Effects of iodine doping on small molecule organic semiconductors for high charge carrier mobility and photoconductivity

Abstract

Here we report the effects of iodine doping on small molecule organic semiconductors. Thin films of semiconducting p-DTS(FBTTh2)2 doped with 1–5 wt% iodine were fabricated and their photo-physical, crystallographic, morphological, and electrical properties were systematically analyzed. The doping significantly increased the energetic distance between the highest occupied molecular orbital (HOMO) and Fermi level of p-DTS(FBTTh2)2, typical for p-type doping. In addition, depletion mode transistor measurements showed an increase in the hole concentration with increasing dopant concentration. From grazing incidence X-ray diffraction (GIXD) analyses of iodine-doped p-DTS(FBTTh2)2 films, we observed significant changes in the crystal orientation at the optimal doping ratio of 1 wt%. Atomic force microscopy (AFM) analyses showed morphological changes with respect to dopant concentrations, which were in good agreement with the GIXD results. As a result, accumulation mode transistor measurements demonstrated an increase in the hole mobility by 54% at the optimized doping concentration compared to an undoped device. Furthermore, photoconductive device operation revealed that iodine-doping can induce dramatically enhanced photo-responsivity as high as 2.08 A/W. We demonstrate that iodine doping can be a simple and effective method for enhancing the performance of small molecule-based electronic devices, by optimizing the energy level configuration as well as enhancing intermolecular interactions.