One of the limitations of present organic solar cells is the relatively poor spectral overlap of their absorption bands with the solar spectrum. Semiconducting polymers as poly(3-hexyl thiophene) have a bandgap higher than 2.0 eV (600 nm), thereby limiting the maximum possible absorption of the solar spectrum to about 30%. A way to overcome this limitation is a tandem solar cell where two bulk heterojunction single cells are stacked in series, each with a different bandgap. The combined absorption then covers a broader region of the solar spectrum. So far, solution-processed tandem solar cells have not been realized due to incompatibility of the solvents. We demonstrate a solution-processed polymer tandem cells by stacking two single cells in series. The tandem cell consist of two bulk heterojunction sub cells separated by a thin semitransparent electrode of gold. This middle electrode serves in three different ways; as a charge recombination centre, as a protecting layer for first cell during spin coating of the second cell, and as a semitransparent layer that creates optical cavities, which allows tuning of the optical transmission through the first (bottom) cell to optimize the optical absorption of the second (top) cell. To cover a broader region of the solar spectrum we combined a small bandgap polymer (λ_(max) ～ 850 nm) with a large bandgap polymer (λ_(max) ～ 550 nm). These sub cells are electronically coupled in series, which leads to an open-circuit voltage that equals the sum of each sub cell. A high open-circuit voltage of 1.4 Volt is achieved. The current density of the tandem cell follows the current of the top cell, which has a lower, limiting current. The tandem architecture and proper materials give us the possibility to cover a very broad spectral range of the solar spectrum to make highly efficient organic solar cells in the near future.
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