This illustration shows the inverse temperature crystallization technique used by researchers from KAUST and Oxford University to produce centimeter-scale, highly pure perovskite crystals. Image reproduced with permission from reference 1© 2017 American Chemical Society.In the race to replace silicon in low-cost solar cells, semiconductors known as metal halide perovskites are favored because they can be solution-processed into thin films with excellent photovoltaic efficiency. A collaboration between researchers at King Abdullah University of Science & Technology (KAUST) in Saudi Arabia and Oxford University in the UK has now uncovered a strategy for using surface tension to grow perovskites into centimeter-scale, highly pure crystals. The researchers describe this strategy in a paper in ACS Energy Letters.
In their natural state, perovskites have difficultly transporting solar-generated electricity because they crystallize with randomly-oriented grains. Osman Bakr from KAUST's Solar Center and co-workers are working on ways to dramatically speed up the flow of these charge carriers using inverse temperature crystallization (ITC). This technique uses special organic liquids and thermal energy to force perovskites to solidify into structures resembling single crystals – the optimal arrangement for device purposes.
While ITC produces high-quality perovskites far faster than conventional chemical methods, the curious mechanisms that initiate crystallization in hot organic liquids are poorly understood. Ayan Zhumekenov, a PhD student in Bakr's group, recalls spotting a key piece of evidence during efforts to adapt ITC for large-scale manufacturing. "At some point, we realized that when crystals appeared, it was usually at the solution's surface," he says. "And this was particularly true when we used concentrated solutions."
The KAUST team partnered with Oxford theoreticians to identify how interfaces influence perovskite growth in ITC. They propose that metal halides and solvent molecules initially cling together in tight complexes that begin to stretch and weaken at higher temperatures. With sufficient thermal energy, the complex breaks and perovskites begin to crystallize.
Interestingly, however, the researchers found that complexes located at the solution surface can experience additional forces due to surface tension – the strong cohesive forces that allow certain insects to stride over lakes and ponds. The extra pull provided by the surface makes it much easier to separate the solvent-perovskite complexes and nucleate crystals that float on top of the liquid.
Exploiting this knowledge helped the team to produce centimeter-sized, ultrathin single crystals and then to prototype a photodetector with characteristics comparable to state-of-the-art devices. Although the single crystals are currently fragile and difficult to handle due to their microscale thicknesses, Zhumekenov explains that this method could help to direct perovskite growth on specific substrates.
"Taking into account the roles of interfaces and surface tension could have a fundamental impact," he says, "we can get large-area growth, and it's not limited to specific metal cations – you could have a library of materials with perovskite structures."
This story is adapted from material from KAUST, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.