Optical and electrical properties of semi-conducting calix[5,9]arene thin films with potential applications in organic electronics

Abstract

Indium tin oxide (ITO) substrates have been functionalized by several substituted calix[n]arene (n = 5 or 9) derivatives using spin coating to fabricate organic diode devices. The effect of rim size and side substituents has been investigated by UV–visible absorption spectrophotometry. The energy band gaps of these calixarene derivative thin films have been found in the 1.166–1.450 eV range. The electrical properties of ITO/calix[n]arene/Al diodes have been studied by current–voltage measurement showing an ohmic behaviour at low voltage. The I(V) characteristics could be modelled by a space-charge-limited current (SCLC) mechanism at high applied bias voltage. The ac electrical transport of calix[5,9]arene derivatives has been studied over a wide range of bias voltage and frequency by impedance spectroscopy. The device had been accurately modelled, for a frequency between 100 Hz and 10 MHz, by a single parallel resistor and capacitor network in series with a resistance. A dielectric relaxation time in the ms range and a transport mechanism controlled by an exponential trap distribution were deduced from the fit of the experimental data. The evolution of electrical parameters with chemical structure (rim size and substituent) has been discussed. The conductivity σ(ω) evolution with frequency and bias voltage was studied for ITO/calix[n]arene/Al devices. The dc conductivity σdc for these devices has been determined. The ac conductivity σac showed a variation in angular frequency as A.ωs with a critical exponent s < 1 suggesting a hopping conduction mechanism at high frequency and a microscopic picture of the relaxation and hopping processes has been proposed.