Electrical admittance studies of polymer photovoltaic cells

We investigate the electrical transport properties of ITO/conjugated polymer-fullerene/Al solar cells and the role of defect states with the help of admittance spectroscopy and C(omega)-V measurements in the dark. A characteristic step in the admittance spectrum can be observed in the temperature range 40-320 K, originating from the electrically active defects. The activation energy determined from an Arrhenius plot was found to be 34 meV. The position of the step does not depend on the de bias voltage which may indicate the pinning of the hole Fermi-level at the interface due to the high density of defect states. The diode capacitance as a function of the reverse bias depends strongly on the device preparation conditions and correlates with the device I-V characteristics. We found a completely bias independent capacitance under reverse bias for the cells with low-power efficiency and almost no rectification under illumination. On the other hand, the high-efficiency cells with good rectification under illumination demonstrate an increase of the capacitance with ac bias even at frequencies above 100 kHz. The former devices are found to be fully depleted, whereas in the latter, a formation of the space charge region can be assumed. (C) 2001 Elsevier Science B.V. All rights reserved.