Effects of casting solvents on characteristics of organic EL devices containing poly(l-glutamate) as hole-transport material

The characteristics of EL devices containing α-helical poly(l-glutamate) and having a carbazole side chain (PCELG) were found to strongly depend on the casting solvents. Among EL devices fabricated using chlorine-containing casting solvents such as 1,2-dichloroethane (DCE), 1,1,2,2-tetrachloroethane (TCE), chloroform (CHCl3), and monochlorobenzene (ϕ-Cl), the EL device fabricated using the DCE solvent exhibited the maximum luminance (65.8 cd/m2). The difference between the threshold voltages of the devices ranged up to 6.0 V, despite their fabrication by using solutions with the same composition ratio. The maximal efficiency of the devices fabricated using the DCE solvent was found to be 20 times greater than that of the devices fabricated using TCE solvents. A comparison of the current densities and voltages among devices fabricated using different casting solvents at their maximal efficiencies showed that the maximal efficiencies tended to significantly increase in the order TCE < CHCl3 < DCE solvents at similar voltages (∼15 V), despite a lowering of the current density. The current density was considered to be directly related to the number of carriers injected into the device. The above-mentioned observations suggested that the maximal efficiency in these devices was not the hole and electron injection efficiencies, but the values of some parameters subsequent to carrier injection, such as the recombination rate, amount of excitons generated, and the diffusion length.