Transient space-charge-perturbed currents of N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamineand N,N'-diphenyl-N,N'-bis (3-methylphenyl)-1,1'-biphenyl-4,4'-diamine in diode structures

摘要

The time-of-flight (TOF) experiment is the standard method to measure the charge mobility in organic semiconductors. Typically, this measurement assumes space-charge-free (SCF) transport. However, pure SCF current cannot be achieved in experiment since carriers have charges, which will cause space-charge perturbation. For a long time, the charge transport under space-charge-perturbed (SCP) conditions was not understood for organic semiconductors. Until recently, it is learned from Monte Carlo simulations that the TOF measurement under SCP conditions may lead to lower mobility. Our previous work also suggests that the influence of space-charge perturbation might be overestimated in the past. These results, if verified in experiment, might motivate us to review the requirements of the TOF measurement. Here, the transient SCP currents in organic semiconductors are studied using two commonly used hole transporting materials-N,N'-diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (NPB) and N,N'-diphenyl-N,N'-bis(3-memylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD). The results confirmed the prediction of earlier peak time, longer transit time, and lower charge mobility. Further investigations using Monte Carlo simulations indicate that the enhanced carrier dispersion and low overall carrier movement under SCP conditions are caused by the inhomogeneous electric field. For the systems we studied, it is found that at small SCP degrees, determining the transit time by the "half plateau value" has small precision loss in mobility compared with the SCF condition.