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 conduc tion type of organic semiconductors. Here, conduction characters of azo pig ment films composed of pure 4,4'-2-[N-(beta-hydroxyl)naphthyl]azodiphenyl ( A) and 4,4'-2-{N-[beta-hydroxyl-gamma-(p-nitroanilino)naphthyl]}azodiphenyl (B) as well as A doped with I-2 vapor (AI(2)) and B doped with NH3 gas (BN H3) were explored by means of these techniques. The results of the explorat ion show that film A has n-type character and film B p-type character. On t he basis of the reverse photovoltaic properties, films AI2 and BNH3 were ju dged to be characteristic of p-type and n-type conduction, respectively. It appears that a group rich in electrons, either an electrophobic substituen t (naphthanol) at the end side of the azo pigment molecule (A) or a dopant (NH3), bestows the azo pigments with n-type character, whereas an electroph ilic group, such as the substituent (p-nitroanilino) connected to azo pigme nt molecules (B) and I-2, gives them p-type character. The phenomena are fu lly discussed based on the energy band modes of the Fermi levels of these m aterials.