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26 April 2006 Organic multi-junction solar cells processed from solution with sensitivity from ultraviolet to the near infrared
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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.
© (2006) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Afshin Hadipour, Bert de Boer, Jurjen Wildeman, Floris Kooistra, Jan C. Hummelen, Mathieu G. R. Turbiez, Martijn M. Wienk, Rene A. J. Janssen, and Paul W. M. Blom "Organic multi-junction solar cells processed from solution with sensitivity from ultraviolet to the near infrared", Proc. SPIE 6192, Organic Optoelectronics and Photonics II, 61920D (26 April 2006);

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