We investigate the electrical transport properties ofITO/conjugated polymer-fullerene/Al photovoltaic cells and the role ofdefect states with current-voltage studies, admittance spectroscopy, andelectron spin resonance technique. In the temperature range 293-40 K,the characteristic step in the admittance spectrum can be observedoriginating from the electrically active acceptor level. The activationenergy determined from the Arrhenius plot is 34 meV. The diodecapacitance as a function of the reverse bias is different from theSchottky diode behavior. We found a bias independent capacitance underreverse bias. This indicates that the devices are either fully depleted,or the space charge region exceeds the device thickness. We can clearlyfollow the formation of photogenerated electron hole pairs underillumination of the device absorber by using the electron spin resonancetechnique. Important for the cell performance is that photogeneratedelectron-hole pairs remain in the composites even after thephotoexcitation is off, implying the presence of defect induced trapstates
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