Regional approximation approach to space charge limited tunneling injection in polymeric devices

We developed a theory that explains the charge injection and transport in conjugated polymer films such as those used in organic light emitting diodes. Using the regional approximation the effect of the space charge on the current density of electrons tunneling from metal electrodes to the lowest unoccupied molecular orbital of a polymer film is calculated. The space charge is considered to decrease with increasing distance of the injecting electrode. If the space charge occupies only a limited region between the tunneling distance and the collecting electrode , the current (I) is found to depend on a power law of the applied voltage (V). However, if the space charge occupies all the region between the tunneling distance and the collecting electrode, I is found to vary exponentially on V for lower V values and follow approximately the law of Child at high V. The space charge limited tunneling current (SCTC) theory gives the same results of the space charge limited conduction theory when the energy barrier for charge carrier injection is small or when the polymer layer is thick. The SCTC theory is compared to the experimental data. It is shown that there is a good agreement between theory and experiment, concerning both current magnitude and current versus voltage dependence. (C) 2000 American Institute of Physics. [S0021-8979(00)04806-4].