a Schematic diagram of cross-sectional view of the stack present in polymer light-emitting diode (PLED). The device structure is Indium tin oxide (ITO)/Zinc oxide (ZnO)/Polyethyleneimine ethoxylated (PEIE): 8-quinolinolato lithium (Liq)/Superyellow/Molybdenum oxide (MoOX)/Al. b Comparison of the lifetime without encapsulation and turn-on voltage of previously reported organic light-emitting diode (OLED), which are represented with black diamonds, and PLED, with blue upward triangles. c Current density-luminance-voltage curves of PLED on glass (black curves) and ultraflexible substrate (red curves). d A photograph of the ultraflexible PLED during operation. Operation voltage of PLED was 6 V. Scale bar indicates 1 cm. e Current efficiency characteristics of the PLED on glass (black curves) and ultraflexible substrate (red curves). f Air operational stability of the PLED under constant-current operation in ambient air. Current density to drive PLED was set to be 8 mA/cm2. The black line indicates the PLED on the glass substrate without any encapsulation; the red line indicates the PLED on the glass substrate with parylene encapsulation; and the blue line indicates the PLED on the ultraflexible substrate with parylene encapsulation. The ultraflexible PLEDs were measured in the freestanding state.