Daniel Balazs from the University of Groningen. Photo: Daniel Balazs.
Daniel Balazs from the University of Groningen. Photo: Daniel Balazs.

Quantum dots are nanometer-sized semiconductor particles with potential applications in solar cells and electronics. Scientists at the University of Groningen in the Netherlands, together with colleagues at ETH Zürich in Switzerland, have now discovered a way to improve how charge is transported in lead-sulphur quantum dots, potentially leading to more efficient quantum dot solar cells. They report their results in a paper in Science Advances.

Quantum dots are clusters of some 1000 atoms that act as one large 'super-atom'. The dots, which are synthesized as colloids –suspended in a liquid like a sort of paint – can be organized into thin films with simple solution-based processing techniques. These thin films can turn light into electricity.

However, scientists have also discovered that the electronic properties of quantum dots create a bottleneck. “Especially the conduction of holes, the positive counterpart to negatively-charged electrons,” explains Daniel Balazs, a PhD student in the Photophysics and Optoelectronics group of Maria Loi at the University of Groningen Zernike Institute for Advanced Materials.

Loi's group works with lead-sulphide quantum dots. When a beam of light produces an electron-hole pair in these dots, the electron and hole do not move with the same efficiency through the assembly of dots. This means they can easily recombine, reducing the efficiency of the light-to-energy conversion. Balazs therefore set out to improve the poor hole conductance in the quantum dots, and to find a toolkit to make this class of materials tunable and multifunctional.

“The root of the problem is the lead-sulphur stoichiometry,” he says. In quantum dots, nearly half the atoms are on the surface of the super-atom. In the lead-sulphur system, lead atoms preferentially fill the outer part of the quantum dot, producing a ratio of lead to sulphur of 3:1 rather than 1:1. This excess of lead makes the quantum dot a better conductor of electrons than holes.

In bulk material, transport is generally improved by 'doping' the material: adding small amounts of impurities. However, attempts to add extra sulphur to the quantum dots have so far failed. But now Balazs and Loi have found a way to do this, and thus increase hole mobility without affecting electron mobility.

Many groups have tried combining the addition of sulphur with other production steps. However, this has caused many problems, such as disrupting the assembly of the dots into the thin film. Instead, Balazs first produced ordered thin films of quantum dots and then added activated sulphur. This meant the sulphur atoms could be added to the surface of the quantum dots without affecting the other properties of the film.

“A careful analysis of the chemical and physical processes during the assembly of quantum dot thin films and the addition of extra sulphur were what was needed to get this result,” says Balazs. “That's why our group, with the cooperation of our chemistry colleagues from Zürich, was successful in the end.”

Loi's team is now able to add different amounts of sulphur, allowing them to tune the electric properties of the super-atom assemblies. “We now know that we can improve the efficiency of quantum dot solar cells above the current record of 11%,” says Balazs. “The next step is to show that this method can also make other types of functional devices such as thermoelectric devices.”

This study also underlines the unique properties of quantum dots: they act as one atom with specific electric properties. “And now we can assemble them and can engineer their electrical properties as we wish. That is something which is impossible with bulk materials and it opens new perspectives for electronic and optoelectronic devices.”

This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.