An organic/inorganic heterojunction p-VOPc/n-Si was fabricated and its electrical properties were investigated. Temperature-dependent dark current-voltage (I-V) characteristics of the heterojunction exhibited rectification behaviour with a rectification ratio of 405 at ±1 V and room temperature. The current-voltage characteristics of the cell showed ohmic conduction at low voltages followed by a space charge-limited current (SCLC) conduction dominated by an exponential trap distribution at higher voltages. At room temperature, the series and shunt resistances were found to be approximately 1.4 and 100 kΩ, respectively. Diode ideality factor n was found to be 3.2 at room temperature and dropped to 1.9 at 363 K. Room temperature mobility of vanadyl phthalocyanine (VOPc) was extracted from the I-V characteristics in the SCLC region and was found approximately 15.5×10 −3 cm 2 V −1 s −1 . The effective barrier height, Ф B , was estimated as 0.77 eV. The effect of temperature, on various heterojunction parameters was recorded under dark conditions and at temperatures ranging from 300 to 363 K.

The opto-electronic characteristics of porphyrin-fullerene bulk heterojunction photovoltaic cells of different active layer thicknesses were studied. In order to achieve different active layer thicknesses, the photovoltaic cells were prepared by spin coating the active layer of each cell at a different spin speed. To determine the active layer thickness, average of absorption coefficients of the materials constituting the active layer was used along with the optical density. Active layer thicknesses were also measured by using surface profilometer. Atomic force microscope surface scans revealed that there was no considerable change in active layer surface roughness from 1000 to 1500 rpm. However, a decrease in average grain size with increasing spin speed was observed. Current density as a function of voltage curves at different active layer thicknesses were recorded in dark and under a simulated solar spectrum AM 1.5G (100 mW.cm-2). Incident photon-to-current conversion efficiency spectra at different active layer thicknesses were also determined. The solar cell having active layer thickness of 68 nm (spin coated at 1200 rpm) showed optimum results. The power conversion efficiency of the photovoltaic cell at this thickness was 0.24%.

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