Using carbon dots to increase the efficiency of solar cells and LEDs

Scientists have developed a method of increasing the efficiency of solar cells and light-emitting diodes (LEDs), using carbon dots.

Scientists across the globe are actively working to make the process of generating energy from renewable sources as efficient as possible. Researchers typically work with the active layer of the cells, which is responsible for absorbing luminous energy. These active layers are commonly made from silicon, gallium arsenide and perovskite. The efficiency, cost and durability of a solar cell depend not only on the active layer but also on the auxiliary layers.

The auxiliary layers of a solar cell can belong to the electron-transport or hole-transport type. When sunlight reaches the active layer, pairs of electrons and electron holes are formed. Then, they need to be taken to their corresponding electrodes, the electron-transport layer is responsible for extracting and transferring the negative charge from the active layer, while the hole-transport layer performs the same operations with the positive one.

How can this process be improved?

An international group of scientists have proposed a new method of creating auxiliary layers for solar cells and LEDs using carbon dots, an environmentally friendly material that can easily be obtained both in laboratory and industrial conditions.

“Carbon dots are carbon-based nanoparticles with a diameter of two to ten nanometres. Their surface always contains various functional groups that largely determine the properties of this material. The application of carbon dots in solar batteries isn’t something new, what’s important is the modification of their surface by means of working with the functional groups.

“A different ratio of these groups on the surface determines the electronic configuration of carbon dots. Consequently, tailoring this allows us to get the optimal values of the working functions of the electrodes and the energy levels of the transport layers on which they are applied.

“This makes it possible to obtain optimal configuration with maximum efficiency. This approach is universal for different types of devices, which for the first time has allowed the use of carbon dots for increasing the operational efficiency of light-emitting diodes,” explains Aleksandr Litvin, a senior research associate at ITMO University and a co-author of the research.

How can this be applied to LEDs?

The material can be used for solar cells as well as the auxiliary layers of LEDs. LEDs have a broadly similar structure, but the process there is reversed: electrons and holes don’t have to be removed from the active layer, instead, they are injected into it in order to create electron-hole pairs the recombination of which in the active layer will ensure luminescence.

“Devices have been created, and their properties have been tested. In the case of perovskite-based solar cells, we managed to obtain an increase in efficiency from 17.3% to 19.5%, that is, by almost 13%. In the case of light-emitting diodes, depending on the material of the emission layer, the external quantum efficiency (the ratio of the number of photons emitted by an LED to the number of electrons injected into it) increased by 2.1 – 2.7 times,” concludes Aleksandr Litvin.

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