Weakness in perovskite crystals uncovered | May 22, 2017 Issue - Vol. 95 Issue 21 | Chemical & Engineering News
Volume 95 Issue 21 | p. 6 | News of The Week
Issue Date: May 22, 2017

Weakness in perovskite crystals uncovered

Iodide salt coatings protect the promising solar-cell materials from attack by oxygen and light
Department: Science & Technology
Keywords: solar energy, solar cells, perovskites, degradation, iodide
Oxygen (red) most easily attacks perovskite materials by occupying an iodide (purple) vacancy.
Credit: Nat. Commun.
A computer graphic showing an perovskite crystal with a dioxygen molecule sitting in an iodide vacancy.
Oxygen (red) most easily attacks perovskite materials by occupying an iodide (purple) vacancy.
Credit: Nat. Commun.

Methylammonium lead halide perovskites (CH3NH3PbI3) show great promise for solar-cell materials because they are more efficient at converting sunlight into electricity than current commercial materials are. Perovskite efficiencies can reach 22%, while those of commercial materials are about 15%. However, these perovskites suffer from a serious drawback: They degrade rapidly upon exposure to oxygen and light.

A group including M. Saiful Islam at the University of Bath and Saif A. Haque of Imperial College London now reports experimental and theoretical evidence for the mechanism behind this degradation (Nat. Commun. 2017, DOI: 10.1038/ncomms15218). Their findings have led them to a way to protect these promising materials.

The team previously determined that when light excites these crystals, it creates electrons, which can react with O2 to make reactive superoxide species. These superoxides then wreak havoc on the perovskite crystal structure, breaking it down to PbI2, methylamine, and water.

In the new work, the group uses computational simulations and experiments to show that O2 diffuses into iodide vacancies in perovskite crystal structures. These spaces were the most vulnerable and facilitated the superoxide degradation process.

With this mechanism in mind, the team tried to stabilize the material by coating it with a thin layer of iodide salts, which reduced the number of iodide vacancies. The protection was striking. Without the coating, the material began degrading within hours and became useless within days. However, crystals blanketed with the iodide salts remained stable for three weeks.

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William Rigdon (May 24, 2017 10:10 AM)
It is fairly difficult to defy the laws of thermodynamic equilibrium for the elements. I highly recommend researchers in this field and other electrochemical cells like artificial photosynthesis or electrolyzers to study the importance of the Pourbaix diagrams and regions of stability for metals/metal oxides in order to devise the most stable electrode materials as well as compatible electrolytes. We must operate in or at least near the boundaries of passivation in order to prevent corrosion. Despite long-standing beliefs that only metals can be used for sufficient conductivity, it should now be obvious that metal oxides, metal halides, and other metal complexes that were thought of as insulators or semi-conductors can be utilized when their defect chemistry is tuned appropriately. You need to look no further than the success of lead oxide or nickel oxide and numerous other examples utilized in today's most prevalent battery chemistries.

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