Researchers from Helmholtz-Zentrum Berlin (HZB) and the University of Potsdam have analyzed surfaces and interfaces of CsPbI3 films, produced under different conditions, at BESSY II. They found that annealing in ambient air does not have an adverse effect on the optoelectronic properties of the semiconductor film, but actually results in fewer defects. This could simplify the mass production of inorganic perovskite solar cells.
The best performing perovskite semiconductors contain organic cations such as methylammonium, which cannot tolerate high temperatures and humidity, so their long-term stability is still a challenge. However, methylammonium can be replaced by inorganic cations such as Cesium (Cs). Inorganic halide perovskites with the molecular formula CsPbX3 (where X stands for a halide such as chloride, bromide and iodide) remain stable even at temperatures above 300 °C. CsPbI3 has the best optical properties for photovoltaics (band gap ∼1.7 eV).
Perovskite semiconductors are produced by spin coating or printing from a solution onto a substrate and are typically processed in glove boxes under a controlled atmosphere: There, the solvent is evaporated by heating, after which a thin layer of perovskite crystallizes. This 'controlled environment' significantly increases the cost and complexity of production.
However, CsPbI3 layers can also be annealed under ambient conditions without loss or even with an increase in efficiency of up to 19.8%, which is even better than samples annealed under controlled conditions.
"We investigated the interfaces between CsPbI3 and the adjacent material in detail using a range of methods, from scanning electron microscopy to photoluminescence techniques and photoemission spectroscopy at BESSY II," says Dr. Zafar Iqbal, first author and postdoctoral researcher in Antonio Abate's team.
At BESSY II, the team of Prof. Marcus Bär used hard X-ray photoelectron spectroscopy (HAXPES) to analyze the chemical and electronic structure of the differently annealed CsPbI3 and perovskite/hole transport layer interfaces. "In the samples that were annealed in ambient air, we observed a surface modification that improves the mobility of the charge carriers at the interface," explains Iqbal. Optical spectroscopy showed that annealing in air resulted in fewer defects.
"Our study explains why the annealing of CsPbI3 films in ambient air works well," says Iqbal. This could be very helpful for upscaling processes for potential mass production.
