Charge Transport Study of 2,2',7,7'-Tetrakis(N,N-di-4-methoxyphenyl amino)-9,9'-spirobifluorene Using Impedance Spectroscopy

The conduction mechanism in 2,2',7,7'-tetrakis(N,N-di-4-methoxyphenyl amino)-9,9'-spirobifluoren (spiro-MeOTAD) has been studied by impedance spectroscopy (IS) by using indium tin oxide (ITO)/spiro-MeOTAD/gold (Au) hole only device. The device could be modelled by an equivalent parallel RC network with a low contact resistance R_s in series, in which the bulk geometrical capacitance above resonance frequency of spiro-MeOTAD remains unchanged while the bulk resistance R_p changes with bias voltage. Upon increasing the bias voltage the conductivity of deposited film has increased by one order of magnitude (i.e., from ∼10^-9 to ∼10^-8 Ω^-1 cm^-1) whereas the dielectric relaxation time (τ) decreased from 1.7 ×10^-4 to 1.2 ×10^-5 s. Capacitance spectra of hole-only spiro-MeOTAD devices indicate the validity of the Space charge limited current (SCLC) model with localised states distributed in energy and field-dependent mobility.