Characteristics of electroluminescent (EL) devices were improved dramatically using indium tin oxide (ITO) chemically modified with H-, Cl-, and CF3-terminated benzoyl chlorides. By the use of reactive –COCl groups, ITO surfaces were modified quickly and the work function of the modified ITO was changed widely depending upon the permanent dipole moments introduced in the para- position of benzoyl chloride. We also compared the performance of the EL devices with ITO modified with different binding groups (–SO2Cl, –COCl, and –PO2Cl2) of p-chlorobenzene derivatives. Finally, we examined the correlation between the change in the work function and the performance of the EL devices by the chemical modification and found that the enormous increase in ITO work function up to 0.9 eV is possible using phenylphosphoryl dichloride with a CF3-terminal group in the para-position.
We demonstrate here that luminance increased more than linearly with an increase in current density of tris(8-hydroxyquinoline) aluminum (Alq3)-based electroluminescent (EL) devices and the EL efficiency reached ∼5 cd A−1 at 250 mA cm−2 when electron and hole injection was well balanced. The luminance–current curves were well fitted with a combination of a linear and a quadratic function of the current. The quadratic component can be attributed to additional singlet excited state (1Alq3*) formation through triplet–triplet (T–T) annihilation of triplet excited states (3Alq3*). The requirement of the well-balanced charge injection implies that the long-lived A4lq3* was quenched efficiently by energy transfer to excess and colored Alq3−⋅ anion or Alq3+⋅ cation radicals in the emission zone when the charge injection was unbalanced. The short-lived A3lq3* was not quenched appreciably.
An SbCl5-doped N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) thin film was studied as a hole-injection layer in low-molecular-weight organic electroluminescent (EL) devices. EL characteristics of devices with a TPD hole-injection layer doped with other oxidizing reagents, such as iodine, FeCl3, and tris(4-bromophenyl)aminium hexachloroantimonate were compared with that of SbCl5-doped TPD. The device with SbCl5-doped TPD on a cleaned indium–tin–oxide (ITO) substrate exhibited the best performance of all the devices studied. The improvement in device performance was attributed to an increase in work function of ITO due to acid formation as a result of hydrolysis of SbCl5 and by thinning the tunneling barrier for hole injection due to formation of the space charge region in highly doped TPD with SbCl5.
The effects of new insulating materials of lithium fluoride and lithium carboxylates at an
aluminum cathodic interface on the performance of organic electroluminescent (EL)
devices are described. Three different cathode interface materials are used: lithium salts of
fluoride, acetate, and benzoate. We compare the current density–voltage (I–V) and the
luminance–voltage (L–V) characteristics of the devices with Al/lithium carboxylates with those of the devices with Al/LiF or an Al cathode. We find that the bilayer cathodes with lithium carboxylates exhibit better device performance than the cathodes with LiF or the Al
cathode itselt. In particular, the device with lithium benzoate gives the best performance of
all the devices studied. The improvements are attributed to the lowering in work function of
the cathode due to lithium metal formation by the higher reactivity of lithium carboxylates
than LiF, with hot Al atoms impinging on these insulating materials.
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