The photo-induced electron transfer processes were investigated in flavin-porphyrin hetero-type Langmuir-Blodgett (LB) films to clarify the photoelectric properties of metal-insulator-metal (MIM) devices composed of LB films sandwiched within aluminium electrodes. A hetero-type MIM device with a flavin and porphyrin molecular heterojunction (MHJ) showed highly efficient photovoltaic effects and high photoconductivity. In contrast, a homo-type MIM device with flavin and porphyrin LB films exhibited low photovoltaic effects and a short-circuit photocurrent density less than a tenth of that of the MHJ device. Furthermore, the transient photocurrent of the MHJ device showed that the time constant for the charge separation (CS) process in the MHJ device was of a sub-nanosecond order. This was more than two orders of magnitude shorter than the time constant for the CS process in the flavin homo-type MIM device. We concluded that the highly efficient photoelectric properties of the MHJ device were mainly attributable to the fast CS process from the photo-excited flavin to the porphyrin at the MHJ. Considering the long distance between the flavin and porphyrin at the MHJ and the moderate free energy difference, the mechanism underlying the fast CS process might be based on the quantum electron tunneling. (C) 2003 Elsevier B.V. All rights reserved.