LAUSR

Organic light‐emitting transistors (OLETs) are emerging as a type of multifunctional devices that integrate the electrical switching and the light‐emitting function. Among the various device structures, multilayer OLETs have shown superior performance due to their flexibility in structure design and material choice. However, multilayer OLETs usually work in the unipolar mode, resulting in extreme unbalance between the holes and electrons, restricting the further improvement of their performance. Here, an investigation of the hole blocking layer (HBL) is presented. The results show that a high electron mobility is not necessarily in a first place when choosing the HBL, while a deeper highest occupied molecular orbital level of the HBL can better facilitate the exciton recombination efficiency. By using bis[(4‐tert‐butylphenyl)‐1,3,4‐oxadiazolyl]phenylene instead of commonly used bathophenanthroline (Bphen) as the HBL, the external quantum efficiency (EQE) is more than doubled for a blue fluorescent OLET, and an EQE as high as 10.3% is achieved in a green phosphorescent OLET with a maximum brightness of ≈8000 cd m−2. The findings in this work highlight the importance of carrier blocking in building high efficiency OLETs and might provide some guidance for their structure design and material choice.