Space-charge and injection limited current in organic diodes: A unified model

•Unified treatment of the space-charge and injection limited currents in organic diodes.•Unified relation between the free charge density at the metal–organic interface (boundary condition) and the current density.•Theoretical and experimental validation of the model with well established pure-injection models.•Reduced number of parameters in the model and reduced computational time.•Tool to characterize fundamental aspects of organic devices: impurities, trapping effects, degradation, or interface treatment.

Space-charge and injection limited currents in organic diodes have been analyzed using a unified model. Both currents have been modeled using the transport equations combined with a proper value of the free charge density at the metal–organic interface. The method has been applied to diodes with different organic materials, metal contacts and lengths of the organic materials. This unified model accurately reproduces published current–voltage curves for a variety of diode structures operating at different temperatures and voltages. The results of the method, in the injection regime, have been compared with the results of pure injection models developed by other authors. The reduction of the computational time and the number of parameters in our model are important advantages of our procedure. Moreover, it is an alternative where the injection models start to fail: at low applied voltages, close to the Ohmic regime, and for low heights of the energy barrier at the interface. The treatment is complemented with a compact model that relates the current density j with the free charge-carrier density at the interface pf(0): pf(0) = K1jm + K2, where the parameter K1 depends on the barrier height at the interface, m depends on the organic material and K2 controls the flat zone at low currents to include the dependence with thermal carriers and impurities.

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