Hopping Model of Charge-Carrier Transport in Organic Nanoparticle Systems

Abstract

In spite of a large amount of work having been done on the description of charge-carrier transport in organic materials for last decades, the processes that determine charge transport in realistic organic electronic devices are still not completely understood, but their comprehension is definitely the key for designing materials with improved properties and, thereby, for a further increase in the performance of the devices. In this review, we will present an overview of the current achievements regarding the theoretical description of the charge transport in disordered organic semiconductors with emphasis on charge transport behaviors at large carrier concentrations as realized in organic field-effect transistors (OFETs). A particular focus is given to the effective medium approximation (EMA) analytical method, which was applied to describe the carrier concentration-, electric field-, and temperature-dependent charge transport in organic materials that are used as active layers in OFET devices. In particular, we show that the establishment of the apparent Meyer-Neldel rule (MNR) is a characteristic signature of hopping charge transport in a random system with variable carrier concentration irrespective of their polaronic character. The EMA model provides compact analytical relations which can be readily used for the evaluation of energetic disorder parameter in organic semiconductor layers from experimentally accessible data on temperature-dependent mobility in OFET devices. It was also found that in multiple-grain organic films very strong local electric fields can be generated at grain boundaries (GB), resulting in the electric field-dependent OFET mobility at low (average) lateral electric field in the transistor channel. The EMA theory is found to be in good agreement with previous computer simulation results and has been applied to describe recent experimental measurements of the temperature-dependent electron mobility in a C60-based OFET for different carrier concentrations and different lateral (source-drain) electric fields. Finally, we compare our theory with alternative models suggested previously to explain the MNR behavior for charge transport in organic semiconductors.