Effect of Cathode Metal Evaporation Rate on the Deep Trapped Hole Formation in Bulk Heterojunction Organic Solar Cells

ABSTRACTIn bulk heterojunction organic solar cells, open-circuit voltage (Voc) is mainly dependent on the lowest unoccupied molecular orbital and the highest occupied molecular orbital of the donor/acceptor polymer pair in the active layer. However, there are other factors that contribute to considerable reduction in the Voc. The active layer/cathode interface is one of these factors. Previous studies show that e-beam evaporation of the cathode metal contact forms deep interface trap holes in the active layer which increases the Voc of the solar cells. Although these studies show the effect of deeply trapped holes on the Voc, several attempts to elucidate the mechanism behind this effect revealed their subtle and elusive nature. In this work, the effect of cathode contact annealing rate on the overall efficiency is studied. Three different sets of devices were fabricated with varying cathode evaporation rates of 0.1Å/s, 1Å/s and 5Å/s. The results show that at low evaporation rates, atoms in the cathode materials lack adequate energy to form deeply trapped holes. Additionally, above a certain value, the evaporation rate does not have a significant effect on the formation of deeply trapped holes. We also demonstrate that power conversion efficiencies of the devices can be maximized by maintaining the evaporation rate within a specific range.