Bandgap Engineering for Ultralow‐Voltage Operation of Organic Light‐Emitting Diodes

The minimum voltage required for a light‐emitting diode (LED) to emit light has been suggested to depend on the bandgap of the emitter. However, the correlation between the bandgap of the emitter and the operating voltage in organic LEDs (OLEDs) has remained unclear because OLEDs require a complex multilayer configuration that includes many materials other than the emitter. Here, a simple equation describing the minimum voltage required for OLED operation, expressed by using the bandgap of the emitter and other energies relevant to OLEDs, is reported. The equation is derived from the characteristics of simple‐structure OLEDs consisting of only electron‐donating (D) and electron‐accepting (A) molecules showing exciplex emission from the green to near‐infrared regions. The minimum voltage required for OLED operation is demonstrated to be more than 0.7 V lower than the emitter's bandgap divided by the elementary charge. The relatively high exciton binding energy, which is unique to organic materials, contributes to lowering the operating voltage. On the basis of these findings, D/A molecules have been newly designed for the ultralow‐voltage operation of OLEDs, and green and blue OLEDs that emit light at ≈1.5 V are realized via triplet–triplet annihilation photon upconversion.