Hole Mobilities in Trivalent Metal Phthalocyanine Thin Films. 2. Anomalous Mobility Temperature Dependence in the Low-Temperature Region 213−93 K for Chloroaluminum, Chlorogallium, and Chloroindium Phthalocyanine Thin Films

In the first paper of the titled two-part study, the temperature and field dependencies of hole mobility in an upper temperature range of T m = 333−213 K for ClAlPc thin films were determined as a function of structural organization using the time-of-flight technique. The present paper reports the results of mobility measurements on extending the measurement temperature range down to ∼90 K in chloroaluminum (ClAlPc), chlorogallium (ClGaPc), and chloroindium phthalocyanine (ClInPc) thin films vacuum sublimed on substrates maintained at either T s = 23 or 90 °C. The mobility behavior undergoes an exceptional reversal in temperature dependence at low temperatures, beginning to rise on further cooling below ∼200 K. The reversal is systematically studied, with mobility measurements as a function of temperature ( T m = 295−93 K) and of field ( E = 134−302 kV/cm) at all temperatures and is shown to be widely reproducible. Mobilities at 93 K equaled or surpassed their values at room temperature, with indicative high-field ( E = 222 kV/cm) values of 7.9 × 10 -5, 7.4 × 10 -5, and 7.25 × 10 -5 V/cm 2 for ClAlPc, ClGaPc, and ClInPc, respectively. The effect is reversible, with a hysteresis of several hours. The photocurrent transients themselves undergo a reversal in trends associated with temperature, their temporal profiles becoming comparable in the low-temperature region 193−93 K with those observed at the higher temperatures. A color change with temperature is correlated to the mobility reversal. High-energy broadening in the absorbance spectra for all films combines with the mobility reversal to indicate a structural reorganization on lowering temperatures that causes greater overlap of the phthalocyanine macrocycles. Trends in mobility dependencies are shown to be consistent with the proposed increase in molecular orbital interactions. The temperature-induced nature of the reorganization is seen to preclude rigorous treatment of the mobility depedencies in the low-temperature range according to the mathematical formalism of disorder due to Bässler and co-workers. Qualitatively, however, the behavior of holes in ClAlPc-type materials remains explainable in terms of activated charge transport in the entire temperature range. Reversal of the mobility temperature dependence is not observed at low temperature for copper phthalocyanine.