Bright and efficient blue polymer light emitting diodes with reduced operating voltages processed entirely at low-temperature

Abstract

Hybrid blue polymer light emitting diodes (PLEDs) with high efficiencies, luminance >20 000 cd m⁻² and low operating voltages are obtained using processing temperatures ≤150 °C. By briefly applying an electric field across the device prior to measuring (pre-biasing), the PLEDs with unannealed Zn_1−xMg_xO/Cs₂CO₃ injectors have maximum luminances three times higher and operating voltages 26% lower than the previous state-of-the-art, which used ZnO cathodes processed at 400 °C. The high performance of our PLEDs is shown to be linked to the filling of trap states in the unannealed oxide cathode. Further reductions in the operating voltage are obtained through reductions in the electron-injection barrier by incorporating Mg into the ZnO cathode, as revealed by electroabsorption spectroscopy. Device characterization also shows that achieving efficient PLEDs requires the use of an interlayer (in our case Cs₂CO₃) to prevent non-radiative recombination at the cathode. The architecture and device processing methods we develop allow us to produce PLEDs with 80 nm thick emitters that have a turn-on voltage of only 3.7 V. This work takes a major step towards cheap, efficient flexible PLEDs for displays and lighting.