  Despite widespread interest, ultrathin and highly flexible light-emitting
devices that can be seamlessly integrated and used for flexible displays,
wearables, and as bioimplants remain elusive. Organic light-emitting diodes
(OLEDs) with $\mu$m-scale thickness and exceptional flexibility have been
demonstrated but show insufficient stability in air and moist environments due
to a lack of suitable encapsulation barriers. Here, we demonstrate an efficient
and stable OLED with a total thickness of $\approx$12 $\mu$m that can be fully
immersed in water or cell nutrient media for weeks without suffering
substantial degradation. The active layers of the device are embedded between
conformal barriers formed by alternating layers of parylene-C and metal oxides
that are deposited through a low temperature chemical vapour process. These
barriers also confer stability of the OLED to repeated bending and to extensive
postprocessing, e.g. via reactive gas plasmas, organic solvents, and
photolithography. This unprecedented robustness opens up a wide range of novel
possibilities for ultrathin OLEDs.
