Ultralow-voltage operation of light-emitting diodes

Abstract

For a light-emitting diode (LED) to generate light, the minimum voltage required is widely considered to be the emitter’s bandgap divided by the elementary charge. Here we show for many classes of LEDs, including those based on perovskite, organic, quantum-dot and III–V semiconductors, light emission can be observed at record-low voltages of 36–60% of their bandgaps, exhibiting a large apparent energy gain of 0.6–1.4 eV per photon. For 17 types of LEDs with different modes of charge injection and recombination (dark saturation currents of ~10−39–10−15 mA cm−2), their emission intensity-voltage curves under low voltages show similar behaviours. These observations and their consistency with the diode simulations suggest the ultralow-voltage electroluminescence arises from a universal origin—the radiative recombination of non-thermal-equilibrium band-edge carriers whose populations are determined by the Fermi-Dirac function perturbed by a small external bias. These results indicate the potential of low-voltage LEDs for communications, computational and energy applications.

Dennard Scaling in Optoelectronics: Scientific Challenges and Countermeasures of Micro‐LEDs for Displays

“…Appreciable light emission can be observed even if the potential energy of carriers decreases further to 2.2 eV, which means that carriers can somehow overcome the potential barrier for injection into the active region, which is not offset completely by the applied voltage. [ 15–17 ] This phenomenon is not unique to Micro‐LEDs and can also be found in SSL LEDs and even FETs. [ 18 ] More specifically, a drain current can be registered even though the applied gate voltage is smaller than threshold voltage, which is called “subthreshold leakage.” However, things get more complicated for light‐emitting devices because a considerable carrier density should be established in the active region so that the generation rate of photons can exceed self‐absorption losses.…”

Sub-Bandgap-Voltage Electroluminescence of Light-Emitting Diodes

“…Di and co-workers used high-sensitivity photodetectors to study the threshold voltages in LEDs and measured the apparent threshold voltages at which the minimum photon flux can be detected. 12 Due to the ultrahigh sensitivity, the apparent threshold voltages can be further decreased from ∼0.8E g /e to 0.5E g /e or even lower (here E g is the bandgap of the EML and e is the elementary charge), providing solid evidence of the universality of sub-bandgapvoltage EL in LEDs. What's more, derived from the Fermi− Dirac distribution, Di et al 12 constructed a relationship between EL intensity and applied voltage to explain the origin of sub-bandgap-voltage turn-on, and suggested that EL could occur even at voltages approaching zero.…”

“…The Auger model is frequently applied to explain the sub-bandgap-voltage ELs in quantum dot-, , polymer-, or perovskite-based LEDs. ,− However, we should mention that this model should not be a dominant mechanism. Generally speaking, high carrier density is needed when the Auger process dominates.…”

“…Therefore, we suggest a more accurate and universal quantity, that is, threshold voltage, which is related to the photon flux. Di and co-workers used high-sensitivity photodetectors to study the threshold voltages in LEDs and measured the apparent threshold voltages at which the minimum photon flux can be detected . Due to the ultra-high sensitivity, the apparent threshold voltages can be further decreased from ∼0.8 E g / e to 0.5 E g / e or even lower (here E g is the bandgap of the EML and e is the elementary charge), providing solid evidence of the universality of sub-bandgap-voltage EL in LEDs.…”

“…Due to the ultra-high sensitivity, the apparent threshold voltages can be further decreased from ∼0.8 E g / e to 0.5 E g / e or even lower (here E g is the bandgap of the EML and e is the elementary charge), providing solid evidence of the universality of sub-bandgap-voltage EL in LEDs. What’s more, derived from the Fermi–Dirac distribution, Di et al constructed a relationship between EL intensity and applied voltage to explain the origin of sub-bandgap-voltage turn-on, and suggested that EL could occur even at voltages approaching zero. Therefore, they have given a different insight into the sub-bandgap-voltage EL but based on the same basis, i.e., the thermodynamics.…”