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

•Cathode sputtering effects the performance of organic photovoltaics.•Defects produced by sputtering and 150–180°C annealing have the same g-values.•Paramagnetic defects formation during sputtering is associated to polymer amorphization.•Defects are associated to decline in regioregularity of photoactive polymers.•To limit thermal load is vital to use sputtering as a cathode growth technique.