Spectroscopy on polymer-fullerene composites and photovoltaic cells

We investigate the electrical transport properties of ITO/conjugated polymer-fullerene/Al photovoltaic cells and the role of defect states with the help of admittance spectroscopy and magnetic resonance technique. In the temperature range 293-40K, a characteristic step in the admittance spectrum can be observed originating from the electrically active acceptor level. The activation energy determined from an Arrhenius plot is 34meV, The diode capacitance as a function of the reverse bias is different from the Schottky diode behaviour. 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. An efficient generation of charge carriers and their fate in these systems have been studied by electron spin resonance (ESR). We can clearly follow the formation of photo-generated electron-hole pairs under illumination of the device absorber. Using high-frequency (95 GHz) light induced ESR it was possible to separate these two contributions to the spectrum on the basis of their g-parameters, and to resolve the g-anisotropy of the radicals. The P+-polaron possesses an axial symmetry whereas for C-60 a lower, rhombic symmetry was observed. Important for the cell performance is that photo-generated electron-hole pairs remain in the composites even after the photo-excitation has been switched off, implying the presence of defect-induced trap states.