Solution-processed organic tandem solar cells with embedded optical spacers

We demonstrate a solution-processed polymer tandem solar cell in which the two photoactive single cells are separated by an optical spacer. The use of an optical spacer allows for an independent optimization of both the electronic and optical properties of the tandem cell. The optical transmission window of the bottom cell is optimized to match the optical absorption of the top cell by varying the layer thickness of the optical spacer. The two bulk heterojunction subcells have complementary absorption maxima at lambda(max)similar to 850 nm for the top cell and lambda(max)similar to 550 nm for the bottom cell. The subcells are electronically coupled in series or in parallel using four electrical contacts. The series configuration leads to an open-circuit voltage of >1 V, which is equal to the sum of both subcells. The parallel configuration leads to a high short-circuit current of 92 A/m(2), which is equal to the sum of both subcells. The parallel configuration results in a much higher efficiency compared to the series configuration. (C) 2007 American Institute of Physics.