Recombination zone in white organic light emitting diodes with blue and orange emitting layers

White fluorescent OLED devices with a 10 nm thick blue-emitting layer and a 31 nm thick orange-emitting layer have been fabricated, where the blue-emitting layer is stacked on a hole transport layer. An interlayer was inserted between the two emitting layers. The thickness of the interlayer was changed among 0.3, 0.4, and 1.0 nm. White emission with CIE coordinates close to (0.33, 0.33) was observed from all the OLEDs. OLED with 0.3 nm thick interlayer gives the highest maximum luminous efficiency (11 cd/A), power efficiency (9 lm/W), and external quantum efficiency (5.02%). The external quantum efficiency becomes low with increasing the interlayer thickness from 0 nm to 1.0 nm. When the location of the blue- and orange-emitting layers is reversed, white emission was not obtained because of too weak blue emission. It is suggested that the electron–hole recombination zone decreases nearly exponentially with a distance from the hole transport layer.

► White organic light emitting diodes with interlayer inserted between blue- and orange-emitting layers were fabricated.
► Maximum external quantum efficiency decreases with increasing the interlayer thickness.
► Width of electron–hole recombination zone decreases nearly exponentially with distance from hole transport layer.
► Width of recombination zone becomes narrow with increasing applied voltage.
► White emission was obtained by adjusting layer thickness of emitting layer adjacent to hole transport layer.