PHOTOVOLTAIC EFFECT IN SYMMETRICAL CELLS OF A LIQUID-CRYSTAL PORPHYRIN

An unusual photovoltaic effect has been observed in symmetrical cells consisting of indium-tin oxide (ITO) electrodes and a liquid crystal porphyrin (LCP, zinc octakis(β-octyloxyethyl)porphyrin). The illuminated electrode acts as a photoanode; the direction of current flow reverses upon reversal of the direction of illumination. Stable photocurrents of up to ca. 0.4 mA/cm2 have been measured under illumination with a 150-W Xe lamp (intensity ca. 150 mW/cm2). The photocurrent increases linearly with incident light intensity (Io) at all wavelengths up to Io > 1015 photons s-1 cm-2. This photovoltaic effect is interpreted as resulting from exciton dissociation at the illuminated electrode leading to the preferential photoinjection of electrons into the ITO electrode and holes into the porphyrin. This appears to be the first unambiguous example of a photovoltaic cell controlled entirely by interfacial kinetics. The predominance of the photoinjection process in these capillary-filled liquid crystal (solid-phase) films, and its relative absence in similar cells containing evaporated films of porphyrins and phthalocyanines, is attributed to the single-crystal-like character of the LCP films. The wavelength dependence of the photocurrent (action spectrum) is a function of the cell thickness, correlating with the absorption spectrum for thin cells, while it is inversely correlated for thicker cells. This is attributed to an effective cell resistance that is determined by the number and spatial distribution of carriers photogenerated in the bulk LCP. An approximate mathematical model of these two processes, photoinjection at the interface coupled with photoconductivity in the bulk LCP, is proposed which reproduces the experimental results. © 1990 American Chemical Society.