Organic multi-junction solar cells processed from solution with sensitivity from ultraviolet to the near infrared

One of the limitations of present org. 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 max. 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, soln.-processed tandem solar cells have not been realized due to incompatibility of the solvents. We demonstrate a soln.-processed polymer tandem cells by stacking two single cells in series. The tandem cell consist of two bulk heterojunction subcells sepd. by a thin semitransparent electrode of gold. This middle electrode serves in three different ways; as a charge recombination center, 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 (lmax .apprx. 850 nm) with a large bandgap polymer (lmax .apprx. 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 V is achieved. The c.d. 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 org. solar cells in the near future.