Investigation of the luminescent properties of metal quinolates (Mqx) for use in OLED devices
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Since Tang and VanSlyke developed the first organic light emitting diode (OLED) in the late 80’s using tris-(8-hydroxyquinoline) aluminium (Alq3) as both the emissive and electron transporting layer, a lot of research has been done on Alq3 and other metal quinolates (Mqx). The optical, morphological and electrical properties of these Mqx have been studied extensively. Alq3 has, however, a disadvantage as it tends to degrade when stored under atmospheric conditions. These degraded products are non-luminescent and lead to poor device performance. A good understanding of what happens during the degradation process and ways of eliminating this process are needed. In this study different Mqx compounds were synthesized and their degradation behavior was studied to see what effect it has on their luminescent properties. One way to tune the emissive colour of Alq3 is to introduce electron-withdrawing or electron-donating groups (EWG and EDG) onto the hydroxyquinoline ligands. These groups have an effect on the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. In this study Alq3 powders were synthesized with an EDG (-CH3) substituted at position 5 and 7 ((5,7-dimethyl-8-hydroxyquinoline) aluminium) (5,7Me-Alq3) and EWG (-Cl) at position 5 ((5-chloro-8-hydroxyquinoline) aluminium) (5Cl-Alq3). A broad absorption band at ~ 380 nm was observed for un-substituted Alq3. The bands of the substituted samples were red shifted. The un-substituted Alq3 showed a high intensity emission peak at 500 nm. The 5Cl-Alq3 and 5,7Me-Alq3 samples showed a red shift of 33 and 56 nm respectively. Optical absorption and cyclic voltammetry measurements were done on the samples. The optical band gap was determined from these measurements. The band gap did not vary with more than 0.2 eV from the theoretical value of Alq3. The photon degradation of the samples was also investigated and the 5,7Me-Alq3 sample showed the least degradation to the UV irradiation over the 24 h of continuous irradiation. By encapsulating the Alq3 molecule with glass (SiO2) or a polymer-like polymethyl methacrylate (PMMA), the oxygen and moisture responsible for degradation have a lesser effect on the degradation of the Alq3 molecule. The as prepared SiO2-Alq3 sample’s emission was blue shifted by 10 nm from that of Alq3. The sample was subjected to UV irradiation and after 24 hours, no luminescence intensity was detected. According to literature the SiO2 will decompose into Si and O species under UV irradiation. These O species have reacted with the Alq3 to form non-luminescent products. The Alq3:PMMA samples showed a maximum emission at 515 nm. There was a decrease in luminescence intensity when the sample was irradiated with UV photons. This was due to the decomposition of PMMA into elemental species and the O again reacted with the Alq3 molecule to form non-luminescent products. However, the intensity stabilized after 100 h of irradiation. X-ray photoelectron spectroscopy (XPS) and infra red (IR) measurements were done on the as-prepared and degraded Alq3 samples. It revealed that the Al-O and Al-N bonds stayed intact, but C-O and C=O bonds formed during degradation, indicating that the phenoxide ring ruptures during degradation. It is known that the luminescent centre of the molecule is located on the quinoline rings and the rupturing of one of these rings will destroy this centre, leading to a decrease in luminescence intensity. When the Al3+ ion was replaced with a Zn2+ ion to form Znq2, it showed higher emission intensity and, compared to Alq3, did not degrade as fast. This might be due to the fact that Znq2 only has two quinoline rings. The effect of solvent molecules, in the solid state crystal lattice, on the photoluminescence properties of synthesized mer-[In(qn)3].H2O. 0.5 CH3OH was studied. Single crystals were obtained through a recrystallization process and single crystal x-ray diffraction (XRD) was performed to obtain the unit cell structure. The main absorption peaks were assigned to ligand centered electronic transitions, while the solid state photoluminescence excitation peak at 440 nm was assigned to the 0-0 vibronic state of In(qn)3. Broad emission at 510 nm was observed and was ascribed to the relaxation of an excited electron from the S1-S0 level. A powder sample was annealed at 130 °C for two hours. A decrease in intensity was observed and could possibly be assigned to a loss of solvent species. To study the photon degradation, the sample was irradiated with an UV lamp for ~ 15 hours. The emission data was collected and the change in photoluminescence intensity with time was monitored. High resolution XPS scans of the O-1s peak revealed that after annealing, the binding energy shifted to lower energies indicating a possible loss of the H2O and CH3OH present in the crystal. The O-1s peak of the degraded sample indicated the possible formation of C=O (~ 532.5 eV), C-O-H and O=C-O-H (~ 530.5 eV) on the phenoxide ring. Commercial Alq3 is normally used in the fabrication of OLEDs. In this study Alq3 was synthesized using a co-precipitation method and it was purified using temperature gradient sublimation. The Alq3 was then used to fabricate a simple two layer OLED with a device structure: ITO/NPB/Alq3/Cs2CO3:Al. The electroluminescence (EL) spectrum of the device consisted of a broad band with a maximum at ~520 nm and was similar to the photoluminescence (PL) spectrum observed from the synthesized Alq3 powder. The luminance (L)–current density (J)–voltage (V) characteristics of the device showed a turn on voltage of ~ 2 V, which was lower than the current density of the device fabricated using the commercial Alq3. The external quantum efficiency (ηEQE) and the power conversion efficiency (ηP) of the device were 1% and 2 lm/W, respectively.