By BizLED Bureau
July 19, 2017: German researchers have claimed that use of copper as a fluorescent material will assist to manufacture of inexpensive and environmentally friendly organic light-emitting diodes (OLEDs). Team of scientists at Karlsruhe Institute of Technology (KIT), CYNORA have now measured the underlying quantum mechanics phenomenon of intersystem crossing in a copper complex. The Science Advances journal witnessed the results of this fundamental work to enhance the energy efficiency of OLEDs.
OLEDs are hailed as upcoming source of light. They produce brilliant colors and high contrasts and homogeneously emit light in all observant directions. Now, it is a possibility to manufacture transparent and flexible OLEDs, design options result and new application , such as flat light sources on window panes or displays that can be rolled up. OLEDs functionality consists of an ultra-thin layer of organic materials, which serve as emitter located between two electrodes. When voltage is applied, holes (positive charges) from the anode and electrons from the cathode are injected into the emitter, where they form hole- electron pairs. These so-called quasiparticles and excitons in excited state. They release energy when they decay into their initial state again.
Excitons may possibly assume two different states: Triplet excitons release their energy in the form of heat whereas singlet excitons decay immediately and emit light. Usually, 75% triplets and 25% singlets are encountered in OLEDs. Triplet excitons have to be used to generate light to enhance energy efficiency of an OLED. In conventional LEDs heavy metals, such As iridium and platinum are added for this purpose for conventional OLED production methods, these materials are expensive and have a limited availability.
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It is much cheaper and environmentally compatible to use copper complexes as emitter materials. Thermally activated delayed fluorescence (TADF) ensures high light yields and high efficiency: Triplet excitons are transformed into singlet excitons which in turn emit photons. TADF is based on the quantum mechanics phenomenon of intersystem crossing (ISC), a transition from one electronic excitation state to another one of changed multiplicity, i.e. from singlet to triplet or vice versa. In organic molecules, this process is determined by spin-orbit coupling. This is the interaction of the orbital angular momentum of an electron in an atom with the spin of the electron. In this way, all excitons, triplets and singlets, can be used for the generation of light. Copper luminescent material reaches an efficiency of 100% with TADF.
Larissa Bergmann and Stefan Bräse of KIT?s Institute of Organic Chemistry (IOC), with researchers from OLED technology company CYNORA and the University of St Andrews, United Kingdom, measured the speed of intersystem crossing in a highly luminescent, thermally activated delayed fluorescence copper(I) complex in the solid state. The results are being shared in the Science Advances journal. The scientists successfully determined a time constant of intersystem crossing from singlet to triplet of 27 picoseconds (27 trillionths of a second). And the reverse process known as reverse intersystem crossing, from triplet to singlet is slower and leads to a TADF lasting for an average of 11.5 microseconds. These measurements improve the mechanisms understanding, leading to TADF and facilitate the specific development of TADF materials for highly energy-efficient OLEDs.