Hole and Electron Transport in Chloroaluminum Phthalocyanine Thin Films

Abstract

Thin films (∼1.2 μm) of chloroaluminum phthalocyanine (ClAlPc) are vacuum sublimed at sublimation rates of 200 and 1000 Å/min on substrates maintained at two temperatures, Ts = 25 and 95 °C. Hole mobilities are very well described by the disorder formalism of Bässler and co-workers. As sublimation rate decreases or Ts increases, σ, the width of density of states, decreases to a minimum of 0.052 eV. “Intrinsic” mobility, μ00, increases concomitantly, as do measured mobilities, reaching ∼1 × 10-3 cm2/(V s). Positional disorder, Σ, is less affected by the sublimation conditions. Results, as discussed in the context of previous work, indicate an increased organization of molecules in slowly deposited films on heated substrates. An anomalous increase of mobilities, as temperature decreases below a reversal temperature, is confirmed for all samples. The reversal temperature itself increases with increased film organization, from 213 to 253 K, consistent with greater coplanar overlap of Pc macrocycles on cooling. Electron transport is observed in these amorphous/polycrystalline phthalocyanine thin films, without specific film treatments. Mobilities under primary vacuum and their dependencies are comparable for electrons and holes. Similar σ and μ00 as for holes but larger Σ are obtained in a given sample. A negative temperature dependence of the mobilities is also observed for electrons at low temperatures. Measurements of electron mobilities in ClAlPc films are impeded by the presence of air and favored by a better molecular organization in the films.