Cathode deposition, paramagnetic defect formation and performance degradation in polymer-fullerene solar cells

Although changes in morphology and defect distribution at the cathode/active layer interface are presumed to limit the efficiency of sputtered cathodes for organic solar cells, little information is available on the nature of defects involved and on possible strategies to mitigate these effects. We demonstrate the sputter-induced formation of paramagnetic centers associated to the change in conformation and amorphization of P3HT in the proximity of the cathodes, which strongly affects the performance of polymer-fullerene bulk heterojunction photovoltaics. Similarity in g-values and X-ray diffraction analysis suggest that defects produced at the cathode/organic interface by sputtering are of the same nature as those produced by thermal annealing at 150-180 °C, and that thin calcium layers thermally evaporated prior to cathode sputtering act as thermal sinks mitigating the defect formation by impinging sputtered ions.