Researchers at Imperial College London have examined the influence of light and oxygen on the stability of CH3NH3PbI3 perovskite-based photoactive layers. They discovered why the cells degrade so quickly, and suggested a mechanism for slowing the decline.
Until now, researchers believed that water played a dominant role in breaking down the cells, but Dr. Saif Haque and colleagues from the Department of Chemistry at Imperial College London have discovered that cells degrade even in dry air by the action of oxygen and light.
“It was thought that water was the only problem, but we have shown that there is another way these cells break down. Knowing the mechanism will help us overcome a major stumbling block for this exciting new technology,” said Dr. Haque.
When excited by light, the perovskite layer releases electrons. These then react with free oxygen in the air, forming highly reactive ‘superoxides’, which in turn attack the perovskite layer and break it down. A by-product of this reaction is water, which further speeds up the degradation.
The researchers’ proposed solution is adding a layer of material that quickly extracts the electrons released by the perovskite layer before they can react with the oxygen. The team are working to determine exactly how this intervention affects the long-term stability of the cells, but early tests have shown that adding a layer of titanium oxide with a large surface area significantly reduces the amount of superoxide formed.
Another way to tackle the problem would be replacing the organic cation component of the perovskite layer with a less reactive material that is not as prone to attack by superoxides.
