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Perovskite: unleash the light of a new LED technology

2023-09-01

Light management is the key to overcoming the efficiency bottleneck of perovskite LEDs.

Perovskite LEDs represent a new generation of light source technology for display, lighting and communications. They combine the flexibility and thinness of OLEDs, with the broadband color tunability similar to III-V LEDs. With years of active research worldwide, encouraging breakthroughs were made in improving the efficiencyand, more recently, stability of perovskite LEDs to rival the performance of the mainstream LED technologies.

External quantum efficiencies of over 20% were achieved for perovskite LEDs of various colors. While the internal efficiencies for some high-performance perovskite LEDs are close to 100%, around 80% of the light generated within the LEDs still cannot escape the fate of being trapped, absorbed and wasted. This problem limits the maximum external efficiency achievable to 20-30%, hindering the development of perovskite LEDs.

Recently, a team of international researchers from China, UK, Korea, Greece, Germany and Sweden published an important review paper entitled “Light management for perovskite light-emitting diodes” in Nature Nanotechnology, pointing out that the key to overcoming the efficiency bottleneck of perovskite LEDs is the management of light (Zhao et al, Nat. Nanotechnol. (2023), www.nature.com/articles/s41565-023-01482-4). After analyzing the optical losses in the currently best-performing devices, the authors outlined a range of strategies for extracting more photons and enhancing LED efficiencies. The corresponding authors of the paper are Prof. DI Dawei (Zhejiang University), Prof. Richard Friend (University of Cambridge) and Prof. Abd. Rashid bin Mohd Yusoff (POSTECH). Prof. ZHAO Baodan (Zhejiang University) and Dr. Maria Vasilopoulou (National Center for Scientific Research “Demokritos”) are the first authors of the paper.

Figure 1: Overview of light management in perovskite LEDs. Source: Nature Nanotechnology (2023).

To improve the efficiency of perovskite LEDs, the main challenge lies in the non-ideal outcoupling of light, as the most of the internally generated photons are trapped in the LED device stack. These photons can subsequently lose energy via parasitic absorption of the LED functional materials and electrodes. Once the physical origins of these losses are clearly identified, it is possible to overcome the present efficiency limitations of perovskite LEDs through light management strategies, which can be briefly divided into two categories: the tailoring of intrinsic optical properties of the perovskite and other functional layers, as well as the modification of extrinsic properties related to the device structures.

Figure 2: Optical power distributions and losses in perovskite LEDs. Source: Nature Nanotechnology (2023).

Prof. DI Dawei, corresponding author of the paperat Zhejiang University, said: “An important advantage of perovskite LEDs over OLEDs and other thin-film LEDs is that they offer new possibilities for the management of light”, said Di. “For example, the strong optical absorption, the small Stokes shift between absorption and emission, and the high luminescence efficiencies of the perovskites allow highly effective photon recycling. This process rescues the photons that are otherwise wasted in the device structure, gives them a second chance and converts them into useful light output.”

Figure 3: Analyses of the photon recycling effect in perovskite LEDs. Source: Nature Nanotechnology (2023).

Prof. ZHAO Baodan, the first author of the paper, said: “Apart from photon recycling, some useful structural and optical properties can be easily achieved with the perovskite materials, including controlled transition dipole moment orientations, tunable refractive indices, and the simple formation of light scattering structures”, said Zhao. “These factors can help to raise the efficiencies to new heights.”

Figure 4: Light scatterers and outcouplers for perovskite LEDs. Source: Nature Nanotechnology (2023).

The new possibilities of light extraction indicate a brighter future for perovskite LEDs, which could provide a strong technical edge over OLEDs in terms of the highest efficiencies attainable. The authors anticipate that ultrahigh external efficiencies approaching 100% may even become possible in perovskite LEDs when the optical losses are effectively minimized, highlighting the exceptional potential of this emerging LED technology.