20 November 2017

Researchers at Linköping University and Umeå University have succeeded in increasing the efficiency of light-emitting electrochemical cells, LECs or LEECs, so their luminance equals that of a panel of organic light-emitting diodes (OLEDs), and their efficiency is on a par with a fluorescent lamp.

Light-emitting electrochemical cell
A light-emitting electrochemical cell is a thin, flexible and lightweight source of light. Technically it resembles an organic light-emitting diode (OLED), which consists of a number of layers positioned between two electrodes. When voltage is applied to the material it lights up; you send electrons and holes (positive charges) into the active layer, and out come photons.
LEC:s can have a lot of different coloursLEC:s can have a lot of different colours Photo credit: Shi Tang och Andreas Sandström/UmU
Electrochemical cells can also be produced in a wide range of colours and as large surfaces.
However so far their efficiency has been too low; some researchers have even argued that it is impossible to produce electrochemical cells that are both bright and efficient.

Nature Communications

In an article in Nature Communications, the researchers present cells with a luminance as high as 2,000 cd/m2, which is equal to modern OLED lighting panels. Their efficiency, i.e. the proportion of electrons that are converted to photons, is 27.5 per cent, which equals the efficiency of a fluorescent lamp.

Furthermore, electrochemical cells are much less expensive to manufacture than OLEDs. To produce the layers for OLEDs you need an absolutely clean environment and a near-perfect vacuum. On the other hand, the layers in the electrochemical cells are formed spontaneously, as soon as voltage is applied, which makes that they can be printed in a printing press under normal ambient conditions.

Exciton collides

The problem the researchers have solved can be simplified as follows:
When an electron that is sent into the active layer finds a hole, an exciton is formed. The exciton floats around for a while before it emits its energy in the form of a photon. If the exciton collides with some other charge, it transfers its energy to this instead, and heat is formed, rather than light. Here, efficiency will be low. This phenomenon was scientifically proven a few years ago by LiU professor Martijn Kemerink and his doctoral student Stephan van Reenen, from the Department of Complex Materials and Devices. Now, together with the research group at Umeå University, led by Professor Ludvig Edman from the Department of Physics there, they solved the problem.

They found a way to dope the material so that traps are formed. The excitons get caught in these, and are prevented from floating around. However, the transport of electrons and holes into the active layer is also slowed down by the traps - the traps appear also in the layers where they aren’t wanted. But by carefully adjusting the number of traps, the depth of the traps and the concentration of the doping material, the researchers were able to neutralise the traps where they aren’t wanted, and could get them to work only in the light-emitting layer.

One problem to solve

Professor Martijn KemerinkProfessor Martijn Kemerink Photo credit: Monica Westman“The idea came from Ludvig Edman, and since then, through experiments and simulations, we have together developed a combination of materials that has increased the efficiency of the electrochemical cells five times,” says Martijn Kemerink.

There is, however, still one problem facing the researchers. The life of an electrochemical cell is too short: about one week. But research is underway to solve this problem as well.

“With the performance we have achieved, high luminance, high efficiency, lots of design options and low cost, the new light source should be able to be a serious competitor to the most efficient light sources available today,” Prof Kemerink concludes.

S. Tang, A. Sandström, P. Lundberg, T. Lanz, C. Larsen, S. van Reenen, M. Kemerink and L. Edman: Design rules for light-emitting electrochemical cells delivering bright luminance at 27.5 percent external quantum efficiency. Nature Communications 8. 1190 (2017). doi:10.1038/s41467-017-01339-0.

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