Spectroscopy on polymer-fullerene photovoltaic cells

We investigate the electrical transport properties of ITO/conjugated polymer-fullerene/Al photovoltaic cells and the role of defect states with current-voltage studies, admittance spectroscopy, and electron spin resonance technique. In the temperature range 293-40 K, the characteristic step in the admittance spectrum can be observed originating from the electrically active acceptor level. The activation energy determined from the Arrhenius plot is 34 meV. The diode capacitance as a function of the reverse bias is different from the Schottky diode behavior. We found a bias independent capacitance under reverse bias. This indicates that the devices are either fully depleted, or the space charge region exceeds the device thickness. We can clearly follow the formation of photogenerated electron hole pairs under illumination of the device absorber by using the electron spin resonance technique. Important for the cell performance is that photogenerated electron-hole pairs remain in the composites even after the photoexcitation is off, implying the presence of defect induced trap states