For a long time now, scientists have been searching for materials with which light sources can be produced more quickly and cheaply in the future, while at the same time conserving the environment and resources. PSI researchers may have found a promising approach here.
The compound under investigation is a yellowish solid. If you dissolve it in a liquid or apply a thin layer of it to an electrode and then apply an electric current, it glows an intense green. This is because the molecules absorb the energy supplied to them and gradually emit it again in the form of light. This process is called electroluminescence. Light emitting diodes are based on this principle.
The green luminescent substance is a hot candidate for producing OLEDs, organic light-emitting diodes. OLEDs have been used in the displays of smartphones, for example, for about three years. In the meantime, the first flexible television screens with these materials are also coming onto the market. OLEDs also make cost-effective large-area room lighting possible.
Previous approaches expensive and inefficient
However, you first have to find the right materials. This is because many of the substances suitable for OLEDs contain expensive metals such as iridium, which prevents their use on a large scale and over large areas. Without such additives, however, the materials can only actually emit a small portion of the energy supplied to them as light, while the rest is lost as oscillation energy, for example.
The aim of current research is to find more efficient materials for more cost-effective and environmentally friendly displays and large-area lighting. Inexpensive and readily available metals such as copper promise progress here.
The solution: CuPCP
Researchers have now examined the copper-bearing compound CuPCP in more detail. Four copper atoms are located in the middle of each molecule, surrounded by carbon and phosphorus atoms. Copper is a relatively inexpensive metal, and the compound itself can be easily produced in large quantities - ideal conditions for large-scale use.
"We wanted to understand what the excited state of the compound looks like," says Grigory Smolentsev, physicist in the Operando spectroscopy research group. In other words: How does the substance change when it absorbs energy? Does this change the structure of the molecule, for example? How is the charge distributed among the individual atoms after excitation? "This tells us what the probable energy losses are that are not released as light," adds Smolentsev, "and this shows us how we can perhaps minimize these losses.
Using two large-scale research facilities at PSI - the Synchrotron Light Source Switzerland SLS and the X-ray free-electron laser SwissFEL - and the European Synchrotron Radiation Facility in Grenoble, France, Smolentsev's researchers investigated the short-lived excited states of the copper compound.
Measurements confirm efficiency
The measurements confirmed that the substance is a good candidate for OLEDs due to its chemical structure. The quantum chemical properties of the compound make a high light yield possible. One reason for this is that the molecule is relatively stiff and its 3D structure changes only slightly when excited. Researchers can now set about further optimizing the substance for use in OLEDs.
Moreover, the measurements at the three major research facilities - at PSI and in Grenoble - were not only aimed at studying this one copper-containing compound. There was more to it than that: the experimental data obtained in this way help to improve the theoretical calculations of molecules. "In the future, it will be easier to predict which compounds are suitable for OLEDs and which are less so," says Grigory Smolentsev. "The measurement data will help chemists to understand which part of the molecule stands in the way of high efficiency. And, of course, how the compound can be improved to increase its light output."