Effect of orientation of porphyrin single-crystal slices on optoelectronic properties

Millimeter-sized organic single-crystal slices of methylpyrroporphyrin XXI ethyl ester (MPPEE) have been prepared by capillary filling at its melting point (similar to 250 degrees C) between two parallel pieces of glass coated with indium-tin oxide (ITO) and separated by about 2-3 mu m. Crystal orientation was characterized with polarized light under an optical microscope. MPPEE single-crystal slices with different orientations appeared as dramatically different colors between two crossed polarizers. The relationship between crystal orientation and optoelectronic properties was investigated by monitoring the photocurrent flow through ITO/MPPEE/ITO symmetrical sandwich cells. The short-circuit photocurrent strongly depended on crystal orientation, varying by more than 1 order of magnitude. When a light spot (similar to 150 mu m in diameter) was scanned across adjacent MPPEE single-crystal domains with different orientations, short-circuit photocurrents generated at different spots within the same crystal domain were essentially constant, but they changed substantially among different domains. The photocurrent changed abruptly at domain boundaries.