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The effects of oxygen doping on the charge transport and photovoltaic properties of ␣-nickel phthalocyanine ͑␣-NiPc͒ based devices are investigated using in situ and ex situ I -V measurements. I -V characteristics for devices employing gold contacts indicate ohmic conduction at low voltages, followed by space-charge-limited conduction in higher fields. Upon exposure of NiPc to dry air an increase in the hole concentration (p 0 ) from 8.5ϫ1010 to 2.6ϫ10 15 m Ϫ3 is observed. When the top gold ohmic cathode is replaced by lead, Schottky type behavior is evident with the junction exhibiting photovoltaic effect. The energy conversion efficiency of the cell increases following exposure to oxygen. These results suggest that fabrication of air stable electronic devices based on NiPc is feasible.

Sandwich structures of Au/NiPc/Au were fabricated by sequential vacuum thermal sublimation onto borosilicate glass substrates. dc electrical measurements were performed for several NiPc thin films of different thickness in the range (d 0:25±1.09 mm). The relative permittivity was measured at a low frequency of 100 Hz, yielding a value of e r 2:74. This value is considerably lower in comparison with previously published data for various phthalocyanine materials. Current density±voltage characteristics at room temperature reveal an Ohmic conduction in the lower voltage range, followed by SCLC in the higher range, where the latter was found to be controlled by an exponential trap distribution. The plot of current density versus thickness provides further evidence of this effect. The results are analyzed in terms of exponential trap distribution yielding the following values: The temperature parameter characterizing the exponential trap distribution T t 1165 K, the concentration of thermally generated holes p 0 1:40 Â 10 16 m ± ±3 , trap concentration per unit energy range P 0 4:71 Â 10 43 J ± ±1 m ± ±3 and total concentration of traps N t e 7:57 Â 10 23 m ± ±3 .

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Sandwich configurations consisting of hole-only and electron-only devices are employed to study the transport properties of polymer/fullerene blended material. Upon thermal annealing at 150 °C and subsequent device characterisations a direct correlation between solar cell device power conversion efficiency and charge carrier mobilities is identified. The highest power conversion efficiency reported herein resulted from the electron to hole mobility ratio of 4.78. This ratio further reduces to 0.357 at 175 °C as a result of the deterioration in electron mobility at the same time as enhancement in hole carrier mobility with no further improvement in PCE. Effects of thermal annealing on trap density, reveals that the trap assisted recombination is more dominant as compared to the bimolecular recombination. Furthermore such a mechanism enhances as a result of increasing trap density at higher annealing temperatures. Effect of light intensity on the open circuit voltage VOC further verifies that the dominant recombination process is through trap assisted recombination.

Subject classification: 73.40.Sx; S12Multilayer sandwich structures of Au/NiPc/Pb were fabricated in-situ utilising a sequential deposition technique. Electrical measurements were performed on both in-situ and oxygen-doped samples. Under forward bias conditions, at low voltages, Ohmic conduction, and at higher voltages SCLC were identified. However, in the reverse bias, a transition from electrode limited to bulk limited conduction process was evident. Depletion region width as well as the potential barrier height (j b ) at the NiPc/Pb interface were calculated from the reverse J-V characteristics yielding values of 183 nm and 1.03 eV, respectively. After exposure to dry air a strong rectifying effect was observed. The latter is suggested to be associated with the change in the work function of NiPc as a result of oxygen adsorption. The potential barrier height for oxygen-doped samples was calculated yielding a value in the range of 0.955-0.96 eV. Hole and trap parameters, for both in-situ and oxygen-doped sample devices were also evaluated. Derived values suggested that trap concentration associated with higher voltage characteristic is significantly higher for the oxygen-doped sample. This type of behaviour is strongly believed to be due to an oxidisation process occurring near the NiPc/Pb interface.

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