  The radiative recombination of injected charge carriers gives rise to
electroluminescence (EL), a central process for light-emitting diode (LED)
operation. It is often presumed in some emerging fields of optoelectronics,
including perovskite and organic LEDs, that the minimum voltage required for
light emission is the semiconductor bandgap divided by the elementary charge.
Here we show for many classes of LEDs, including those based on metal halide
perovskite, organic, chalcogenide quantum-dot and commercial III-V
semiconductors, photon emission can be generally observed at record-low driving
voltages of 36%-60% of their bandgaps, corresponding to a large apparent energy
gain of 0.6-1.4 eV per emitted photon. Importantly, for various classes of LEDs
with very different modes of charge injection and recombination (dark
saturation current densities ranging from ~10^-35 to ~10^-21 mA/cm2), their EL
intensity-voltage curves under low voltages exhibit similar behaviors,
revealing a universal origin of ultralow-voltage device operation. Finally, we
demonstrate as a proof-of-concept that perovskite LEDs can transmit data
efficiently to a silicon detector at 1V, a voltage below the silicon bandgap.
Our work provides a fresh insight into the operational limits of
electroluminescent diodes, highlighting the significant potential of
integrating low-voltage LEDs with silicon electronics for next-generation
communications and computational applications.
