Ensuring the long‐term stability of high‐performance organic light‐emitting diodes (OLEDs) has remained a great challenge due to their limited lifetime and durability. Herein, a novel functional interlayer consisting of a poly(amic acid)‐polyimide copolymer is introduced for use in OLEDs. It is shown that an OLED sample with a polyimide‐copolymer interlayer exhibits high peak brightness of nearly 96 000 cd m−2 and efficiency of ≈92 cd A−1, much higher than those (≈73 000 cd m−2 and ≈83 cd A−1) of a well‐organized reference OLED. Moreover, the growth of dark spots is strongly suppressed in the sample OLED and the device lifetime is extended considerably. Further, highly stable and uniform large‐area OLEDs are successfully produced when using the interlayer. These improvements are ascribed not only to the excellent film‐forming and hole‐transferring properties but also to the inner passivating capability of the polyimide‐copolymer interlayer. The results here suggest that the introduction of an inner passivating/encapsulating hole‐transferable polyimide‐copolymer interlayer together with conventional external encapsulation technology represents a promising breakthrough that enhances the longevity of high‐performance next‐generation OLEDs.