Exploiting diffusion currents at Ohmic contacts for trap characterization in organic semiconductors

• We investigate diffusion at Ohmic contacts using drift–diffusion simulations.
• For exponential traps, we find significant deviations from J∼Vl+1J∼Vl+1.
• The power law exponent changes with intrinsic layer thickness.
• This change is characteristic for the trap parameters.
• Trap parameters can be determined from experimental J–V curves.

Studying space-charge limited currents enables fundamental insight into the properties of charge carrier transport. However, in unipolar devices with Ohmic contacts, diffusion of charge carriers from the contacts into the intrinsic layer can dominate the current–voltage (J–V  ) characteristics, especially when the devices are thin as in organic electronic devices (∼∼100 nm). Thus, the common approximation of drift-only trap-limited currents (J∼Vl+1J∼Vl+1) caused by an exponential distribution of traps is not applicable for determination of the trap distribution. Here, we show by numerical drift–diffusion simulations of unipolar devices with p-doped injection layers (p–i–p devices), how diffusion currents affect the J–V power law depending on the intrinsic layer thickness for typical transport parameters of organic semiconductors. As the thickness dependence of the power law is characteristic of the trap distribution, the distribution can be determined from a simple variation of the device thickness.

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