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Spiro-OMeTAD Single Crystals Reveal Secret to Enhanced Charge-Carrier Transport

Scientists at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia have achieved a major breakthrough in the fields of perovskite and solid-state dye-sensitized solar cells by unlocking untapped potential in the performance spiro-OMeTAD. 

Spiro-OMeTAD has been the dominant hole conductor for perovskite and dye-sensitized solar cell technology. However, its performance has been stagnant despite researchers’ efforts to exploit the material to its maximum. 

Now KAUST has shown that improving the crystallinity of the hole transport layer is the key to more efficient devices. They grew single crystals of the pure material for the first time and discovered that spiro-OMeTAD's single-crystal structure has a hole mobility three orders of magnitude greater than that of its thin-film form.

“Spiro-OMeTAD demonstrated its initial promise in solar cells in 1998, when it was used as a solid-state hole-conductor in place of the traditional liquid electrolytes of dye-sensitized solar cells,” says study leader Osman Bakr, who is a professor of engineering at KAUST and has been selected as an Innovator under 35 in the Arab World by MIT Technology Review Arab Edition.

“Despite its dominance, spiro-OMeTAD possessed a low charge-carrier mobility and its crystal structure remained unknown, thus researchers where unable to elucidate the charge transport pathways in the material, which is crucial for understanding how to further improve its electrical properties,” Bakr says, adding that fellow researchers had started to give up on spiro-OMeTAD and rather focus on designing new, better hole-transporters. The assumption that spiro-OMeTAD’s performance — born out of the inability to crystallize the material and reveal its ultimate potential and intermolecular packing structure — proved a mistake. Until the KAUST researchers grew pure single crystals of spiro-OMeTAD...

“We used an antisolvent vapour-triggered crystallization (AVC) method by the judicious choice of ‘good’ and ‘bad’ solvents,” Bakr reveals. “Considering that each spiro-OMeTAD molecule contains two π–conjugated fluorene fragments, intermolecular π–π stacking may occur spontaneously upon crystallization.” To exclude all possible disruptive intermolecular (π–π) interactions between spiro-OMeTAD and the solvent molecules, he says they used dimethylsulfoxide (DMSO) as the solvent instead of the commonly used chlorobenzene. And methanol, which is fully miscible with DMSO, was selected as the antisolvent. “Slow diffusion of the methanol into the spiro-OMeTAD solution in DMSO gradually reduces the solubility of spiro-OMeTAD and eventually triggers crystallization once a super-saturation state is reached,” explains Bakr. 

Study could impact design of future generations of perovskite and solid-state dye-sensitized solar cells 

“Our findings provide a straightforward and assured way to greatly enhance the mobility of the hole-transporting layer, and therefore the overall performance of perovskite and solid state dye-sensitized solar cells, by improving the crystallinity of the spiro-OMeTAD layer instead of relying on the effort- and time-consuming approach of designing and synthesizing new molecules with uncertain outcome,” says Bakr.

According to the expert, improving the crystallinity of the hole transport layer is the key to further breakthroughs in solar device engineering. “Improving the mobility of the hole-transport layer has been a key driver for perovskite and solid-state dye-sensitized solar cells achieving high efficiencies — including the latest >20% power conversion efficiencies achieved by perovskites,” Bakr says. “Considering the perfect match of band structures (i.e. energy alignment) of spiro-OMeTAD and perovskite materials, achieving a three-order-of-magnitude mobility enhancement over ploy-crystalline thin films by improving the crystallinity spiro-OMeTAD is expect to pave the way for further impressive growth in the efficiency of perovskite solar cells.”

With this breakthrough study under their belt, the KAUST team is also looking ahead at even more work to be done to advancing the efficiency of perovskite solar cells. “One of the most exciting aspects of our study is that it shows that simple approaches for improving the crystallinity of a material such as spiro-OMeTAD can lead not only to unexpectedly large improvements in the electrical properties, but also allows us to uncover the complete crystal structure of the material,” Bakr says. “As a result, with the crystal structure at hand, theoretical and computational studies can be preformed to describe the charge hopping mechanism of the material and pin-point precisely the parts of the chemical structure that could be modified to vastly improve the electrical properties.” 

Given the large difference between the properties of single crystals and polytcrystalline thin films, he concludes the findings also indicate that there is still a lot of room left for advancing the efficiency of perovskite solar cells by engineering the crystallinity of the different layers.

The paper “Spiro-OMeTAD single crystals: Remarkably enhanced charge-carrier transport via mesoscale ordering” has been published in Science Advances.

Written by Sandra Henderson, Research Editor, Solar Novus Today

Labels: King Abdullah University of Science and Technology,KAUST,perovskite solar cells,solid-state dye-sensitized solar cells,spiro-OMeTAD,hole conductors,spiro-OMeTAD single crystals,Osman Bakr

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