|Colloidal quantum dot solar cell. Credit:T. Sargent, Univ. Toronto|
Colloidal quantum dots (CQDs) could potentially be an ideal semiconductor material for solar cell panels. They are cheaper to produce than standard semiconductors and they can be painted on a wide range of materials. But the high surface-to-volume ratio of CQDs has posed a continuous problem. With so many small dots making up a film on a solar cell, there are countless surface sites where electrons might get stuck in midgap traps within the semiconductor bandgap, decreasing the effective energy conversion of CQD cells.
Traditionally, the midgap traps are filled – or passivated – by long organic ligands during CQD synthesis. But since long ligands are insulators, they don’t make for a good conducting film. To make the film, the long ligands are stripped off and replaced with shorter versions, but the replacement is not universal. Parts of the bandgap are left unpassivated, leaving sites in which electrons can be trapped.
Now, a team from the University of Toronto led by Professor Ted Sargent, along with scientists from the King Abdullah University of Science & Technology in Saudi Arabia, has created a new technique to plug up the surface defects using a hybrid combination of organic and inorganic materials. This hybrid technique created a record 7% efficiency for a CQD device, paving the way for a cheaper, efficient solar cell.
To passivate the maximum number of midgap traps, after synthesis, while the dots were cooling, a metal halide salt solution of CdCl2 dissolved in coordinating solvents was added to the reaction flask. When the Cd and Cl separate in solution, the chlorine is able to bind into the smallest trenches on the surface of the CQDs. The remaining long ligands are removed during the solid state processing of the CQDs and replaced with shorter ligands of mercaptopropionic acid (MPA). Between the short ligands and the chlorine binding, a much larger portion of the surface is passivated than with organic ligands alone.
“We’ve shown that using combinations of organic and inorganic materials can lead to creative ways to passivate a surface trap,” says Susanna Thon, a co-lead author on the paper. “And this shows that there is a path forward through materials chemistry that can help up us achieve higher efficiency in practical devices.”
The hybrid approach of passivation using small organic ligands and inorganic compounds also aided in increasing the density of CQDs in the film. This has also been a hurdle in creating efficient solar cells as empty space is detrimental to electronic transport. The team speculates that the bidentate organic linkers increase density by crosslinking the films. Also, because they are packed in so tightly there are more CQDs within a given film thickness, which means more light absorption per unit of surface area.
The hybrid technique brought about a 7% certified efficiency, a 37% increase over the previous record of 5.1% efficiency. While that still does not quite measure up to near 20% efficiency of traditional silicon semiconductors in commercial solar panels available now, Thon hopes that the low cost of making CQDs may actually make this form of solar panel more cost effective in the future, when CQD cell efficiency reaches 10%, likely in the next few years.