Effect of different substituents on conducting character of azos and local states of azo/TiOPc composites

The conducting character of the azo pigments of 4,4'-2-{N-[beta-hydroxyl-gamma-(p-methoxylanilino) naphthyl]}azodiphenyl (DCRL) and 4,4'-2-{N-[beta-hydroxyl-gamma-(m-nitroanilino)naphthyl]} azodiphenyl (DCBS) is examined by surface photovoltaic spectrum (SPS) and field induced surface photovoltaic spectrum (FISPS). The different responding behaviors to external fields indicate that DCRL and DCBS are an n-type and a p-type semiconductor, respectively. Electron withdrawing (donating) groups -NO2 (-OCH3) pull (push) the electrons from (to) central azo backbone and lower (boost up) the Fermi level thereby determine its conduction character. When they are incorporated into a p-type semiconducting film of titanyl-oxo-phthalocyanine (TiOPc), both TiOPc/DCRL and TiOPc/DCBS composites show surface photovoltaic signals significantly reduced in the magnitude in comparison with that of pure TiOPc, suggesting that the built-in field between TiOPc/electrode junction is sensitive to azo compounds. With respect to TiOPc/DCRL composite (20% DCRL by wt.), in near-IR spectral region, the FISPS demonstrates symmetric responses to both positive and negative fields, which are associated with local states. However no characteristic local state responses are observed in TiOPc/DCBS composite (20% DCBS by wt.). The formation of donor/acceptor system between p-type TiOPc and n-type DCRL is a reasonable ascription to the observed phenomenon. (C) 2002 Elsevier Science B.V. All rights reserved.