Lithium−Quinolate Complexes as Emitter and Interface Materials in Organic Light-Emitting Diodes

We synthesized lithium−quinolate complexes, 8-hydroxyquinolinolatolithium (Liq) and 2-methyl-8-hydroxyquinolinolatolithium (LiMeq), as emitter and electron injection/transport materials to be used in conventional two-layer organic light-emitting diodes in combination with N,N'-bis(p-methoxyphenyl)-N,N'-diphenylbenzidine (DMeOTPD) as hole transport material (HTL). The lithium complexes were also examined as interface materials in combination with 8-hydroxyquinolinolato-Al(III) (Alq3) as emitter material. The device efficiency with these complexes was optimized by combinatorial methods. We also compared the electron injection, transport, and emission properties of Li complexes with the well-known emitter Alq3 in the same experiment by taking advantage of the combinatorial approach. The Li quinolates are found to be efficient emitter molecules. But the efficiencies of lithium quinolate devices are lower than that of Alq3 devices. Contrary to the Alq3 emission, the Li quinolates exhibit a bathochromic shift of emission compared to the respective photoluminescence spectra. No clear evidence of exciplex formation was seen by comparing the photoluminescence spectrum of an equimolar mixture of Li quinolate and DMeOTPD with the observed electroluminescence spectrum. However, the lithium complexes increase the efficiency of an optimized indium−tin oxide (ITO)/DMeOTPD/Alq3/Al device considerably when used as a thin interface layer between Alq3 and aluminum. The improvement of device characteristics with lithium quinolates is similar to that obtained with LiF salt.