In situ investigation of degradation at organometal halide perovskite surfaces by X-ray photoelectron spectroscopy at realistic water vapour pressure
Journal: Chemical Communications
Publication Date: 19 April, 2017
Shedding new light on the degradation of next generation solar cells
So-called ”organic perovskites” have shown immense promise as cheap light harvesters for a new generation of solar cells, with device efficiencies rising in the last 5 years to more than 22% – now close to that of the conventional material, silicon. However, exploitation of these materials is currently prevented by their instability when exposed to water and air, which leads to rapid degradation. Understanding the mechanism of this reaction would allow researchers to identify methods by which the process may be retarded or even stopped. The difficulty is that the material goes off before the reaction of water at its surfaces can be studied!
Now a cross-disciplinary team of researchers from Physics, Chemistry and Materials Science at the University of Manchester has utilized a new technique, Near-Ambient-Pressure X-Ray Photoelectron Spectroscopy, to measure the reaction of pristine perovskite with water for the first time. The key was to manufacture the perovskite inside the spectrometer, without any contact with air. This allowed them to unambiguously determine the degradation mechanism. These results will help synthetic chemists to design new light harvesting materials, which will not easily decompose when exposed to water, so their full potential can be exploited.
- X-ray photoelectron spectroscopy (XPS) uses the photoelectric effect, for which Einstein was awarded his nobel prize in 1921. We shine X-rays onto the sample and measure the energy of the electrons liberated.
- Normally, this experiment is done in a very high vacuum (like that in interstellar space!), as the electrons get absorbed by any gas around the sample and don’t make it into the analyser. This makes it really hard to study surface reactions.
- In the new technique of near-ambient-pressure XPS, the sample is studied in a reaction cell at more normal pressures – and the electrons that escape enter a very sensitive analyser which has many pumping stages to reduce the pressure rapidly in the analysis zone. There are only around 30 of these highly specialised instruments around the world. Our facility is based in the Photon Science Institute at the University of Manchester.
- In this case, the experiment showed that the perovskite decomposes directly into several simple molecules, which has allowed us to distinguish unambiguously between the different mechanisms proposed for the degradation reaction.