Spectroscopy on polymer-fullerene composites and photovoltaic cells

We investigate the electrical transport properties of ITO/conjugated polyme r-fullerene/Al photovoltaic cells and the role of defect states with the he lp of admittance spectroscopy and magnetic resonance technique. In the temp erature range 293-40K, a characteristic step in the admittance spectrum can be observed originating from the electrically active acceptor level. The a ctivation energy determined from an Arrhenius plot is 34meV, The diode capa citance as a function of the reverse bias is different from the Schottky di ode behaviour. We found a bias independent capacitance under reverse bias. This indicates that the devices are either fully depleted, or the space cha rge 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 el ectron-hole pairs under illumination of the device absorber. Using high-fre quency (95 GHz) light induced ESR it was possible to separate these two con tributions to the spectrum on the basis of their g-parameters, and to resol ve the g-anisotropy of the radicals. The P+-polaron possesses an axial symm etry 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, impl ying the presence of defect-induced trap states.