Time-of-flight measurements in inhomogeneous electric fields

Time-of-flight (TOF) measurements are commonly accepted as one of the most reliable experimental technique to determine the mobility and density-of-states (DOS) distribution in disordered organic and inorganic materials. However, interpretation of TOF data is bound to several a priori assumptions one of which is the assumption of an essentially constant and homogeneous field distribution in the bulk of a sample. This condition can be fulfilled in a wide-band-gap intrinsic semiconductor supplied with blocking contacts. Dark charge injection from contacts and, notably, background doping immediately invalidate this assumption. In the present work we analyze the effects of inhomogeneous field distribution on the TOF photocurrent transients. In particular, it will be shown that the post-transit photocurrent analysis of the data, obtained with a steeply decreasing field, can yield an overestimated value of the carrier mobility and an apparent DOS distribution strongly deviating from the true one.