We suggest a recipe on how to determine the density of states (DOS) in disordered organic semiconductors from the measured dependence of the charge carrier mobility on the concentration of carriers n. The recipe is based on a theory for the concentration-dependent mobility. As an example, we apply our theoretical results to experimental data obtained on two polymers and show that from the class of trial DOS functions g(ε)∝exp{-(ε/σ)(p)}, only those with p>1.8 can explain the experimental results. In particular, we claim that the concentration-independent mobility at low n evidences that the DOS cannot be purely exponential, which is in contrast to numerous recent assumptions in the literature.

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The concept of the transport energy (TE) has proven to be one of the most powerful theoretical approaches to describe charge transport in organic semiconductors. In the recent paper L. Li, G. Meller, and H. Kosina [Appl. Phys. Lett. 92, 013307 (2008)] have studied the effect of the partially filled localized states on the position of the TE level. We show that the position of the TE is essentially different to the one suggested by L. Li, G. Meller, and H. Kosina [Appl. Phys. Lett. 92, 013307 (2008)] We further modify the standard TE approach taking into account the percolation nature of the transport path. Our calculations show that the TE becomes dependent on the concentration of charge carriers n at much higher n values than those, at which the carrier mobility already strongly depends on n. Hence the calculations of the concentration-dependent carrier mobility cannot be performed within the approach, in which only the concentration dependence of the TE is taken into account.