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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 subcells 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 (λmax ~ 850 nm) with a large bandgap
polymer (λmax ~ 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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