Electroluminescent devices based on rare-earth tetrakis β-diketonate complexes

Abstract

In this paper the synthesis, photoluminescence and electroluminescence investigation of the novel tetrakis β-diketonate of rare-earth complexes such as, M[Eu(dbm) 4] and M[Tb(acac) 4] with a variety of cationic ligands, M = Li +, Na + and K + have been investigated. The emission spectra of the Eu 3+ and Tb 3+ complexes displayed characteristic narrow bands arising from intraconfigurational transitions of trivalent rare-earth ions and exhibited red color emission for the Eu 3+ ion ( 5D 0→ 7F J , J = 0–6) and green for the Tb 3+ ion ( 5D 4→ 7F J , J = 6–0). The lack of the broaden emission bands arising from the ligands suggests the efficient intramolecular energy transfer from the dbm and acac ligands to Eu 3+ and Tb 3+ ions, respectively. In accordance to the expected, the values of PL quantum efficiency ( η) of the emitting 5D 0 state of the tetrakis(β-diketonate) complexes of Eu 3+ were higher compared with those tris-complexes. Therefore, organic electroluminescent (EL) devices were fabricated with the structure as follows: indium tin oxide (ITO)/hole transport layer (HTL) NPB or MTCD/emitter layer M[RE(β-diketonate) 4] complexes)/Aluminum (Al). All the films were deposited by thermal evaporation carried out in a high vacuum environment system. The OLED light emission was independent of driving voltage, indicating that the combination of charge carriers generates excitons within the M[RE(β-diketonate) 4] layers, and the energy is efficiently transferred to RE 3+ ion. As a best result, a pure red and green electroluminescent emission was observed from the Eu 3+ and Tb 3+ devices, confirmed by ( X, Y) color coordinates.