OLEDs-Exciplex Emission

Bis(dimesitylboryl)-2,2 -bithiophene (BMB-2T, 242) forms a stable amorphous glass and emits pure blue color with a high fluorescence QE of 86% in THF solution [270]. However, an OLED with ITO/m-MTDATA/TPD/BMB-2T/Mg Ag emits with a broad emission due to an exciplex with TPD. The exciplex can be prevented by insertion of a thin layer of 1,3,5-tris(biphenyl-4-yl)benzene (TBB) between TPD and BMB-2T, leading to a pure blue emission. It seems that the boron complex or boron-containing compounds easily form an exciplex with common HTMs. Other similar blue emitter materials also demonstrate such behavior. 

In the OLEDs there is a high possibility that exciplex formation occurs at the ETL/EML or HTL/EML interfaces because HTL and electron transport layer (ETL) usually have an electron-donating and an electron-accepting nature, respectively. There have been some researches on the application of exciplexes for the tuning of emission colors (Li et al 2006, Liang and Choy 2006) and white emitting OLEDs (Tong et al 2007). Extensive studies on exited bi-molecular complexes and their application in electrophosphorescent devices have been done by Kalinowski et al (2007) and Cocchi et al (2006). 

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