PHOTOGENERATION OF CHARGE-CARRIERS IN ANISOTROPIC MULTILAYER STRUCTURES OF PHTHALOCYANINATO-POLYSILOXANE

The photoinduced generation and the transport of charge carriers have been investigated in sandwich structures of Langmuir-Blogdett films of phthalocyaninato-polysiloxane between aluminum and gold electrodes. T he diodelike current-voltage characteristics as well as the photovolta ic properties are directly related to the asymmetric electrode configu ration. Under short-circuit conditions the photoactive region is locat ed close to the aluminum electrode. By applying an external bias, the photoactive region is enlarged throughout the whole film thickness, an d collection efficiencies of up to 0.3% are observed. Recombination ef fects have been studied by investigating the intensity dependence of t he photocurrent. Nearly square root behavior under short circuit condi tions is due to efficient charge carrier recombination whereas a linea r intensity dependence is observed under reverse bias, indicating char ge carrier diffusion to the electrodes. The transition between these t wo regimes is analyzed with respect to the charge carrier mobility yie lding a value as small as 10(-11) cm(2)/V s. The anisotrope molecular orientation in the film causes a photoconductivity clearly depending o n the polarization of the incident light. The formation of a microcavi ty in these structures strongly influences the spectral properties and it allows, in agreement with theoretical calculations, to obtain an i mproved efficiency in a selected region of low film absorption. Atmosp heric oxygen doping results in clear increase in the dark current, but the charge generation process remains nearly unaffected. (C) 1998 Ame rican Institute of Physics. [S0021-8979(98)05718-1]