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

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 sigma(omega) evolution with frequency and bias voltage was studied for ITO/calix[n]arene/Al devices. The dc conductivity sigma(dc) for these devices has been determined. The ac conductivity sigma(ac) showed a variation in angular frequency as A.omega(s) 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.