Drift-diffusion simulation of leakage currents in unintentionally doped organic semiconductors with non-uniform interfaces

Organic electronic devices frequently employ intrinsic semiconductors as active layer. The choice of different materials for the charge injecting and extracting interfaces gives rise to a finite contact potential. Injection of holes against the built-in electrical field results in a diffusion-limited exponential current–voltage characteristic at low bias. Leakage currents, however, seek paths with preferable negative built-in voltages arising from non-uniform interface properties. Along such paths holes are spilled into the intrinsic layer from the charge extracting contact whose Fermi energy is pinned in the lower half of the semiconductor’s band gap. We show here that the drift movement of holes opposed by diffusion can lead to a sublinear increase in the injected current with voltage. Charge injection is assisted by a built-in electrical field, depending mainly on the dopant density and the potential barrier at the injecting contact. Variability of these important properties can arise unintentionally, e.g., due to ambient processing, and low bias currents can serve as simple monitor. As a model system, measured current voltage characteristics of ambient processed poly(3-hexylthiophene-2,5-diyl) (P3HT) films are compared to numeric simulations solving the coupled nonlinear Poisson and drift-diffusion differential equations illustrating the basic principle.