How Do Light and Heat Affect Photoinduced Halide Segregation?

By Surbhi JainSep 26 2022Reviewed by Susha Cheriyedath, M.Sc.

In an article recently published in the journal ACS Energy Letters, researchers discussed the effects of light and heat on photoinduced segregation behavior in 2D mixed halide perovskite.

Study: Photoinduced Segregation Behavior in 2D Mixed Halide Perovskite: Effects of Light and Heat. Image Credit: sutadimages/Shutterstock.com

Background

Bandgaps in metal halide perovskites can be variably tuned, and the stability of multijunction perovskite-Si tandem cells is a major concern. However, it is frequently noted that under light illumination, mixed halide perovskites (MHPs) separate into iodide-rich and bromide-rich domains.

Although the relationship between vacancy and photoinduced halide segregation (PHS) has been demonstrated, nothing is known about the mechanisms behind PHS. Various models have been researched to define PHS. With low light levels, there are also significant variations in the predicted illumination values for the suppression of illumination, in addition to variations in the forecast of its temperature dependency and terminal composition.

The significance of the oxidation of iodide species and hole trapping has also recently come to light. Despite the extensive research into PHS in 3D perovskites, nothing is known about PHS and ion migration in layered Dion–Jacobson (DJ) perovskites and/or Ruddlesden–Popper (RP). Despite the intricacy, it is crucial to understand the processes that take place in 2D MPH.

To properly exploit the use of a 2D MHP to design PHS-resistant devices, it is imperative to better comprehend PHS. Unfortunately, the issues with the various PHS models created for 3D perovskites also affect their 2D equivalents. Due to varying experimental conditions, it is impossible to determine the segregation resistance by comparing the activation energies in various MHPs. It is widely acknowledged that there are still several unresolved issues with PHS and that the study is made more difficult by inconsistent data in the literature.

About the Study

In this study, the authors examined the effects of light and temperature on PHS in 2D MHPs using a phenylpropylammonium (PPA) spacer. 2D PPA-based MHPs displayed complex segregation behavior that was dependent on temperature and illumination intensity, with the suppression of segregation observed at high temperatures, which were attributed to the highly exothermic nature of the process as well as moderate illumination intensities. The finding highlighted the significance of additional processes present in the proposed material, which displayed distinctly different behavior when compared to 2D MHPs with other aromatic cations.

The team studied the PHS dependency on illumination intensity and temperature in a 2D MHP to explain the divergent events. To track the halide composition corresponding to a 2D MHP having a phenylpropylammonium (PPA) spacer during illumination, in situ absorption measurement was performed. The PPA cation was used as an aromatic spacer with a longer alkyl chain tail. It was demonstrated that layered perovskites with aliphatic and aromatic spacer cations exhibit distinctly different behaviors and that PHS and ion migration were strongly dependent on the used spacer cation.

The researchers demonstrated that for the light intensity of 6.5 mW/cm², the PPA-based perovskite displayed considerably different behavior from both BZA and PEA. For PEA, suppressed segregation was observed at both 25 and 84 °C, whereas for BZA, no suppression was observed at either temperature. For PHS material at the tested lighting power in the range of 6.5–100 mW/cm² and temperature in the range from 25–84 °C, a complex dependence between the segregation and temperature was observed. Then, using the Arrhenius and Van't Hoff equations, the segregation process was explained in terms of kinetics and thermodynamics, respectively.

Observations

At 6.5 mW cm^-2, the predicted reaction Gibbs free energy changed from negative to positive with rising temperature. For 9.5 and 12 mW cm^-2, the reaction entropy decreased in the light-dominated region (LDR) to 5 ± 25 and -2 ± 33 J K^-1 mol^-1, respectively. For 6.5 mW cm^-2, the reaction entropy was 478 ± 28 J K^-1 mol^-1, and for 9.5 and 12 mW cm^-2, it was -1020 ± 190 and -597 ± 249 J K^-1 mol^-1, respectively. Enthalpy changes of -344 kJ mol^-1 and -197 kJ mol^-1 were observed for light intensities of 9.5 and 12 mW cm^-2, respectively, which was consistent with the behavior seen for lower illumination intensity.

In the range of 25 to 71 °C, the equilibrium for 9.5 and 12.0 mW cm^-2 was moderately temperature dependent, which was consistent with the observation of two separate zones in Arrhenius plots. Under 6.5 mW cm^-2, the segregation activation energy was 32.0 ± 3.8 kJ mol^-1. Due to its extremely exothermic nature, photoinduced segregation was hampered at high temperatures during the PHS process, which was predominantly temperature-dependent at moderate intensities.

When the intensity increased to a moderate level of around 0.1 suns, other mechanisms started to take control, and more complex behavior was shown with two separate regions that could be distinguished by their temperature dependency. As a result, shifting of the equilibrium to the mixed state was observed with excitations of 9.5 and 12 mW/cm².

The segregation rate reduced with increasing temperature at an illumination intensity of 1 sun, contrary to the observed behavior at a low light intensity of 6.5 mW/cm². However, at powers above 0.5 sun, segregation was dominant.

Conclusions

In conclusion, this study used an in situ absorption measurement to investigate the PHS process in the PPA₂Pb(Br_0.5I_0.5)₄ film. Without any interruption from remixing in the dark, the segregation process was successfully caught under Xe light. Although segregation rates rise with temperature, the obtained data suggested that PHS could be suppressed by temperature increases. The differential effects of temperature and stimulation intensity on PHS were also observed.

The authors mentioned that the intricate relationship between intensity and temperature emphasizes the need for in situ characterization approaches as well as for additional research using other spacer cations to improve the understanding of PHS in layered halide perovskites.

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References

Leung, T. L., Ren, Z., Syed, A. A., et al. Photoinduced Segregation Behavior in 2D Mixed Halide Perovskite: Effects of Light and Heat. ACS Energy Letters, 7, 3500-3508 (2022). https://pubs.acs.org/doi/10.1021/acsenergylett.2c01688

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