LED technology breaks performance barrier

Light-emitting diodes made from perovskite semiconductors have reached a milestone in the efficiency with which they emit light — potentially ushering in a new platform for lighting and display technology.
Paul Meredith is in the Department of Physics, Swansea University, Singleton Park, Swansea SA2 8PP, UK.

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Ardalan Armin is in the Department of Physics, Swansea University, Singleton Park, Swansea SA2 8PP, UK.

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Light-emitting diodes (LEDs) have revolutionized lighting and displays, not least because they use energy more efficiently than any previous light-emitting technology. Micro-LEDs made from inorganic, ‘compound’ semiconductors are emerging that deliver unprecedented resolution for displays, whereas organic semiconductor LEDs (OLEDs) provide unparalleled colour quality and near-180° viewing angles, and could potentially be used to develop flexible, lightweight displays. In this issue of Nature, two papers1,2 report what could be the birth of a new family of LEDs based on semiconductors called perovskites. Remarkably, the efficiencies with which the perovskite LEDs (PLEDs) produce light from electrons already rival those of the best-performing OLEDs3, and have been achieved in less than four years since the report4 of the first PLED — suggesting that there is plenty of room for even further improvement in their performance.

Perovskites have shot to scientific stardom in the past few years, mostly because they show great promise for solar cells5, but their potential for use in other applications, such as light sensors6 and LEDs4, is rapidly emerging. Crucially, perovskites can be processed from solution (for example, using low-cost, low-tech printing methods), and work well in the designs for optoelectronic devices that are easiest to make. This might allow perovskite-based devices that have large areas (several square centimetres) to be made extremely cheaply, and with low embodied energy (the total energy involved in the entire life cycle of a device).

Cao et al.1 and Lin et al.2 have independently developed PLEDs that break an important technological barrier: the external quantum efficiency (EQE) of the devices, which quantifies the number of photons produced per electron consumed, is greater than 20%. There are several similarities between the devices reported by the two groups. Perhaps most notably, the active (emissive) perovskite layer is about 200 nanometres thick in both cases, and is sandwiched between two relatively simple electrodes. This design is called a planar structure, and is the most basic manifestation of diodes made from thin films of materials (Fig. 1). The electrodes are appropriately modified to ensure that electrons and holes (quasiparticles formed by the absence of electrons in atomic lattices) are efficiently pumped into the perovskite. As in all LEDs, when electrons meet holes, they can release energy in the form of photons through a process known as radiative recombination.

Figure 1 | Improved light-emitting diodes (LEDs) based on perovskite semiconductors. a, LEDs have previously been made from perovskites by sandwiching a thin layer of the semiconductor between a gold electrode and a transparent electrode. However, only about 20% of the light generated in the perovskite escapes from the device. b, Cao et al.1 report perovskite LEDs (PLEDs) in which the semiconductor layer consists of separated submicrometre-sized crystals, partitioned from the gold electrode by a thin layer of an organic material. This design increases the amount of light that escapes. c, Lin et al.2 report PLEDs based on a different perovskite, in which the semiconductor crystals are partly enclosed by an organic compound and the gold electrode is replaced by an aluminium one. This device optimizes the efficiency with which charges (not shown) that are pumped into the perovskite are converted into photons.

Another similarity between the devices is that the perovskite layers were prepared using solutions, from which the semiconductors crystallized to form the emissive components of the LEDs. Cao et al. used a perovskite known as formamidinium lead iodide (FAPI), mixed with an amino-acid additive (aminovaleric acid) to control the size and orientation of the resultant perovskite crystals. FAPI has been quite widely explored as a semiconductor for solar cells, but Lin et al. report a new composite material in which crystals of the perovskite CsPbBr3 (Cs, caesium; Pb, lead; Br, bromine) are partly enclosed by a shell of an organic compound (methyl ammonium bromide; MABr).

Achieving high EQEs in any LED requires the elimination of non-radiative losses — electron–hole-recombination pathways that do not produce photons. Both Cao and colleagues’ and Lin and colleagues’ PLEDs deliver on this equally well. But the two groups also used other, subtly different methods to improve the EQE.

Cao et al. targeted the outcoupling problem, which is well known to those working with thin-film LEDs (such as PLEDs and OLEDs). The outcoupling problem is that the optical physics of planar diodes causes 70–80% of the light generated by the semiconductor to be trapped in the device. Various strategies have attempted to address this issue in OLEDs, such as using diffraction gratings7 and buckling the device8.

But Cao and colleagues took a simpler approach: they optimized their perovskite-processing conditions so that the emissive layer spontaneously forms as distinct submicrometre-scale crystal platelets (Fig. 1). The authors’ computational modelling shows that this submicrometre structuring increases the fraction of light that makes it out of the emissive layer to 30%, compared with 22% for an equivalent ‘flat-layer’ perovskite device (a device in which the perovskite layer does not have submicrometre structuring). In combination with the reduction in non-radiative losses, this results in an EQE of 20.7%.

By contrast, Lin et al. used a flat emissive layer, but tried to optimize the balance of electrons and holes injected into the perovskite, to make the most efficient use of every charge. This seems to be facilitated by the MABr shells that enclose the perovskite crystals. The resulting PLEDs have an EQE of 20.3%.

But caution is advised before ordering your PLED ultrahigh-definition television. OLEDs, and indeed all optoelectronic devices based on organic semiconductors, suffered for many years from stability issues. The first polymer OLEDs9 could emit light for only seconds, and subsequent advances were needed to ensure that smartphone screens and OLED televisions last for tens of thousands of hours. The lifetime of LEDs can be measured by the T50 metric, which is the time for the performance of the device to drop by half. The T50 values of Cao and colleagues’ and Lin and colleagues’ PLEDs are currently modest: 20 hours and 100 hours, respectively.

Furthermore, displays require a minimum of three colours (and preferably more) to create high-quality colour images. Developing a range of colours for OLEDs was a big challenge. Cao and co-workers’ PLED emits in the near-infrared region of the electromagnetic spectrum, and Lin and co-workers’ PLED emits green light — which is definitely a good start. Multiple colours of PLEDs could be generated by altering the composition of the devices, but the same developmental journey as was needed for OLEDs lies ahead.

The two papers also highlight problems that occur every time new optoelectronic materials emerge as a technological platform: inconsistent characterization and a lack of standards. Because Cao and colleagues’ PLED emits light from outside the visible spectrum, they report the metrics of their devices radiometrically — they use a measure that simply takes into account the total emitted power. By contrast, Lin and colleagues describe the emission of their green PLED using photometric measures, which are weighted by the response of the human eye. The two groups also report the peak EQEs at different brightnesses, and therefore at different driving currents. This makes direct comparison somewhat problematic.

Caveats aside, the two papers are a milestone in PLED development. For now, LEDs based on compound semiconductors remain the dominant technology: they outclass the competition in many respects, including cost, efficiency, colour and brightness. They will be hard to beat. But that should not stop the pioneers of perovskite (or, indeed, organic) LEDs from trying.

Nature 562, 197-198 (2018)

doi: 10.1038/d41586-018-06923-y


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