates this oxidation process by increasing the rate of charge carrier generation, which facilitates the oxidation reaction.²³

Ultraviolet-visible (UV-vis) spectra in Fig. 1(b) reveal a pronounced absorption peak at 500 nm for the magenta liquid, which corresponds to the characteristic signal of molecular I₂.³⁶Fig. 1(c) illustrates the absorbance trends, and by integrating the data from Fig. 1, we can both qualitatively and quantitatively confirm that perovskite films degrade under oxygen-rich conditions, with light exposure further accelerating the degradation rate. Specifically, the I₂ content in perovskite films with various transport layers under light-O₂ is approximately ten times higher than that observed under light-N₂. However, under N₂, the films did not exhibit significant I₂ evolution after prolonged light exposure. Similarly, when the samples are under O₂, the I₂ content is eight times greater than when under N₂. These results demonstrate the fast degradation rate under oxygen-rich conditions.

We further investigated the redox processes of perovskite films on different substrates (PEDOT:PSS and NiO_x) under UV irradiation. 10-hour toluene immersion tests were conducted under four distinct atmospheric conditions: N₂, ultraviolet-irradiated nitrogen (UV-N₂), O₂, and ultraviolet-irradiated oxygen (UV-O₂). As illustrated in Fig. S1, a negligible color change was observed under N₂, while a light pink coloration appeared under UV-N₂. This is consistent with previous studies indicating that, even in an inert atmosphere, UV exposure can excite charge carriers in perovskite films, leading to defect formation, ion migration, and subsequent structural degradation.³⁷ Nevertheless, the discoloration under UV-N₂ was markedly less intense than that observed in oxygen-rich atmospheres. Fig. 1(d) quantitatively reveals that the I₂ concentration under UV-O₂ is approximately 3.1 times higher than that under UV-N₂, underscoring the synergistic impact of oxygen and UV light in accelerating film degradation.³⁸ Even when exposed to O₂, the I₂ content exceeded that under UV-N₂. Both qualitative and quantitative analyses consistently confirm that oxygen is the dominant factor driving perovskite degradation, irrespective of light exposure.

To investigate the synergistic effect of moisture and oxygen on perovskite degradation, we immersed films in a toluene solution containing 0.2% (v/v) water and exposed them for 10 hours under N₂, light-N₂, O₂, and light-O₂. In the presence of water, the degradation trends are consistent with those observed under anhydrous conditions (Fig. S3a and c), quantitative analysis (Fig. S3d) reveals that under light-O₂, the water present synergistically accelerates degradation, increasing the I₂ concentration by approximately 7.2% relative to the anhydrous environment. Under O₂, there is about a 2% increase in I₂ concentration. This conclusion is highly consistent with the findings of our study, further confirming that oxygen serves as the primary driver of perovskite lattice degradation, while water acts as a synergistic accelerator, facilitating the disruption of the crystal structure and organic components, thereby expediting the overall degradation of the material.³⁹ To further investigate this, we extended our study to pure FAPbI₃ and a Sn–Pb alloy perovskite (Cs_0.05(FA_0.7MA_0.5)_0.95Sn_0.15Pb_0.85I₃) under identical conditions. Both compositions show degradation trends consistent with Cs/FA/MA perovskite but generate slightly more molecular I₂ under O₂ and light-O₂ (Fig. S4e). The accelerated degradation observed in FAPbI₃ is primarily attributed to residual PbI₂ resulting from its two-step fabrication process, which may act as reactive sites facilitating degradation.^40,41 In the Sn–Pb alloy, the enhanced degradation rate is likely due to the oxidation of Sn²⁺ ions, promoting the formation of reactive iodine species.⁴² These findings confirm that the degradation mechanism proposed herein is consistent across representative perovskite systems, thereby reinforcing its broad applicability.

As shown in Fig. 2(a), Raman spectra of perovskite films exposed to oxygen-rich conditions exhibit a prominent peak at 169 cm⁻¹, which can be ascribed to the formation of molecular I₂.²³ Additionally, the Raman peaks at 114 cm⁻¹ and 141 cm⁻¹, which are attributed to I₃⁻,³⁵ exhibit significantly higher intensity compared to the peaks observed in films treated in an N₂ atmosphere. In situ Raman spectroscopy (Fig. S5) demonstrates that the combined application of stepwise heating and UV illumination under an O₂ atmosphere accelerates degradation and allows for the direct detection of molecular I₂, providing additional support for the proposed oxidation mechanism. This observation evidences that the major redox products are I₃⁻ and I₂ species in perovskite films under oxygen-rich conditions.

Fig. 2 (a) Raman spectra of perovskite subjected to different conditions for 10 hours. (b) Pb 4f XPS spectra subjected to different conditions for 10 hours. (c) I 3d XPS spectra subjected to different conditions for 10 hours.

X-ray photoelectron spectroscopy (XPS) was conducted to investigate the evolution of the Pb 4f and I 3d peaks in the perovskite films under four different treatment conditions. As shown in Fig. 2(b), the pristine perovskite film exhibits the characteristic Pb²⁺ binding energy at 138.00 eV (Pb 4f_7/2),⁴³ indicating a stable, undegraded state. Under light-N₂, the Pb 4f peak shifts slightly to ∼138.15 eV, suggesting minimal chemical changes and weak degradation in the absence of oxygen. In contrast, under light-O₂ conditions, the Pb 4f_7/2 peak further shifts to ∼138.40 eV, likely associated with oxygen-induced degradation. The binding energy shift under light-O₂ is more obvious than that under light-N₂, demonstrating the synergistic effect of light and oxygen in accelerating the degradation of the perovskite film. The observed shift toward higher binding energy indicates a reduced electron density around Pb²⁺, associated with lattice degradation and Pb–I bond instability, while the impact under inert conditions is negligible.⁴⁴ As shown in Fig. 2(c), the I 3d spectrum of the pristine perovskite film exhibits the characteristic binding energy of I⁻ at 619.00 eV (I 3d_5/2),⁴⁵ indicating a chemically stable state. Under light-N₂, the peak shifts slightly to 619.09 eV, reflecting minor changes with minimal degradation. Under light-O₂ conditions, the I 3d peak shifts further to 619.25 eV, which is approximately 0.25 eV higher than the pristine state, indicating the larger chemical changes induced by the synergistic effect of light and oxygen. During this process, I⁻ undergoes electron loss via photo-induced oxidation in the presence of oxygen, thereby significantly accelerating the degradation of iodine species within the perovskite lattice.^46,47

To evaluate the performance and stability of the devices under different conditions, we fabricated ITO/MeO-2PACz/perovskite/[6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM)/BCP/Ag solar cells. These devices were placed under the four predefined environmental conditions: N₂, light-N₂, O₂, and light-O₂, and their current density–voltage (J–V) characteristics were subsequently measured under AM 1.5G simulated sunlight. For the unencapsulated device, the PCE exhibited a slight improvement of 1.22% under N₂, while moderate degradation (1.47% loss) was observed under light-N₂, indicating minimal impact in the absence of oxygen. However, exposure to O₂ led to a significant efficiency drop of 6.69%, which was further aggravated under light-O₂, with the PCE declining by 18.91% (Fig. 3c). To mitigate oxygen-induced degradation, encapsulated devices were further evaluated under the same environmental conditions. As shown in Fig. 3(b) and (d), encapsulation effectively isolated the perovskite films from oxygen, resulting in negligible efficiency changes after 10 hours of exposure to all four environments. The operational stability of the devices was further evaluated by tracking the maximum power point (MPP). As shown in Fig. 3(e), under light-O₂, unencapsulated devices degraded rapidly to 50% of their initial efficiency within just 5 hours during MPP tracking, whereas encapsulated devices retained 99% and 98% of their initial efficiency under light-N₂ and light-O₂, respectively, even after 200 hours. The MPP tracking results clearly demonstrate that encapsulation is an effective strategy to mitigate oxygen-induced degradation.

Fig. 3 (a) and (b) J–V characteristics of unencapsulated (a) and encapsulated (b) perovskite solar cells with the architecture ITO/MeO-2PACz/perovskite/PCBM/BCP/Ag, measured under AM 1.5G illumination. (c) and (d) PCE of unencapsulated devices (c) and encapsulated devices (d) under different environmental conditions. (e) Maximum power point (MPP) tracking of encapsulated and unencapsulated devices measured under continuous illumination conditions. Devices were tested under light-O₂ and light-N₂ atmospheres at 55 °C with an illumination intensity of 100 mW cm⁻².

XRD patterns in Fig. S6 reveal a significant acceleration of perovskite film degradation under O₂ and light-O₂ conditions, as evidenced by the intensified PbI₂ diffraction peak at ∼11.8°.^48,49 SEM images of films aged for 10 hours under N₂ and light-N₂ (Fig. S4) show no significant morphological changes. Samples aged in O₂ (Fig. S5a) exhibited mild surface roughening and the appearance of PbI₂ nanoparticles. More pronounced degradation is observed under light-O₂ (Fig. S5b), featuring increased surface roughness, void formation, and more PbI₂ accumulation, consistent with the XRD data.⁵⁰ These morphological and structural observations further illustrate the compounding effect of oxygen and illumination, reinforcing their central role in triggering decomposition pathways. In particular, light exposure in oxidative environments facilitates the formation of reactive oxygen species (ROS),⁵¹ which leads to a significant reduction in device performance.

A deeper investigation into the carrier dynamics and defect characteristics of the material under oxygen-rich conditions was conducted by performing steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. The excitation light for both PL and TRPL measurements was incident from the air side. As depicted in Fig. 4(a), the perovskite films exposed to N₂ and light-N₂ atmospheres exhibited strong and sharp PL emission centered at approximately 790 nm,^52,53 indicating high radiative recombination efficiency and low defect density. Conversely, the samples exposed to O₂ environments, regardless of light exposure, exhibit a pronounced reduction in PL intensity along with a noticeable blue shift of the emission peak. This blue shift is indicative of an increase in bandgap energy, which can be attributed to oxygen-induced degradation of the perovskite lattice. It should be noted that I⁻ ions within the structure are susceptible to oxidation by O₂ molecules under photoexcitation, according to the reaction:

Fig. 4 (a) Steady-state and (b) time-resolved PL spectra of perovskite films under different conditions. The incident excitation light was irradiated from the air side. (c) Schematic illustrating the degradation behavior of perovskite films under different environmental conditions: N₂, light-N₂, O₂, and light-O₂.

The loss of I⁻ and the generation of I₂ disrupt the integrity of the [PbI₆]⁴⁻ octahedral framework, resulting in defect formation, and the deterioration of the film's optoelectronic quality.⁵⁴ A substantial decline in PL intensity is observed upon O₂ exposure, reducing it to approximately two-thirds of that under N₂. This effect becomes markedly more severe when illumination is introduced, with the PL intensity decreasing nearly fivefold.

Fig. 4(b) displays the TRPL decay curves of perovskite films under different conditions. The perovskite films showed negligible changes in average carrier lifetime under N₂ and light-N₂ conditions, consistent with the degradation trends discussed earlier. Upon exposure to O₂, the carrier lifetime markedly decreases to ∼86 ns, suggesting a substantial enhancement of nonradiative recombination pathways. This effect becomes even more severe under light-O₂ conditions, where the lifetime further shortens to ∼52 ns, amounting to a reduction of nearly three-quarters compared to the N₂ baseline.^55–57 Overall, these results underscore the pivotal role of oxygen in accelerating perovskite degradation. Regardless of illumination, oxygen exposure leads to increased defect densities, reduced carrier lifetimes, and bandgap widening, all of which collectively deteriorate the material's optoelectronic performance.

Conclusions

In summary, we investigated the redox reactions occurring in PSCs with different perovskite compositions and interfaces by exposing the devices to four distinct environmental conditions: N₂, light-N₂, O₂ and light-O₂. However, we found that neither the transport layers, nor the spectral components of light significantly contribute to the redox reaction, once the oxygen is absent. We unequivocally identify that oxygen is the dominant factor driving oxidative degradation in PSCs. Under oxygen-rich conditions, halide perovskites undergo accelerated photolytic decomposition and ion migration, facilitating the formation of defect species such as molecular I₂, and I₃⁻ species. These defects markedly impair charge transport and increase non-radiative recombination pathways, thereby degrading device performance.

The results emphasize that the role of oxygen must be stringently regulated in redox-related studies, as it profoundly influences interfacial reactivity and redox kinetics. For instance, during toluene soaking experiments, trace amounts of oxygen are often unavoidable and can significantly alter the chemical environment at the interface.

Furthermore, during device fabrication, oxygen molecules may be adsorbed onto material surfaces, thereby promoting undesired oxidation reactions. Such surface-adsorbed oxygen can act as an intrinsic driving force for redox processes, particularly under device operational conditions or even post-encapsulation, ultimately compromising the long-term operational stability of the system. This work provides critical mechanistic insights into the redox kinetics of perovskite materials and offers guidance for the rational design of PSCs with enhanced long-term stability.

Author contributions

S. Yang and Y. Hou directed the research. R. Yang and Y. Liu performed the experiments and data analyses. Y. Zhu provided assistance with the SEM characterization. Q. Li provided assistance with the Raman characterization. R. Y. and X. L. wrote the manuscript. All the authors participated in writing and editing the manuscript and contributed their efforts to the discussion.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data for the plots presented in this paper and other findings of this study are available from the corresponding author upon reasonable request.

All data supporting the findings of this study are available in the Supplementary Information accompanying this manuscript. Supplementary Information: Detailed descriptions of materials, characterization methods, synthetic procedures for perovskites in different systems, and all relevant experimental details. It includes UV–Vis, Raman, XRD, surface SEM, cross-sectional SEM, and additional supporting figures and data. See DOI: https://doi.org/10.1039/d5qm00466g.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (22379044), Shanghai Pilot Program for Basic Research (22TQ1400100-5), Shanghai Municipal Natural Science Foundation (25ZR1401081), the Fundamental Research Funds for the Central Universities (JKD01251505, JKVD1251041), the Postdoctoral Fellowship Program of CPSF (GZC20250071), Shanghai Engineering Research Center of Hierarchical Nanomaterials (18DZ2252400), and Shanghai Titan Natural Science Development Foundation and Shanghai Frontiers Science Center of Optogenetic Techniques for Cell Metabolism (Shanghai Municipal Education Commission).

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Footnote

† Rui Yang and Xinyi Liu contributed equally to this work.

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