Organic Light Emitting Diodes(OLED) are built up in layers. The layers of an OLED are responsible for injecting and transporting the charge carriers.
As in semiconductors, electrons and spurious electrons (holes) serve as charge carriers. The electrons are generated in the cathode and injected into the Electron Transport Layer( ETL); the holes in the anode and Hole Injection Layer( HIL) and drift via the Hole Transport Layer ( HTL) to the Emission Layer( EML), where they recombine with the electrons, emitting light. The Hole Transport Layer thus has a transport function for the holes, not only in OLEDs but also in perovskite solar cells.