Comparative study of using different alkali metal alkylcarboxylates as electron injection materials in OLEDs

Abstract

A comparative study of using alkali metal alkylcarboxylates as electron injection materials for different electron transfer layers in OLEDs is carried out based on the next two directions. Firstly, by fixing the (CH₃)₃CCOO⁻ anionic group and gradually changing the alkali metal ions; (CH₃)₃CCOOLi, (CH₃)₃CCOONa, (CH₃)₃CCOOK, (CH₃)₃CCOORb and (CH₃)₃CCOOCs are used as electron injection materials in OLEDs. In a multilayer 10-(benzo[d]thiazol-2-yl)-2,2,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-pyrano[2,3-f]pyrido[3,2,1-ij]quinolin-11-one (C-545T) doped OLED, when tris(8-quinolino-lato)aluminium (Alq₃) is used as the electron transport layer (ETL), the devices with these alkali metal pivalate complexes as the EIL can all achieve high luminous efficiencies of up to 20 cd A⁻¹. However, when 2-methyl-9,10-di(naphthalen-2-yl)anthracene (m-ADN) is used as the ETL, the electron injection property and device efficiency are determined by the activities of the alkali metal, and only the devices with (CH₃)₃CCOORb and (CH₃)₃CCOOCs as the EIL can achieve high efficiency. Secondly, by fixing the Cs⁺ ion and changing the alkyl group; (CH₃)₃CCOOCs, (CH₃)₂CHCOOCs, CH₃CH₂COOCs, CH₃COOCs, Cs₂CO₃ and CsF are used as electron injection materials. No matter whether Alq₃ or m-ADN is used as the ETL, the devices with these organic cesium complexes as the electron injection layer exhibit similar electron injection property even when changing the alkyl chain. However, for the devices with m-ADN as the ETL, and Cs₂CO₃ or CsF as the EIL, the stability of device in the air is poor, which leads to a much lower luminous efficiency.