Polymer light-emitting diodes with a phenyleneethynylene derivative as a novel hole blocking layer for efficiency enhancements

This paper reports on the use of an electron transport layer (ETL) in polymer light-emitting diodes based on poly(2,5-bis(3′,7′-dimethyl-octyloxy)1,4-phenylene-vinylene) (BDMO-PPV). This ETL is inserted between BDMO-PPV and a calcium cathode as a hole blocking layer (HBL). A novel phenyleneethynylene derivative (ImPE) is proposed and compared to well-known materials such as tris(8-hydroxyquinoline) aluminum (Alq3) and bathocuproïne (BCP). Efficient hole blocking is achieved leading to yield improvements at low luminances. With a 8 nm-thick ImPE layer, at 1 cd/m2, the power efficiency reaches 1.2 lm/W whereas a BDMO-PPV-only PLED exhibits a 0.13 lm/W power efficiency. ImPE enables to reach higher performances than Alq3 for low luminances (<20 cd/m2). However, for luminances higher than 350 cd/m2, it is demonstrated that the hole blocking in no more efficient because of a too strong electric field.