Role of structural disorder in charge transport properties of cobalt phthalocyanine thin films grown by molecular-beam epitaxy

The charge transport properties of 100 nm thick cobalt phthalocyanine (CoPc) films grown on single crystal Al2O3 (0 0 0 1 oriented) and quartz substrates using molecular-beam epitaxy, have been investigated as a function of applied bias (±50 V) and temperature (300–75 K). Films grown on Al2O3 are highly ordered and exhibited non-hysteretic current–voltage (J–V) characteristics. The J–V characteristics in this case are well described by space-charge-limited conduction (SCLC) mechanism. On the other hand, films grown on quartz substrates are highly disordered and exhibited hysteretic J–V characteristics. Analyses of complex hysteretic J–V’s in disordered films indicated a transition from trap-controlled SCLC mechanism to Poole–Frankel emission with lowering temperature. Various parameters, such as, hole density, trap concentration and space charge limited motilities have been estimated. X-ray photoelectron spectroscopy data show that charge trapping centers in the films grown on quartz substrates are created by chemisorbed oxygen.