Charge Transport Investigations in Novel Triarylamine Derivatives Based Organic Devices

Abstract

Research on new organic semiconductor materials and their optoelectronic properties has been growing in prominence due to diverse potential applications. This thesis explores the charge transport properties of some novel triarylamine based organic semiconductors in hole-only devices to examine their suitability for OLED applications. Devices fabricated by physical vapor deposition are characterized using Impedance Spectroscopy as well as frequency and/or voltage dependent measurements of current density, capacitance and conductivity. The thickness dependent hole injection efficiency of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) film in devices of N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′- diamine (α-NPD) shows that the hole injection and transport properties reach a maximum for F4TCNQ film of optimum thickness of ~ 4 nm. The hole mobility of α-NPD (~ 10-5-10-4 cm2V-1s-1) is observed to increase over several orders of magnitude with its thickness and is found to be in good agreement with the literature. Following these studies, the electrical and optical properties of hole-only devices are investigated using triphenylamine (TPA) derivatives with either methyl or tert-butyl as side groups. This substitution is seen to improve charge transport properties. In addition to the above mentioned measurements, the role of side groups on the electrical and optical properties of the TPA derivatives is also investigated through UV-Vis and Photoluminescence (PL) spectroscopy. The frequency dependent analysis of charge carrier hopping rate reveals a crossover from dc to ac conductivity in the presence of a small ac bias. The new TPA molecules investigated are seen to possess hole mobility in the order of 10-7-10-6 cm2V-1s-1 and Photo Luminescence (PL) efficiency (~ 30%) which indicate their suitability for OLED applications.