Solution electrochemical studies have been conducted of the principle lumophores, dopants, and hole-transport agents of aluminum-quinolate(Alq3)-based organic light-emitting diodes (OLEDs) along with the characterization of their electrogenerated chemiluminescence (ECL). In acetonitrile/benzene solvent mixtures, Alq3 shows single one-electron reduction and oxidation processes, with a separation between the first oxidation and first reduction potentials, ΔE electrochemical = 3.03 V, close to the estimates of energy difference between HOMO and LUMO levels obtained from absorbance spectra of thin films of Alq3, ΔE optical = 3.17 eV. A new sulfonamide derivative of Alq3, (Al(qs)3), showed a positive shift (ca. 0.32 V) in the first reduction potential versus the parent molecule, and resolution of the overall reduction process into three successive, chemically reversible, one-electron reductions. Two successive one-electron oxidations are seen for 4,4‘-bis(m-tolyphenylamino)biphenyl (TPD), a hole-transporting material in many bilayer OLEDs, and for TPDF2, a fluorinated version of TPD, with TPDF2 oxidation occurring 0.1 V positive of that for TPD. Electrogenerated chemiluminescence reactions (Alq3 -•/TPD+• (or TPDF2 +•) and Al(qs)3 -•/TPD+• (or TPDF2 +•)) were found to produce emission spectra from Alq3*s or Al(qs)3*s states which were nearly identical to those seen from OLEDs based upon these molecules. Emission intensities increased with the increasing potential difference between the relevant redox couples. The diisoamyl derivative of quinacridone (DIQA), a quinacridone dopant for certain Alq3-based OLEDs, undergoes two successive one-electron reductions and two successive one-electron oxidations. The ECL reactions DIQA-•/DIQA+•, DIQA+•/Alq3 -•, DIQA+•/Al(qs)3 -•, DIQA-•/TPD+• and DIQA-•/TPDF2 +• all produce the same singlet emissive state, DIQA*s, and the same emission spectral response seen in quinacridone and DIQA-doped OLEDs.

The offsets between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) at the 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA)/chloroindium phthalocyanine (ClInPc) and PTCDA/zinc phthalocyanine (ZnPc) organic heterojunctions were estimated from a combination of X-ray and ultraviolet photoemission (XPS,UPS) measurements. This combined method allows an improved determination of the electronic structure of such organic/organic‘ interfaces due to the separate determination of the band bending (charge redistribution) following heterojunction formation. Both interfaces have large offsets in the onset for photoemission from their HOMO levels (PTCDA/ZnPc:  0.88 eV; PTCDA/CnlInPc:  0.93 eV). Using thin film absorbance data, the corresponding offsets in LUMO levels were estimated to be 0.66 eV (PTCDA/ZnPc) and 0.34 (PTCDA/ClInPc). The ZnPc/PTCDA interface showed a significant interface dipole (0.25 eV) while the ClInPc/PTCDA contact was essentially dipole free.

A soluble derivative of quinacridone, N,N′-di-isoamyl quinacridone (DIQA), has been synthesized and used to study the mechanisms of Förster energy transfer and charge transfer in organic light-emitting diodes (OLEDs) based on 8-hydroxyquinoline (Alq3). Quantum efficiencies and spectra were measured for both photoluminescence (PL) and electroluminescence (EL) for films of poly(9-vinylcarbazole) (PVK) doped with Alq3 and DIQA. Both PL and EL showed an efficiency enhancement in films of PVK:Alq3:DIQA compared to films of PVK:Alq3. However, the optimal DIQA doping concentration was found to be lower for EL than for PL. Examination of the spectra revealed that more emission originated from DIQA for EL than for PL at a given doping level. We conclude that Förster energy transfer from Alq3 to DIQA occurs in both cases of PL and EL, but that charge transfer to DIQA occurs in the operation of the OLED resulting in additional pathways to DIQA emission. Ultraviolet photoelectron spectroscopy measurements showed that electron transfer from Alq3 to DIQA, hole transfer from PVK to DIQA, and hole transfer from Alq3 to DIQA are all energetically favorable processes. These results suggest that charge transfer is an important mechanism in the efficiency enhancement seen in OLEDs based on a host–dopant scheme, and that both the electronic properties and the optical properties of the dopant material are important parameters for device optimization.

Ultraviolet photoelectron spectroscopy was performed on phthalocyanine samples in the gas phase and as thin vapor-deposited solid films. The results are compared to those of molecular orbital calculations and electrochemical redox experiments in solutions and as solid films. Electron-withdrawing groups or substituents are introduced to the phthalocyanine system to increase the ionization potential and electron affinity. An almost parallel shift of photoelectron spectra is obtained. Unsubstituted phthalocyaninatozinc (PcZn), complexes of tetrapyridotetraazaporphyrins (TPyTAPZn, TPyTAPVO) and hexadecafluorophthalocyanines (F16PcZn, F16PcVO, F16PcAlF) have been studied. The alignment of energy levels in organic heterojunctions prepared from the substituted phthalocyanines and the unsubstituted zinc complex is discussed in detail. The offset between the highest occupied electronic levels, the establishment of a surface dipole, and the redistribution of charge in the heterointerface region beyond molecular contact is shown.