Electrical admittance studies of polymer photovoltaic cells

We investigate the electrical transport properties of ITO/conjugated polyme r-fullerene/Al solar cells and the role of defect states with the help of a dmittance spectroscopy and C(omega)-V measurements in the dark. A character istic step in the admittance spectrum can be observed in the temperature ra nge 40-320 K, originating from the electrically active defects. The activat ion energy determined from an Arrhenius plot was found to be 34 meV. The po sition of the step does not depend on the de bias voltage which may indicat e the pinning of the hole Fermi-level at the interface due to the high dens ity of defect states. The diode capacitance as a function of the reverse bi as depends strongly on the device preparation conditions and correlates wit h the device I-V characteristics. We found a completely bias independent ca pacitance under reverse bias for the cells with low-power efficiency and al most no rectification under illumination. On the other hand, the high-effic iency cells with good rectification under illumination demonstrate an incre ase 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 for mation of the space charge region can be assumed. (C) 2001 Elsevier Science B.V. All rights reserved.