Study on the low vacuum process for polymer solar cell with $TiO_x$ interlayer

Abstract

Polymer solar cells are an attractive worldwide research target because of their low cost, light weight, mechanical flexibility, large area and good processability. The performance of polymer solar cells has been improved by using new materials and new structure in devices. Many factors influence the performance of solar cells, including the absorption spectrum of the active layer, the HOMO and LUMO level of donors and acceptors, the work functions of the electrodes, and the morphology and charge carrier mobility in the active layer. We focused on the Al electrode deposition step. Al electrode is usually deposited at high vacuum. But reaching high vacuum in evaporation chamber is accompanied with high cost and inconvenience such as expensive equipment including high performance vacuum pump and time for reaching high vacuum. So, we studied polymer solar cells with Al electrode deposited at low vacuum. It is well known that aluminum oxide is formed when aluminum is deposited at low vacuum. Because of the formation of aluminum oxide, Al film deposited at low vacuum has rough surface as compared with Al film deposited at high vacuum. The rough Al surface decreases the performance of polymer solar cells by making the interface between the active layer and the Al electrode defective. In order to overcome this problem, $TiO_x$ which is efficient electron transport material was introduced. If the $TiO_x$ layer is inserted between the active layer and the Al electrode, electron path way in the defective interface deposited at low vacuum can be maintained. By using this process, the device fabricated at low vacuum could show almost same performance as the device fabricated at high vacuum.