Charge transport in thin films of molecular semiconductors as investigated by measurements of thermoelectric power and electrical conductivity

Abstract

Thin films (~ 100 nm) of molecular semiconductors were prepared by physical vapour deposition on quartz substrates equipped with gold electrodes that allowed simultaneous measurement of the electrical conductivity (specific conductivity σ) under an applied electric field and of the thermoelectric power (Seebeck coefficient S) under an applied temperature gradient. The measurements were performed in vacuo and during exposure to molecular oxygen. The thin film samples were chosen as representatives of three characteristic groups of molecular semiconducting materials: an unsubstituted phthalocyanine (phthalo-cyaninatozinc(II) (PcZn)), a modified phthalocyanine with an electron-withdrawing ligand (tetrapyrido(2,3-b;2′3′-g;2″3″-1;2‴3‴-q) tetra-azaporphyrinatozinc(II) (TPyTAPZn)) and a perylene pigment (N,N′-dimethyl-3,4,9,10-perylenetetracar☐ylic acid di-imide (MePTCDI)). The p-type character of PcZn as reported from a number of studies is confirmed whereas the films of TPyTAPZn and MePTCDI showed n-type behaviour. These results are inferred from the sign of S as well as from the influence of oxygen on σ. Both S and σ of the three materials were studied under variation of the film temperature. Activation energies for S and for σ are derived and lead to a discussion of charge carrier generation and charge carrier transport in each film. It is concluded that both a delocalized charge carrier transport as well as a thermally activated hopping mechanism have to be considered to discuss the electrical properties of the samples.