Effects of thermal annealing on polymer photovoltaic cells with buffer layers and in situ formation of interfacial layer for enhancing power conversion efficiency

We have investigated the effects of thermal annealing before and after cathode deposition on poly(3-hexylthiophene) (P3HT)/[6,6]–phenyl C61-butyric acid methyl ester (PCBM) blend photovoltaic cells with different cathode buffer layers. The introduction of cathode buffer layer such as lithium fluoride (LiF) and calcium oxide (CaO) in pre-annealing cells can increase the open-circuit voltage (Voc) and the power conversion efficiency (PCE). Post thermal annealing after cathode deposition further enhanced the PCE of the cells with LiF/Al cathode, whereas the performance of the cell with CaO/Al cathode is decreased. It is found that the Voc and PCE can be largely enhanced when the post-annealed cell with Ca/Al cathode is exposed in air. This may attribute to the formation of CaO buffer layer, which favors increasing shunt resistance and Voc. Finally, a PCE of 4.60% is calculated under AM1.5G solar illumination at 100 mW/cm2.