Current-limiting mechanisms in polymer diodes

We examined the current-voltage (JV) characteristics of poly[2-methoxy,5-(3,7-dimethyloctyloxy)]-1,4phenylenevinylene (MDMO-PPV) diodes in the framework of the model for space-charge-limited currents (SCLCs). The thickness of the MDMO-PPV layer was varied between samples, and the effect of using different metals for the cathode on the device performance was investigated. Since the SCLC model alone could not explain the experimental data, modifications to it were individually investigated: (1) a double Gaussian density of states to account for deep traps, (2) injection of electrons from the back contact resulting in a recombination current, and (3) the influence of the built-in potential resulting from the asymmetric work functions of the electrodes. Although the JV curves of organic diodes are often discussed in literature in the context of SCLC in the presence of traps, the introduction of deep traps into the SCLC model could not explain the observed cathode dependence. Considering recombination processes in the model could account for the cathode dependence of the experimental data, but not for the behavior observed when the thickness of the semiconducting layer was varied. The SCLC model in combination with a built-in potential and a parallel resistance was able to explain both the thickness and cathode dependence of the JV characteristics. (c) 2006 American Institute of Physics.