Enhancing Tandem Perovskite LEDs Through Photon Recycling

Light-emitting diodes (LEDs) have long been the focus of efforts to push forward performance in brightness, efficiency, and durability. One emerging strategy is the tandem LED—where multiple light-emitting units are stacked vertically in series. The benefit: you combine the light output from each layer and reduce current density per layer, which helps minimize degradation and boosts overall luminance. 

Recently, perovskite materials have gained traction as the emissive layers in LED devices, thanks to their impressive characteristics: high photoluminescence quantum yields, adjustable bandgaps, and solution-processable fabrication methods. These make perovskite LEDs (PeLEDs) exciting for applications in displays and lighting.  

But while single-unit perovskite LEDs have made strong progress, building effective tandem structures has been difficult. Key obstacles: managing recombination at the interface, ensuring efficient charge transport across layers, and optimizing how photons behave in stacked devices. 

A particularly advantageous trait of perovskite materials is their small Stokes shift—which means the difference between absorption and emission energy is minimal. This characteristic allows for significant photon recycling within the device: photons emitted by one layer can be re-absorbed by a neighbouring layer and re-emitted, thereby enhancing overall light output.  

In tandem LED architectures, efficient photon recycling addresses a major issue: lots of light gets trapped in waveguide modes or lost within the structure. Recycling enables that light to be used again, boosting extraction efficiency. But harnessing this requires clever engineering of the interlayer between stacked emissive units—in particular optimizing optical coupling while preserving electrical isolation.  

In a recent breakthrough, researchers demonstrated fully solution-processed tandem perovskite LEDs, integrating two stacked perovskite light emitters. They achieved not only the additive effect of stacking but also benefit from this photon recycling mechanism.  

The results are noteworthy: the device exhibited a low turn-on voltage of 3.2 V (indicating efficient charge injection and minimal losses), and a peak external quantum efficiency (EQE) of 45.5%—which significantly exceeded what you’d get from simply summing the individual single-unit LEDs. An average peak EQE of around 40.9% was also reported across multiple devices.  

Operational stability is also promising: the device showed a half-lifetime of 64 hours at an initial radiance of 70 W sr⁻¹ m⁻². That moves tandem perovskite LEDs closer to practical use in lighting and display systems. 

A key technical enabler was the interlayer engineering: the layer separating the two emissive perovskite units was made very thin and highly transparent to maximize photon transmission and recycling, yet maintained electrical decoupling to prevent leakage between layers. This careful balance was achieved via solution processing methods, which suggests more cost-effective and scalable fabrication. 

Moreover, the inherent material properties—particularly the small Stokes shift—enhanced photon recycling: the photons emitted by one layer are at energies highly re-absorbable by the adjacent layer, enabling multiple absorption/re-emission cycles that effectively increase the radiative lifetime and light output.  

What does this mean for the future? Tandem perovskite LEDs with optimized photon recycling are poised to revolutionize solid-state lighting and displays by offering high brightness, narrow spectral widths (excellent color), high energy efficiency, and improved longevity—all with the potential for lower production costs thanks to solution processing. 

Beyond just lighting, the underlying design and photon-management strategies could also influence other optoelectronic areas—such as photovoltaics—where tandem stacking and photon recycling are used to push efficiency limits. The design blueprint from this work may be applicable in layered perovskite architectures across devices.