Osman Bakr, KAUST Assistant Professor of Material Science and Engineering, led a team of researchers from KAUST’s Solar and Photovoltaics Engineering Research Center (SPERC) and the University of Toronto in developing a novel approach to growing a type of single crystal hybrid organic-inorganic perovskite at room temperature to further understand the material’s remarkable photovoltaic properties. The researchers' work was recently published in the prestigious journal Science.

Natural perovskite is a mineral with the chemical formula CaTiO3. Synthetic perovskites, which are promising compounds for use in photovoltaic technologies, take their name from the fact that they have the same crystalline structure as the original mineral. The efficiency – or the amount of sunlight converted into electricity in solar cells based on these materials – has jumped from 2% to 20% over the past five years.

“That is a huge jump if you consider the history of photovoltaics,” Prof. Bakr said. He noted that it took researchers working with silicon, a material commonly used in photovoltaics, around 30 years to reach 20% efficiency.

Prof. Bakr and the team examined organolead trihalide MAPbX3 perovskites, a material that “seemed magical in some way,” he explained. Solar cells made using this material have reached 20.1% certified power conversion efficiencies, but it has been difficult for scientists to discover the material’s fundamental properties and performance limits, and thus understand its true photovoltaic potential.

“The form of the material used in photovoltaics has a lot of disorder, and there are differences in the structure that have made it difficult to determine the actual properties of the material,” said Prof. Bakr. “Our goal as a research team was to understand the real properties of this material by making it in the most purified and perfect form possible, and that was as single crystals.”

The researchers had to synthesize their crystals in a large enough quantity and of a big enough size to make them of practical use. To do this, they developed a method called antisolvent vapor-assisted crystallization (AVC), and used a different solvent and antisolvent than those commonly employed in the process. This allowed for the growth of high-quality and large-sized crack-free MAPbX3 crystals at room temperature.

The volumes of their single grown crystals exceeded 100 mm3, and the crystals had “amazingly high quality,” Prof. Bakr noted, as measured by a property called the crystals’ trap-state density, which was exceptionally low at approximately 109 – 1010 cm-3.

“This density is a million times lower than the density previously reported in perovskite materials, and is comparable with trap-state density in the highest photovoltaic-quality single crystal silicon, which is grown at very high temperatures,” he explained.

The crystals also had charge carrier diffusion lengths exceeding 10 µm, “another property explaining their amazing performance in solar cells,” Prof. Bakr said. “The charge carrier diffusion length is orders of magnitude longer than previously reported in thin films, and confirms a practical strategy for further efficient device engineering by improving the crystallinity of perovskite thin films used in thin film-based photovoltaic technology.”

Prof. Bakr and the team believe their work has many implications for current and future photovoltaic and other technologies.

“Improving the purity of the material opens up new avenues to use it in a wider range of semiconductor and optoelectronic devices,” Prof. Bakr said. “We can now start to consider its use in applications that require extremely high purity, high quality and high crystallinity semiconductors. Our work also shows that perovskites have immense and yet largely untapped potential for use in related applications, such as lasers, photodetectors and light-emitting diodes.”

- by Caitlin Clark, KAUST News