A group of American scientists discovered that LED semiconductors that are slightly bent to an atomic thickness can emit light with an efficiency close to 100%, and avoid a decrease in efficiency as the brightness increases-which usually plagues these LEDs.
From smartphone screens to low-energy lighting, light-emitting diodes (LEDs) have changed the world many times. But the efficiency of LEDs tends to decrease as the brightness increases-a problem that is particularly troublesome for a new and interesting two-dimensional semiconductor material, the so-called transition metal dihalides (TMDs). The significant efficiency drop of these atomically thin materials at high brightness hinders their application in practical applications.
Now, researchers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory may have found a very simple way to bypass the efficiency barriers that these LEDs are prone to encounter.
The team has proven that applying a mechanical strain of less than 1% on the TMD can change the electronic structure of the material, and even at high brightness levels, it is sufficient to achieve nearly 100% light emission efficiency (photoluminescence quantum yield). The research team believes that this result can enable a new generation of LED equipment to avoid efficiency erosion caused by increased brightness.
In all organic and some inorganic LEDs, the drop in efficiency at high brightness is rooted in a phenomenon called exciton-exciton annihilation (EEA).
When an energy source such as an electric current or a laser beam excites a semiconductor, it kicks negatively charged electrons from the valence band of the semiconductor into the conduction band, leaving positively charged electron holes.
In semiconductors with the correct properties, electron-hole pairs still exist in the form of neutral quasi-particles called excitons. The subsequent radiation recombination of electrons and holes in the excitons results in the emission of photons, thereby producing visible light emission from the LED.
At low exciton density, almost all excitons have enough space for radiation recombination, and the quantum yield of TMD LED is close to 100%. However, as the brightness of the LED increases and the density of excitons increases, the excitons begin to collide and erase each other, resulting in non-radiative attenuation, or EEA, dissipated in the form of heat. Result: The photoluminescence efficiency of this ultra-thin material decreases as the brightness increases.
The number of non-radiative EEAs largely depends on the details of the semiconductor energy band structure. The Berkeley research team found that, especially for TMD semiconductors, the number of EEAs is enhanced by the van Hove singularity.
The van Hove singularity is a slight distortion in the energy structure of a semiconductor, which increases the density of states (the number of possible energy states that can be occupied) at that point.
In order to solve the EEA problem under high exciton density, Berkeley researchers studied methods to adjust the energy band structure of TMD materials. They found that applying uniaxial strain – literally stretching the material slightly – worked well.
In their experiments, the team installed many different TMDs, including single-layer WS2, WSe2, and MoS2. On a flexible plastic substrate, a hexagonal boron nitride layer (as a gate insulator) and a graphene layer (as a gate) were added electrode). Then, the researchers applied a voltage bias on the device, excited the material with a laser beam to generate excitons, and measured the photoluminescence quantum yield of the material as the laser intensity (and exciton density) increased.
The team found that for unstrained TMD, as expected, the quantum yield decays as the exciton density increases. However, slightly bending the flexible substrate and applying a tensile strain of 0.2% to the TMD will result in a significant reduction in the amount of roll-off. When the tensile strain is 0.4%, there is no effective efficiency drop under high brightness, and the material can maintain nearly 100% photoluminescence quantum yield regardless of the exciton density.
The team’s analysis shows that the effect of tension on quantum yield is related to the existence of “saddle points” in the semiconductor energy band structure-similar to the mountain channel in its energy landscape. In unstrained materials, the saddle point, that is, the region of the Van Hove singularity, is located near the favorable energy of the exciton-producing exciton annihilation, thereby enhancing the level of exciton annihilation. Slightly bending the material can reshape the band structure and fully move the saddle point so that the van Hove singularity is not conducive to exciton annihilation. This, in turn, allows more exciton radiation recombination and increases the quantum yield of photoluminescence.
Although most of the team’s experiments involve mechanical peeling of various two-dimensional material sheets, the researchers can also prove the beneficial effect of strain on the quantum yield of large-area (centimeter-level) WS2 sheets. Grown by an extended chemical vapor deposition process. Researchers believe that this additional discovery points to the prospect of a new generation of LEDs that are not affected by efficiency loss attenuation at high brightness.