Application of Surface Photovoltage Technique to the Determination of Conduction Types of Azo Pigment Films

Surface photovoltage spectrum (SPS) and field-induced surface photovoltage spectrum (FISPS) are valuable techniques for the determination of pn conduction type of organic semiconductors. Here, conduction characters of azo pigment films composed of pure 4,4‘-2-[ N −(β-hydroxyl)naphthyl]azodiphenyl ( A ) and 4,4‘-2-{ N -[β-hydroxyl-γ-( p -nitroanilino)naphthyl]}azodiphenyl ( B ) as well as A doped with I 2 vapor ( A I 2 ) and B doped with NH 3 gas ( B NH 3 ) were explored by means of these techniques. The results of the exploration show that film A has n-type character and film B p-type character. On the basis of the reverse photovoltaic properties, films A I 2 and B NH 3 were judged to be characteristic of p-type and n-type conduction, respectively. It appears that a group rich in electrons, either an electrophobic substituent (naphthanol) at the end side of the azo pigment molecule ( A ) or a dopant (NH 3 ), bestows the azo pigments with n-type character, whereas an electrophilic group, such as the substituent ( p -nitroanilino) connected to azo pigment molecules ( B ) and I 2, gives them p-type character. The phenomena are fully discussed based on the energy band modes of the Fermi levels of these materials.