Smartphones, laptops, and lighting applications rely on light emitting diodes (LEDs) to shine brightly. But the brighter they shine, the more inefficient they become, releasing more energy as heat instead of light. Researchers have demonstrated an approach for achieving near 100% light emission efficiency at all brightness levels.
The approach focuses on stretching or compressing a thin semiconductor film in a way that favorably changes its electronic structure. The team identified how the semiconductor’s electronic structure dictated interaction among the energetic particles within the material. Those particles sometimes collide and annihilate each other, losing energy as heat instead of emitting light in the process. Changing the material’s electronic structure reduced the likelihood for annihilation and led to a near-perfect conversion of energy to light, even at high brightness.
The discovery was made using a single, 3-atom-thick layer of a type of semiconductor material, a transition metal dichalcogenide, that was subjected to mechanical strain. These thin materials have a unique crystal structure that gives rise to unique electronic and optical properties: When their atoms are excited either by passing an electric current or shining light, energetic particles called excitons are created. Excitons can release their energy either by emitting light or heat. The efficiency with which excitons emit light as opposed to heat is an important metric that determines the ultimate performance of LEDs. But achieving high performance requires precisely the right conditions.
For the high exciton concentration at which optical and electronic devices typically operate, though, too many excitons annihilate each other. The new work suggests that the trick to achieve high performance for high concentrations lays in tweaking the material’s band structure, an electronic property that controls how excitons interact with each other and could reduce the probability of exciton annihilation. When more excited particles are created, the balance tilts toward creating more heat instead of light.
The researchers started by placing a thin semiconductor (tungsten disulfide, or WS2) film atop a flexible plastic substrate. By bending the plastic substrate, they applied a small amount of strain to the film. At the same time, they focused a laser beam with different intensities on the film, with a more intense beam leading to a higher concentration of excitons — a high “brightness” setting in an electronic device.
Detailed optical microscope measurements allowed the researchers to observe the number of photons emitted by the material as a fraction of the photons it had absorbed from the laser. They found that the material emitted light at nearly perfect efficiency at all brightness levels through appropriate strain. To further understand the material’s behavior under strain, the team performed analytical modeling. They found that the heat-losing collisions between excitons are enhanced due to “saddle points” — regions where an energy surface curves in a way that resembles a mountain pass between two peaks — found naturally in the single-layer semiconductor’s band structure.
Applying the mechanical strain led the energy of that process to change slightly, drawing the excitons away from the saddle points. As a result, the particles’ tendency to collide was reduced and the reduction in efficiency at high concentrations of charged particles ceased to be a problem.