Enhanced photocurrent in Al porphyrin Schottky barrier cell with heterodimer consisting of metal-free porphyrin and zinc porphyrin

An Al/porphyrin Schottky barrier cell with an equimolar mixed solid of a we ak electron-donating zinc porphyrin such as 5,10,15,20-tetra(2,5-dimethoxyp henyl)porphyrinatozinc (Zntpp(OMe)(2)) and a weak electron-accepting metal- free porphyrin such as 5,10,15-triphenyl-20-(3-pyridyl)porphyrin (H(2)pyp(3 )p) shows much larger photocurrent compared to the cells with the pure Zntp p(OMe)(2) solid and with the pure H(2)pyp(3)p solid. Relatively large value s, short-circuit photocurrent quantum yield of 18.9%, open-circuit photovol tage of 0.79 V, fill factor of 0.23, and energy conversion yield of 1.21% a re obtained for the cell with the mixed solid when illuminated by a monochr omatic light of 445 nm with light intensity of 7 mu W cm(-2). The series re sistance of the mixed solid cell is as large as that of the pure Zntpp(OMe) (2) cell; the potential gradient in the depletion layer near the Al/mixed s olid interface is smaller than that near the Al/pure Zntpp(OMe)(2) solid in terface; and the exciton diffusion lengths in the pure Zntpp(OMe)(2) solid, the pure H(2)pyp(3)p solid, and the mixed solid are almost the same. Thus, the photocurrent enhancement for the mixed solid is not explained by these factors. The enhancement is interpreted by the ease of a photoinduced intr amolecular electron transfer of a porphyrin heterodimer produced by an axia l coordination of pyridyl group in H(2)pyp(3)p to zinc ion in Zntpp(OMe)(2) in the mixed solid because the charge-separated dimer can separate to a fr ee electron and a free hole under a large electric field formed near the Al /porphyrin interface much more easily than the excited pure porphyrins.