Highly Transparent and Colorless Organic Light‐Emitting Diodes

Herein, transparent organic light‐emitting diodes (T‐OLEDs) are presented with excellent light‐emission performance achieved by virtue of optically transparent functional layers. In this T‐OLED, indium tin oxide (ITO) thin films are used not only as a transparent bottom anode but also as a transparent top cathode. Blue polymer poly(9,9‐di‐ n ‐octylfluorenyl‐2,7‐diyl) (PFO) is used as a host material for the emission layer (EML) of T‐OLEDs, to achieve an optical light transmittance of 70% in the visible range of the light spectrum. By controlled blending of the commercial emitting material Super Yellow with PFO, the light‐emission efficiency of EML is significantly improved through a Förster resonance energy transfer (FRET) radiative recombination between electronic charge carriers. The organic functional layers of T‐OLEDs are protected from plasma damage during direct‐current magnetron sputtering deposition of the ITO cathode, using a spin‐coated bilayer electron‐transport coating of zinc oxide nanoparticle layers and cesium carbonate‐doped polyethyleneimine ethoxylated (d‐PEIE). As a result, T‐OLED exhibits a combined luminance of 1600 cd m −2 from the top ITO cathode (700 cd m −2 ) and bottom glass surfaces (900 cd m −2 ). The T‐OLED shows a device transmittance of 70% in the visible range and a current efficiency twice as high as that of pure PFO‐type T‐OLED.