Solar Cell Efficiency Questions Finally Finds an Answer

The enigmatic material – spiro-OMeTAD has been cloak-and-dagger for around two decades. Despite research efforts, its utility of this material has constrained till hole-transporting material in perovskite and dye-sensitized solar cells.  But latest study has resurrected the potential of spiro-OMeTAD determining the future of solar cells.

This time, a team have grown single crys tals of the pure material, and in doing so, they have made the surprising discovery that spiro-OMeTAD’s single-crystal structure has a hole mobility that is three orders of magnitude greater than that of its thin-film form (which is currently used in solar cells).

In the new study, the researchers have found out a way to grow pure single crystals of spiro-OMeTAD by dissolving the spiro-OMeTAD in a carefully chosen solvent. They then placed this vial inside a larger vial containing an antisolvent, in which spiro-OMeTAD does not dissolve as well, and allowed the antisolvent vapor to slowly diffuse into the inner vial. Eventually the solution in the inner vial becomes supersaturated, so that not all of the spiro-OMeTAD can stay dissolved, causing the spiro-OMeTAD to crystallize. The researchers then performed a variety of measurements on the crystals to investigate their charge transport mechanisms and other properties.

“This reports are a major breakthrough for the fields of perovskite and solid-state dye-sensitized solar cells by finally clarifying the potential performance of the material and showing that improving the crystallinity of the hole transport layer is the key strategy for further breakthroughs in device engineering of these solar cells,” Osman Bakr, a professor of engineering at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia and leader of the study, told Phys.org.

The findings suggest that, at least in the short term, the time-consuming process of designing and synthesizing radically new organic hole conductors as replacements to spiro-OMeTAD is not mandatory.

Generally, perovskite solar cells and dye-sensitized solar cells are made of three critical layers. Two of these layers—the electron-transporting layer and the light-absorbing layer—are well-understood structurally. However, the mesoscale packing structure of the hole-transporting layer, which is usually spiro-OMeTAD, has so far eluded researchers, and consequently its charge transport mechanisms have remained a mystery.

Making this revolutionary breakthrough, the method used here to grow single crystals cannot be performed at a large scale, the researchers predict that similar methods that use an antisolvent to trigger crystallization could be used to enhance the crystallinity of the thin-layer spiro-OMeTAD, improving its hole mobility in order to make more efficient solar cells.

Further commenting on the development, Bakr said, “These astonishing findings open a new direction for the development of perovskite solar cells and dye-sensitized solar cells by showing the still untapped potential of spiro-OMeTAD.” “They unravel a key mystery that has confounded the photovoltaic community for the last 17 years”, added Bakr.