Photon Upconversion With MOFs

Photon upconversion: energy transfer between the molecules is based on electron exchange (Source: M. Oldenburg / KIT)

Photon upconversion: energy transfer between the molecules is based on electron exchange (Source: M. Oldenburg / KIT)

The upcon­version of photons allows for a more efficient use of light: Two photons are converted into a single photon having higher energy. Researchers at KIT now showed for the first time that the inner inter­faces between surface-mounted metal-organic frameworks (SURMOFs) are suited perfectly for this purpose. They turned green light blue. The result opens up new oppor­tunities for opto­electronic appli­cations such as solar cells or LEDs.

Metal-organic frame­works (MOFs) are highly ordered molecular systems that consist of metallic clusters and organic ligands. At the Institute of Functional Inter­faces IFG of KIT, researchers developed MOFs that grow epi­taxially on the surfaces of substrates. These SURMOFs can be produced from various materials and be customized using different pore sizes and chemical func­tionalities so that they are suited for a broad range of appli­cations, e.g. for sensors, catalysts, diaphragms, in medical device techno­logy or as intelligent storage elements.

Another field of appli­cation is opto­electronics, i.e. components that are capable of converting light into electrical energy or vice versa. Many of these components work on the basis of semi­conductors. “The SURMOFs combine the advantages of organic and inorganic semi­conductors,” Christof Wöll, Director of IFG, explains. “They feature chemical diversity and crystal­linity, allowing us to create ordered hetero­structures.” In many opto­electronic components, a so-called hetero­junction – this is an interfacing layer between two different semi­conductor materials. It controls the energy transfer between the various excited states. Researches of the KIT Institute of Micro­structure Techno­logy IMT now created a new piggyback SURMOF in which a second SURMOF grew epitaxially, layer by layer, on a first one. At this heterojunction, it was possible to achieve photon upcon­version, trans­forming two low-energy photons into a single photon with higher energy. “This process turns green light blue. Blue light has a shorter wave­length and yields more energy. This is very important for photo­voltaics appli­cations,” explains Bryce Richards, Director of IMT.

The photon upcon­version process is based on the so-called triplet-triplet annihi­lation. Two molecules are involved: a sensi­tizer molecule that absorbs photons and creates triplet excited states, and an emitter molecule that takes over the triplet excited states and, by using triplet-triplet annihi­lation, sends out a photon that yields a higher energy than the photons that were ori­ginally absorbed. “The challenge was to create this process as efficiently as possible,” explains Ian Howard, leader of a junior research group at IMT. “We matched the sensi­tizer and emitter layers in a way to obtain a low con­version threshold and a higher light effi­ciency at the same time.”

Since the triplet transfer is based on the exchange of electrons, the photon upcon­version process revealed by the researchers includes an electron transfer across the interface between the two SURMOFs. This suggests the assump­tion that SURMOF-SURMOF hetero­junctions are suitable for many opto­electronic appli­cations such as LEDs and solar cells. One of the limi­tations for the effi­ciency of today’s solar cells is due to the fact that they can only use photons with a certain minimum energy for electric power generation. By using up conversion, photo­voltaic systems could become much more efficient. (Source: KIT)

Reference: M. Oldenburg et al.: Photon Upconversion at Crystalline Organic-Organic Heterojunctions. Adv. Mat., online 8 August 2016; DOI: 10.1002/adma.201601718

Links: KIT Institute of Microstructure Technology IMT, Karlsruhe, Germany • KIT Institute of Functional Interfaces IFG, Karlsruhe, Germany

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