Effect of film and device annealing in polymer:polymer solar cells with a LiF nanolayer

We report the effect of thermal annealing on the performance of polymer:polymer solar cells with a lithium fluoride (LiF) nanolayer inserted between active layer and electron-collecting electrode. The active layer was prepared using blend films of regioregular poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT). Film annealing and device annealing were separately performed at 150 °C for 30 min to understand the influence of the existence of LiF nanolayer during thermal annealing. Results showed that both film and device annealing did considerably improve the power conversion efficiency (PCE) of P3HT:F8BT solar cells though the PCE was higher in the case of device annealing. The surface analysis suggested that the improved device performance by thermal annealing is attributed partly to the formation of planar p–n junction structure in the P3HT:F8BT blend film during thermal annealing.