Synthesis of lithium-quinolate complexes and their use as emitter and interface materials in OLEDs

We synthesized lithium quinolate complexes, 8- hydroxyquinolinolato lithium (Liq) and 2-methyl-8- hydroxyquinolinolato lithium (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'-diphenyl benzidine (DMeOTPD) as hole transport material (HTL). The lithium complexes were also examined as interface materials in combination with 8- hydroxyquinolinolato Al(III) as emitter material. The device efficiency using these complexes were optimized using combinatorial methods. We also compared the electron injection, transport and emission properties of Li-complexes with the well known emitter Alq<SUB>3</SUB> in the same experiment 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 Alq<SUB>3</SUB> devices. Contrary to the Alq<SUB>3</SUB> emission, the Li-quinolates exhibit bathochromic shift of emission compared to the respective photoluminescence spectra. No clear evidence for exciplex formation was seen by comparing the photoluminescence spectrum of an equimolar mixture of Li-quinolate and DMeOTPD with the observed EL spectrum. However, the lithium complexes increase the efficiency of an optimized ITO/DMeOTPD/Alq<SUB>3</SUB>/Al device considerably when used as a thin interface layer between Alq<SUB>3</SUB> and aluminum. The improvement of device characteristics using lithium quinolates is similar to that obtained using LiF salt. The mechanism of improvement of efficiency using a lithium complex interface layer seems to be of chemical doping nature.