25 April 2018 Excitons in 2D materials: Breaking strong attraction

Artistic Illustration of the Excitons

ICFO researchers unveil how excitons in 2D materials split into free carriers in next-generation photodetectors and solar cells. Some of the most important technologies of our time, such as photovoltaics and optical communication systems, rely on the conversion of light into electricity. The amazing properties of newly discovered 2D materials are envisioned to dramatically boost this process.

When light is absorbed by a semiconducting 2D material, the electrons and holes are so attracted to one another by Coulomb forces that they bind into quasi-particles called excitons. These excitons have many useful properties, but in order to generate electricity, they need to be split into free charge carriers. How exactly such strongly bound excitons could dissociate was still puzzling scientists.

In a recent study published in Nature Communications, ICFO researchers Mathieu Massicotte, Fabien Vialla, Peter Schmidt, Mark Lundeberg and Diana Davydovskaya, led by ICREA Professor at ICFO Frank Koppens, have, for the first time, observed and identified the process by which excitons break into free carriers.

With the help of theoretical groups led by Vladimir Fal’ko from the University of Manchester, Kristian S. Thygesen from the Technical University of Denmark and Thomas G. Pedersen from Aalborg University, the ICFO researchers have been able to solve this riddle by measuring the current generated in an atomically thin p-n junction by ultrafast laser pulses. This allowed them to track the exciton dissociation process over time and attribute it to a quantum mechanical process called tunnel ionization.

Regarding the experiment, Mathieu Massicotte emphasized that “from a scientific perspective, one thing that was surprising to us was how fast these strong excitons were splitting, even when we applied relatively small electric field. We found that quantum tunneling really gives a boost to this dissociation process”.

With respect to the outcomes of the study, Prof. Koppens adds that “these new results provide clear guidelines that will help unlock the tremendous potential of 2D materials for optoelectronic applications”.