Late bloomers: copper complexes in organic LEDs

Abstract

In recent years, displays based on organic LEDs (OLEDs) were successfully introduced into consumer electronics as a new, efficient means of converting electricity into visible light. However, the real potential of OLED technology has yet to be unveiled. The dream of using OLEDs for lighting, smart packaging, and other mass-market applications is still in the future, and in fact is even contradicted by the partial reliance of modern commercial OLEDs on metal-organic, phosphorescent iridium compounds as emitting materials. Iridium is one of the rarest metals in the earth’s crust (abundance: 0.0003ppm).1 The reason people still choose it is efficiency. Using triplet harvesting (a technique for converting excitation energy into light), a theoretical efficiency of 100% can be achieved,2 whereas the maximum efficiency of classic fluorescent emitters (such as aluminum oxinate) is limited to 25% as predicted by quantum statistics (see Figure 1).3 Nevertheless, the high price, low abundance, and the special photophysical properties of iridium emitters hamper the manufacturing of affordable large-scale devices as well as the production of large quantities of OLEDs. Consequently, OLED displays have to date been restricted to high-price applications such as smartphones and tablet PCs. Recently, however, new avenues to more abundant emitter materials have begun to open. The so-called singlet-harvesting approach4 (see Figure 1) allows for substitution of phosphorescent heavy-metal emitters with ones that use no metal at all5 or more abundant metals such as copper6 (abundance: 68ppm).1 The idea that copper(I) complexes are now seen as a new, promising class of emitters is interesting, considering that the first OLEDs incorporating such materials were announced mere weeks7, 8 after the groups of Forrest and Thompson published the two papers2, 9 introducing the triplet-harvesting approach Figure 1. Triplet harvesting and singlet harvesting. In an organic LED (OLED), electrical energy is transformed into so-called excitons. Due to quantum statistics, 25% of the excitons have singlet character, while 75% have triplet character. The triplet-harvesting approach (left) transforms all incoming excitons into triplet excitons and uses those to generate light (transition from T1 to S0/. The recently established singlet-harvesting approach (right) transforms all excitons into singlet excitons for the same purpose (transition from S1 to S0/. E: Energy difference. kb: Boltzmann constant. T: Temperature.