<p>Flexible red
OLEDs based on a quadruple layer stack in-between electrodes with 160 mm<sup>2</sup>
active area were fabricated in ambient air on PET via slot-die coating. For the
OLED structure PET/ITO/PEDOT:PSS/PVK/PFO:tPDI<sub>2</sub>N-EH/ZnO/Ag the ink
formulations and coating parameters for each layer were systematically
evaluated and optimized. The air-stable red-light emitting material tPDI<sub>2</sub>N-EH
was successfully utilized as blended homogeneous film with PFO for the emitting
layer. The use of an organic hole-transport layer (PVK) and inorganic electron
injection layer (ZnO) significantly improved the brightness of the reference
device from 4 cd/m<sup>2</sup> to 303 cd/m<sup>2</sup>. Surface analysis using AFM
measurements showed that PVK interlayer reduced the surface roughness of the hole
injection layer (PEDT:PSS) from 0.45 nm to 0.17 nm, which improved the ability
to form uniform emitting layers on top. In addition, the ZnO interlayer improved
the average roughness of the device from 1.26 nm to 0.85 nm and reduced the
turn-on voltage of the device from 5.0 V to 2.8 V.</p>