How are the Thermal Properties of Organometal Halide Perovskites Affected by K-Doping?

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Jun 24 2022

In an article recently published in the journal ACS Applied Energy Materials, researchers discussed the thermal stability properties of K-doped organometal halide perovskite for application in photovoltaic materials.

Study: Thermal Stability of K-Doped Organometal Halide Perovskite for Photovoltaic Materials. Image Credit: Perutskyi Petro/Shutterstock.com

Background

Since they were originally used as light-absorbing components for solid-state solar cells with excellent power conversion efficiencies (PCEs), organometal halide perovskites (OHPs) based on methylammonium lead triiodide (MAPbI₃) have attracted significant interest as next-generation photovoltaic (PV) materials.

OHP solar cells have been extensively explored for use in tandem solar cells, where PCEs significantly higher than the Shockley–Queisser efficiency limit for the single-junction solar cells are anticipated because the band gaps (E_g) of OHPs may be adjusted by modifying their compositions. OHPs are the most suitable materials for next-generation solar cells, but their poor thermal and environmental stability has been a significant barrier to their widespread use.

Utilizing encapsulating materials, less reactive hole and electron transport layers, and carbon-based electrodes have all been researched as ways to increase the stability of OHP layers. It has been demonstrated that the stability and PCE of MAPbI₃ can be improved by substituting a few of the monovalent methylammonium ions (MA+) with one or more of the cesium ions (Cs+), formamidinium ions (FA+), or rubidium ions (Rb+), and some of the monovalent iodide ions (I-) with chloride ions (Cl-), bromide ions (Br-), or fluoride ions (F-).

About the Study

In this study, the authors discussed the impact of K-doping on the thermal stability of mixed OHPs. Additionally, the in-situ X-ray diffraction measurement data were used with the Johnson-Mehl-Avrami (JMA) model.

The team created the MAPbI₃ precursor solution by combining 632 μL of N,N-dimethylformamide (DMF), and 71 mL of dimethyl sulfoxide (DMSO) with 159 mg of MAI and 461 mg of PbI₂. In 1 mL of a DMSO and DMF mixed solvent with a volume ratio of 1:4, 529 mg of PbI₂, 188 mg of FAI, 74.3 mg of PbBr₂, and 22.7 mg of MABr were dissolved to create the mixed OHP precursor solution.

The researchers examined the OHPs deposited on quartz substrates in an N₂ atmosphere. The impact of K-doping on OHPs was quantitatively demonstrated by applying the JMA model to analyze the data obtained from isothermal in situ X-ray diffraction (XRD).

Observations

Each OHP had similar n values that were close to 1.5, which was the value that described a diffusion-controlled spherical crystal growth mode. The findings showed that after heating for 10,000 hours at 85°C, K-doped OHPs were remarkably resilient and scarcely broken down. K-doping had a major impact on the thermal stability of OHPs because the E_a value for the decomposition of FA_0.85MA_0.15Pb(I_0.85Br_0.15)₃ with K doping was enhanced to 128.4 ± 5.2 kJ/mol.

It was discovered that the E_a values for the decomposition of MAPbI₃, FA_0.85MA_0.15Pb(I_0.85Br_0.15)₃, and K-doped FA_0.85MA_0.15Pb(I_0.85Br_0.15)₃ were 97.7 ± 6.0, 92.9 ± 9.8, and 128.4 ± 5.2 kJ/mol, respectively.

Even though the perovskite peak at 13.8° in the mixed OHP decreased over time and the PbI₂ peak at 12.5° increased, the rate of change was much lower than that of MAPbI₃, and more than 12 hours were required for all the perovskites to disintegrate.

K-doping further improved the heat resistance of mixed OHPs. The mixed OHPs with and without K-doping exhibited nearly identical X-ray diffraction (XRD) patterns, with peaks at about 12.5°, which indicated the presence of a tiny quantity of PbI₂ in both samples from the start. The mixed OHPs with and without K-doping had activation energies of 128.4 ± 5.2 and 92.9 ± 9.8 kJ/mol, respectively. The findings showed that K-doped OHPs could be stabilized to the point where they were not disintegrated even after 10,000 hours of heating at 85 °C.

After 10,000 hours of exposure to 85 °C, K-doping increased the activation energy from 92.9 ± 9.8 to 128.4 ± 5.2 kJ/mol, which showed that the OHP seldom decomposed.

Conclusions

In conclusion, this study elucidated that isothermal in situ XRD measurements are useful for assessing the durability of OHPs and that K-doping could increase the durability of OHPs for practical application.

The authors mentioned that future research will be able to pinpoint the key components underlying the superior durability of perovskite solar cells through evaluation using stacked films with electron and hole transport layers that are more similar to those of actual devices and measurement in the presence of oxygen and moisture.

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