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 it s 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 und er an optical microscope. MPPEE single-crystal slices with different o rientations appeared as dramatically different colors between two cros sed polarizers. The relationship between crystal orientation and optoe lectronic properties was investigated by monitoring the photocurrent f low through ITO/MPPEE/ITO symmetrical sandwich cells. The short-circui t photocurrent strongly depended on crystal orientation, varying by mo re than 1 order of magnitude. When a light spot (similar to 150 mu m i n diameter) was scanned across adjacent MPPEE single-crystal domains w ith different orientations, short-circuit photocurrents generated at d ifferent spots within the same crystal domain were essentially constan t, but they changed substantially among different domains. The photocu rrent changed abruptly at domain boundaries.