We designed and synthesized a new host material with a highly soluble and thermally stable indenocarbazole derivative that can make green phosphorescent organic light-emitting diodes (PHOLEDs) in a solution process.

It is very difficult to realize highly efficient and stable solution processed organic light emitting diodes (OLEDs) due to an interface mixing behavior during continuous wet processes. To eliminate this problem, we need to form a cross‐linkable hole transport layer (x‐HTL) before depositing an emitting layer (EML). However, we cannot clearly solve interface mixing problem although we use such x‐HTL during process. Especially, we found that the crosslinking of HTL causes serious reduction of hole mobility, which results in shift of exciton formation zone toward HTL / EML interface. Thus, we need to focus on EML composition because the interface mixing could be accelerated if the thermal stability of materials in EML is very poor. In other words, small molecules in EML often diffuse into the preceding HTL although it's crosslinked. Besides, we also found that the thermal stability of host materials in EML is very important for this interface mixing behavior.