Electrode Considerations for the Optical Enhancement of Organic Bulk Heterojunction Solar Cells

In this paper, we consider the optical effects in conventional and inverted bulk heterojunction organic solar cells associated with various electrodes, and perform a systematic study on the anode and cathode buffer layers commonly used in high performance devices. In the devices produced here, we determine that parasitic absorption by low work function metals such as calcium reduces photocurrent by 25%, and that parasitic reflection at interfaces between the transparent conducting oxide and metal oxide buffer layers can reduce photocurrent by more than 10%. We also quantify the impact of an optical spacer, and determine that this accounts for only a fraction of the improvement that can be gained through alternative electrode optimization routes. It is therefore our intention that this study serves as a guideline for the optimization of the electrodes of organic thin film photovoltaic devices. The optical enhancement in conventional and inverted bulk heterojunction organic solar cells is systematically studied with respect to the anode and cathode buffer layers (BL). By removing parasitic absorption caused by low work function metals such as calcium, the photocurrent is increased by 25% leading to an improved efficiency of 3.9% for structures without calcium as a BL compared to reference cell performance of 3.2% using calcium as an electron accepting BL. Furthermore, the impact of the optical properties of transition metal oxides on device performance is assessed.