HOLE AND ELECTRON-TRANSPORT IN CHLOROALUMINUM PHTHALOCYANINE THIN-FILMS

Thin films (similar to 1.2 mu M) Of chloroaluminum phthalocyanine (ClA lPc) are vacuum sublimed at sublimation rates of 200 and 1000 Angstrom /min on substrates maintained at two temperatures, T-s = 25 and 95 deg rees C. Hole mobilities are very well described by the disorder formal ism of Bassler and co-workers, As sublimation rate decreases or T-s in creases, sigma, the width of density of stares, decreases to a minimum of 0.052 eV. ''Intrinsic'' mobility, mu(00) increases concomitantly, as do measured mobilities, reaching similar to 1 x 10(-3) cm(2)/(V s). Positional disorder, Sigma, is less affected by the sublimation condi tions. Results, as discussed in the context of previous work, indicate an increased organization of molecules in slowly deposited films on h eated 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 organiza tion, from 213 to 253 K, consistent with greater coplanar overlap of P c macrocycles on cooling. Electron transport is observed in these amor phous/polycrystalline phthalocyanine thin films, without specific film treatments. Mobilities under primary vacuum and their dependencies an comparable for electrons and holes. Similar sigma and mu(00) as for h oles but larger Sigma are obtained in a given sample. A negative tempe rature 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 o rganization in the films.