Au/tetraphenylporphyrin (TPP)/n-type silicon (n-Si)/Al heterojunction solar cell was constructed and prepared by growing TPP film on n-Si wafer using thermal evaporation technique, the dark current-voltage and dark capacitance-voltage characteristics of the cell were measured over temperature range from 291 to 373 K. The parameters and mechanisms of conduction of heterojunction diode have been, studied, current-voltage characteristics indicated an ohmic conduction at voltages < 100 mV and at voltages > 1.3 V Space charge-limited conduction and multi-step tunneling mechanisms occur consequentially at (0.11-1.25) V The capacitance-voltage measurements showed that the diode is linearly graded junction and the width of depletion layer, impurity gradient and built in voltage were estimated. The current-voltage characteristics under illumination have also been investigated and photovoltaic properties of Au/TPP/n-Si/Al solar cell were evaluated. (c) 2005 Elsevier B.V. All rights reserved.