This is an electron microscopy image of the strongly restructured surface of a hybrid organic-inorganic perovskite crystal after treatment with benzylamine. On top of the etched 3D crystal, traces of what appears to be the 2D perovskite can be seen. Image: Loi Lab/University of Groningen.Recent advances in solar cell technology use polycrystalline perovskite films as the active layer, achieving a conversion efficiency of as much as 24.2%. Hybrid organic-inorganic perovskites are especially successful, and they have been used in optoelectronic devices such as solar cells, photodetectors, light-emitting diodes and lasers.
But the surface of hybrid perovskites is prone to defects, or surface traps, where charge carriers are trapped in the semiconducting material. To solve this problem and reduce the number of traps, the crystal surface must be passivated.
This involves treating the perovskites with chemical solutions, vapors or atmospheric gases to remove defects that make the material less effective. Benzylamine is one particularly effective molecule for this purpose. A detailed understanding of the physical and chemical mechanisms by which these treatments work is key to increasing the collection of charge carriers in perovskite solar cells.
In a paper in Applied Physics Reviews, a team of researchers from Germany and the Netherlands describe their work testing hybrid organic-inorganic perovskite crystals treated with benzylamine to investigate the mechanisms by which the surface of the crystal is passivated, and the trap states reduced.
"This molecule has been used in polycrystalline fields in solar cells, and people have demonstrated that the solar cells were improved," said Maria Loi from the University of Groningen in the Netherlands. "We wanted to study, in a clean system, why the solar cells were improving and understand why adding this molecule makes the devices better."
Their experiments revealed that benzylamine enters into the surface of the crystal to create a new, two-dimensional (2D) material – 2D perovskite – on the surface of the three-dimensional (3D) crystal. Where this 2D version forms and later breaks away from the surface, a terraced etching pattern occurs.
"The main purpose was to passivate the surface to reduce defect states," Loi said. "To our surprise, we found out the surface was modified, which was not an expected mechanism. People report that this molecule can improve the quality of devices, but nobody has reported that, in reality, it was creating a two-dimensional layer and could also restructure the material."
The authors also discovered that a combination of benzylamine and atmospheric gases is most effective for passivation. That could mean, Loi said, that more than one type of trap state exists. Further investigation of multiple types of trap states could allow precise tuning of the mechanisms involved in preparing crystals for efficient optoelectronic devices.
This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.