Narrow Band Tails in Organic Semiconductor Crystals

In order to study the electronic states in the bulk of high quality organic semiconductor crystals, we have applied the technique of temperature-dependent SCLC measurements. The result is a quantitative density of states curve extending from typically 0.5 eV to (0.05--0.1) eV above the valence band mobility edge. We have studied dozens of crystals of pentacene, rubrene and related compounds, and find in the purest samples a very low DOS in the range of $10^{15}$cm$^{-3}$eV$^{-1}$ and a characteristic exponential distribution parameter of $\sim$180 meV. Close to the mobility edge, a rapidly rising DOS is found with a narrow typical energy of $\sim$10 meV. Extrapolated to the mobility edge, these curves meet the DOS derived from electronic structure calculations. The band tails are significantly narrower than in amorphous Si:H, or derived for pentacene in previous measurements, where surface or interface effects might play a significant role.