Hybrid organic-inorganic heterojunction solar cell, Au/tetraphenylporphyrin (TPP)/p-Si/Al, was fabricated. The TPP films were deposited by thermal evaporation technique onto p-type silicon single crystal wafer. The current-voltage characteristics of the heterojunction diode have been studied at a temperature range of 298 - 390 K and the voltage applied during measurements varied from -1.5 to 2 V. The device showed a rectification behavior like a diode under different temperatures. It was found that the conduction mechanisms of the diode are controlled by the thermionic emission at forward voltage bias <= 0.5 V and the single trap level space charge limited conduction (SCLC) mechanism at forward voltage bias >0.5 V. Dependence of the I-V characteristics on temperature, illumination and X-ray irradiation dose of 50 kGy for such a device have been studied. The dependence of photovoltaic parameters on annealing temperatures, illumination conditions and irradiation dose has been estimated. The calculated parameters are: series and shunt resistances, ideality factor, barrier potential, open-circuit voltage, short-circuit current, fill factor and efficiency. (C) 2014 Elsevier Ltd. All rights reserved.