Low-cost caesium phosphate as n-dopant for organic light-emitting diodes.

We demonstrate the usage of caesium phosphate (Cs3PO4) as a low-cost n-dopant for conductivity enhancements of the electron transport layer in organic light-emitting diodes (OLEDs). Up to 9 orders of magnitude, reaching 2.3 × 10-5 S/cm with only 10% dopant concentration in 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline single-carrier devices, were achieved. Luminance current voltage and impedance spectroscopy measurements confirmed the increased conductivities of the single carrier devices in green light-emitting diodes. Constant operation voltages of below 4 V were realized independently of the electron transport layer (ETL) thickness, which was varied from 50-150 nm. Dielectric constants were modeled by usage of the least squares method. Measured and calculated capacitances based on the geometric properties and modeled dielectric constants varied by less than 1.3% for doped OLEDs, confirming the highly conductive nature of Cs3PO4-doped ETLs and demonstrating that a highly conductive electron transport layer can be understood as an extended cathode. [ABSTRACT FROM AUTHOR]