Matching Photocurrents of Sub‐cells in Double‐Junction Organic Solar Cells via Coupling Between Surface Plasmon Polaritons and Microcavity Modes

and processing techniques [ 4 ] has been carried out to improve the power conversion effi ciency (PCE) of organic solar cells. So far, effi ciencies of ∼ 8% for small molecule [ 5 ] and ∼ 10% for polymer devices [ 6 ] have been acheived. However, further improvements are still required for their commercial application. In particular, the open-circuit voltage (V oc ) needs to be improved for their ideal application in driving low-power electronic devices, typically to ∼ 4.2 V. [ 7 ] Tandem confi gurations, in which two or more single cells are stacked in series, have been accepted as a successful and universal strategy to increase the V oc of the devices. [ 8 ] A record of up to 7 V for six-junction solar cells has been reported. [ 9 ] However, the photocurrent of the tandem cells is usually lower than that of the single-junction cells, because less light is absorbed by the back sub-cell compared to the front one in a typical tandem device, which creates an imbalance in the photocurrent generated by the back and front sub-cells, and limits the PCE of the overall device. Therefore, matching the photocurrent of the back and front sub-cells is crucial for a high PCE of the tandem organic solar cell. Appropriate choice of materials with different bandgaps can overcome this problem to a certain degree. [ 10 ] However, this approach is diffi cult to apply to multi-junction organic solar cells, especially for small molecule-based cells, due to the limited choice of the small-molecule donor materials that have signifi cantly different absorption profi les. Careful design of the optical fi eld and photovoltaic contributions from the sub-cells is another strategy to optimize the tandem cell. Surface plasmon polariton (SPP)-induced fi eld enhancement has been believed to be a highly attractive solution to enhance optical obsorption in an organic solar cell without increasing the thickness of its active layers. [ 11 ] SPPs are described as confi ned light waves