Thermal annealing dependence of charge injection and transport in the P3HT:graphene nanocomposite based devices

Abstract

The present study explores the effect of annealing temperature on the charge injection and transport properties of the devices composed of the active layer of poly (3-hexylthiophene) (P3HT):graphene(G) nanocomposites. Changes in the molecular ordering of the P3HT domain induced by the thermal annealing were examined through UV–visible spectroscopy, Raman spectroscopy, and X-ray diffraction studies. The change in the surface morphology upon annealing was observed by field emission scanning electron microscope. Increase in annealing temperature promotes molecular ordering, reduce trap density (from 3.30 × 1016 cm−3for as-cast to 4.06 × 1015 cm−3 after annealing at 200 °C) and trap width (from 0.32 eV to 0.09 eV), resulting in a significant improvement in the mobility (from 4.82 × 10−7 cm2 V−1 s−1 to 1.85 × 10−4 cm2 V−1 s−1). However, junction parameters and diode behavior only enhanced up to annealing temperature of 80 °C. Further increase in annealing temperature leads to a significant increase in the leakage current, and consequently degrades the diode performance. Furthermore, under forward bias, thermionic emission governs the injection mechanism for an as-cast device and the device annealed at 80 °C, while Fowler Nordheim (FN) tunneling controls charge injection at the higher annealing temperature. Under reverse bias, a substantial reduction in tunneling barrier height at high annealing temperature facilitates large hole injection, giving rise to huge leakage current that deteriorates device performance.