Light-emitting devices made from organic materials have the potential to be thin, flexible and lightweight, and might therefore be used in a variety of applications — including foldable display screens, ‘smart’ wallpaper incorporating digital devices, and windows that could be converted into illuminated panels at the flick of a switch. On page 536, Ai et al.1 report the development of organic light-emitting diodes (OLEDs) that use free radicals as the emitter and convert electrons into light with high efficiency. The efficiencies of other types of OLED are generally limited by quantum-mechanical effects, but radical-based OLEDs (ROLEDs) don’t have this constraint, owing to the electronic state of the radicals. The authors’ ROLEDs have the highest emission efficiency obtained so far among LEDs that emit light in the deep-red and infrared regions of the electro-magnetic spectrum.
Several types of LED are being actively developed because they are expected to produce displays that have higher brightness, colour purity, contrast and resolution than conventional lighting devices, while using less energy. OLEDs, in particular, have become familiar in the past decade, because they are used in the displays of mobile phones and televisions. Such displays perform better in several respects (such as contrast and colour reproducibility) than do liquid-crystal displays, which are currently used in many electronic devices.
OLEDs were first reported2 in 1987, and typically have a multilayered structure: a layer of material that contains light-emitting molecules is sandwiched between layers that transport electrons and holes (positively charged quasiparticles formed by the absence of electrons in atomic lattices), which, in turn, are sandwiched by electrodes as the outermost layers (Fig. 1). Additional layers that enable efficient injection of holes and electrons from the electrodes into the transport layers are also sometimes used. When an electric field is applied between the two electrodes, holes and electrons are injected and merge (recombine) on emitter molecules in the light-emitting layer to generate photons. The structure of the emitter molecule determines the colour of the emission.
One problem that still needs to be overcome for OLEDs is their low efficiency, which is quantified by the external quantum efficiency (EQE) — the ratio of the number of photons that leave the device to the number of electrons injected into it on the application of an electric field. The EQE is, in turn, proportional to two factors: the internal quantum efficiency (IQE), which is the efficiency with which photons are generated in the light-emitting layer from injected electrons; and the light outcoupling efficiency, which is the ratio of the number of photons that exit the device to the number generated within it. The value of the outcoupling efficiency is typically 20–30% (ref. 3)3. Quantum mechanics dictates that the IQE of conventional OLEDs based on fluorescent molecules is limited to 25% (ref. 4)4. The remaining 75% of efficiency is lost through recombination pathways that don’t result in light emission. The EQEs of such OLEDs are therefore 5–6% at best.
Several groundbreaking methods have been established to solve the efficiency problem. For example, the IQE of OLEDs has been increased to nearly 100% by using phosphor-escence (rather than fluorescence) as the light-emitting process5, or by using a heat-activated light-emitting mechanism known as delayed fluorescence6. These strategies overcome the problem for devices based on conventional fluorescent molecules, but Ai et al. now report an innovative alternative method: they use organic radical molecules that exploit a different light-emitting mechanism, thereby enabling an IQE of almost 100%.
So what are organic radicals? Most organic molecules have an even number of electrons, in which each electron pairs up with another one, forming what is known as a closed-shell state. Organic radicals, however, have an odd number of electrons, and have one or more unpaired electrons in ‘open-shell’ states. Such radicals are highly reactive and therefore chemically unstable, and are typically generated transiently during chemical reactions. But the reactivity of radicals can be suppressed by modifying their molecular structures, and some are stable enough to be handled under air at room temperature.
In the context of light emission, it has long been thought that almost all stable radicals are non-emissive and inhibit emission from other sources. Nevertheless, stable light-emitting radicals have been available7,8 since 2006, raising the possibility that they could be used in lighting materials and devices. Importantly, it was proposed9 that ROLEDs would have high IQEs because, owing to the radicals’ open-shell electronic states, they don’t exhibit the energy-loss pathways that cause problems in conventional OLEDs.
The first ROLED was reported10 in 2015 by researchers from one of the groups that contributed to the current paper, and it had an EQE of 2.4%. A year later, the same group showed experimentally3 that it should be possible for ROLEDs to achieve an IQE of 100% — a milestone in the history of this LED class. Ai et al. now report another key step in the evolution of ROLEDs: they have developed two stable radicals that emit brightly in the deep-red and infrared regions of the spectrum, and they use them in devices that not only achieve almost 100% IQE, but also have an excellent EQE of 27%. This is the highest EQE among all LEDs that emit similar colours, and is largely a consequence of the efficiency with which electrons are converted into light on the radicals.
The high efficiency of Ai and colleagues’ device is impressive, but ROLEDs in general currently emit light in only a limited range of colours. This is because just a small number of stable light-emitting radicals have been reported, and only those that have a particular type of chemical structure (known as an electron-donating group) deliver high EQEs when used in ROLEDs. Moreover, the electronic characteristics of light-emitting radicals suggest that these molecules will not be good at emitting blue (high-energy) light. A crucial next step will be to establish molecular design principles that enable organic radicals to be tuned to produce a wide range of colours — Ai and co-workers’ radicals are not the first to emit deep-red and infrared light, and so have not extended the colour range.
Nonetheless, Ai and co-workers have demonstrated an innovative method for increasing the EQE of OLEDs, which could not have been achieved through simple developments of conventional fluorescent OLEDs. The authors’ method also increases the number of radicals that can be used in ROLEDs. Given that they were discovered only a few years ago, there is probably plenty of potential for even further improvement — a challenge that offers great opportunities for materials scientists. In this field, radical progress truly promises a bright future.
Nature 563, 480-481 (2018)