To investigate the relationship between molecular orientation and current voltage (J-V) characteristics, molecular orientation and J-V characteristics have been simultaneously measured in some indium tin oxide (ITO)/X/Al structures. X were thin films fabricated from poly(3-hexylthiophene) (P3HT), coumarin6 dispersed in poly(N-vinylcarvazole), or biomolecular hemin (Hm). The results have shown that the orientation change of the P3HT chain causes a reproducible loop of the J-V characteristics in P3HT thin film, and that the peak observed in the J-V characteristics in Hm is associated with irreversible molecular orientation change. (C) 2008 Elsevier B.V. All rights reserved.