Time-of-Flight Investigation of Charge Carrier Mobilities in Oligoacene Single Crystals

Abstract

Organic semiconductors remain an active area of research due to their unique mechanical and opto-electronic properties. The charge transport properties of organic semiconductors are dependent on their molecular packing structures. A fundamental understanding of the charge transport and device physics on a microscopic scale remains a central focus of discussion. Models and theories have been based on the understanding derived from inorganic systems, but these tend not to hold for organic semiconductors. Single crystals of small conjugated oligoacenes, with high chemical purity and molecular structural order, can be model systems in the study of the relationship between molecular packing and carrier charge transport. The ability to probe intrinsic charge transport, not influenced by environmental factors or measurement techniques, plays a fundamental role in gaining a deeper understanding of the factors affecting charge transport. Time of flight (TOF) is an experimental technique used for charge carrier mobility studies that minimizes the external factors affecting charge transport. TOF also has the potential to study both bulk (vertical) and surface (lateral) charge carrier transport in organic semiconductors. This work reports the charge carrier mobility in single crystals of tetracene and rubrene using vertical and lateral field TOF (LFTOF). TOF instrumentation was designed and constructed. Room temperature vertical TOF hole mobility results in the c-direction for tetracene single crystals were acquired as a function of electric field (µc ≈ 1.3 cm2/Vs at 296 K). Bulk TOF hole charge carrier in rubrene single crystal as a function of temperature and electric field were acquired with an average value of 0.29 cm2/Vs at 296 K increasing to 0.70 cm2/Vs at 180 K and demonstrated an inverse power law temperature dependence, ‘band-like’ transport, in the c-axis direction. The use of LFTOF to study transport on the surface of single crystal organic semiconductors was demonstrated. LFTOF hole mobilities of 0.8 cm2/Vs at 296 K were in the range of reported field effect transistor mobility results. An overview of organic semiconductors and traditional transport models along with emerging transport models for organic semiconductors is presented.