Non-Equilibrium Charge Transport in Disordered Organic Films

Abstract

Charge transport in disordered organic layers has been intensively investigated in recent years both experimentally and theoretically. In these investigations, basic studies are needed for practical and technological developments in modern organic electronics and photonics. The concept of Gaussian disorder model (GDM) i. e. the temperatureand fieldassisted tunneling (hopping) of charge carriers between localized states (LSs), forms the background for understanding of the physical nature of transport (Bassler, 1993; Novikov et al., 1998). Energetic disorder is described by Gaussian distribution of energies of LSs. The ubiquitous experimental option for investigating charge transport is the timeofflight (TOF) experiment, the observable being the transient current in the organic layer. Excess charge carriers are generated in an organic film by light in course of TOF experiments. These carriers are not yet in quasiequilibrium shortly after their generation, notably if there is an excess energy during excitation (Bassler, 1993). This circumstance together with a strong variance of transition rate of carriers between LSs in disordered materials causes a decrease of the average mobility with time, while the spatial dispersion of carriers relative to their mean position is anomalously large, i.e. the transport occurs in nonequilibrium conditions. Usually this case is referred to dispersive transport (Bassler, 1993; Arkhipov & Bassler, 1993b), whereas at long time transport is characterized by timeindependent mobility and diffusion coefficients. The latter transport mode, referred to quasiequilibrium, or Gaussian transport, is the topic of recent works (Arkhipov et al., 2001a; Schmechel, 2002; Fishchuk et al., 2002; Pasveer et al., 2005). It is often realized in materials with moderate energetic disorder. Indeed, the TOF transients of 1 m   thick samples at room temperature bear out a well-developed plateau. This circumstance, however, does not always imply that transport is completely quasiequilibrium. An unambiguous signature of the deviation is the anomalously large dispersion of formally nondispersive TOF signals and the concomitant scaling of the tails of TOF signal as a function of sample thickness and electric field strength. Moreover, quasiequilibrium transport is questionable for the case of thin (<100 nm) organic films, suitable for organic lightemitting diodes (OLEDs), when the transit time is short enough (Blom & Vissenberg, 1998). 4